Hold on tight. The week leading up to Thanksgiving will be a roller coaster. We launch into this weekend colder than normal, but then enjoy a shot of milder air to start next week. Approaching Thanksgiving, we have another step down with temperatures falling back below normal Wednesday and Thursday.

The forecast beyond Thanksgiving is a very low confidence one. The models are giving too many different solutions to have much confidence in any of them. Therefore, the forecast is for normal temperatures. That means highs running in the upper 40s to low 50s and lows in the mid-30s around the city.

Highlights

November 21-27

Temperatures: Near normal

Precipitation: Near normal

Chances of snow: Below average

Friday will be another winter-like day with temperatures only rising into the mid-30s and lows falling back into the mid-20s. Temperatures are expected to moderate a little on Saturday with highs rising back into the middle 40s.

Sunday through Tuesday temperatures are expected to run above normal but a frontal system approaching Sunday is likely to produce rain which could linger into Monday. Sunday’s maximum temperatures could be a little tricky as winds off the ocean and rain could lock readings in the low to mid- 50s. Temperatures will feel balmy on Monday, especially if the sun peaks out, as they rise into the 60s, maybe even the upper 60s.

Temperatures will start cooling down on Tuesday but highs still are likely to reach into the 50s.

Behind Tuesday’s cold front, temperatures are expected to fall back below normal on Wednesday and Thanksgiving Thursday though not nearly as cold as this past week. High temperatures are then expected to reach the mid-40s. Lows will probably run in the upper 20s to mid-30s. The good news about Wednesday and Thursday that, while the weather may be brisk, it looks relatively benign.

November 28-December 3

Temperatures: Normal to slightly below normal.

Precipitation: Near normal

Chances of snow: Below average

Confidence: Below average

The average high temperatures during the week typically average in the low 50s and the low temperatures generally run in the 30s, dropping to 36 by December 3. I suspect that there will be days in the 40s and 50s but have little confidence in being able to predict which days will be the warmer ones and which will be cooler than normal. The forecast leans heavily towards climatology as the operational model forecasts have differed significantly from one another over the past couple of days. The ensemble members also are displaying considerable spread among their solutions making this forecast a very low confidence guess.



GFS ensemble mean forecast for the next 16 days (WeatherBell.com)

Technical discussion

Today’s GFS forecasts of the surface pattern for Sunday (below left) and Wednesday help illustrate why we are slated for warming starting Sunday and also why temperatures by Wednesday should again settle back into the below normal range.



(WeatherBell.com, adapted by author)

On Sunday, a low is forecast to track northeastward towards the Great Lakes area at the same time a high pressure system will be located off the East Coast. That configuration will provide a strong southerly gradient of pressure which will help pull warm air towards the area. The complicating factor on Sunday is that there will be a warm frontal boundary to our south across the Carolinas and our surface winds will probably have a southeastward component. Therefore the low-level wind trajectories will be off the ocean. Later in the winter that would really do a number on us but this time of year may not have too much of an impact except to keep us cloudy and rainy if the warm front shifts northward a little quicker than forecast. With sun we’d probably hit the upper 50s, without it, we may stay in the low-to-mid 50s especially if the rain moves in during the day.

The map valid Wednesday afternoon (above right) has the front well to our south and east (the black line off the coast). Instead of being well above freezing, our 850mb temperatures (around 5000 ft) have fallen to -6C. The model also suggests that there may be a reinforcing cold front dropping southeastward through Iowa. Thanksgiving looks chilly.

The forecast through Thanksgiving can be made with a modicum of confidence. The vertical lines one the box and whickers diagram below show very little spread in temperature forecasts for Sunday and Monday’s big warm up. Note how short the vertical lines are.

Once beyond Monday’s warm up, they differ somewhat on the timing of the frontal passage and how quickly it will move through on Tuesday (note the longer vertical blue lines at around 156 hrs). However, for the time range, the ensemble members converge towards forecasts that are in pretty good agreement about the cool down for next Wednesday and Thursday (Thanksgiving).

After next Thursday, the forecast is a big question mark. Why am I so uncertain?

If you at the box and whiskers diagram above, the operational European model forecast (the black line) diverges sharply from the ensemble mean (the green line) by the 28th. The operational European run has been fairly consistent in taking another low to our north which would place us in southerly winds ahead of the front. By contrast, the GFS and a number of the ensemble members differ and do not show the next low going way to our north. The GFS has kept the upper level trough farther to the east than the European model which provides it with a colder solution.

The European ensemble members from last night exhibit considerable differences between the various solutions. They really split apart beyond 10 days. The three figures below display the 5-day mean 500 mb height departures from normal.

Each figure shows the anomaly pattern of a cluster of ensembles that exhibit similar patterns. Blue areas indicate where heights are forecast to be below normal (cold pools of air) and yellow and red shades indicate where each cluster is predicting above normal heights (warm pools of air). Where these anomalies are located on 500 mb forecast are important in driving what type of weather we get.

Unfortunately, ensemble forecasts are grouped into the four clusters. The three most common clusters are shown above.

54 percent of the members had a pattern with above normal heights in the West with an implied trough in the East between the ridging over the West and another positive anomaly around and just east of 50W. Such a pattern probably would offer us temperatures that averaged a little below normal.

However, the two other clusters which represent 40% or the members exhibit above normal heights in the East which would suggest a warmer possibly wetter pattern than indicated by the largest clustering.

The least populated cluster would lead to precipitation averaging quite a bit above normal while the other clusters would probably be near or maybe little below normal. The various clusters are hinting at different scenarios.

The lack of strong convergence among the ensemble member solutions suggests that any definitive forecast made during the week following Thanksgiving is little more than a roll of the dice. For that reason I’ve leaned heavily towards climatology. There is way too much uncertainty to make any semi-confident prediction of how the week might pan out.