A tale of two weather models

Published 7:00 am Saturday, September 5, 2020

By Skip Rigney

You may have heard that a big cool down was headed our way in the upcoming week. Unfortunately, the chances of that appear to be shrinking.
On the bright side, we should notice a slightly better feel to the air tonight and Sunday thanks to a weak “backdoor” cool front. That means that the front is approaching us from the north and northeast rather than coming out of Louisiana, as the vast majority of cool fronts do.
The arrival of drier air will allow the atmosphere to more efficiently lose heat overnight, which will shave two or three degrees off of Sunday morning’s low temperature. Nothing dramatic, but noticeable.
Meanwhile, forecasters across the U.S. have their eyes on a pool of cold air building over northern Canada. In an example of how the atmosphere is connected over long distances, events in east Asia will provide the energy needed to dislodge the Canadian cold air and send it plunging into the United States.
This past Wednesday Typhoon Maysak brought South Korea 155 mile per hour winds, the strongest to ever hit that nation. As the remnants of the typhoon continued north into China and Russia, some of the tremendous energy from the storm fed into the atmosphere’s upper level winds changing the flow downstream high above the Pacific Ocean.
By Sunday morning, at an altitude of five to seven miles, westerly jet stream winds of 150 mph will reach the northwest Canadian coastline. Those winds will then make a right hand turn and begin to drag the cold Canadian surface air southward toward the United States.
The two best medium-range computer weather models in the world, the U.S. National Weather Service’s (NWS) Global Forecast System (GFS) and the European Center’s for Medium Range Weather Forecasting (ECMWF) model, agree that by Monday unseasonably cold air will be flooding across the U.S.-Canadian border from Montana to Minnesota.
That’s where the GFS and ECMWF model predictions begin to diverge. For the past week, the GFS has calculated that the cold air would continue spreading southward and eastward all the way to the Gulf, bringing a strong cold front through south Mississippi on Wednesday or Thursday, and bringing low temperatures into the upper 50s for the first time since May.
The ECMWF model runs, on the other hand, have consistently shown the core of the cold air stuck far to our west during the upcoming week, with a warm bubble of air holding fast over the southeastern United States.
On Friday morning forecasters at the NWS’s Weather Prediction Center noted that they’ve seen this before. Last fall there were some cases of the GFS in the 5-10 day forecast period bringing cold air too quickly eastward compared to the ECMWF mid-range predictions. As the forecast period became shorter the model solutions tended to meet in the middle, but would end up “verifying closer to the original ECMWF idea.”
Unfortunately, for those of us who look forward to cooler weather, the last few runs of the GFS have been trending toward the ECMWF solution.
So, while it’s still possible that we could have a big cool down by Thursday or Friday, don’t get your hopes up. Instead, let’s enjoy a little drier air tonight and Sunday.

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