We look to have a spectacular first half of the holiday weekend across the lower Great Lakes/upper Ohio Valley region, with a strongly positive NAO and positive EPO pumping up a ridge over the central portion of the country. However, the pattern has changed from what dominated the first half of the month (and provided some spectacular stretches of weather) and may continue to change.
For the second half of October into November, an extremely strong Scandinavian ridge was in place over northern Europe. This resulted in wavelengths that caused a downstream –PNA and troughing into western N. American. This forced ridging or at least a zonal pattern over the eastern half of the country, resulting in well above normal temperatures over the region and for a change, a near to below normal month precipitation wise.
However, the current hemispheric 500mb map above shows that the ridge over Scandinavia/northern Europe has flattened significantly. The large negative 500mb height anomalies have shifted from the NE Atlantic to over S. Greenland, and from the western N. Pole to off the northwestern N. American coast. Now, the result for the region is similar, for this weekend at least, more great weather. However, the models are in relatively decent agreement in shifting this Alaskan vortex into a position that allows for some ridging over the western US, and perhaps supports some cooler weather over the lakes/OV.
In the short term, one shortwave will dive down into the Plains Friday into Saturday. The models still differ significantly with the evolution of this feature. The GFS, its ensembles along with the UKMET appear to be northern stream dominant and close off the trough over the Midwest/Great Lakes, supporting some snow showers over the region Sunday into Monday. The Euro, its ensembles and the CMC appear to be southern stream dominant and show a cut off low over the Deep South. Both models gradually move the cut off north, with the Euro not showing much northern stream energy phasing with the cutoff and just a weak low riding up the Appellations with little if any snow. The 0z Canadian shows a much stronger shortwave diving into the upper Midwest Monday, which then phases with the cut off low and results in a relatively deep low riding through the eastern lakes. This results in a swath of accumulating snow over eastern MI and perhaps northern OH, into southern Ontario.
Given the La Nina, I would initially want to give the nod to the GFS/UK and the dominant northern stream. However, with the very strong vortex over Alaska it may be hard to get enough amplification of the northern stream over the next few days to see a cutoff storm over the lakes. This will be very interesting to watch over the next couple days, as the very active Pacific jet stream will give the models fits. A wide array of solutions remains possible.
Moving on, the GFS/ECM ensembles both show the Alaskan vortex moving NW and weakening significantly over the next week:
Both sets of ensembles appear to show this pattern locking in, meaning that perhaps the pattern is undergoing a shift right now. Both sets of ensembles want to set up some sort of semi-persistent troughing over the Great Lakes/Ohio Valley into early December.
This pattern probably won’t favor a major storm over the region once it locks in (with an outside shot next week as the shift occurs), but it looks like we probably won’t dry torch to start December, and occasional clippers/cold shots may give some shots of light snow, especially downwind of the lakes.