The Washington PostDemocracy Dies in Darkness

Two week outlook: Very cold early next week, then a journey back towards normal

For those looking for snow, the best I can offer is a possible tease tonight. Yes, temperatures are likely to run well below normal through next Thursday or Friday but accumulating snow remains unlikely.

Another system is likely to spread light precipitation across the region towards Monday but will help pull just enough warming northward to give us mostly rain leaving snow lovers the faint  hope of a few non-accumulating flakes falling as the system exits the region.

Behind that system, we should get the coldest shot of air of the season.   The good news for winter weather haters is that temperatures are expected to moderate towards next weekend (November 22 and 23) with the moderating trend continuing towards Thanksgiving.  I’m leaning towards temperatures averaging near to slightly above normal temperatures during the last 5 days leading up to Turkey day and towards near normal precipitation.


November 14-20

  • Temperatures:   much below normal
  • Precipitation: slightly below normal
  • Chances of snow: Low

Instead of the maximum temperatures running in the 56 to 58F range, high temperatures through this Saturday are expected to run in the low-to-mid 40s and lows are expected to drop into the 30s tonight and into the 20s Friday night before rising back into the 30s on Saturday.

Sunday and Monday daytime highs are expected to climb back into the upper 40s to lower 50s with Monday.   Low temperatures may only settle back into the low 40s Sunday night.

A front will be pushing through the area Monday night or Tuesday ushering in the coldest air of the season.  Daytime temperatures Tuesday and Wednesday may stay locked in the 30s with night time temperatures Wednesday night falling into the mid-20s near the city and perhaps some upper teens in the colder suburbs.

During this period, we have a couple of shots at light precipitation, one tonight and another around Monday. Tonight, there is an outside chance of the precipitation ending as a few flakes of snow.  However, accumulating snow is unlikely during the period.

November 21-27

  • Temperatures: near normal to slightly above normal
  • Precipitation: near normal
  • Chances of snow: low

The forecast becomes much more uncertain as we progress ahead in time, more uncertain than normal.

The pattern looks like it will start to relax and that temperatures will moderate back towards the normal for the period.  Temperatures could even creep above normal a few days before Thanksgiving.

Normal max temperatures start out averaging 55 F at the beginning of the period and then drop to 53 by Thanksgiving while the normal low averages 39 at the period’s start and then dips to 37 by Turkey day.  Model forecasts diverge significantly by next Friday with the operational European model bringing one more shot of unseasonably cold air into the region while most ensemble members have the maximum temperatures rising to around 50.  Beyond next Friday, the temperatures are expected to rise back towards normal to even a little above normal.

Precipitation is tricky as the flow starts becoming more southwesterly aloft which usually means that the pattern will become more conducive to moisture being pulled northward from the Gulf of Mexico.  For that reason I’m inclined towards precipitation being near normal.

Technical discussion

The figure below does a good job illustrating the strong agreement between the ensemble member forecasts and the operational European model through next Thursday and how cold the maximum temperatures are likely to be.

Monday (the time period between 108-120 hours) is the one period where vertical blue lines show quite a bit spread reflecting the differences in timing of the next front and variations as to whether a low will be tracking far enough north to pull some warm air into the region.  One member actually had the high rising to near 65 while the ensemble mean and operational European run kept the high under 50.  The latter looks more likely.

Once the front crashes through, the ensemble members are remarkably consistent in predicting temperatures will drop well below normal and they suggest it will feel very much like we’re sitting in mid-winter.

 The box and whiskers diagram (above) also shows how muddled the forecasts gets by November 21.Note that just after 204 hours on the figure above that the black line indicating the European operational run diverges strongly from the green line, the ensemble mean forecast.  At the same time the length of the vertical blue lines lengthens.  That divergence in solutions suggests that there is rising uncertainty about the forecast even though the pattern suggests temperatures will moderate towards Thanksgiving.

You can see why the pattern is expected to be mostly a cold through next Thursday, often with temperatures well below normal by looking at the 5-day mean European ensemble forecasts below.

Medium range forecasters like to look at 5-day mean pattern because the larger scale waves in the atmosphere are easier to forecast than the smaller more transient features.  The 5-day mean 500 mb (approximately 18,000 ft) patttern is shown on the left.  The pattern still holds onto a strong upper level ridge over Alaska and western Canada with above normal heights (the red area, representing a warm pool).  A potent upper level trough is located downstream of the upper ridge with below normal heights (the blue area,  a cold pool) extending well into the Southeast.   That combination produces steering flow from the northern parts of Canada southward into the Great Lakes region.

That northwesterly flow aloft helps drive cold air southward into Midwest and eventually the East.  The 5-day European ensemble mean temperature anomaly forecasts at 850 mb (around 5000 ft) for the same period are shown on the right hand figure above.  The average temperature difference from normal forecast for our area is around 14.5F (8C) below normal for the 5-day period.  With such a cold pattern in place, the operational European model forecast that keeps the high temperature next Wednesday in the mid-to-upper 30s is not surprising and looks reasonable.

Now let’s compare that cold pattern with the one being predicted during the ensuing 5 days leading up to Thanksgiving.

The 5-day European ensemble mean 500 mb forecast above  is quite different from the cold one.  The trough and below normal heights that were centered over eastern Canada and extended southward into our area have swung eastward towards southern Greenland and have been replaced by above normal heights over the Northeast.  The above normal heights that were across Alaska have receded northward and the flow across Canada is much flatter than it was during the previous 5 days.

More importantly, the lack of a strong ridge is likely to keep any really cold air from dropping southeastward from Canada into the U.S.  The area of blue shading across Pacific Northwest that extends southward into the South suggests that some air of Pacific origins may enter the country.   The mean pattern suggests some southern stream energy may start wetting up the south which could eventually impact us.   All the above suggest that the pattern will not be as cold across our area during the latter 5 day period than the previous 5 days.

The 5-day mean European ensemble 850 mb temperature departure from normal forecasts for the same period are shown above (right).  Above normal temperatures forecast across much of the country (the warm colors) including our area.  However, the temperature anomalies are relative weak possibly due to the fact that the various ensemble member forecasts are starting to differ quite a bit but also due to the fact that the flow is becoming more westerly than north to south (meridional) which tends to not produce extremes.

In summary, we should start out with temperatures running well below normal much of the next week but then temperatures should moderate as the pattern relaxes and our flow aloft starts becoming more southwesterly.  Despite the first week’s cold, the pattern does not look favorable for snow, especially the 5 days leading up to Thanksgiving.