About the chance of wintry weather Saturday, and the outlook beyond

December 11, 2013
cwg_Junker

Another round of winter weather is possible on Saturday but how it plays out and whether we have a wintry mix, a cold rain or both is a big question mark.  Beyond that, over the next two weeks, we see a slow transition from a cold pattern to a milder one, before possibly flipping cold again towards Christmas .

Saturday storm overview

The overall pattern Saturday is similar to the one associated with our snow and freezing rain event Sunday, but the air mass does not look quite as cold.

The models are predicting that another strong Canadian high pressure center will be located in a favorable spot to funnel cold air towards the region as a low pressure system tracks towards the Ohio Valley before likely reforming along the Atlantic coast.   The low will provide moisture while the high pressure area keeps temperatures just cold enough to threaten the region with more wintry weather.

Unfortunately, the models differ on the details about how cold the surface temperatures might be when the precipitation arrives, and how quickly milder air streams in and scours out the cold.

This is another one of those tricky cold air damming events in which the air near the surface is sometimes colder than forecast by the models even as warm air can come in fairly quickly aloft.  Therefore, we often end up with a mix of precipitation types  prior to any changeover to rain.  Usually, the western and northwestern locations like Leesburg and Frederick end up with more wintry weather than the District and points east.  More often than not, these type of events are a nuisance rather than disruptive.

Right now, our best bet is a period of mixed precipitation Saturday morning, changing to plain rain in the afternoon and night.   But, with the storm 4 days a way, details in the timing, amount, and type of precipitation will probably change some.

The model forecasting dilemma

The NAM and most of the 9Z SREF model members favor temperatures remaining above freezing through most of the event and suggest that the storm would be primarily rain while the latest European and GFS models suggest some mixed precipitation before a change to light rain.  Last night’s European model conveyed a more snowy scenario. Such model diversity makes any definitive stab at forecasting how much winter weather this system will produce this early in the game a rather futile endeavor.

Last night’s European model forecast for Saturday is a poster boy for cold air damming and offered the most wintry look of the various model simulations in the last day or so.  Its depiction of a big surface high over Quebec (below left) and a trough of low pressure across the North Carolina screams that cold air could hang over the area for most of the event.   It simulated temperatures at 5,000 feet (850 mb) are around -4C at 7 p.m. Saturday (below, right) – plenty cold enough for snow in D.C.


(WeatherBell.com)

However, last night’s operational European model was probably an outlier.  The inset on the figure above (on the right panel) shows the European ensemble mean forecast for the same time as the European model.  The European ensemble mean forecast  temperatures at 5,000 feet (850 mb) are decidedly warmer than the operational model.  The bulk of the European ensemble members (alternative versions of the operational model, with initial conditions tweaked) predict  a brief period of snow but would also argue that most of the precipitation across Washington would fall as something other than snow, probably as rain or, to our west, freezing rain. Last night’s European model is pretty much a worst case scenario. It’s  possible – but probably not the most likely scenario.

This morning’s NAM model would suggest little in the way of winter weather as it raises temperatures to above freezing across the area fairly quickly.  The GFS (see below) is a compromise between the two more extreme solutions.   It advertises a quick shot of light snow at the onset quickly changing to rain across the District and for points east.  However, it hangs on to freezing rain for the far western suburbs (see below) through much of the day.  Right now that looks the most likely solution but any of the three different solutions remain in play.


(WeatherBell.com)

The new European model from this morning (just out) supports the GFS solution and is not like the cold, snowy version of the same model from last night.

So what’s the bottom line about Saturday?

Precipitation is expected to move into the area Saturday morning possibly beginning as snow which will probably eventually change to freezing rain and then rain. How long the cold air hangs in place is very much in doubt.  The best chances for impactful winter weather are in areas well west of D.C. but even folks in the District need to monitor the storm over the next two days.

What about a general two week outlook that includes Christmas week?

Temperatures through the middle of next week should be on the cool side.  They’ll be quite cold the next couple of days but with the temperatures rising to near normal by the middle of next week.  Then look for moderating temperatures, probably rising above normal for several days before the next cold shot arrives.

The GFS ensemble (GEFS) display the warming that is expected during week leading up to Christmas.  However, ensemble members have a large spread (the length of the vertical blue and green line) on the box and whisker diagram below.  That’s pretty typical for a week two forecast but is also warning that all such long range outlooks are cloaked in lots of uncertainty especially in the timing of fronts.  That said, the warm up looks like a pretty good bet.  However,  the duration of our warmer than normal period is up in the air as is the duration and timing of of the next cold shot that the models are indicating might arrive towards Christmas day.


Temperature forecast from the GFS ensemble mean out 16 days shown by black line – with green bars showing temperature range from different ensemble members. The gray line in the background is the historical average. (WeatherBell.com)

The 500 mean height pattern (below left) helps explain why we expect a warm spell to come to fruition and also gives a hint of why the warm air may be transient (short-lived). The dip in the jet stream or (blue area) is located over the western U.S. during the 5-day mean period ending Dec 24.  Examining a mean pattern helps make the location of the longer wave features stand out.  The dip in the West promises to cause our winds aloft to average from the southwest direction during those 5 days.  The southwest winds aloft produce warming temperatures; as such, the temperatures at 5,000 feet (850 mb, below right) average above normal for that 5 day period.   Such a pattern also favors storms tracking to our west – in any storm’s warm (rainy) sector.


(WeatherBell.com)

The complicating factor is the ridging or orange area poking into Alaska on the 500 mb (altitude of 18,000 feet, roughly)  forecast and the implied northwesterly flow across western Canada  into the northern plains.  Such a pattern usually leads to the development of another cold high pressure system that then plunges  south into the northern Plains.  Eventually some of that cold air gets to us after a storm passes to our north and drags a cold front through – maybe with some rain.  That is what the GEFS ensemble temperature forecast on the box and whisker diagram above seems to be playing.

After the front comes through and drives colder air into the region, we’ll have to watch to see if a wave develops along it – bringing another round of messy, overrunning precipitation around or shortly after Christmas.

For this two week period, we’ll probably average normal to a little above normal precipitation with the best chances of above normal precipitation being to our north.

Comments
Show Comments
Most Read Local
Next Story
Ian Livingston · December 11, 2013