Tanner Verstegen

Mini Case Study: When the Low Level Jet is no collocated to the Instability.

On 16-17 July 2016 a

nocturnally driven thunderstorm

complex was shaping up across the

Upper Midwest and pressing east.

Exactly where it was going was

unclear and the model guidance was

only so-so for the event. The forecast

in question was from Wisconsin, into

Lower Michigan (Consumers Energy).

Looking at the theta-E gradient, along

with the CAPE gradient would suggest

a southeast track (see images below).

Even the bow segments and shape of

the complex might suggest such

propagation, but the low level jet was

suggesting another solution.

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Models in general, did handle initialize the cluster okay, but lacked several differences. The run to run

consistency was not evident between various model runs and other models. Global models had QPF

spread vastly through the Midwest and Great Lakes and it was not clear where thunderstorms or any

strong thunderstorms would line up. See below for HRRR and HRRRx (which had been running in unison

at the time)

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The issue with the forecast was that the nose of the low level jet (where force lift would occur) did not

align with the local maximum for instability. Based on the LLJ, the forecast for Lower Michigan had been

going EEI-2/EEI-3 wind gusts possible with thunderstorms rolling through late in the overnight hours. For

the purpose of aesthetics, the HRRR forecasted 850mb winds are attached below, along with the Storm

Prediction Center analysis at 850mb as the thunderstorms were near the Mississippi River.

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Putting it all together, one could make a case for a more eastward progression of these thunderstorms

(forecasted earlier in the day for Lower Michigan) and a case for a more southeast track.

In the end, both factors played a role and the precipition field was essentially stretched from north to

south (See radar on next page). However, the thunderstorms set up along the CAPE gradient and

continued to produce severe weather, while the northern edge turned into a soaking rain which was

driven by the associated lift upon the nose of the LLJ. Not to go unmentioned, some of the CAPE was

advected northeast (see probe mouse-over below), but not enough to produce lightning.

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The storm reports also told a similar story, all of the reports stayed through Iowa and north Illinois, and

far lower Michigan:

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Summary:

While it may seem like a subtle difference, the location of these two factors led to a more challanging

forecast by meteorologists and computer models. The take away from this study is that the

thunderstorm complex DID follow the CAPE and Theta-E gradient, NOT the location of the LLJ, but new

development on the nose of the LLJ DID occur, yet in the form of moderate rain. While the convergence

caused by the deceleration of the flow did cause ascent to create rain in Wisconsin, the lact of instability

did not allow for additional thunderstorm development in this case.

These are just findings of one case, but maybe/hopefully they can be used as a proxy or analog of a

future forecast in the future. Based on these results, a simple forecast flow chart goes as follows:

Assuming LLJ and CAPE max is downstream of

T-Storm

Locate LLJ Nose Location, Is there CAPE?

NO: Steady rain here

YES: Additional thunderstorms possible

Locate CAPE Gradient, Is the a LLJ Nose Here

NO: Thunderstorms may continue to

propagate through here

YES: Thunderstorms will propagate here, with

additional development