Back in August when the most asked question seemed to be about when the first widespread frost of Autumn would potentially occur, I would answer with September 20-25 would be my best guess. In fact, within this blog in late August I made a similar “projection” about that time period. Of course when talking about September, there was going to likely be earlier time frames with lows in the 30s, which was the case and in fact, some frost was reported on the mornings of September 10 and 11. Pockets of frost are typical of most Septembers (even August) before the first widespread frost of the season. This year was no exception.
Looking forward to the first widespread event, it no longer appears the September 20-25 time frame will end up being the period for that first widespread frost. The situation foreseen in late August was for the active typhoons in the central and western Pacific ocean (which has ended for now), would eventually cause a large ridge to form over the northern Pacific ocean by the middle of September as the “energy” from the typhoon(s) recurved toward Alaska. This in turn will “force” a trough to form downstream from this ridge into the western part of the United States. Below is a northern hemisphere 500 mb (18,000 feet) height and anomaly map from 7:00 PM CDT September 15. The expected pattern projection did occur and with that trough, North Dakota is now in a southwesterly flow (mid and upper level wind blowing from the southwest).
The projection from this point was for a storm to form near around September 18-20, bring rain/thunder to the state, followed by a high pressure center to build in from Canada and considering the time of year, frost potential would be high. Although there is a storm system currently in Wyoming that will be moving north, the cold front that is moving through the area today (preceding the Wyoming storm), will in turn keep most of the moisture availability away from that Rocky Mountain low pressure center.
Therefore, in the next 24 hours there will be some rain / thunder but it looks mostly light and hit and miss.
Once this system moves through, there will be another storm system passing to our south and to our north in this “southwest flow” pattern, not allowing any polar or arctic air to move southward. Some lows on the upper side of the 30s will be possible on Friday and Saturday morning in particular, but most locations again, avoiding a frost, and with the next storm expected to go north of North Dakota early next week, that will mean the next high pressure (clear sky / light wind), will likely stay to our north and east around the September 24-25 time frame. These scenarios would greatly lessen (or eliminate) any frost potential from September 20-25 time period.
Part of the logic of the next polar high staying to our north and east will be that although the overall upper level wind will stay somewhat southwesterly, next week (loop above) will be actually be more west/southwest, which won’t allow deep troughing or ridging necessary for a strong push of colder air into the United States during this time frame. This is an example of “seeing” the pattern change 3 to 4 weeks in advance, but yet, the end result is not exactly as was expected. So being right and wrong at the same time. Therefore, the next widespread frost looks to be holding off until the end of next week at the earliest which means September 25-30 scenario (or later) for freezing temperature to be occurring this year for a majority of this area.
In the short term, still more days above average than below over the next 6-10 days, precipitation, although some areas will record some, mostly light is expected based on the projected pattern during this time frame.
Daryl Ritchison