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Temperature Verification
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Risk of Snow Showers tomorrow, cold for Friday

Current Weather:
Temperatures currently in the lower 40s should drop into the mid 20s overnight under mainly clear skies and light winds in and around the Beltway. Farther to the north and west, temperatures are presently in the mid to upper 30s, and should fall into the upper teens overnight.

Forecasts For the Next Few Days:


By 8 or 9 AM tomorrow, clouds should begin streaming in from the west, as energy associated with a tiny pocket of mid and upper level energy slides east from the Midwest. High temperatures tomorrow will hover in the lower to mid 30s in and around DC, which will support a few snow showers, which may develop during the late morning and into the late afternoon hours. Some minor snow accumulation is possible on grassy and elevated surfaces, but most roadways should remain snow-free due to above-freezing road temperatures.

Probability of Snow: 40%

Image at right: NAM forecast precipitation for Thursday afternoon showing the possibility of some light snow showers around the metro region. Image courtesy of the Penn State E-Wall

Cold, Canadian air is expected to pour into the region overnight Thursday as a cold front sweeps through the Mid Atlantic. Temperatures early Friday morning will range from 15 around Hagerstown and surrounding areas, to near 20 near Washington, D.C.

Clouds should scour out by mid morning, leaving us with mainly sunny to skies. High temperatures will be in the upper 20s and lower 30s.

Clouds will increase late Friday night and into Saturday morning as a very weak upper level disturbance rotates through the region. At this point, there does not appear to be enough lift to get any precipitation over the mountains. Temperatures will hover around 40.

Paying Attention to What the Forecaster Actually Says:
With the recent forecasts of snow in the metro region, it has gotten to me when people ask me, "hey, the weather channel said there was supposed to be snow last night. How come there was none/How come it didn't stick?"

It seems incredible to me that, with all the precision and time forecasters put into their work, the vast majority of the public really doesn't pay attention to some of the crucial details. Here's a portion of the text forecast for Montgomery county for Thursday from the National Weather Service:

Seems simple enough, right? Now, take a quick look at the image to the right. It's The Weather Channel's forecast for Thursday. If it DIDN'T snow at all tomorrow, I would bet that most people would say, "the meteorologists got it wrong again. Where was the snow??"

Very few public viewers actually take the time to read the forecast text. Notice that little percentage at the bottom? In the NWS text forecast, this percentage is 40%, and in The Weather Channel's it's 30%. These are known as Probabilities of Precipitation.

Most people think of POP as the percentage of getting precipitation on that day. That, unfortunately, is only one part of the formula. POP is based on two things:

1) The probability that any precipitation will fall in the time period and
2) The predicted areal coverage if precipitation actually develops

So take this hypothetical situation (it would more than likely never happen, but what the heck):
Washington, D.C. records a POP of 20% for 100 consecutive days. If this POP were accurate over the long term, then Washington should have experienced some type of precipitation for 20 of those 100 days (not a great number, right?) The likelihood of precipitation being recorded when the POP is 20% is extremely low.

So, the next time you read or hear a forecast, it would be wise to also look or listen for the Probability of Precipitation. Weather isn't exactly and exact science.
By Lee Carlaw On Wednesday, January 23, 2008 At 5:19 PM

Nuisance Snow storm tomorrow

Current Conditions:
Temperatures are currently hovering in the mid to upper 30s across most of the region, with Reagan National as the warm spot at 41 degrees last hour. Temperatures are expected to fall into the low to mid 20s overnight as clouds thicken from the south and west, as a large developing surface low moves northeastward.

The snowfall forecast for tomorrow is a tricky one, since surface temperatures, once again, will be very marginal for accumulations.

Winter Storm Breakdown:
Snow should begin to fall by 12PM or so in the District, earlier to the southwest, and later to the north and east. The surface temperature profile will be critical to this forecast. At this point, our best estimate is that the snow will change to sleet and then rain by 4-6PM as warm air filters in from the east.

By the time the precipitation change-over occurs, however, there is the potential for a Trace to 2 inches of snow in and around the District, and 1-3 inches out to the north and west of town, where temperatures will likely remain colder.

If temperatures fall just a few degrees below forecast levels, a significant change to snowfall totals would be needed. At this point, however, I am not convinced areas in and around the District stay below 32 for very long after 4 or 5PM.

Snowfall Forecast:
I think most locations, even out towards Martinsburg, WV and Harisonburg, VA will change over to rain, it will just take longer, thus the higher snowfall totals out that way. There will likely also be a period of freezing rain in the lower portions of the Shenandoah and out towards Garret County, MD. I anticipate a light glaze of ice around the aforementioned areas.

By Lee Carlaw On Wednesday, January 16, 2008 At 5:34 PM

Colder weather in the future

Current Conditions:
Temperatures around the region are currently hovering near 40 degrees, but a weak surface trough has just cleared the area, which should usher in slightly colder temperatures for tonight.

Washington, D.C. reported 0.03" of rain with the storm system last night, and BWI only picked up about 0.07" from a thin line of showers.

Many areas to the north, were anticipating snowfall totals at or above 12", but most locations only received (generally) between 3 and 6 inches. Numerical computer models failed to accurately predict the dynamics of this coastal low, which lead to significant forecast busts across the Northeast.

Tonight and Tomorrow:
The sun is breaking out in many locations as a large winter storm, currently hammering upstate Maine with heavy snow, slowly pulls away to the northeast (see image above).

Skies will continue to clear, gradually overnight. Temperatures will fall into the mid to upper 20s around town, and near 20 farther to the north and west.

Expect partly cloudy conditions tomorrow, with temperatures rising in the lower 40s by mid afternoon. Breezy northwest winds will make it feel like it's in the upper 30s, however.

Entering a Cold and Stormy Pattern--Snow is again a Possibility:
A few models have been hinting at the possibility of yet another coastal storm developing Thursday night and Friday morning. While precipitation may start off as either a mix of rain and snow, or pure snow Thursday night, thermal profiles would seem to favor everything changing over to rain by Friday morning.

(Image at right: Surface precipitation and temperatures about 10,000 feet up. Notice how DC is above freezing at this level (+2C or so) which, with surface temperatures above freezing, would translate into rain for the Mid Atlantic).

So at this point, here's the forecast: Brief period of rain/snow Thursday night, quickly changing to rain by midnight. Things could change, but significant changes look unlikely at this point.

On the heels of the aforementioned storm is yet another disturbance which has the potential to drop snow on the region. While not a classic set-up, current forecasts offer us a slightly higher probability of snowfall Saturday through Sunday evening.

After these storms move by, a massive outbreak of Canadian Cold air is forecast to overrun much of the United States in below-average temperatures from around Sunday, January 20th, through the end of the month.
By Lee Carlaw On Monday, January 14, 2008 At 2:55 PM

Coastal Storm Possible on Monday?

Where's the Storm Now?
The feature we expect to develop into a coastal storm is currently a highly disorganized area of cloudiness associated with impulses of upper air energy over the northeastern Atlantic Ocean (see image at right). These little "packets" of energy are expected to slide southeastward, following the jet stream, and then combine--or phase--to produce a surface low pressure system in the Northwestern Gulf of Mexico sometime Saturday night.

This much of the forecast is pretty much a 100% certainty at this point.

What happens next, however, is difficult to assess with great confidence right now, as there are too many different things that have to click just right to produce a significant winter storm for our region.

What the models are saying:
Global Forecast System (GFS): We use this model the most to diagnose weather patterns over the United States. It has a decent track record (over the past several years), but has not been performing too well this winter. The past two runs of this model reveal a pretty much ideal situation for a significant snowstorm in the major I-95 Cities, from Washington, D.C. to Boston, MA.

North American Mesoscale (NAM): One of the worst performing numerical weather models beyond 48 hours this year. Nevertheless, it has begun to move towards a solutions that mirrors that of the aforementioned GFS. Surface temperatures will likely be an issue (mid to upper 30s).

Canadian Model: I feel this model has performed very well so far this winter. While it develops a significant surface low, it's roughly 200-300 miles off the coast--too far away to dump any appreciable precipitation on the region. (See image at right: bottom right panel is surface precipitation valid late Sunday night).

The European Model: By far, one of the best out there for medium-range forecasting, but is rather poor inside of 84 hours. Previous runs have developed a very nice storm for us, but this morning's run develops the low even farther east than the Canadian.

Current Thinking:
We are confident there will be a storm of some kind Sunday night into Monday. Whether it remains cold enough for snow, however, is still up in the air. Our airmass is not particularly cold, and any coastal low that develops will have to move within a swath 100 to 150 miles offshore to give us any hope of receiving snowfall.

At this point, temperatures look to remain in the upper 30s to near 40 for most of us, which would lead me towards a rain, or rain/snow mix solution for Sunday night/Monday. If the GFS does verify, however, much of the precip would be snow. Stay tuned.
By Lee Carlaw On Thursday, January 10, 2008 At 4:06 PM

Funky DC weather

Most of the precipitation with this winter storm (at least before Sunday morning) will fall in the form of sleet or freezing rain. Areas in extreme Northwestern Maryland have the best chances of seeing accumulating snow, but even there, warm surface temperatures may turn things over to rain briefly during the afternoon.

As for the District and Baltimore, this now looks like a primarily rain event. We may see a few flakes or sleet pellets before the storm ends tomorrow afternoon as cold air filters down from the northwest.

A Winter Weather Advisory is in effect from 3AM to Noon for areas north and west of the District including Montgomery, Loudon and Howard Counties. The National Weather Service anticipates icy conditions tomorrow morning.
Icy situation tomorrow morning:
Models over the past few days have trended cooler with a developing low pressure system in the southern United States. Temperatures are expected to cool into the low to mid 30s north and west of the District. As the upper levels warm above freezing, sleet and freezing rain are slated to develop around the aforementioned areas sometime tomorrow morning.

I don't really see this storm being much of a wintry event for the immediate metro regions, but northern sections of Montgomery and Loudon counties and points north and west could be in for a slippery ride tomorrow. Anywhere from a trace to one tenth of an inch of ice accumulation is expected before everything turns over the rain by midday.

After this storm exits the region tomorrow evening, our full attention turns to a potential large coastal storm.

To Snow or not to Snow:
Numerical model runs on Tuesday afternoon turned most Washington snow-lovers euphoric. The normally trustworthy GFS pumped out an eye-popping 18-24"+ of snow for most I-95 cities on Saturday through Sunday. Recent runs, however, have brought the storm farther inland, and instead of a monster pure snow event, forecasts snow changing over to plain rain.

But, there are some very promising signs that things are about to change. The newest computer model runs hint at the possibility that the low scoots a bit farther east, bringing a prolonged period of snow back into the equation. It's still too early, and I'm too uncertain, to make any calls at this point, but the potential for a large winter storm is still a very real possibility.
By Lee Carlaw On Wednesday, December 12, 2007 At 5:09 PM
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