Most Read: Local

Posted at 02:05 PM ET, 02/11/2013

Pre-Valentine’s Day storm looks more wet than white; weekend storm?


Could we have a white Valentine’s Day? The odds don’t look so good. Snow lovers may need to hang their hats on an iffy weekend storm.

While the models track Wednesday’s system on a course that - ordinarily - would give us snow this time of year, this particular storm looks more likely to bring rain. Temperatures above freezing really put a damper on accumulating snow prospects.

Likelihood Wednesday storm occurs?

Some uncertainty remains about the exact track of the storm and whether the storm will pass too far south to give us appreciable precipitation, but the model trend is now favoring at least light to moderate amounts.

Wednesday precipitation timing?

Unless we are fringed and the bulk of the precipitation stays south, the precipitation is expected to begin Wednesday afternoon and end sometime in the wee hours Thursday morning.

Wednesday precipitation type?

Most of the precipitation is expected to fall as rain as the temperatures Wednesday afternoon are expected to rise into the upper 30s and low 40s and are not expected to fall below freezing through the event. However, snow sometimes falls with temperatures in the mid-30s so rain changing to a brief period of snow cannot be completely ruled out towards the end of the storm.

While it still looks like a long shot, there is a slight chance of enough snow to produce minor accumulations on grass especially if we get a period of moderate to heavy snow. The far north and west suburbs - as usual - are most likely to witness snow.

Weekend storm?

Some of the models are now hinting at the potential for snow this weekend. This latter threat is wrapped in uncertainty as there are considerable differences in the models’ projections of the weather in that time frame. Tomorrow or Wednesday, I’ll take a closer look at Saturday and Sunday though I offer some preliminary thoughts below.

Technical discussion

Wednesday system

The models continue to differ on how wet the pre-Valentine’s storm will be and on the track.

The NAM has been the most aggressive model over the past several runs and had been the warmest. But today’s run shifted the low track southward somewhat while still keeping the area in over half an inch of liquid equivalent precipitation. Still, it simulates more of the precipitation as rain than snow.

The GFS simulates around half the precipitation as the NAM and it suggests that most, if not all, of the precipitation would fall as rain.

The latest European model is close to a blend of the two models.

While the track of the storm, especially on the today’s GFS model (see below), would normally be a pretty good one for getting snow, this storm’s snow potential is being hampered by the lack of a cold surface high to the north or west.


GFS model forecast of sea level pressure and the near-surface (2m) freezing line (in blue)

Instead of a high pressure system to our north, there is a trough of low pressure (the dashed black line extending from the surface low over western North Carolina northward to the Great Lakes). For snow lovers, temperatures at 1 p.m. Wednesday are above freezing all the way to Lake Erie. There is no cold air anywhere near the area. The storm almost has to manufacture its own cold air.

Without a really strong upper level system like we had with the “Commutageddon” storm in January of 2011, the chances of manufacturing enough cold air to produce accumulating snow in the Washington area looks slim. To get snow we would need a strong upper system capable of producing strong lifting. That doesn’t seem to be in the cards.

More on the marginal temperatures

The forecasts of the near ground temperatures from the various SREF ensemble members on the plume diagram below illustrate the steep hill that this storm has to climb to become a snowstorm.


Remember that ensemble members have their initial conditions or internal physics tweaked slightly to get a feel for the uncertainty about various aspects of the storm. In this case, the most important factor governing whether we see snow or not is whether the temperatures at and near the surface will be cold enough to snow. The period when most of the precipitation is expected to fall is annotated on the figure.

Note when the precipitation starts, all the members are well above freezing. Only towards the end of the event does one member fall below freezing and it is a bit of an outlier. Most members including the ensemble mean (the black line) comfortably warm enough to preclude snow. Only a handful of others (those with the temperatures falling towards 35) would support the rain changing to sloppy wet snow. The air mass with this storm is not very cold and, without, the surface high is unlikely to produce much snow.

A President’s Day weekend storm?

The models differ on whether we get a snowstorm over the weekend. The European model is in the no-storm camp and today’s GFS in the yes.

The forecast from the GFS is shown below.

The set up on the GFS is much more favorable for snow than the one for Wednesday’s storm. The model is showing a strong high pressure system to our northwest and a low developing to our south - a really nice set up for feeding cold air into the area at the same time precipitation is falling.


Today’s GFS 500 mb (18,000 feet) simulation of heights and vorticity (left) and sea level pressure, precipitation and near-surface temperatures (blue line) valid 7 a.m. Sunday morning.

Unfortunately for snow fans, the development of the low pressure sytem is predicated on piece of upper level energy and spin digging southward strongly and then taking on a negative tilt right as it reaches the coast. More often than not, such scenarios don’t work as there usually is another impulse behind it that often robs some of the energy from the leading impulse. Still, the system is worth monitoring.

I’ll write an an article tomorrow or Wednesday on the outlook for the next two weeks and will also take another look at this weekend’s snow potential (if warranted).

By  |  02:05 PM ET, 02/11/2013

Categories:  Latest, Winter Storms

 
Read what others are saying
     

    © 2011 The Washington Post Company