Thanksgiving week looks warm and wet followed by cold

My last post introduced the idea of a cold and possibly stormy last 10 days of November. The order and timing of events speculated on in that post is becoming more clear, although uncertainties remain.

Teleconnections are still favoring a cold period relative to the means during the last 10 days of November. The EPO (negative) and PNA (positive) have high confidence due to recent model verification while the NAO (negative) has had poor model verification recently (e.g. the models missed the positive NAO during the last two weeks of October).

The key features that will determine the order of events for next week start with a strong storm that will bring wind, rain, and snow into southern Alaska early next week. Warm-air advection ahead of this storm will raise downstream heights, which will amplify the jet stream and shorten the wavelength. This will force arctic air currently in western Canada southward into the northern Rockies. Shortwave energy ejecting from the base of the trough will lead to a moderate snowstorm from the front range of the Rockies into the northern Plains and into the western Great Lakes in the Tuesday-Wednesday period. This open wave solution from the 12z GFS is similar to that of the ECMWF and is a much weaker solution than the more aggresive closed-wave solution the GFS displayed 24 hours ago. The weaker, open-wave solution here is preferred due to the positive tilt of the storm, which will limit the deepening of the center of low pressure.

While it has not been consistently shown on the models, a secondary wave may develop along the frontal boundary as the cold front is impeded by the stubborn southeastern ridge in the Wednesday-Thursday period. This would result in greater moisture convergence from the Gulf and the possibility of significant rainfall across the Mid-South. The likelihood of severe weather looks limited with this system at the moment but this should clarify over the next few days. The coldest airmass of the season will overspread the region Thanksgiving weekend and high temperatures may not get out of the 30s across the Bluegrass with low 40s across the south. There is a chance for more snow flurries across the Bluegrass after the cold front sweeps through the region.

The long range (11/24 – 11/30) looks cold as well as both the GFS and ECMWF bring a another trough into the northern Rockies. This would allow more Arctic highs to swing across the Great Lakes during the period and keep temperatures well below the end of November mean of low 50s/mid 30s.

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