FORECAST VALID FROM SEPTEMBER 1ST TO DECEMBER 1ST (Meteorological Autumn)
It’s that time of the year again, when the kids go back to school, the leaves are beginning to change (much earlier than usual) and cooler air masses are beginning to take over, the hot and dry weather we had over the summer is now becoming a distant memory as much of the heat that covered most of the nation is being suppressed and forced to the South and West , more confined to the southwestern and western US.
We have been talking about the summer pattern break down since early August, and its been slow and steady as El Nino slowly and steadily develops, and we have made a lot of progress , from July when everywhere in the US besides the Pacific NW was above to well above average, to August where the real heat shifted west and was more so confined to the Rocky mountains points west, with occasional but not steady hot humid spells east of the Rockies, and that brings me to the first factor and a major factor in this forecast
The PNA has been, is , and will continue to stay positive throughout this autumn season, this is indicative of ridging in the west and thus heat and warmth in the western US. There will be times when the PNA ridge collapses and warmth will spread east but I expect a ridge of high pressure to be dominant overall out west, and that will lead to a bit of a dip in the jet stream along the eastern Rockies and points east.
I don’t expect this dip in the jet stream or in other words , this trough to be dominantly deep and amplified over the easter 2/3 rds of the nation but more so weak and broad, especially in the September and early parts of October.
Beyond mid October, I expect the pattern to become more amplified at times ,with a deeper trough in the eastern half of the nation as we begin to see El Nino really ramp up and take hold, and I expect the chances of coastal storms to increase by the second, and of course the summertime Bermuda and Atlantic ridge to begin to weaken. But even in late October and November, I don’t foresee a deep and dominant trough over the east and still periods of warmth, pretty much your typical autumn weather with ups and downs. I do expect some shots of cold and below normal
The Arctic Oscillation is beginning to go negative, as the polar vortex begins to descend south, aided by typical impacts of El Nino and the positive PNA phase which means it will be much easier for Canadian air masses to drop south occasionally this fall season and so I expect our share of cold shots .
And finally , te final thing I am going to cover is the North Atlantic Oscillation
Right away we see it has been dominant negative and about to dip down again, it doesn’t have much impact during the summer months, but now as e head into fall, it will begin to have a much bigger impact and signs point to it staying negative, and as long as its negative (which means a dominant ridge of high pressure is over or around Greenland) it will slow down the pattern, help troughs stay in the east and enhance chances of coastal storms this fall, but again especially for the first half of fall, I don’t expect troughs to be real deep because of how strong the Bermuda high is this year, it will continue to occasionally have influence on the east coast (in the form of warm and humid spells) most likely through majority of this autumn
Here is what I expect dominantly through autumn, with the black line being a rough estimate of where I expect the jet stream, now like stated above, there will be times where the trough over the east is deeper and stronger with cool to cold shots and times where the PNA western US ridge breaks down and we warm up in the east to above average levels, and times (especially early in the season) where the Bermuda high expands west and brings in warmth and humidity (as we have seen plenty of this first week of September). But overall I expect a pattern change starting this weekend (September 7-9th) that will lead to a trough being dominant in the east, weak at times and strong at times. I also expect the heart of these troughs to be over the Midwest, and with that so will the coldest anomalies, which is why I have below average temperatures in the upper Plains and Midwest.
Overall the balance between the troughs/cold shots and warm ups I expect temperatures to end around average in the NE , interior areas of the NE could end slightly below normal. If the Atlantic ridge and Bermuda high was not so strong I would expect more in the way of cold shots and thus below average temperatures.
Precipitation to be near or slightly above average for the forecast area, as we are entering and will continue to be in an active pattern with the developing El Nino and eventually strengthening southern or sub tropical jet stream.
Its way to early to say for sure but a lot of signals point to a POTENTIALLY VERY INTERESTING winter coming up for the eastern US , hint, as in interesting I mean the OPPOSITE of last winter. I’m going to start piecing things together throughout the fall and come out with occasional thoughts on how I think winter will play out, as winter is my favorite season of the year!!