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Wintry Mess for the Workweek

Winter Storm Warning DC North and West 4AM Sunday to 4AM Monday for 1-2 inches of snow/sleet accumulation and 1/4 to 1/2" of ice.

Freezing Rain Advisory DC South and East 4AM Sunday to 4AM Monday. Up to 1/5" ice accumulation possible.

After a fine, but chilly, Saturday afternoon, clouds will begin to increase from the west overnight as a developing storm system rotates into the upper Mid West. Latest visible satellite imagery reveals a large expanse of cloud cover in the heart of the Continental United States and a clearly defined cold frontal boundary slicing through eastern Oklahoma and Texas.

All of this is headed eastward, being steered by a 100-110 mph jet stream.

The forecast over the next 48 to 54 hours is incredibly tricky--just like the majority of winter storms this season.

Precip onset:
The North American Mesoscale model brings isentropic precipitation (ie, overrunning precip) into the DC metro region shortly after 7AM as does the GFS. So, I am confident that a wintry-mix of freezing rain, sleet, and snow overspreads the District between 7 and 10 AM.

Precip Type:
Probably the most uncertain aspect of this storm. It appears that cold air will not be quite as strong as it was during the last winter storm, as high pressure is situated in the Hudson Bay in Canada. Nonetheless, a strong Cold Air Damming signal is present in forecast models as cold air bottles up east of the Appalachian Mountains.

The image to the right is a skew-t plot of temperatures and dewpoints. It's basically a vertical slice of the atmosphere, but the isotherms (lines of equal temperatures) are skewed from right to left. By 7PM Sunday evening, the GFS is clearly the warmer of the two models. If the GFS were to verify at this time, all of the frozen precipitation in the District will have turned over to plain rain. The NAM, on the other hand, is much colder, and still barely supports frozen precipitation.

I believe most of the precipitation (say, 80-90%) will be frozen, that is, either freezing rain, sleet, snow, or a mixture of all three. It will start as as snow with possibly a little sleet mixed in. By Sunday evening, enough warm air has intruded into the upper atmosphere to turn everything over to freezing rain.

This may turn out to be a fairly significant ice storm for the region, primarily north and west of the District. Places in St. Mary's county or southern PG county will likely go above freezing Sunday evening and Monday morning, turning everything over to plain rain, limiting ice accumulations.

The morning rush on Monday may be significantly impacted by this winter storm.
By Lee Carlaw On Saturday, February 24, 2007 At 1:14 PM

Clipper drops Snow; Rain is the next Threat

Potential Weather Hazard:
A powerful storm system is expected to develop in the Midwest by next weekend, and will likely bring heavy rainfall to the region early next week.

Tonight and Beyond:
After a nice period of moderate snow earlier this evening associated with a Clipper system, skies will begin to clear, allowing temperatures to dip into the mid and upper teens. Gusty northwest winds will combine with the cold surface temperatures to produce wind chill values in the single digits.

Tomorrow, mostly sunny skies and diminishing winds will accompany high temperatures in the upper 30s to near 40 degrees to the south. Winds overnight Monday will then turn to the southwest, ushering in moisture and warmer temperatures ahead of a developing weak storm system that will pass to our north on Wednesday.

Rain showers should begin to overspread the immediate metro region between 2PM Tuesday afternoon, and 8PM in the evening. Showers will continue off and on through the night, before tapering from west to east Wednesday morning and early afternoon.

Sunday afternoon run of the WRF/NMM numerical weather model revealing rain showers in the region late Tuesday night. Image courtesy of PSU E-Wall

Significant Rainfall a Possibility Next Week:
A more significant storm system (appears it will be a rain system at this point), is forecast to move into the region next Sunday. An intense area of low pressure looks to develop somewhere in the Midwest on Saturday before rotating northeastward into southeastern Canada. A few models develop a scenario in which the DC metro and/or Mid Atlantic area experience torrential rainfall and the possibility of severe weather before the frontal passage.

Stay Tuned.
By Lee Carlaw On Sunday, February 18, 2007 At 6:48 PM

Chilly weather to Remain

Cold, Windy, and Still shoveling out:
Even with the region's latest snowstorm long gone (in fact, it's become quite a snowstorm for Nova Scotia and eastern Quebec), bone-chilling weather lingers across the Mid Atlantic and Northeast. Winds gusting upwards of 30 mph in some locations are creating wind chills in the low to mid teens, with below zero readings in extreme western Maryland and southwestern PA.

Due to the significant amount of sleet and freezing rain that accompanied this week's storm, temperatures in the low to mid 30s and an increasing sun elevation have failed to do much in the way of melting the wintry mess. This has forced many local school systems to shut down for the third day in a row, much to the delight of students around the region. Look for temperatures to moderate somewhat tomorrow afternoon under partly cloudy conditions. High temperatures should reach into the upper 30s and lower 40s across the area, with the exception of areas in northern Maryland, where temperatures will struggle to break the freezing mark.

A Clipper System for the weekend:
A rather vigorous clipper system, seen on regional satellite and radar, is currently rotating southeastward out of the Midwest, accompanied by snow showers as far south as the Oklahoma border.

Numerical models have been locked on to this latest wintry system for the past few days, and spread light snow showers into the northwestern suburbs late tomorrow afternoon and evening. Most of the time, clippers have a very difficult time spreading precipitation into the immediate metro area because of the moderately tall peaks of the Appalachian Mountains, which act as a sort of brick wall. This factor, coupled with the already moisture-starved system, usually spell "no snow" for the District.

However, it appears that this latest clipper will dive far enough south of the Capital to spread light snow showers into the metro region late tomorrow night. Dry air is abundant at the surface, but I think precipitation will gradually be able to work through this impediment through the night. Expect generally less than an inch of snowfall accumulation with this system.

Snowstorm Post-Mortem:
This week's snowstorm was, bluntly, one of the more difficult forecast that have faced area meteorologists this season. Marginal surface temperatures, disagreeing computer models, warming mid layers (which brought freezing rain and sleet into the forecast) all lead to a very challenging winter forecast.

Generally, I think my forecast worked out pretty well in most locations. Granted, I did not expect such significant ice accumulations across south central Maryland, where upwards of half an inch to three quarters of an inch of ice accrued on almost everything. The higher forecast snowfall amounts in northwestern MD did not verify (where I had envisioned 6-12" of snow, only about 6-8" accumulated in very isolated pockets).

All in all, however, my forecast verified pretty well in most areas, including the District, where 2-3 inches of snow, sleet, and freezing rain accumulated. My "all snow line," on the other hand, was completely off, as freezing rain and sleet made it all the way into southeastern New York and Northwestern PA.

The image at right is the total snowfall/ice map with data from the local National Weather Service office in Sterling, VA. The pinkish line represents where significant ice accumulations occurred. Areas within the pink area received anywhere from .5 to .75 inches of freezing rain.

Overall subjective grade: B-
By Lee Carlaw On Friday, February 16, 2007 At 4:27 PM

Exciting Times--Potential for a Major East Coast Storm

Monday Evening Update:
18z Numerical Model Guidance run this afternoon would seem to favor a slightly colder scenario for the DC metropolitan region. Previous runs turned precipitation over to plain rain Wednesday afternoon as a warm nose of surface air shot northeastward. However, the 18z GFS run was slightly colder around the region for the duration of this storm, certainly a good sign for winter-weather lovers.

Still, for those of you who enjoy snow, you're probably going to have to wait for the next storm (if there is one). The snow currently falling across the area is associated with an upper level disturbance out ahead of the main weather system currently sitting in northern Texas .

Very little in the way of accumulation is expected tonight in the immediate metro area. Temperatures will slowly fall into the upper 20s and lower 30s.

----Previous Post-------

Potential For Significant Snowstorm in Mid Atlantic:
After a brief hiccup in the numerical modeling of this latest winter storm (OOz run of the Global Forecast System painted and absolutely horrific snowfall forecast for the region), guidance seems to be on track once again. The Latest GFS run pumps out close to 1.25 inches of liquid precipitation, which, under the standard 10:1 snow to rain ratio, would yield a 12 inch snowfall for the region.

While there are still significant differences in the vertical temperature profile of the atmosphere during this event, I have enough confidence to outline potentials with this potential storm.

  1. I am confident that light to moderate snow will break out across the region from west to east sometime late Monday night or early Tuesday morning. This precipitation will be associated with warm air in the upper levels sliding over a dense dome of cold air near the surface.
  2. The second part of the storm, likely the most significant sow-maker for us, should arrive sometime Tuesday night into Wednesday morning.
  3. All Precipitation should exit the region by Wednesday night
Precipitation-Type Issues:
From a climatological standpoint, major winter storms rarely finish without some kind of precipitation change-over, either from snow to freezing rain, or from snow to sleet, at least in the Mid Atlantic. The North American Mesoscale Model (NAM), which honestly does not perform that well at 84 hours, is indicating the potential for a change-over from snow to sleet early Wednesday morning as low pressure systems get cranking to our south.

The GFS is much colder, on the other hand, and keeps all of the precipitation in the form of snow from DC north and west. Areas closer to the Bay in Southeastern Maryland will likely change over to a mixture of rain/snow/and sleet on Wednesday.

If the snow does in fact change over to sleet and/or freezing rain, snow totals would be significantly lower than if we managed to remain all snow. At this point, I think the majority of the storm will be snow, but I cannot rule out the possibility that areas as far north as Hagerstown change over to sleet at some point during the storm.

Even with the usual disparity among computer model guidance, there is a relatively high risk of a significant winter event in the Mid Atlantic early next week. I am very confident that a snowfall of at least 4 inches is in order for the Washington, D.C. metropolitan region.
By Lee Carlaw On Saturday, February 10, 2007 At 5:29 PM

Technical Thoughts

Below are my technical thoughts from last night. Little has changed in my thinking with respect to next week's potential winter storm:

Twenty-Four day 500mb upper air analysis from 00Z February 2 reveals a weather pattern dominated by a large positive PNA ridge that, at some points, stretched almost all the way to the North Pole. A massive polar vortex has remained relatively persistent over southern Canada, driving very low atmospheric heights into the CONUS. Currently, 500mb height anomalies are around -350 to -360 meters over the Canadian Maritimes.

The aforementioned positive PNA ridge has slowly been trending towards a more neutral state as successive upper lows rotate towards the Pacific Northwest. Nonetheless, it appears as if near normal to slightly below normal temperatures will remain the norm over the northeastern portion of the country for at least the next week or so as the PV spins over eastern Canada.

12Z Ensemble GFS/MREF members reveal very similar solutions through at leas the next 144 hours. A piece of upper air energy looks likely to eject northward from the Laptev Sea within the next 36 to 48 hours and attempt to partially phase with the current Polar lobe over Newfoundland. This new polar low will likely play a significant part in a potential major winter storm for the Southeast and Mid Atlantic states early next week.

As mentioned above, most GFS/ MRF ensemble members seem to agree (uncharacteristically at such an extended range) on some sort of modified Miller A low pressure system developing in the south central United States next Sunday evening/Monday morning, with a large area of isentropic lift and associated precipitation developing along and to the east of the low pressure center. The aforementioned polar low that should traverse the North Pole in the next few days will likely aid in suppressing the potential storm system far enough south to keep the precipitation-type for the immediate metro region primarily snow.

Synoptically, the surface set-up looks to closely resemble that of the President’s Day snowstorm of 2003, which dumped upwards of three feet of snow across the region. While this storm does not look to be accompanied by as strong a 500mb low, the surface set-up is very similar, which, along with the almost unanimous model support from the ECMWF/GFS/MRF/CANADIAN models, leads me to believe there is a very good potential for a significant winter weather event in the Mid Atlantic next week.

Granted, the cosmetics of this storm are very uncertain at this point, as they would be with any potential storm over 5 days out, but signs are pointing in the snow-lovers’ favor. I am a little concerned with the set-up of the PNA ridge and NAO, which has basically been positive the entire winter, as well as the position of the polar low in southern Canada (which may build so far south it shunts precipitation south of the region), but these concerns will be smoothed out as time progresses. Needless to say, this system bears very careful attention over the weekend.

By Lee Carlaw On Thursday, February 08, 2007 At 5:49 PM
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