Weather pattern trends wintry towards Thanksgiving

Cooler than normal temperatures are likely for the period from Sunday, November 24 through December 1.

Temperature forecast difference from average through Thanksgiving (WeatherBell.com)

Temperature forecast difference from average (middle black line) through Thanksgiving into early December from the GFS ensemble, initialized at 6 UTC (1 a.m. EST) Tuesday morning. The x-axis labels indicate the difference from average at 6 UTC (1 a.m.) daily from November 20 to December 6 (WeatherBell.com)

During this upcoming period, high temperatures are expected to average in the 40s with lows mostly in the mid-30s.  This compares to average highs in the low-to-mid 50s and average lows in the upper 30s.

Highs and lows forecast by the GFS model ensemble through Thanksgiving (WeatherBell.com)

Highs and lows forecast by the GFS model ensemble through Thanksgiving; the middle number (with the “u” symbol) is the average temperature for the day (WeatherBell.com)

Highs averaging in the 40s don’t preclude having days with highs in the 30s or 50s.  But, at such long time ranges, supplying accurate forecasts of the timing of fronts and when there will be brief warm-ups and cool-downs is difficult.

The pattern is not a dry one and suggests there will be a couple of chances at getting precipitation.  Again, it’s too early to try to glean any details about any particular storm.  However, those traveling during the Thanksgiving week would be well-advised to keep abreast of forecasts along their route.  The models indicate that there is a threat of precipitation – most likely (but not certainly) rain – sometime between Wednesday and Friday of next week.  For those interested in a more detailed, nerdy look at the forecast, read on.

Why should temperatures average cooler than normal across the area?

Last week, I discussed two of the prime players in determining our temperatures and precipitation tendencies.

One of those bellwethers, the Pacific North American, is now being forecast to transition into its positive mode.   A positive PNA pattern typically brings colder and wetter than normal weather to our region.

We can already see cold air building in northwest Canada. Lows this morning ranged from -36 to -40 in spots.

Temperatures today in northwest Canada and Alaska (WeatherBug)

Temperatures today in northwest Canada and Alaska (WeatherBug)

Let’s look at the guidance suggesting the change to a positive PNA pattern.

Classic positive PNA pattern developing before Thanksgiving.

Classic positive PNA pattern developing before Thanksgiving.

The ensemble mean shown to the right is produced by averaging a number of model runs that have had their initial conditions tweaked and then taking a 5-day average of the variables simulated.   Averaging the pattern for 5-day periods helps dampen out the smaller scale features which are hardest to forecast while retaining the larger scale features that are usually easier to predict.  That’s still not a guarantee that the product will be get the pattern right but does improve the odds.   Such a product will not tell you which days will be the warmest or coldest but will often provide s signal of how the temperatures during that period will compare to the norm.

The 500 mb lines on the map to the right show the classic positive PNA configuration.  Above normal heights (red outlined area) are located over western North America indicating that the jet stream is buckling and the jet is displaced northward from its normal position across the West.  The area outlined in blue across the eastern U.S. indicates where the 500 mb heights are below normal with the upper level jet displaced south of its usual position.   On such a map the upper level winds roughly parallel the height lines (the thin black lines) so that weather systems over Canada dive southward into the eastern half of U.S. setting the stage for our below normal temperatures.

The positive PNA pattern has prompted CPC to put out a colder and wetter than normal outlook for Thanksgiving week.  Their forecast probabilities suggest they are fairly confident in this prediction.

How confident are you about the below normal temperature forecasts? 

Like CPC, I’m fairly confident that temperatures will average below normal during the November 24 to December 1 time frame but much less confident about individual days.

Below is a product that displays the forecast temperatures for Sterling, Virginia from each GEFS ensemble member based on last night’s data.  Note that all predict temperatures will be quite chilly once the front passes this Saturday or Sunday.  However they differ significantly on whether the cold air starts arriving Saturday or Sunday.  They also suggest some slight moderation of the air mass towards the middle of next week.  Unfortunately, the product does not display temperature beyond next Wednesday.  However, most of the GEFS ensemble for Thanksgiving day look to be cool to near normal for highs.

Forecasts of surface temperatures from the GFS model ensemble (GEFS) through November 27

Forecasts of surface temperatures from the GFS model ensemble (GEFS) through November 27

Another approach to the temperature question is to look at the model output statistics from last night’s GFS and GFS ensemble (GEFS).  Such statistical forecasts correct the raw model temperature output based on regression equations (statistically they attempt to correct model forecast biases).  The MOS guidance suggests that after this weekend high temperatures on colder days will probably be in the low 40s while they nudge up into the upper 40s or low 50s on the warmer days.

Uncertainty in the precipitation forecast

Atmospheric chaos really shows up in the precipitation forecasts next week.

Below is a graphic showing the accumulated precipitation from November 20 through Thanksgiving eve.  All the models agree that there will be at least light precipitation with the front that comes through the area sometime this weekend.  However, the models differ significantly about when subsequent precipitation might fall.  Two members bring more precipitation through on Tuesday (Nov. 26) while others wait until the day before Thanksgiving; others offer little additional precipitation through at least Nov. 27.

Yes, a few members do display a little snow. But, in my view, away from the mountains snow remains a long shot as any precipitation that comes through Wednesday or Thursday will probably be associated with a system that has a low that tracks across to our north.  A northerly track almost always leads to enough low level warming to keep us in rain rather than snow, especially this early in the “winter” season.

On the other hand, both last night’s European and today’s GFS models indicate there will be a southern stream system tracking across the Gulf States towards our region.   A number of ensemble member support this idea (see below).  However, they differ quite bit on the handling of the system. Some bring it into the region on Thanksgiving while others have the precipitation on Friday; still others keep the precipitation suppressed as the storm scoots out to the south.

This morning’s GEFS ensemble member forecasts of the surface pattern 7 a.m. Thanksgiving morning. 

The models including a number of GEFS members suggest there is a legitimate threat of precipitation sometime between Wednesday and Friday so travelers should keep abreast of later forecasts which should be able to better describe and quantify the threat.

gefs-thanksgiving

The bottom line

We look like we’ll be entering a period where temperatures are on the cool side giving us an early taste of winter.  The period also looks unsettled with a couple of chances of getting precipitation, probably in the form of rain.

Also on Capital Weather Gang

D.C. area forecast: Chilly today, then slow moderation until Sunday's Polar plunge