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Nice blog by Evan Duffy of AccuWeather! Checking out his video now.. didn't realize the PV was responsible for the cold in the NorthWest. I guess it is since it was elongated. Interesting
Quote:
Tuesday, November 07, 2017 8:49:02 AM Polar vortex responsible for recent cold may strike again
Recently an elongation of the Polar Vortex (PV) was partially responsible for the major cold in the Northwest. When the PV elongates, cold polar air is displaced and allowed to travel towards the mid-latitudes. We often look for these events during winter following a sudden stratospheric warming event. Outside of the winter months other complex factors interact with the PV and can sometimes lead to displacement. Check out the recent temperature departures in Great Falls, Mont.!:
The PNA index was also partially responsible, with a major negative PNA opening the door for the Arctic air to make a home in the Northwest. A positive PNA would have probably provided an opening in the Central states for that cold.
Model runs have recently shown why the cold disappears. The PV settles back over the pole and the AO index takes a negative run. The PNA also levels off, although slowly.
The six- to 10-day 500 mb pattern corresponds quite well with these changes. With no strong signal, the atmosphere almost always defaults to zonal flow and the northern branch of the jet stream responds appropriately. The southern branch is predisposed right now to some slight ridging between Texas and Florida thanks to the currently warm SST anomalies of the Gulf of Mexico and the East Coast of the united States.
Looking forward, there have been signs that there could be another PV elongation event during the 11- to 15-day period. With some pretty solid consistency, a few of the global models have indicated a Siberia-Canadian Prairies axis elongation (See the 10 mb height progression above). The AO would likely go positive in this pattern and open up the opportunity for some colder air to reach southward. The question is where would these cold shots head?
The PNA and WPO both head back towards neutral during the period, the EPO remains neutral, and the NAO is largely neutral. None of these signals really suggest any good path into the U.S.
But those above assumptions are based on the GFS ensembles which tend to flatten things too much in the 11- to 15-day. Some of the global models are suggesting the PNA recovers faster to positive. This is what the GFS operational shows and opens the door for significant cold air in the East.
So going forward, we are on watch for another cold air outbreak before Thanksgiving. When the GFS ensembles head into the six- to 10-day range the key will be to watch for any signs of the ridge shifting to the West Coast, while making sure the PV elongation is still forecast to occur.
On a side note, if these cold air outbreaks continue at times into November and December, then the snowpack could continue to build south away from the pole at a solid rate, setting up January to be the cold month we have been calling for it to be.
Clearfield is getting a good snow? It's definitely snowing good in northern PA then, Clearfield is in the Susquehanna valley (mountains and plateaus nearby are ~1000+ feet higher)
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