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                   Wednesday, May 16, 2012 - 6:55AM CST

Event forecast update for May 16-26

By DAN ROBINSON
Editor/Photographer
25 Years of Storm Observing
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Regarding the Plains, there are a couple of storm forecasting items to talk about today. The first is an anemic trough that is slated to affect the central and northern Plains Friday and Saturday.

As you can see there, although we'll have some southwest flow for a time where we need it, it's just too weak to do us much good after the long-term ridging preceding it. We need a stronger trough to help create a good southerly fetch from the Gulf to transport deep moisture, then that same trough needs to provide strong mid and upper level winds above that moisture to create directional shear needed for supercells.

While this configuration can produce chaseable tornadic storms given a few difficult-to-predict pulses of stronger flow, it needs the moisture component at the surface to be robust enough to a.) break the cap, b.) prevent too much mixing out, and c.) provide lower-based storms. I don't believe we'll have enough moisture to make for a great event on Saturday, but I'll concede that there *could* be a tornadic supercell somewhere in this setup. Is it worth a long trip? I don't think so. We're looking at one, maybe two if we're lucky, conditional storm observation days over Kansas and Nebraska. For me, I'm still not impressed enough to take a couple of vacation days to go for it.

Now let's look beyond the weekend. After many days of agreement on a storm observer's worst possible forecast of a persistent 'death ridge', the models have been thrown into some confusion about what the last half of May has in store during the past 36 hours. This can be either good news or bad news, as eventually we'll see some pattern emerge from the haze. Will it be a good one, or a continuation of the doldrums of late?

I like the massive westerly jet over the Pacific. That's the type of energy that can do some damage to the current boring pattern entrenched over the Plains, providing those winds can amplify into a nice trough as they come on shore and plow eastward. The fantasyland GFS (past 180 hours) has it doing just that, but the Euro model has it staying rather flat and skipping over the ridge to the north like a flat stone on a lake surface. Unfortunately, the Euro model has traditionally been much more reliable in the long ranges, so I'm inclined to lean in its direction, as painful as that may be. The Euro's solution shows the broken record continuing, with the good flow staying too far north and deep moisture too far south to do any good.

What's worse, the Euro's depiction of temperatures in the southwestern US becoming a summer-like furnace does not bode well for the rest of the season. That region is where our EML (cap) comes from, and a 'nuclear' cap plus weak upper flow and struggling surface moisture simply means no storms when and where you'd want them.

The good news, again, is that the Euro hasn't been 100% consistent with this pattern (and the GFS disagrees), so things could still change. However, I'm simply not optimistic yet about a Plains trip for as far out as I can reasonably see.

The following table plots the chance of a Plains weather expedition happening in a particular date range (Midwestern storm observation days are not factored into this table):

2012 Plains Storm Expeditions - Probabilities as of May 16
May 16-225%
May 23-3110%
June 1-725%

Midwest Event 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 16
May 162%
May 17-192%
May 20-2160%

Midwestern storm forecast discussion (Update based on 5/16 00z data): The Midwest should stay fairly quiet through the end of the week. On Sunday and Monday, an approaching cold front underneath a little bit of upper support from the (weak) trough traversing the country should give our region a good shot at storms. As models depict it now, a lightning expedition or two looks likely during this time.

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