The days before Thanksgiving are some of the busiest travel days of the year, meaning that the weather has an even larger impact than usual. Accordingly, we have already been keeping an eye on model guidance for this period. While it’s still too far out to forecast with confidence, our November outlook did show some cooling to near average later this month, with an overall warm month in the means. Not coincidentally, there does appear to be some kind of pattern change on the model guidance that has piqued our interest.
Forecast models and their ensembles have consistently showed above normal 500mb heights developing early next week over the Northeast Pacific. These heights extend up to Alaska, which causes the Eastern Pacific Oscillation (EPO) to go into its negative phase. Another ridge initially over the eastern US this will be forced to build into the Davis Strait and Greenland — temporarily bringing the North Atlantic Oscillation (NAO) down into its negative phase as well. This pattern will bring one or two cold shots into the Western and Central US on Thanksgiving week. The main questions are: a) will this airmass make it into the Eastern US, b) if so, how much (if any) modification will there be, and c) how long will this colder regime last?
The image below of Sunday’s 12z GEFS (GFS Ensembles) shows the -EPO and -NAO. But the polar vortex is still over the North Pole, keeping the Arctic Oscillation (AO) more positive. Typically, we want to see this polar vortex further south, underneath the -NAO ridge to support sustained cold. At this time, most ensemble means indicate a trough in the Northeast with some below average temperatures possible, but not an Arctic outbreak.
The -EPO being oriented further west than usual could also lead to a -PNA pattern with a Southeast US ridge. This could keep us near the baroclinic gradient, with a storm track from the Rockies to the Northeast/Great Lakes/Mid-Atlantic. Sunday’s 12z ECMWF shows a frontal boundary with cold temperatures and a weak wave sliding to our south early in Thanksgiving week. Then the Southeast ridge builds by the Wednesday before Thanksgiving Day. If correct, it could be warmer and drier again again. The 12z ECMWF ensemble mean being less amplified with the Southeast ridge than the operational model could be an indication that the projection below is too strong (and thus too warm), however.
One key factor we’ve discussed in our winter forecast is tropical forcing the near the Dateline. This typically supports a trough around or south of the Aleutians Islands. This pumps a ridge downstream over Northeast Pacific and Alaska and can lead to colder weather patterns in the Eastern US. The most recent 7-day OLR (Outgoing Longwave Radiation) anomalies show that the forcing has indeed returned close to the Dateline, as negative anomalies indicate more clouds (leading to less outgoing longwave radiation), which is a byproduct of low-level convergence and tropical forcing.
Another key factor is the current +PDO or warm sea-surface temperatures in the Northeast Pacific that could enhance this ridge, due to latent heat release. The last two winters saw a very robust -EPO, somewhat due to the warm sea-surface anomalies over the Gulf of Alaska. This year, the anomalies are not quite as warm, but they are still warm enough to potentially be a factor in the weather pattern going forward.
Additionally, as previously discussed for our premium members, the stratospheric polar vortex is strong. This could cause the 500mb polar vortex to remain strong and/or re-consolidate near the North Pole or Alaska, which can interfere with the strength of poleward ridging. This may play a part in keeping the -EPO and/or -NAO transient. In that case, the cold air that comes east would be modified and not last more than a couple of days.
So at this time, we think it could be volatile for parts of the CONUS during Thanksgiving week, with plenty of anomalously cold temperatures somewhere in the Northern tier and anomalously warm temperatures in the southern tier. This could lead to a large see-saw between cold temperatures and warm temperatures. The storm track will most likely take a northeast trajectory from the Southern Rockies to the Great Lakes region. But due to the volatility and the fact that Thanksgiving is still around ten days away, we aren’t confident yet on the exact sensible weather in our area for the busy travel week. There is the risk for particularly cold weather at least during a portion of Thanksgiving week given the -EPO pattern developing, but its magnitude and duration remain in question; and if the cold plunges into the West, we could also have a period of warmth within this colder regime.
Stay tuned for more updates here as we get closer to big holiday!
The body of this post was written by Miguel Pierre, with a few additional thoughts and edits by Doug Simonian