THE BOTTOM LINE
The weather models overnight-- both the operational European and operational GFS continue to show a major cold air outbreak for portions of the Plains and Midwest coming in shortly after or around the equinox -- September 22 -- 25. The cold air appears to be strong enough to pose either real Frost Threat or a Frost scare. This is NOT what I would call a hyped event.
THE FORECAST/ sequence of events
1. The first big trough moves in the western Canada this weekend which develops a significant storm for Central Canada early next week. The trough moves into the Upper Plains and Midwest which develops a SECOND Low pressure area over IA that moves into the eastern the Great Lakes by 9/22.
2. That Low goes BOOM and moves into eastern Quebec. The stronger this Low becomes the more cold air will get pushed into the Midwest behind it on 9/23 - 9/24 - 9/25 - 9/26. The European and GFS model YESTERDAY afternoon had this Low over western Québec / Eastern Ontario which allowed for a much deeper penetration of the very cold air (for late Sept) to move into almost all the Plains and WCB with the 32° line pushing into Central Missouri and Kentucky and upper 20s over much of the WCB on 9/24 and 9/25.
3. The 0z WED European model is not THAT cold because he Low that develops over the eastern Great Lakes tracks into eastern Québec... NOT into western Québec/ eastern ONT. The Model still shows a significant frost over much of the Midwest on the morning of the 24th and the 25th. As I said above the intensity and the track of this Low over the eastern Great Lakes will be a big determining factor as to how much cold air gets pulled into the Midwest and for how long. Given that the development of this Low is still 6-7 days away... things are likely to change a bit!!.
4. If you are reading other weather forecast services that are telling you that the models are split or there is a lot of Model confusion as to the frost potential... that should be a clue that they probably don't know what they're talking about. Over the past few days the 0z GFS run has depicted a weather map that is significantly different than the vastly superior 0z European model... only to see the very next run of the GFS -- the 6z run-- turn dramatically towards the European model solution. Good forecasters see these difference in the models and noticed the model trends... bad forecasters will throw up their hands and say maybe .
5. The European ensemble support for the operational European is pretty strong and the 0z GFS ensemble is actually colder than the operational 0z GFS by a significant margin. There may be a second cold Blast /frost at the end of the month.
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When everyone is thinking the same ... then someone isn't thinking --- George S Patton