After the warm-up, what next? A cooler trend mid-month, with snow chances increasing slightly

January 10
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The long range outlook is more up in the air than the last couple.

Our warmer than normal period that starts tomorrow will probably be short-lived as we transition back towards a below normal temperature pattern sometime around January 15 or 16. But the pattern is not likely to be nearly as cold as the last couple of cold shots. Instead, temperatures will probably average slightly below normal highs and lows (43 and 28 at Reagan National and 42 and 24 at Dulles Airport) for this time of year. However, there will probably be a few warmer than normal days mixed in with the more frequent below normal ones. Late in the outlook period the models are diverging on the possible evolution of the pattern.


(WeatherBell.com)

Precipitation is expected to be near or slightly normal. There does look like there will be a window of opportunity to get snow sometime between January 15 and 19, with the probability of getting an inch or more of snow during any one of 5 days being around 35 percent (slightly above normal).

After January 19, the pattern for snow goes downhill. The pattern slips to one that is slightly below normal for snow despite probably being on the chilly side. However, if models are correct, temperatures towards the end of the period (January 24) could start rising again. Right now, that is up in the air.

January 15-19: the most favorable period for getting snow during the next couple of weeks

This five day period looks like it would average having below normal temperatures (see graph above). The 5-day European ensemble mean 500 mb pattern forecast (below) from last night covering the period has a ridge (red area) located over the west which has forced the upper level jet northward into western Canada and a trough and dip in the jet stream (blue area) over the east. Remember that the upper level winds are roughly parallel the lines on the map implying that our weather systems will be approaching us from Canada; from not nearly as far north as during our last cold outbreak but far enough north to keep our highs mostly in the 30s to low 40s. The mean trough near the East Coast also provides some potential for lows to try to develop along the coast as upper level systems dive southeastward towards the coast.


European model showing average pattern at 500 mb (18,000 feet) between Jan 15 and 20 (WeatherBell.com)

Today’s GFS model is now trying to forecast such development next Wednesday. Today’s European has also come aboard but both models suggest there will be surface temperature issues for accumulating snow. Don’t get wrapped up in the specifics of any model this far out. I’m only showing the GFS (below) to illustrate some models are now trying to develop a low near the coast during the five day period.


(WeatherBell.com)

Last night’s GEFS (GFS ensemble) plume diagram for Dulles Airport (below) shows the two general threats for snow during the period but also indicates how much uncertainty remains for each of them. Such a diagram demonstrates how small differences in data or initial conditions can lead to significant differences in a forecast. Each line on the chart represents a GFS model run that has identical physics but slightly different initial conditions.


The lines are green when precipitation would fall as rain and blue as snow. When lines are parallel to the x-axis and are flat-lining, no precipitation is falling. When the line is climbing, precipitation is falling and the steepness of the climb tells you the intensity of the precipitation.

Note that once the lines turn from green to blue (around the 15th), there are two times when some of the lines start climbing indicating that some of the models are predicting snow. However, at those same times, other lines are flat so those ensemble members are dry, probably reflecting the fact that the model simulation formed no storm or developed it too far offshore to give us snow.

We’ll be watching both potential threats over the next couple of days. My guess is that the probability of getting at least an inch during one of the 5 days (Jan 15-19) is about 35 percent, slightly above normal.

Jan 20-24 period: cloaked in uncertainty but still probably chilly but not frigid

The European ensemble mean and GEFS for this five day period differs quite a bit across the Northeast with the European model keeping a cool look a little longer than the GEFS. Both ensemble mean maps have above normal 500 mb heights across the western U.S. and much of western Canada but they differ along the East Coast with the European ensemble mean holding onto a stronger trough, especially over the Southeast (not shown). The dip in the jet stream over the East allows cooler than normal air to hang in over this region during the period. Reflecting the dip in the jet, the European ensemble mean from last night was forecasting 850mb temperatures (altitude of 5,000 feet) to average below normal across much of the East during the January 20-24 period (see below).


(WeatherBell.com)

The GEFS super ensemble mean and its analogs (similar patterns that have occurred in the past) agree that the period will start off being cool but its analogs also suggest there is potential for warm air to start creeping into the area sometime during the period. I’ve composited (averaged) the observed temperature departures from ensemble mean analogs (below). I wouldn’t get hung up on the timing of the analog’s cold versus warm periods. At such long time ranges, trying to be specific about timing is pretty foolish. Basically, the GEFS superensmble mean analogs suggest the period will be near or slightly below normal.


The analogs from  GEFS and European model were not very bullish on snow. Each only identified two snowstorms within the five day periods suggesting that after the 19th, the pattern is not one that is conducive to getting a significant snowstorm (famous last words).

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