I’ve written about persistence forecasting before: that is, throw away the models and when you haven’t had rain a long time… don’t believe any scenario with significant rain until it actually happens or at least you see radar all lit up to your west! That will largely apply in this upcoming system. The Twin Cities hasn’t had a rainfall higher than 0.11” since August 27th, that’s remarkable and would be almost exactly 8 weeks come Sunday.
With all that said, this is the best set up we’ve had probably in 8 weeks for significant moisture. Will it land ideally on the areas of severe and extreme drought in southern Minnesota? It’s hard to say that just yet, but it’s not impossible.
My anecdotal theory that the storm tracks could be changing is that these extreme swings in temperature patterns may finally be shaking things up. Look at our roller coaster since September 18th:
Wide swings are normal in the fall in Minnesota, but this year has seen swings in highs and lows that are about 8% more extreme. That’s not nothing. The polar vortex has had a second ‘squeeze event,’ which resulted in our latest cold spell.
On the right is a look at the stratospheric pattern earlier this week. The polar vortex, when it’s strong and stable (and we don’t see big cold snaps) should be nearly perfectly circular rather than oblong. Excessive heat out of the west and out of Europe have literally been putting the pressure on the vortex.
What this does at the very least is sort of like shaking up an etch a sketch, allowing stuck patterns to at least move elsewhere or become ‘unstuck.’ There is research that shows we are seeing more and more of these disruptions to the polar vortex as a result of climate change.
Regardless, the pattern is becoming more active over the upper Midwest. We have a well oriented, energized jet stream for one:
Secondly, not only will it be very warm on Sunday with highs in the 70s (which is 20 degrees above normal):
But we have a surge of Gulf moisture moving north. Dew points could top out near 60 late Sunday into Sunday night, which is pretty muggy for late October:
The moisture values for Sunday night will be in the 2 to 4 standard deviations above normal range for the date, based upon the 1991-2020 climatology. If you extrapolate that alone to climatological precipitation values, you come up with 0.50” to 1.00” of rain.
There is even the threat of severe weather. The European model’s forecast lightning shows a line/cluster of storms moving in from the southwest late Sunday evening.
The ingredients for severe storms late in autumn is different than mid summer. Instability is sometimes easier to achieve as the atmosphere aloft is usually colder and intense energy can move in from the south (temperature and dew point combination).
CAPE (convective available potential energy) isn’t terribly impressive, but it’s enough for thunderstorms and indicative of instability. The issue here will be that this activity will come very late in the evening and overnight, which is often a deal breaker this late in the season. The best possibilities may be some large hail. The storm prediction center has ‘low predictability’ on any talk of severe weather that far out at this point and for good reason. Storm chasers are getting excited about it, but they usually over blow every opportunity. J
We will be lucky enough to be on the warm side of this system, but since it is late October, it will be a classic fall storm with snow on the north end. The black hills, up through Manitoba can expect snow, while we see showers and thunderstorms.
Trying to pin rainfall amounts is tricky business this far out, but many models do create widespread 1 to 2 inch values. They hardly disagree on where however. The more conservative, and sane ensembles of the models seem to be focusing more to our north or east on the heaviest chances.
Here’s the European ensemble (many different runs of the same model with ‘tweaks’ and averaging the outcomes) for the probability of 0.50” or more Sunday night into Tuesday night:
Either way, fingers crossed we get some significant rain. We desperately need it and no matter what, this will not end the drought but could help make a significant start.
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