Much to the dismay of storm chasers across Tornado Alley, the 2018 severe weather season is off to a dismal start. On May 25th we're about at the midway point in the climatological peak season for tornadoes which generally spans the months of April, May, and June. This morning, the preliminary 2018 tornado count stands at ~ 59% of average year-to-date with only 311 tornadoes reports compared to a typical year which would have seen over 500 to this point.
Largely to blame is a ridge of high pressure which has shoved the jet stream well north into Canada, an alignment that's more typical of mid to late summertime. While ample moisture from the Gulf of Mexico has more or less remained present across much of Tornado Alley, support from the jet stream in the form of the necessary wind shear to create rotating supercell thunderstorms capable of tornadoes has not.
All that said, the tide may be turning as we head into June. Let's not attempt to close the book on a slow year just yet, as Mother Nature is more than capable of snapping back to the climatological norm in a hurry. There is mounting evidence that there may at the very least be a late season push.
Memorial Day Weekend, a favorite among storm chasers appears to remain relatively quiet. Only isolated pockets of severe storms are expected during the last week of May, none of which are expected to make a dent in the 2018 tornado count.
I do not currently anticipate a sudden complete reversal from the current ridge into a classic Tornado Alley setup featuring a large trough over the Rocky Mountains which would prime the more typical areas from Kansas into Oklahoma and Texas. But, there is growing consensus among long-range models that we'll see some combination of a slow westward migration and/or flattening of the ridge which would allow for upper level jet stream support to return to at least northern portions of Tornado Alley.
The loop below shows the evolution of 500mb geopotential heights from late May into the first 10 days of June. What you'll see is the large 'Omega' shape, or the ridge of high pressure that has shoved the tight gradient in heights well into Canada begin to flatten out and slowly migrate westward. This leaves the circled region from the Dakotas into the Corn Belt in a region of "northwest flow" which lends itself toward a 'Ring of Fire' pattern where frequent storm systems travel along the outside of this ridge. The circled area would be the favored region for severe weather at the end of the loop in early June:
This is illustrated by 850 mb temperatures as well. At the beginning of the loop with the large ridge of high pressure present over the Central US, summer heat has spread well into Canada. Over time as the ridge flattens and migrates westward we see a return of cooler temperatures to the northern US, and a boundary setting up between the early summer heat in the Central/Southern Plains, and cooler temperatures in the Upper Midwest/Great Lakes. This circled area along the boundary between heat/cooler temps would be the favored area for the development of storm systems and frequent clusters of thunderstorms:
One more illustration in the form of 250 mb winds, or the uppermost reaches of the jet stream.
First, let's look at the alignment today. The brighter blues and greens showing the strongest jet stream support are well into the northernmost territories of Canada, with only minor support from the sub-tropical jet stream over southern New Mexico/west Texas. Again, this keeps the Central US and much of Tornado Alley under sunshine and summer heat:
Let's fast-forward to the first week of June. Much better! The large ridge is gone, and while there may not be a large trough, or dip in the jet stream over the west coast, we've at least got a more zonal flow that will allow for shortwaves, or pockets of faster jet stream winds capable of coupling with plentiful June moisture and instability to kick off storm systems and perhaps some of the larger severe weather days of the 2018 storm season:
I've chosen to show graphics from the ECMWF or European long-range model thus far, but there is support for this shift elsewhere.
The GFS for example, also shows a favorable pattern shift. We see a westward migration in the ridge which would favor an increase in severe weather in the circled area:
The Canadian GEM also shows the potential for northwest flow, and an uptick in severe weather across the Northern Plains/Midwest in the first 10 days of June:
Currently, the area that seems most likely to see a significant uptick in severe weather production covers the Northern Plains into Upper and Mid-Mississippi River Valleys, or the Dakotas into the Corn Belt states of Iowa, Illinois and Indiana.
It's much to early to begin speculation on individual storm systems or local regions under the gun on specific dates. That said, beginning as early as the first weekend of June there seems to be some early signal at a revival of the 2018 severe weather season perhaps with an even further uptick in activity beyond that into the June 5-15 time range. We're getting way out there into La-La-Land where the long-range models have struggled mightily to nail down pattern evolution dating back to the end of winter. All that said, the evidence is mounting and my confidence is growing in a June spike in tornado activity across the northern half of the Great Plains into the Midwest.