More Details on Monday Risk

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By Scott Roberts

We’ve really just entered the time span of the models which will end up giving us the best idea of what we’re looking at for this event. I’m going to break it down by some of the factors I outlined this morning.

Dryline Position

  • GFS: approximately K-14, or a line from Ellsworth to Anthony. Very sharp south of US 50. (Example: 68 at Attica, 37 at Sharon, a distance of about 10 miles.)
  • NAM 3k: Somewhat further west, perhaps as far as a Coldwater to Greensburg to Hays line (US 183). The challenge is, the storms are on the boundary between the rich, mid-60’s dew point air and the mid-50’s air — still somewhat moist. The real boundary to the dry air is many miles west, then we go suddenly from the low 50’s to the low 30’s.
  • HRW FVF3: Also showing storms along the boundary between the rich moisture and the lower-quality moisture, with a broad area of 50’s back to Highway 183, or approximately from Hays to Coldwater and then the sudden jump into the 30’s.

Here’s a comparison. Click on any one to enlarge it. (Really, the important information is the pattern, not the exact placement in relation to one town or another.)

Moisture

 

In May, any of the blues and purples in the dew point graphics are enough to get storms going. That means dew points higher than 60 degrees. Later in the month it might take another 5-8 degrees, but right now 60 is decent-quality moisture.

Capping

While I didn’t talk about this factor this morning, it does play a role — especially when it comes to getting the storms started — initiation. Consider it like the lid on a a pot of water on the stove. If you don’t put a lid on, the boiling water becomes steam and doesn’t condense. Put on the lid and pressure builds up underneath it. Ever had a pot of macaroni boil over? The quickest way to stop it is to put the lid back on the pot!

For severe storms, you want “some” cap so the rising air bubbles some and develops more strength. But you don’t want “too much” because the updrafts will never get tall enough. How much is “some” and “too much?” The values vary day to day, and I suspect the research meteorologists have a pretty good idea for this combination of temperature and humidity what the “ideals” are. I have a more instinctual take on it, and can’t put a number to it.

That said, I believe all three models indicate the potential for meaningful severe weather. We keep solid capping in place through the mid-day, and really don’t get it fully eroded in south central Kansas until 3pm or so. That’s about the right timing.

Shear

The models vary somewhat in the placement of the best shear. Without jumping way deep in the weeds, at this point we believe if the GFS is closest to reality come Monday afternoon, the main risk will be giant hail in south central Kansas, along with a few tornadoes. If the NAM 3k scenario plays closer to reality, the threat would include large hail, but the chance for strong tornadoes would increase substantially. The FV3 (as Brad calls it “the always-optimistic FV3”) moves from a primarily supercell to a primarily squall line event where rain and hail from one storm “seed” its neighbor, limiting hail size. But a line of decently scattered supercells would potentially spread the larger hail risk out. The thing to watch in a line scenario is what we call “tail-end Charlie,” the southernmost storm which has the best air to feed on.

This is what the predicted radar looks like on the FV3 for 5pm:

If that scenario plays out, the primarily area for tornadoes would be along the state line from Medicine Lodge to Wellington.

Bottom Line

These are the predicted radar presentations at 7pm Monday forecast by the GFS, NAM 3k, and NAM runs at 7am (12z) today. Click any of them to enlarge.

My take, with Brad and Matt’s concurrence, is that an outbreak day looks to be a strong possibility. There is still question about whether it is an outbreak of very large hail, tornadoes, or both. That should be better resolved after the 0z (7pm Saturday) model runs, when it comes into the window of the rest of the model suites. Given that it takes several hours for those model runs to start showing up online and that we’re going into Sunday morning, I plan to do our next full Outlook about this time tomorrow. My best instinct is we’ll be looking at an upgrade to MODERATE (level 4 on the 5-level scale) for a portion of the current ENHANCED (level 3) area when the day 2 outlook is issued overnight.

I should point out: in nearly any scenario, the Wichita/Hutchinson metro area is under the gun for all severe weather risks sometime after 3pm.

Safety Reminders

TODAY’S TO-DO LIST TO PREPARE FOR MONDAY: see our post earlier today

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