Three storms next week, but snow prospects really cloudy

cwg_JunkerThree storms are on tap over the next 10 days and none of them are going to be easy forecasts.  Snow lovers should definitely manage expectations, as there is some chance there’s not a lot at the end of the day.

All three storms have precipitation type forecasting challenges due to relatively mild air streaming in from the southwest at high altitudes.   Strong storms in such flow almost always track to our north putting us on the warm side of the storm – with a mix of precipitation changing to rain.  That’s the most likely scenario next Tuesday night into Wednesday and Friday into next weekend (Feb. 8 and 9). Weaker storms, however, can sometimes lead to snow when a cold front slides to our south providing a boundary for warm, moist air to overrun the cold air in the low layers.  Such a scenario *might* unfold Monday, but it’s a terribly complex forecast.

(For the record, let’s squash the rumor of a huge snowstorm/blizzard next weekend.  We don’t think it’s going to happen.)

Monday’s *possible* snow threat

The set up early Monday, with a cold front pressing south while warm, moist air comes over top of it, could produce snow. But, if the push of cold air is not strong enough, we’ll have rain rather than snow.  If the front pushes too far south, we’ll just get light, nuisance precipitation (if any) – as the storm largely misses us.

The models

A slower southward push of cold air coupled with a slightly stronger upper level system would probably lead to the storm developing farther north giving us more precipitation (the European model solutions).  The last three European model runs illustrate how small changes in the strength of the approaching upper level impulse and the timing of the front might have on our chances of seeing snow versus rain.  Last night’s European model (top left below) spread plenty of precipitation across us but never allowed the cold air to push far enough south to support accumulating snow.  Yesterday’s European model (top right below) kept the low farther south and produced a fairly significant snowstorm over us.   Today’s run is between last night’s and yesterday’s.

(WeatherBell.com, adapted by CWG)

(WeatherBell.com, adapted by CWG)

The NAM model (latest run shown above, bottom left) has been waffling between a complete whiff (latest run above) and a significant storm (last night).

The GFS (latest run shown above, bottom right) has generally been vacillating between a complete miss and a nuisance storm (latest run).  Today’s run keeps the front and storm  well to our south but the model does spit out around 0.08” of liquid equivalent – essentially a dusting.  Today’s Canadian model is in between yesterday’s European model and today’s.  Lots of precipitation but with marginal temperatures for snow.

The NAM and GFS forecast above are only for 3-hourly periods rather than 6-hour periods like in the two European model forecasts above it.  However, they do illustrate the differences in the northward extent of the precipitation with a front that is farther to the south than either European model.  By 1 p.m. the GFS did spread light precipitation into the area, but temperatures are rising above freezing.

Today’s 09Z SREF plume diagram illustrates the ranges of precipitation forecasts of 21 model runs with slightly different initial conditions.  Where the green (rain) and blue (lines) increase in height on the diagram, that’s indicative of falling precipitation in those particular simulations.  The steeper and higher the lines rise, the heavier the precipitation and more amounts are climbing. While there are a few heavier members similar to the European runs, most members predicted markedly less precipitation with a number flat lining and keeping it dry on Monday.  While the average of all members is .22” (around 2 inches of snow), the median is only 0.06” in the same ball park as this morning GFS.

sref-mon

 Conclusions

While all solutions are still in play, the track of the weak upper level center suggests the wetter models may be on to something.  Still, the weather god’s decided to throw us one of those complicated situations where all you can say right now is there is a chance of rain or snow, both, or a miss.  Even if the wetter models are right, the warm temperatures expected Saturday and Sunday would complicate the snow accumulation forecast – especially in the city.  Events in which rain transitions to snow are very tricky and often disappoint snow lovers.

Our low confidence best bet right now – highly subject to change: for our colder suburbs to the north and west: a 60 percent chance of rain, likely changing to a period of snow north and west of the city early Monday morning, with light accumulations possible;  around the city and south and east: a 60 percent chance of rain, possibly changing to snow before ending mid-to-late morning with little or no accumulation.

Tuesday night to Wednesday storm

The models are now in strong agreement that the low slated to impact us on Wednesday will track well to our north and west keeping us on the warm side of the storm.  Because of the similarities between the various models I’ve only posted the latest GFS forecast (below) to illustrate the track and how the low level temperatures warm.  The freezing line early Wednesday morning (4 a.m.) dips into the western sections of our area but then rapidly shifts north.  Low level cold air is sometimes tenacious and holds on longer than forecast so there still is a chance of the precipitation starting as sleet and freezing rain over the western and northern suburbs so the system still needs to be watched but most of the precipitation is expected to fall as rain.

gfs-mon

 Friday into Saturday’s storm

The storm for Friday is looking less and less like a snow storm as the basic pattern of having a trough (dip in the jet stream) in the West and a ridge (bump in the jet stream) in the East holds.  Such a configuration generally forces the jet stream to our north resulting in storm tracking to our north rather than south.  While there are some difference in the location of the green lines representing jet stream simulations (below), all have a dip in the west and a hump or ridge in the east.  With such a configuration any major storm is likely be a big precipitation maker but will end up with the primary low passing to our north.

sat-spag

The vast majority of ensemble members (model simulations) now take the storm to our north..  A few, however, do track a weak low to our south like last night’s European model.  These solutions would offer the best chances for snow.

With its southern track, last night’s European model held onto low level cold air while the deep southwesterly flow warmed the temperatures at 5000 to 7000 feet above freezing offering more of freezing rain and sleet than snow threat. The huge 1040 mb high pressure system that moves across the Northeast prior to the storm does offer freezing rain possibilities if the precipitation comes in quickly enough before the high gets off the coast and starts pumping marine air into the region.   A northern track doesn’t completely rule out some snow at the onset of precipitation nor does it rule out the potential for freezing rain.  Even with a northern track. that’s a real threat.

The bottom line is the storm is looking less and less like a significant snow threat.  Rain or a mixed precipitation event are more likely possibilities than an all snow event.   However, at such a long lead time, it’s foolish to completely rule out any possibility concerning a storm towards next weekend.

Also on Capital Weather Gang

School snow shutdowns: D.C. area versus the rest of the country