If it is a Monday or Tuesday it must be time for one of the GFS model runs to go hog wild with the cold and the trough in the eastern US. I say that sarcastically because it seems that the 0z or 12z runs of the operational GFS over the past few weeks but only on a Monday or Tuesday. Sure enough here we are early Tuesday morning and we see yet another major trough and pattern amplification which appears to be over done by the operational GFS on the 0z and the 6z runs at day 7 and day 8... that results in the Midwest east of the Mississippi River as well as the Northeast US seeing their 1st truly cold and mass of the season.

Clearly this sort of overreaction ....which is not supported by the ensemble one of the GFS or by the European or Canadian weather models ...is probably going to be treated with a good deal of skepticism. Of course there is a chance that the 0z and 6z runs of the GFS could turn out to be correct. The Mechanism which causes this possible cold air outbreak to occur does show up on all of the weather models.

The large slow-moving Upper Low in the jet stream now over the Plains will begin to move the East this weekend and merge with a new piece of energy coming out of central Canada. This will form a large and powerful Low pressure area at the surface bringing the first major autumn storm to the Great Lakes and Eastern Canada. It also results in a huge trough over the East coast and Eastern Canada and this trough allows the cold air to move southward on the GFS model.

However these other models show more energy coming in from the Pacific very rapidly which restricts the amount of cold air which can come southward. In other words the pattern is moving too fast for the cold air to really settling over the Midwest and bring about a significant Frost. The morning of the 29th will see strong winds as low pressure area intensifies over eastern Canada and the High comes in from central Canada... so the winds will prevent any sort of significant frost from occurring over the Midwest.

The coldest morning will probably be on the 30th over the Midwest... but NOT over the Plains. The cold air appears to be coming in too far to the east to really affect the Plains. Also the heart of the cold air appears to be too far to the north on the morning of the 30th... with the center of the High staying well N of the northern Manitoba. If the pattern were to relax enough one could surmise that the morning of October 1 would be the real morning to watch the potential for frost-- but that is just the issue. All of the weather models show a new system that slams into the West coast on the 28th and 29th which races eastward into the Plains states by the evening of the 30th... so cold air simply does not have enough time to settle into position.

The 11-15 day looks rather uncertain but at this time there is no strong signal all for any sort of cold or outbreak.

DT
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