After spending the past week in southern Maine with family, it's a bit shocking to return to the DC metro region. I was actually enjoying the cooler temperatures and scattered snow showers New England is so famous for.

I awoke this morning bright and early (around 6 0'clock) to catch a mid morning flight out of PWM, and was able to snatch a gorgeous sunrise characterized by firery red and orange hues, and brings to mind the old adage:

"Red sky at night, sailors delight, Red sky in morning, sailors take warning."

As the sun rises in the east, its rays strike suspended droplets of water, ice, and supercooled water droplets to the west, implying a storm system may be in the works as the clouds move from west to east. And lo and behold, the forecast out of Portland, Maine calls for increasing cloudiness and off-and-on chances for snow through Tuesday.

The "Welcome to 2006" weather forecast:
As we close out an extraordinarily active weather year, the forecast around the Mid Atlantic remains fairly placid as low pressure in the Upper Midwest begins to transfer energy off the East Coast later tonight. A nice little snowstorm will set up for the folks in the Northeast as a second area of low pressure develops a couple hundred miles east of Martha's Vineyard.

We will be left with decreasing cloudiness-at least east of the mountains-which should allow overnight temperatures to drop into the lower 30s. Clouds will hang tough a bit longer on the western slopes of the Appalachians as winds needle in from the west until tomorrow morning as a surface high slides in from the south.

The cloud forecast for Time Square, NY is tricky. Most indications point towards an overcast sky to ring in the New Year, but any showers should be isolated and sporadic. So the festivities in Manhattan should go on sans complications from Mother Nature.

An Historic Tropical season:
And it still keeps on rolling along, even though hurricane season ended over a month ago. Tropical Storm Zeta has been spinning about in the southeastern Atlantic Ocean, a couple hundred miles from the Cape Verde Islands.

Temperatures today made it into the lower 50s under mainly sunny skies and calm winds. This is the first time in about 20 days that DC eclipsed the 50 degree mark.

I haven't been posting too much (not enough in my opinion), and part of the reason is this dull weather pattern we're in.

The upper level wind pattern (around 15-20,000 feet or so) has remained more or less zonal, or a west to east flow of the winds, which promotes a stagnant weather setup across the eastern half of the US, as very little energy is moved across the Continent.

The image at right is a depiction of the upper level wind pattern (left half) and reveals an almost purely west to east progression of the steering currents.

However, things are about to change as some energy ejects from southwestern Canada, and dives headlong into the Midwest tomorrow. This energy should begin to carve out a pretty significant trough in the mid levels of the atmosphere, and will send a low pressure system our way for Christmas. At this point, precipitation looks to remain all rain for the metro region, with some mixing possible farther north and west of the city early Monday morning.

Temperatures through next week will remain above average (44 degrees in Washington) as highs soar into the lower to mid 50s.

Something to Watch next week:
While significant timing differences exist between numerical weather models, it is becoming apparent that something has "got to give" in this weather pattern very soon. Long range guidance suggests that we are heading into a pattern change/re-load to start the 2006 season, and we can simply not make pattern reversals from 50-60 degree weather, to below normal temps without some atmospheric bombshell affecting the United States.

Models are hinting that this bombshell is developing along the eastern portion of Asia as we speak in the form of a monster trough (and associated below normal temperatures). Both the European Model (ECMWF) and Global Forecast System (GFS) move this developing trough eastward, and by the end of next week, show this dip in the jet stream inducing a large surface cyclone along the Eastern Seaboard (this should automatically send the fireworks off in any Washingtonian's head).

Forecast skill at this timeframe (around 144-200 hours out) is just not there, and confidence at this point remains extremely low. The aforementioned model solutions are a big change from last night's runs, which depicted near to above normal temperatures for the Mid Atlantic, with no hint of any surface low along the coast.

At this point, December 30-31st event is pure speculation, but definitely requires some careful monitoring.

There won't be any posts for the next week until I return from my "winter vacation." For those of you traveling this Christmas, stay safe and enjoy the Holidays.

Welcome to Winter, Washington!
At 1:35 PM EST, the sun's rays hit the earth in line with the Tropic of Capricorn (which lies at 23.5 degrees South), the lowest angle of the year. This signals the shortest day of the year in the Northern Hemisphere (9 hours and 26 minutes at Reagan National Airport), but lengthening days are in the offings as the sun's rays slowly begin there move northward.

Image right: Graphic showing the Winter Solstice, as the Sun rays strike the earth exactly over the Tropic of Capricorn at 25.5 degrees South Latitude.

The weather over the next several days looks to be relatively uneventful as high pressure mingles over the area. High temperatures will begin a return to above-normal levels by the end of the week as southwesterly flow at the surface draws warmer air into the region. Temperatures to close out the week will approach 50 degrees for the first time in nearly 16 days.

Outlook:
Regional Satellites and surface observations reveal mostly cloudy conditions across most of the Metro region this hour, as a lobe of upper level vorticity (or "spin" in the atmosphere) that stretches from NY to the Midwest traverses slowly eastward. I expect a majority of the clouds to dissipate later this afternoon as subsidence (sinking air) moves into the area. High temperatures will approach 40 degrees in most locations today under partly cloudy, to mostly sunny skies.

Skies will clear overnight, allowing temperatures to plummet into the upper teens and lower 20s, by sunrise. Be sure to wear a heavy jacket if venturing out tomorrow morning.

A Snowy Christmas?
With Winter officially beginning in two days, thoughts of a White Christmas are prominent among many Washingtonian's. The "official" definition of a White Christmas as defined by the National Weather Service is: "1 inch of snow on the ground during December 25th".

To the right is a graphic (pretty self-explanatory) displaying the probability of at least 1 inch of snow on the ground Christmas Day. The Washington, DC metropolitan region generally does not see a true "white Christmas," but it can happen.

By next Saturday, a developing Clipper Low diving southeastward out of Canada will begin to spread rain (and possibly snow showers) across the metro region. At this point, surface temperatures should remain far too high to support any snowfall accumulation in the Mid Atlantic Region (sorry snow lover's). No White Christmas this year.

An Arctic Re-load in our Future:
Over the past 24 hours, computer models have pumped out colder solutions the close out 2005. A deep trough looks like it will develop by December 27-28th, accompanied by colder conditions, and a stormier weather pattern. Now there's something to look forward to...

Weather Outlook:
The low pressure system discussed below has decided to take the low-road, and has been surpressed to the southeast of the Metro region. A few showers linger around the Norfolk area at this time, but should rapidly translate eastward over the next couple of hours. As for us, visible satellite images indicate generally sunny conditions across the region, with a few clouds hanging over portions of Lower Southern MD.

We should be left with a fine close to the weekend as highs soar into the mid and upper 40s.

The Effects of the Coriolis Force:
I'm sure many of you have either heard or learned about this phenomenon in elementary or middle school--the apparent shifting of objects in the Northern hemisphere to the right, and visa versa in the south. But most science teachers cannot explain the full concept of this driving mechanism behind the most terrifying storms on earth.

The earth spins at a considerable speed to the east, which is greatest at the Equator, where the circumference of the earth is nearly 25,000 miles. At 0 degrees latitude, the earth is spinning at a mind-bending 1,037.5 miles per hour. As you move north (or south), the rotation of the earth becomes slower, and falls to nothing at the poles.

Now, because the earth rotates in a clockwise fashion, any object not directly in contact with the surface will be moved either to the right or left depending on 1) which hemisphere it's in and 2) which direction it's moving (north/south from the equator).

If an object moves north from the equator, it will be shoved to the east as it approaches higher latitudes. Likewise, an object moving south from the North Pole will be deflected to the west as it approaches lower latitudes.

Back in the First World War, the Kaiser artillery division constructed a massive cannon named Big Bertha, capable of firing 210mm howitzer shells over 60 miles from the German front line to Paris. The German gunners were given coordinates directly over the French capital, but the shells consistently kept missing their target by nearly 1 mile to the west. Without knowing it, the German's had fallen victim to the Coriolis Force.

When: Sunday, December 19th @ 4:15
Weather Impact: Minimal
Commentary: Due to the late kickoff, temperatures will be hovering in the lower 40s under partly cloud skies. By the end of the 4th quarter, temperatures will have fallen into the mid 30s. Be sure to wear an extra jacket/gloves/hat etc. to stay warm

Short Term:
Tonight: After a treat to near-normal temperatures this afternoon, temperatures will take a nose dive into the upper 20s/lower 30s tonight under mostly cloudy skies. Even though a strong area of high pressure sits over the region, a very intense jet streak (embedded faster flow within the mean upper level jet) is screaming over our heads at 30,000 feet, spreading a dense cirrus deck into the Mid Atlantic.

Tomorrow: By tomorrow afternoon, many of the cirrus clouds should have thinned, yielding a partly cloudy afternoon with high temperatures near climatological values (mid 40s).

A Snowy Redskin's Game?
As the 'skins return to FedEx Field Sunday afternoon, Mother nature may be concocting an event to add some pizzaz to the already heated rivalry between the Redskins and Cowboys.

When: Sunday, December 18-Monday, December 19
Impact:
Confidence: Medium
Commentary: An area of low pressure will begin organizing along the western Gulf Coast Tomorrow, and track eastward into the Southeastern states by Sunday morning. Presently, the atmosphere appears like it will be cold enough to support a plain snow event for the District, changing to rain the closer to get to the Delmarva. Latest indications are we will remain on the northern edges of this low pressure system, which would limit snowfall potential to below 3 inches. The timing may turn out being perfect for some wintry scenes out of Landover this Sunday.


The map at right is a graphical depiction for Sunday's snow event. If this low were to track 100 miles farther west than currently forecast, we could be looking at a decent snow event come Sunday night/Monday. But for now, I am only expecting a minor impact from this system on the immediate metro region.

Extended Outlook:Continued Cold, or a warmer future?

Next 5 Days (Monday 19-Saturday 24)
Precipitation: Below Normal (Average is 0.5")
Temperatures: Near to slightly below average (Average High: 45/Low: 30)
The weather pattern evolving over the next week will be a stagnant one, featuring a large upper level trough over southeastern Canada, and a ridge over the West Coast. This type of atmosphere generally favors a more placid pattern with below normal precip. We may see a few flurries here and there by the end of the week as very weak upper level "wavelets" pass near the Mid Atlantic.

Following 5 Days (Sunday 25-Friday 30)
Precipitation: Near Normal
Temperatures: Slightly Below Normal (Average High: 44/Low: 29)
Come Christmas time, I am expect near to slightly below normal temperatures. Some models are hinting at another ramp-up of the southern Jet Stream, which would favor more storms sliding along the Southeastern States by the end of December. Granted, forecast confidence at this range is quite low, and there remain significant timing differences between global models.

The Power of Easterly Winds:
If you've been a Washingtonian for awhile (and by this I mean living near the city for more than 10 years), you probably have some solid background knowledge on this area's climatology. And if you're a true weather nerd, and pay extra close attention to local meteorologists during winter storms, you know that an easterly breeze off the Chesapeake Bay and Atlantic Ocean means almost certain death to wintry precipitation in and around the metro regions.

Case in point, winds have been blowing out of the east pretty much all day today--directly off the comparatively warmer waters of the Bay Atlantic Ocean. Temperatures at Reagan National Airport have risen 3 degrees in the last 3 hours, turning a pure freezing rain to a cold rain at 36 degrees.

Temperatures are expected to continue rising overnight as easterly winds persist, changing the wintry mix to a plain rain into Frederick and Loudon Counties. Areas in southern PA will see anywhere from 3-8 inches of snow and ice accumulation before this system wraps up tomorrow morning.

Nonetheless, roadways are still slick in and around the DC area, and parts of north central MD have already seen close to .5 inches of ice accumulation. Use extra caution tonight, especially on the secondary roads and overpasses.

Ice Storm Warning: It's been a long time since ICE STORM WARNINGS have been hoisted, but areas north and west of Charlottesville to Sterling to Baltimore are under the gun for significant ice accumulations between 1/4 and 1/2 inch by Friday AM.

Winter Weather Advisory: Includes the immediate metro regions for ice accumulations generally less than 1/4 inch. Remember, any ice accumulation can make for extremely hazardous travel.

Within the next 12-18 hours, a wintry melange of snow/freezing rain/and ice pellets will combine to form one heck of an early-morning commute for the Washington, DC megalopolis. Precipitation will break out across the area early tomorrow morning (before the morning commute really gets underway), and road surfaces will all be below freezing due to the excessively cold temperatures we've been experiencing.

After downplaying this event for quite some time, I'm becoming more concerned significant travel hazards could develop tomorrow for those of you venturing out on the roadways around 7-9AM.

This is, without a doubt, the hardest winter storm to predict so far this season. So many factors are coming into play, it's basically impossible to pin down accurate ice and snowfall amounts. I have tried to do justice to the many precip types expected to fall around the area. I like the NWS's handle in this event, and generally followed their Warning/Advisory lines for this forecast.

5-9AM: Snow and freezing rain break out from SW to NE across the region, accumulating quickly on power lines, secondary roads, cars, etc.

9-11AM:Most precip changes over to freezing rain.

11-2PM:Precip southeast of the .25-5" swath changes over to plain rain, but likely remains frozen north and west of town

2PM-7AM(FRI): Rain continues around the metro regions, icing north and west of the cities changes over to a rain/sleet/freezing rain mess. Precip ends from SW to NE


Snowstorm Impact: School Forecast:

Without beginning another post, this will just be an update on our present situation. Overall, I like my ongoing forecast--a very tough one at that--and my thoughts are outlined below:

For the Immediate metro region (including DC, Baltimore and points south and east to St. Mary's and Calvert Counties)

DCweather Snowstorm Timeline

7AM-10AM: Snow/sleet/freezing rain mix begins to fall
10AM-2PM: Wintry precip rapidly begins to turn over to rain as warmer air moves into the area
2PM-8AM (Fri): Rain continues to fall, heavy at times, and should melt any ice that accrued to surfaces during the morning

Storm Impact: School Forecast: (**AM Travel conditions may be a bit hazardous as ice/snow accumulates on roads/sidewalks/etc.**)

PREVIOUS DISCUSSION
Today through Wednesday:
As high pressure parked to our north in southern Canada continues to strengthen, stiff Northerly winds will funnel much colder air into the region, and high temperatures will struggle to eclipse the freezing point this afternoon-evening under mainly sunny skies. On top of this, winds gusting 10 or 15 mph are adding salt to the wound, dipping wind chills into the mid and upper teens!

Tonight will be another brutally cold period as clear skies and calm winds allow what surface warmth snuck down to the surface to radiate back into the atmosphere. Low temperatures in the far western 'burbs may fall into the lower teens (definitely some single-digits out towards Johnstown, PA), and into the mid to upper teens closer to the metro region.

Not much changes in this cold weather pattern for Wednesday--and the only noticeable difference may be a few more high level clouds streaming in from the west in association with a developing low situated in the Upper Midwest. High temperatures tomorrow will also be around 30-34 degrees.

Thoughts on Thursday's "Snowstorm"
There currently remain very minor differences between computer models regarding this week's potential wintry weather bonanza. High pressure to the north will be our supply source for cold surface temperatures, and a significant moisture train from the Gulf of Mexico will be our source for precipitation. I like the idea that precipitation breaks out across the metro region early Thursday morning in the form of snow showers. However, a tongue of warmer temperatures aloft should begin melting any snow as it falls through the layer between 1000 and 3000 feet up. Surface temperatures will likely remain below freezing through the mid morning, which would support some freezing rain/sleet which would likely begin accruing to any sub-32 degree surfaces.

The problem I am having revolves around when (and if) precip switches over to rain by noon Thursday. The NWS in Sterling seems to think most of the precip would remain a wintry variety, which would cause some significant travel problems and school cancellations around the region. Basic Climatology, however, tells me otherwise.

This early in the season, we can't really expect surface temperatures to remain below freezing with easterly surface winds, and warming mid levels. I do not presently believe this next wintry blast will cause too many problems around the immediate metro region. The wintry mix should turn over to a cold rain from DC south and east by noon, limiting problems around the metro (although there may be some travel hazards during the morning hours while snow and freezing rain falls).

Out to the north and west, things get a little dicy. Surface temps would likely support frozen precip through most of the day, and significant ice accumulations are possible north of Loudon-Montgomery-Frederick counties.
Potential Storm Impact:
-Travel problems possible during morning hours
near DC, significant icing possible northwestern MD

Above image: Schematic of a freezing rain environment (similar to that of a sleet environment) as snowflakes melt in a shallow warm layer aloft before refreezing closer to the surface as sub-freezing raindrops. These accrue to roads, telephone lines, trees, etc. causing significant problems for the affected regions

Enjoy the weather while you can because colder air is set to invade the region over the coming days. High temperatures today will manage to break the 40 degree mark under partly cloudy skies and breezy northwesterly winds ranging from 10-20 mph.

Temperature tonight take a nosedive as an impressive Canadian high pressure system traverses the Cumberland Plateau and the Appalachian Mountains. Mainly clear skies and slackening winds (and some leftover snowcover) will aid in quickly cooling down the atmosphere. Lows tomorrow morning will bottom-out in the mid to upper teens in northern MD, and near 20-23 degrees closer to the metro region.

Both Tuesday and Wednesday will offer biting cold temperatures in the lower 30s as the effects from the said Canadian high ooze into the Mid Atlantic.

More Tricky DC forecasts:
As is often the case in the Nation's Capital, the difference between a "good forecast" and a bad one can come down to a matter of miles. Due to our location near the Atlantic Ocean, and Mid-Latitudes, the rain/snow and snow/ice lines often sit right on top of the District--and turns generally easy forecasting into a ghoulish nightmare.

During the afternoon Wednesday, a low pressure system will begin organizing along the Canada/United States boarder in the upper Midwest, while a cold front develops, slicing through the center of the US. By Wednesday evening, another separate low pressure area should develop along the frontal boundary around the Gulf Coast.

At the same time, the aforementioned Canadian High Pressure cell will make itself comfortable in central Quebec. This type of set-up is nearly a classic CAD (Cold Air Damming) example-->

When: December 15th-16th
Impact:
Confidence: Low/Med
Commentary: Precip will likely overspread the region late Thursday AM as a rain/snow mixture. Most of this should change over to rain by midday as warmer surface temperatures move in. By Nightfall, however, an ice/sleet mixture may be in the works for the region as temperatures in the mid levels of the atmosphere warm above freezing. There is below normal confidence with the forecast due to the shallowness of the cold air along the eastern slopes of the Mountains. At this point, I don't see this system causing significant problems around the region--but for those of you who have lived here for awhile know 1/2 inch of snow can cause major problems around here.

After a bit of slipping and sliding on the sidewalks this weekend, high temperatures in the lower 40s (around 10 degrees shy of average) have finally melted most of the slick spots on the secondary roads and walkways.

Outlook:
Tonight, don't be surprised if you see a few snow flurries here and there, especially across northern MD as temperatures dip into the upper teens and lower 20s.

Enjoy the "mild" 40 degree weather tomorrow, because a large Canadian high pressure cell will begin to migrate eastward into the Mid Atlantic on Tuesday. High temperatures will tumble into the low to mid 30s both Tuesday and Wednesday, before moderating slightly by the end of the week.

By midday Thursday, clouds should once again thicken up in response to developing low pressure systems in the upper Midwest, and Gulf Coast. These two disturbances may bring us yet another chance for some wintry precip to close out the week (more below).

Snowstorm picture:
We weren't the only ones to receive some snow fall from last weeks storm. Sebago, Maine received a healthy dose of the white stuff (pictured at right).

And a little farther to the west, in Concord, NH, nearly 16 inches of snow fell between Friday and Saturday afternoon--what a wonderful snowstorm that would have been in the DC metro region...

When: December 15th-16th
Impact:
Confidence: Low/Med
Commentary: Models continue to advertise the potential for a small snow/freezing rain event Thursday PM into Friday. Surface temperatures Thursday night will be cold enough to support snow, but only for a few hours or less. Warmer surface temperatures should invade the region by early Friday morning, changing any precip south of the Mason-Dixon line to rain. Granted, there is some uncertainty surrounding this forecast, but at this point, I feel relatively confident this next event will only pose some "nuisance" issues to the Washington, DC metro region.

After snow finally began to trickle down onto the streets of Washington, DC early this morning, so began one tough Friday morning commute for thousands across the area. Snow rapidly changed over to sleet and freezing rain (as anticipated) creating a messy soup of wintry precip on the secondary and untreated road surfaces.

And with snow comes the ever present thought of a school day, relished by the millions of school kids around the area. As Doug Buchanan of Channel 9 News put it, we can call this snowstorm the storm that didn't keep you away from fun, but kept you out of school.

Outlook tonight:
Temperatures today were able to crest in the upper 30s under mainly sunny skies and brisk 10-20 mph winds. Any snow that managed to melt today under the "balmy" conditions will rapidly re-freeze overnight as winds slacken, and temperatures fall into the upper teens and lower 20s.

Generally placid weather conditions are anticipated through midweek with high temperatures remaining within a degree or two of 40.

Monster Snowstorm for Boston (Figures):
An amazing set of events transpired throughout the day in the northeast as the low that brought us our wintry wonderland today pulled off to the northeast. By midday, the lower pressure system had begun a very rare weather phenomena known as bombogenesis. This is basically cyclogenesis (the creation of a low) to the extreme. The low pressure must loose at least 24 millibars in 24 hours to qualify for bombogenesis--which is precisely what occurred today over eastern Massachusetts.

The barometer in Boston plummeted nearly 17 millibars in only 3 hours, an absolutely incredibly pressure fall that is rarely seen outside of the tropics in hurricanes. On top of this, the low took on the appearance of a hurricane for a period, noted by the "eye-like" structure over Cape Cod. While this storm never qualified as a true hurricane, wind gusts to 80 mph, blinding snows, and vicious cold temperatures will make this a memorable event for the folks living in the northeast.

When: December 14th-16th
Impact:
Confidence: Low
Commentary: Some numerical computer models continue to latch onto the idea that yet another storm system enters the eastern US during the end of next week. Forecast confidence at this point is relatively low, given the extended nature of this event, but things bear watching. At this point, some snow is possible in association with this system, but current indications point towards more of a rain/snow event.

UPDATE:
From what I can gather, a fairly sizeable storm looks likely for the DC metro region overnight tonight. Temperatures are hovering near the freezing mark, and will begin falling into the lower 20s after dusk. This storm is a fast mover and will likely only produce precipitation for 12 hours or so (if that). The heaviest precip moves into the area after Midnight, which will cause significant travel problems tomorrow morning.

***A WINTER STORM WARNING IS IN EFFECT FOR THE DC AND BALTIMORE REGIONS, A HEAVY SNOW WARNING FOR WESTERN MD, AND A WINTER WEATHER ADVISORY FOR LOWER SOUTHERN MD.***

An area of low pressure continues to intensify and organize over the Midwest, extending clouds, showers, and thunderstorms across the enter Southeast region.

High pressure dominating our weather today will back down from a confrontation with this low pressure center later today, as it weakens and moves eastward over the Atlantic Ocean. By 5AM, clouds and snow showers will have overtaken much of the region (including the District and BWI), and may become quite heavy before changing over to a mixture of sleet/freezing rain/snow over the immediate metro region.

Areas out west around Garret County, southwestern PA will likely remain all snow for the duration of this event and snowfall totals may exceed 10 inches in localized areas.

Closer to home, around DC, Montgomery county, and Baltimore, snow will change over to an icy mixture by late morning, and will compact snowfall totals significantly. Nonetheless, significant snowfall accumulation is anticipated in the said region: between 3 and 6 inches.

The Timing is Right for a Horrible Morning Commute:
As snowfall moves into the area overnight, during the coldest periods of the day, snow will likely begin accumulating shortly after it begins to fall. The morning commute around the region will be hazardous, and commuters are urged to use extra caution venturing out to work tomorrow.

And on this topic, timing is good for many school closings across the area.

Outlook:
Skies were clear to partly cloudy today across the Mid Atlantic in the wake of last nights winter storm, which continues to pull rapidly northeast into the Canadian Maratimes. High temperatures averaged in the upper 30s.

Tonight, temperatures will once again dip into the lower 20s under mainly clear skies and dying winds. Add an extra layer as you head out of the house tomorrow morning, and use a bit of caution on the roadways as any snow that melted during the day will refreeze by sunrise.

Tomorrow is basically just a carbon copy of today, although temperatures will manage to surge into the upper 30s to near 40, instead of the mid 30s experienced today.

December 4-5 Winter Storm Recap:


While many asked me today "What happened to all the snow you forecasted?" the snowfall forecasts issued by many meteorologists and hobbyists around the region weren't too far off the beaten track. A general swath of 3-4 inches of snow accumulated within my forecast 3-6 inch swath from eastern WV to eastern PA. This forecast was, however, too high for areas of western MD and most of eastern WV as most locations in the said regions received less than 3 inches of snowfall.

I was also quite pleased with the forecast swath of 6-10 inches made for central VA and south central MD around St. Mary's and Calvert counties. Although a bit on the high side (okay, not a single city has reported >6 inches of snowfall), I was able to pin down the area of highest snowfall totals a good 36 hours in advance of this system.

But most disturbing to me is the fact that most people expected snowfall forecasts for roadways--and I can see why this is--because this obviously has the most impact on individuals. Observations are generally taken on a snow board, which is coated white to reflect most oncoming solar radiation, hence begins to "record" snowfall accumulation right at the start of winter storms.

Forecast Grade: C+
This is mainly because totals could have been scaled down somewhat to around 2-4 inches in the 3-6 inch swath, and 3-5 in my 6-10 inch swath. But overall, I am pleased with the snowfall forecast (which can be viewed at the bottom of the previous post).

And while I'm on this topic, I would like to tip my hat to the folks at the Warning Forecast Office in Sterling, Virginia, who basically nailed this storm system, even though numerical models were of very little use nearing the event. Keep up the good work!

Previous Forecast Calls:
Washington, DC: Forecast: 3" Verify: 2.9"
Baltimore (BWI): Forecast: 5" Verify: 2.8"
St. Mary's: Forecast: 7" Verify: 4-6"


Related Links:
Spotter snowfall reports can be accessed here
Snowfall map from the NWS
Season's First Snowfall (Washington Post)

Another Wintry Blast later this week?
Unlike this past winter event, which featured a lack of numerical model support prior to the onset of precipitation, this latest **potential** Nor'easter has a plethora of guidance support nearly 70 hours before any snow falls from the sky.

The GFS model has been emphatic on develop a large low pressure system off the NC coast Friday morning. The type of Scenario the GFS is painting would lead me to believe a significant snowstorm could be in the works for our region beginning very early Friday morning.

However, as we have seen many times in the past, relying on one computer model is basically forecast suicide, so I turn my attention to the NAM (North American Mesoscale). This model has a tendency to severely overpredict Quantative Precipitation Totals leading up to a winter event, and this is no exception to the rule. This model spits out close to 15 inches of snow for the DC area by Saturday morning, which seems far too high to me at this point. But the synoptics are basically the same as the GFS--large low forming off the NC coast Thursday night, out of the region Friday evening.

With this said, there is still some uncertainty regarding this forecast, although my confidence is decidedly higher than with the previous storm system. The potential exists for significant snow and/or ice accumulations across the Mid Atlantic beginning very early Friday morning, and lasting through much of the day, before tapering to flurries by nightfall. At this time, I am relatively confident in a swath of >5" of snow from DC north and west, with ice accumulations a possibility farther south and east.

8:15PM:
The previously predicted 3-6 inches for the Greater DC area might have been a bit overdone, (figures) although we are approaching 1.5 inches in southern Montgomery County at this hour. Nearly 3 inches of the white stuff has accumulated in Fredericksburg, VA, and most locations around the region are reporting 2 inches of slushy snow at this time.

The radar presentation continues to improve, with distinct "banding" lines appearing throughout central MD. And as expected, the heaviest and steadiest snowfall is located southeast of the city, across St. Mary's county and the Fredericksburg areas.

A general T-2 inch swath is likely in northwestern MD, and eastern WV, while snow totals from 1-4 inches will be prominent closer to home. Be careful heading out on the roads tonight, as temperatures have fallen below the 32 degree mark.

WashingtonPost Article: "Snow Arrives in Washington"
School Closings: WUSA9 News
School Closings: WJLA 7 News
School Closings: NBC 4 News

Winter Storm To Affect the Entire Mid Atlantic

Winter storm warnings are in effect for south Central MD (St. Mary's County), and back southwest towards the southern Shenandoah Valley. The said regions should expect rather extensive snow accumulations of 6 to possibly 10 inches.

Closer to home in the District, my call for 3-6 inches still stands, so I'm basically re-iterating what I have stated in previous posts.

Snowfall Timeline:
10AM-Noon: Snow will continue movie northeastward as it saturates the atmosphere, although it is taking a little longer than anticipated for precipitation to saturate the lower levels. The first flakes should begin falling around the noon hour in the District (give or Take), but no accumulation is anticipated on the roadways. Grassy and sheltered surfaces may see some snow accumulations near 1-3 inches.

Noon-Midnight: Snow continues throughout the region, with some mixing of sleet/freezing rain in lower southern MD. Snow may become heavy at times, and I wouldn't be surprised to see some thundersnow around the area. (Basically a thunderstorm, but "raining" snow--this is really fun to watch).

Midnight-11AM Tuesday: Snow tapers to light snow showers after spurts of heavy snow as the low pressures "Comma Head" passes over the region. All precip is out of the region by the Noon hour on Tuesday.

Again, my forecast reasoning remains the same, and the forecast map below is a carbon copy of the map in the previous post.

Snowfall Accumulation map:

Storm Impact Potential: Medium, snow totals >5" prominent

School Closings: Good chances of delays, and fair chances of closings across the region.

Select Snowfall Predictions:
Washington, DC (KDCA): 3"
Baltimore, MD (BWI): 5"
St. Mary's: 7"
Cumberland: 2.5"

(We'll see how these hold up by Tuesday evening...)


Warning Map Courtesy of WUSA9

Update:
With this latest wintery event less than 24 hours away, computer models still have not completely latched onto a solution. The GFS is becoming an outlier with its surface track farther east over the Atlantic Ocean (which would yield generally less than 3" of snow across the region). Most other models, like the UKMET, NAM, GGEM, MM5, etc. have come into somewhat better agreement in the last few hours supporting a surface track closer to the coast. I have bumped totals in the immediate metro down a notch given my uncertainty, and the potential for the snow to begin closer towards the Mid Morning.

It is possible areas to our southeast get slammed with isolated 6"+ totals. Areas along the Eastern Shore will remain mainly rain, with some mixing possible Monday night. This will keep precip totals generally below 3".

A WINTER WEATHER ADVISORY IS IN EFFECT UNTIL 7 AM EST SUNDAY MORNING FOR AREAS NORTH AND WEST OF THE DISTRICT. THERE IS THE POTENTIAL FOR 1-2 OF SNOW TO ACCUMULATE ACROSS NORTHERN MD AND WV BEFORE EVERYTHING TURNS OVER TO RAIN BY MID MORNING

At publishing time, National Weather Service radar was beginning to pick up echoes over eastern WV and central VA at around 6-7,000 feet off the surface. I have not, however, been able to find any reports of precipitation making it down to the surface as lower levels remain very dry. Dewpoint depressions (simply the difference between the Temperature and Dewpoint, and reveals the relative dryness, or wetness, or the atmosphere) are averaging near 15-25 degrees F, which would indicate an extremely dry surface layer.

Virga and it's relation to the Doughnut Hole:
For this reason, most of the precipitation showing up on the doppler radar is Virga, and is evaporating before it makes it anywhere near the surface. This nifty little schematic reveals how the doppler radar can "sense" falling precip, while nothing is actually hitting the surface. This is also the cause of the famed "doughnut hole" on the radar images often seen before the onset of snow.

Suppose snow is falling through 1000 feet, but evaporating after that. Because the radar beam is tilted (.5 degrees at its minimum), 20-30 miles away from the radar site, the beam will have penetrated the 1kft level. Anything within the 20-30 mile radius will appear to be "high and dry" due to this phenomenon.

Wintry troubles tonight and tomorrow:
Through the next 2-4 hours, snow showers (possibly mixed with some sleet) will begin to overtake the entire region from west to east as a low pressure system develops just west of the Cumberland Plateau. Around the immediate metro, snow showers will rapidly change over to freezing rain/sleet, and eventually pure rain by 4 AM as warmer air noses into the region. Before the changeover occurs, however, there will be a small timeframe in which snow has the potential to accumulate to .5-3 inches across the entire area before rain showers develop by mid morning.

Forecast Confidence: Medium/High

Major Mess Looking Likely Monday and Tuesday:
My basic reasoning from the previous post remains unchanged regarding Monday's system. Computer models have slowed the onset precip time just a tad to around 6-9AM, although this is highly subjective depending on where you live.

I still anticipate a sizeable winter storm flanking the Eastern Seaboard, with most of the precipitation falling in the form of a heavy snow. There are also some indications that would point towards the potential for some thundersnow, which is not all that uncommon with deepening Nor'Easters. Thundersnow can be accompanied by snowfall rates in excess of 4in/hr (although rates closer to 1in/hr look like a stronger possibility).

By the time Tuesday evening rolls around, anywhere from a trace of snow in lower Southern MD, to 4-8 inches in Central MD will be possible. Snow closings may also be in the works, and significant travel delays are a given.

Still, there is a bit of uncertainty revolving around this forecast, but things are coming into much better focus.

The First snowfall Map of the 2005-06 season is issued:
Please bear in mind these are not official forecasts, and should not be treated as such. The following is my preliminary snowfall outlook for our region valid Monday morning through Tuesday evening.

A general swath of 4-8 inches appears to be a good bet around the area, and lesser amounts towards the Eastern Shore, and to our north and west (as you get farther away from the precip shield), although my totals northwest of the area will likely end up being ramped upwards tomorrow.

Forecast Confidence: Medium

After significant model shifts last night towards a much drier storm system next week, recent runs of the NAM, GFS, UKMET, and Canadian models have all shifted their thinking once again. Taken at face value, the GFS would develop a 4-8 inch snowstorm across the Mid Atlantic Monday morning into Tuesday.

However, we must first navigate our way through this first low pressure system, already beginning to throw virga our way. One thing I would like to make clear to everyone is that this first system is basically set in stone save the minor timing differences between models. The second, larger system is what's causing forecasters the big problems.

Storm A: Sunday morning-Sunday evening:

Between midnight and sunrise Sunday, snow showers are expected to break out across the entire region for a few hours, before rapidly changing over to rain by mid morning. Hence, no snowfall accumulation is anticipated at this time.

Storm B: Monday morning-Tuesday:
Now here's the tricky part: Late last night, computer models shifted their forecast tracks south and eastward, which would significantly limit the amount of precipitation the metro area would see. However, as recently as 30 minutes ago, the morning run of the North American Mesoscale Model began shifting its low pressure track even further northwest, which would point towards a major winter weather event early next week.

Given the recent alignment in model thinking, forecast confidence is somewhat higher. Precipitation looks like it will move into the District between 6 and 10 AM Monday. One major concern I have is the potential for low level insulation given the time at which precip is expected to enter the Mid Atlantic--if it moves in later than 12PM or so, low level temps will have warmed into the mid to upper 30s, decreasing potential for accumulating snowfall.

While model differences still run rampant, things are coming together for at least a moderate snowstorm in the Mid Atlantic.

Snowfall Accumulation Probability Map:

The chart at left a rough estimation of my current thoughts with respect to snowfall accumulation through Tuesday evening. Stay tuned to DCweather, Capitalweather.com, or the National Weather Service for more updates on this developing winter storm.

(click for larger image)

Currently:
Weather radar is showing some scattered snow flurries mainly north of the city associated with "orographically enhanced" snow showers affecting the Appalachian Mountain region at this time. Air parcels are being lifted up and over the mountains, and being dumped into the metro region.

With that said, snow flurries and scattered snow showers will be possible across the metro region east of the Blue Ridge through this afternoon. Winds are also gusting to 20-25 mph as a departing low pressure system deepens in the Northeast.

Let it Snow!
I have always been fascinated by winter storms, especially those famed Nor'Easters that ride up the Eastern Seaboard in the middle of Winter and dump crippling snows on the region.

What will transpire over the next 100 hours will likely be the first real "snowstorm" for the metro region. With that in mind let me say this: the weather pattern setting up over the next 3-5 days is incredibly complex, involving several lows developing along a stalled frontal boundary to our south.

General Synopsis:
Computer models are still having a difficult time pinning down this forecast, and my confidence is relatively low. Below are my thoughts on the following storms, organized into a "timeline" of sorts.

System Number 1 (Saturday evening-Sunday): By Sunday evening, a weak low pressure center will be forming around the Cumberland Plateau, and progress rapidly to the northeast towards the Mid Atlantic. By late Saturday night (probably towards dawn), snow showers, possibly mixed with sleet and rain will move into the immediate metro region.

As this first low departs on Sunday, most of the precip should turn over to rain as surface temperatures fly into the 40s. No overall accumulation is expected from this first system.

System Number 2 (Sunday -Tuesday): Early Monday morning, another low pressure system will be developing along and old frontal boundary in the southeast. Sometime around daybreak Monday, snow showers should break out once again across the area. Most of this event is expected to be snow at this point. High temperatures look like they will remain in the low to mid 30s, which should be cold enough to support accumulating snowfall IF SNOW SHOWERS BEGIN TO FALL DURING THE OVERNIGHT HOURS ON SUNDAY.

Snowfall will last through Monday night, and may become heavy at times. There are some indications warmer temps move in aloft overnight Monday, which would turn precip over to sleet. Precip should finally wrap up Tuesday afternoon as the low moves northeast and wraps northwesterly winds into the region.

Now IF this forecast I have outlined above pans out, total snowfall around the region might range between 2-8 inches (I know it's a large range, but there is a considerable amount of uncertainty in this forecast), mainly on grassy surfaces. Totals will be towards the lower end of this spectrum if snow showers move in later than I am thinking...maybe around noon on Monday. Precip would be falling into a considerably warmer airmass, which would likely inhibit much accumulation until Monday evening.

This might all be wishful thinking, but this forecast is supported by a few computer models. There are significant timing differences between the GFS, UKMET, and GGEM models, which is the biggest reason for my lower confidence. Stay with DCweather for updates over the weekend!