Wintry mix to rain most likely scenario for Sunday-Monday storm

December 3, 2013
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A storm is expected to deliver some winter weather across the region on Sunday into Monday.  At the storm’s onset, precipitation could start as snow or sleet but is expected to change to freezing rain and then rain over most areas.

For much of the area, this storm will probably be more of a conversation starter or nuisance than a high-impact winter storm.  However, there still is the threat of an ice storm causing problems for areas well west of the city out towards the Shenandoah Valley.

Discussion

Yesterday Jason noted that if the storm tracks to our west and north, it would cut down on the chances of seeing any accumulating snow.  Today that cautionary note has come to fruition.

The model runs have converged towards a consensus that the storm will track to our west and north putting us under a southerly flow at high altitudes.  Such flow is expected to lead to an elevated warm layer which should turn any snow to sleet, freezing rain and then rain.   How long freezing rain linger before possibly changing to rain is much more problematic.

Right now, it looks like any snow, sleet or freezing rain in and around the city, especially east of I95, would change relatively quickly to rain. However,  freezing rain could linger longer west of the city, especially in the far western suburbs. Even if temperatures hover near freezing a little longer than forecast, a lot of the frozen precipitation may fall during the day  on a relatively warm ground, reducing the risk of ice build-up.

Below, note how this morning’s GFS brings precipitation as a quick shot of snow and frozen precipitation (sleet and freezing rain) along and west of I95 but, by nightfall, has changed most of the area over to rain except the far western suburbs. By Monday morning’s rush hour, most of the region should be above freezing with precipitation tapering off.


(WeatherBell.com)

However, the event is still around 5 days from now and how long the cold air holds  even in the city is far from certain.  Sometimes cold air hold in a little longer than forecast during these kind of events. Also, I’m sure the timing of onset of the precipitation and details about the timing of the changeover will change several times over the next several days.

What we know about the event

An arctic front will press southward across the region Friday night and Saturday.  Behind the front, a strong surface high will build to the east and is expected to be located over southern Quebec as precipitation starts spreading towards our area Sunday.  Such a location is in a favorable spot to facilitate the funneling of cold air southward, i.e. cold air damming,  east of the Appalachians. Having this cold air in place would set the stage for a period of frozen precipitation as there will be a low pressure system approaching from the west.  However, the low is forecast to track towards the Great Lakes region, a track that almost prevents us from getting much snow, as mentioned above.

While there remains some uncertainty about the exact track of the low, the ensemble members of both the GFS and European strongly support the idea of the low tracking to our north and west.  The 06Z GFS ensemble members (below) illustrate how uniform the various members are in warming temperatures at an altitude of about 5,000 feet (850 mb). All the members (see below) have temperatures rising above freezing at 5,000 feet by 1 a.m. Monday.  Most also have the pressures lowering towards the Ohio Valley and Great Lakes. That’s a big tell that a significant snowstorm is not likely for the I95 corridor.


Members of the GFS ensemble showing surface pressure and precipitation at 1 a.m. Monday. The orange line is the freezing line at 850mb (5,000 feet), which often serves as snow/mix line. Click to enlarge.

Could the sleet and snow hold on a little longer than forecast?

Of course they could, but the probability of that happening is pretty low.  Last night’s 06Z GEFS ensemble system does a pretty good job of showing the probability of the cold air holding a little longer than forecast by the operational models.  While the various ensemble members differ somewhat on the evolution of the storm and on its exact track as it lifts to our north and west, the ensembles are pretty emphatic that by 7 p.m. Sunday the 850mb temperatures (around 5000 feet) will have risen to above freezing across the area.

Note on the figure below, the probability of the 850mb temperature remaining below freezing past 7 p.m. Sunday is less than 20% (bottom left).


Probability of temperatures being below freezing at 5,000 feet (850 mb, left) and at the surface (2 meters, right)

(A further caution to snow lovers:  in this kind of pattern with so much southwesterly flow aloft,  the temperatures above 850mb are often a little warmer than the 850mb temps; so the probabilities shown below may actually be a little on the high side.)

What about freezing rain?

That’s much trickier question because surface temperatures especially west of the city are sometimes forecast to rise too quickly when strong cold air damming is present.   The longer the high pressure system stays locked in over the Northeast, the longer the freezing rain may linger out in places like the Shenandoah Valley.  However, when surface pressures are expected to be lower across the Ohio Valley and Great Lakes region than over the Northeast, that almost always yields a low level easterly component to the wind which brings it off the relatively warm Atlantic ocean.  Therefore, surface temperatures will be warmer on the eastern side of the city than the western suburbs and that warm air will try to edge westward with time.

In the image above (right panel), note that east of the city, the probability of freezing rain lingering until 7 p.m. is below 30 percent according to the ensembles but is above 50 percent over the Shenandoah Valley.  These probabilities, especially for our Valley readers, are probably on the low side as the crude model resolution doesn’t handle low level cold air damming that well.

Conclusion

Precipitation will move into the area sometime on Sunday.   Some winter weather is likely for regions along and west of I95 and there is still potential for an ice storm over the Shenandoah Valley.  This looks more like a typical winter tease around the city and points to the east. The precipitation may start as a wintery mix but probably will change fairly rapidly to rain.   However,  cold air damming is tricky and I’ve been surprised during some storms on how tenaciously the cold air holds in at time. So even for the city,  this storm still is worth monitoring.

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Jason Samenow · December 3, 2013