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Posted at 01:57 PM ET, 01/22/2013

Some snow on Friday remains possible and even a dusting late Wednesday night


Two possible snow events are on the horizon. The first is a weak disturbance or “clipper” which could bring a dusting as it crosses the area late Wednesday night or early Thursday morning.

A more significant storm may impact the area during the day on Friday.

Next chance of accumulating snow (of at least 1 inch): Friday - most likely timing afternoon/evening (subject to change)

Chance of accumulating snow (of at least 1 inch): 50 percent

This second storm has more snow potential than last week’s non-storm for two main reasons: 1) most importantly, temperatures will be cold enough that any snow that falls will stick. 2) D.C. appears to be be more firmly in the precipitation shield so we won’t have to worry so much about the storm missing us completely.

However, there are considerable differences in model simulations of how much precipitation the storm will generate and there is a chance D.C. ends up in a snow hole, with more precipitation falling to our east and west. Whatever snow falls, we are confident will stick due to freezing temperatures ahead of the storm.

Possible scenarios for the Friday snowstorm

Scenario 1, a snow lover’s dream - least likely: Low pressure system tracks from Texas to near the West Virginia panhandle before reforming to our south over the Virginia or off the Virginia Capes. Strong warm advection (warm air blowing in on top of cold air) would lead to a period of moderate to heavy snow resulting in 3-6 inches across the area. Surface temperatures would stay cold enough for accumulations on area roads.

Scenario 2, a nuisance snow - less likely: A much weaker and ill -defined surface pattern leads to a very weak low tracking northward towards the Ohio Valley before reforming off the coast. The weaker warm advection pattern means the low remains too weak to tap into moisture from the Gulf of Mexico. That leads to clipper type snowfall amounts with somewhat heavier amounts fall over the mountains and to our east (snow shadow or snow hole effect for D.C.). Likely accumulations would be a trace to an inch or so. With temperatures being so cold, slick spots could still develop on some roads.

Scenario 3, a compromise solution - most likely: The third scenario is based on the idea that a weak but well-defined low will track towards West Virginia panhandle but will not produce as much heavy precipitation as either last night’s European model or this morning’s NAM model. This scenario takes the average from the model ensembles and would lead to a 1-3 inch type event. Right now, I’m slightly more inclined towards this middle of the road scenario than either of the other two. Still, even 1-3 inches of snow at these sub-freezing temperatures would cause potential driving problems.

Technical discussion

The clipper for late Wednesday night or Thursday morning.

A strong upper level disturbance will be crossing the region Wednesday night into early Thursday morning. The models are in consensus that the storm will produce snow over the mountains to our west but differ on whether any snow will make it east of the mountains.

Of the models, the GFS is the most aggressive in predicting that some snow will sneak east of the mountains. The other models are mostly forecasting some clouds and virga (precipitation aloft but not reaching the ground). Based on the strength and track of the upper level impulse, I suspect there will be at least flurries across the area but a period of light snow is also possible if the GFS scenario plays out.


Left: GFS model shows a disturbance passing through the region at 7 a.m. Thursday. Right: GFS model shows a light amount of precipitation falling as snow between 1 a.m. and 7 a.m. Thursday, probably a dusting at most.

The Friday storm

The plume diagram from last night’s GEFS ensemble run (numerous runs of the GFS model with initial conditions tweaked) from 06Z (1 a.m.) does a good job of showing the spread of solutions for Friday’s storm. Each blue line represents a different simulation, precipitation totals (in liquid equivalent) are given on the left.


Of the other models (besides the GEFS), this morning’s NAM, last night’s European and today’s UKMET runs lean more towards the high end of the ensemble spread while the last three operational GFS runs have been towards the lighter end of the spectrum. Today’s European run falls in the middle.

The one point of agreement in all the model runs is that any precipitation that falls between now and Saturday will be in the form of snow. Unfortunately, the range of model simulations for Friday vary from a dusting to a 3 to 6 inch snowstorm.

In the image below, you can get a feel for how the weaker and much more ill-defined surface low simulated by the operational GFS (right) weakens the precipitation field relative to the NAM model (left) 78 hours into the future.


The NAM (left) tracks a well-defined (though weak) low from northern Texas to near the West Virginia panhandle by 1 p.m. Friday. The well-defined low and an upper level wind field that takes on a west to southwesterly orientation help to pull moisture northward. The leads to precipitation amounts towards the higher end of the spectrum shown by the plume diagram (essentially our first scenario).

By contrast, the GFS has a weaker low farther south. This low almost disappears as it comes eastward. The low then jumps off the East Coast by 7 p.m. Friday. This jump pulls what moisture is present east of our area, leaving us in a relative precipitation minimum. In this scenario, D.C.only gets around 0.10” of liquid (our dusting to an inch scenario). My gut feeling is that the GFS is underplaying the precipitation somewhat but not with very much conviction.

Unlike many of the storms during the past couple of years the temperatures for this storm are forecast to be well below freezing at all levels of the atmosphere.


Note on the model sounding for Reagan National Airport (to the right) how the green and red lines are almost on top of each other from the ground up to 500 mb (around 18,000 ft) and that both those lines are considerably to the left of the blue line (freezing line). The proximity of the red and blue lines indicates the atmosphere has saturated and snow is falling (in the model world). That all the lines are to the left of the blue line indicates that temperatures at all levels of the atmosphere are well below freezing. When you combine the cold atmospheric temperatures with surface temperatures forecast to be in the 20s, it argues any snow that falls will be dry and will stick.

Conclusion

A clipper may dust the areas with snow late Wednesday night or early on Thursday. A more significant storm is on the horizon for Friday. Accumulating snow is becoming increasingly likely as temperatures should emain well below freezing during the storm. However, there still is considerable uncertainty about where in the spectrum of dusting to 5 or 6 inches this storm’s snow totals will fall. This remains a storm worth monitoring closely over the next couple of days.

By  |  01:57 PM ET, 01/22/2013

Categories:  Latest, Winter Storms

 
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