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Chase forecast update for May 20-31
Things have taken a turn for the worse regarding the outlook for the remainder of May. Models are now showing only a brief period (2 to 3 days) of strong mid/upper southwest flow over the Plains from the western trough before it retrogrades back to the west. After this, another strong ridge builds in by the weekend and plants itself firmly for as far out as the models show.
The few days of good upper support normally would be something for expeditioners to get excited about, but a good chase setup depends on more than just upper flow. The surface and low-level environment shown by the models (Euro, GFS and now partly the NAM/WRF) is disappointing. Deep moisture still struggles to make it north to underlay the upper jet's strong winds, leaving unseasonably weak amounts of instability in the areas with good shear. Low-level wind fields (850mb) are shown as veered, which typically bring more hot, dry air into the low levels, further diminishing an already anemic, mixing-prone surface moisture profile. And probably the biggest issue with this system is the very strong cap (warm 700mb temperatures) that may very well keep storms from firing at all in any place where directional shear is even marginally good enough for tornadoes.
Is the situation unchaseable? Not at all. If the cap does break in scattered areas that can see good moisture pooling underneath the strong flow, supercells and tornadoes could result. But I think that will be a difficult setup to find, at least the way things are depicted now. There may be multiple potential targets that are great distances from one another, forcing expeditioners to choose one and have no way to reach the other after committing. Each day shows the more likely outcome being "blue-sky busts" with no storms in areas with tornado-adequate parameters, or a storm going up in another faraway target area. Chasers will have to take a gamble: go where storms are more likely but with weak instability, which in this week's case, will be far, far up north (like Minnesota and North Dakota), or stay down in the better instability and risk the 'cap busts'.
I'm really on the fence about observing any of these events in the Plains. There is very high bust potential. For now, I think the upcoming setups will have to be evaluated either the day before or the day of as to whether an expedition will be merited. If the models were to be spot on, the only decent play I see is possibly Wednesday in Iowa - but that would technically be a Midwestern expedition day!
Beyond Friday, frankly the models have the look of 'spring peak tornado season over' with a very strong ridge in place over the Plains and Midwest. The trough to the west seems to be locked in place, only providing upper flow in the extreme north (Montana, North Dakota and Canada) which will likely only have marginal surface moisture to work with. Remember 'peak season over' doesn't mean 'no more tornadoes' - marginal setups can and do produce on rare occasion (as Saturday in Kansas demonstrated). What it does mean is that the easy-to-forecast-and-expedition, high probability events might be mostly over for the season. As with any year, it's not over until it's over. Let's see what June brings - it might just surprise us all.
The following table plots the chance of a Plains weather expedition happening in a particular date range (Midwestern expedition days are not factored into this table):
|2012 Plains Storm Expeditions - Probabilities as of May 20|
|Midwest Chase Probabilities: The following table outlines the probabilities of a Lower Midwest chase (within 5 hours of St. Louis) happening within a particular time frame:
|2012 Midwest Chases - Probabilities beginning May 20|
Midwestern chase forecast discussion (Update based on 5/20 00z data): Storms in and around the St. Louis metro look almost certain on Sunday afternoon-evening, although the parameters are very poor to support much more than some marginal lightning opportunities. Nonetheless, I have the day off and will be out observing. Things will be quiet around St. Louis for possibly a very long time thereafter. I'm looking at a possible Iowan expedition on Wednesday (which I'd consider a Midwestern, not a Plains, chase) if models are correct about parameters there on that date. A portion of 40 knot flow is shown overspreading an area of modest instability centered on the state, and storms are shown firing in the afternoon. It's not a perfect setup, but looks worth a vacation day given its closer proximity. I'm giving it only a 40% chase probability due to the likelihood of things changing drastically in the next few days.
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