A Couple Chances for Severe Weather This Week


Risk Overview

Valid Starting
Monday April 17, 2017 12:00 am
Expires
Saturday April 22, 2017 12:00 am
Primary Risk
Thunderstorms
Other Risks
  • Thunderstorms
  • Hail
  • Wind
  • Tornado

Total Severe Weather Risk


Slight


Potential Impact


Medium

Lead Tracker Scott Roberts‘s Confidence


Likely

Summary

Some models are pointing toward a risk for tornadoes, severe hail and damaging wind Wednesday. Right now most of them have converged on the northern half of Kansas and a little bit of southern Nebraska as the main risk zone, but one fairly reliable model is depicting things much further south. Another model, also fairly reliable, jumped way north in its last run.

The main positives with this setup in comparison to the last few are a better forecast moisture profile and better timing of a mid-level impulse.


Details

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Throughout this briefing I am going to use images from the three models most in agreement at the moment: the GFS, the CFS and the Euro. Let’s compare their forecasts for several important factors in storm development.

Ingredient 1: Moisture

Dew Point Temperature, GFS Model, 7pm Wednesday

Darker blues are showing a couple of nice pools of moisture by Wednesday evening ahead of the advancing dry line.

Moisture is fuel for thunderstorms. Warm, moist air transports energy into areas where storms can develop. Generally it’s a good sign when the atmosphere has two days to recharge moisture form the Gulf after a system fully clears out of the Ark-La-Tex and winds switch back around to the south.

The GFS (upper image, click to enlarge) shows dew points in the upper 60’s from just southwest of Wichita to about the state line, and mid-60’s clear up into the Manhattan area. The kinking area around Concordia is indicative of a forecast triple point just in front of an eastward-moving low. The triple point is generally the most-favored area for tornado development in the whole system.

The CFS shows the same pattern in the moisture, but only pools it up in the vicinity of the triple point, say around Russell. It depicts a looser triple point than the GFS, but overall I’d say the two models agree with each other within 50-75 miles. That’s good enough for 72 hours out.

Neither model depicts a super-sharp dry line, but I think it’ll be sharp enough. The GFS shows a 12-degree spread from Hutchinson to Pratt and mid-60’s to low-50’s working west form Salina to Russell along I-70.
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Ingredient 2: Available Energy

The Euro outlines an area of middle-value CAPE in south central Kansas

The Euro outlines an area of middle-value CAPE in south central Kansas

We watch a measure known as CAPE (Convective Available Potential Energy) to get an idea of the amount of energy a unit (parcel) of air would have if lifted from the ground into the air a distance.

The Euro (ECMWF), as of the 12z Sunday model, forecast what I’d term as mid-value CAPE in central-south central Kansas ahead of the dry line. The 0z Monday model run just came in, and it looks like the Euro has shifted the forecast path of the low substantially north and slows it down more in keeping with the CFS. The shift north in the Euro is almost as far south as the model path is forecast by the NAM. That leads me toward a risk area somewhere in between, where the CFS and GFS are showing.

The CFS forecasts much weaker CAPE, perhaps due to blocking of the best moisture by the dry line bulge.

The CFS is showing a pronounced dry line bulge. If that bulge gets too far ahead of the triple point, would choke off the moisture flow into that area and steal storm energy.

That would move the risk area for much of the severe weather into central Kansas ahead of the bulge, but would also increase the chance of a squall line vs. having supercell storms.

Concurrently the major risks would go from tornadoes and hail to hail and some wind.

I’m still not really versed on the CFS, so I’m unclear on how much credence to give that aspect of the forecast. There’s still quite a bit of time to resolve it.

GFS shows a much larger area of CAPE ahead of the dry line and along the warm front.

The GFS, which as noted above shows the strongest moisture return, translates that into pretty good CAPE for this time of year. I’m having a challenge with the GFS as I always do at this time frame because it tends to be too fast and to overdo moisture. But with it being the solution that’s right in the middle of everything else, I have to give the forecast some credence…

Ingredient 3: Capping

The Euro (12z Sunday forecast) had the entire warm sector capped off. The 0z Monday forecast kept the nearly-unbreakable cap over Kansas.

That pink in the Euro forecast indicates what we often call a “thermonuclear” cap. It brings into very strong question whether any storm can get enough warm air rising fast enough to penetrate the warm layer in the mid-levels and continue to rise. capping has been a very large factor in our recent storm evolutions. The image is from the 12z (7am CDT) Sunday run of the ECMWF. The 0z Monday (7pm Sunday CDT) run keeps Kansas totally under that significant cap.

The technical term for the value you’re seeing is CIN, or Convective Inhibition. Translated, that’s the amount of force a unit (parcel) of air needs to overcome to rise to where it can form a thunderstorm.

There are several areas of grey that coincide with the higher CAPE values in the GFS.

While the GFS capping image looks like a mess, it actually shows values I tend to prefer when looking for a place storms will initiate. Focus on the gray area that’s right ahead of the advancing dry line, right over the top of the best CAPE. In that area CAPE is quite a bit greater than CIN, leading me to believe storms are quite possible in that zone. I don’t like to see CIN numbers around zero in the hours before storms are forecast, as there needs to be a little resistance to the air rising in order for enough energy to develop to “boil” and generate thunderstorms.

Bottom line

On a subjective scale, from a chasing standpoint I’d give Wednesday a 6 or 7. I have a pretty good feeling moisture return will be sufficient, and early enough in the day, to fuel storms. I also have above-average confidence the cap will erode at about the right time of day to allow storms to build into the upper troposphere. Surface wind is not forecast to be as far southeast (backed) as I’d expect in a tornado risk day.  But a mid-level shortwave (disturbance at about 10,000 feet) is forecast to be just west of the dryline at 7pm. That would tend to help storms maintain strength.

Wednesday is far from an outbreak scenario. But it’s one of those days I feel fairly good we’ll have some tornadoes reported along with large hail and damaging wind, especially after dark. Area-wise, I’m leaning toward I-70 and north, east of Salina. Part of that may have to do with my being in Topeka the next three days, and the tendency of the human mind to look at things from “my” viewpoint. We’ll see how things pan out.

Update, 2:30am CDT Monday

SPC Day 3 update just came out. As a non-meteorologist it is a little scary putting out a risk area forecast on a slight-risk day before verifying it against the thoughts of SPC and the NWS forecast offices. It’s good to know today that I didn’t miss something….

SPC Outlook Graphic for 4-19-17, issued 0230 CDT 04-17-17. Click to view full-size.

On the other hand, SPC is playing down the tornado risk at this point and calling for more of the storms to be after dark. SO…I’ll be watching how things develop. If the risk justifies it, I’ll do another briefing Wednesday morning.

Friday risk

Earlier in the weekend SPC highlighted parts of Kansas in a 15% risk area for Friday. That went away yesterday (Sunday). There’s considerable model differences that show up Thursday, Friday and Saturday that make it very challenging to pin down a risk right now.