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Outlook for Wednesday, September 18

Outlook Summary

Thunderstorms capable of producing severe wind gusts and large hail are possible today across parts of the eastern Dakotas and Minnesota into the mid Missouri Valley. A marginal severe threat will also extend southward into parts of the central and southern Plains.

Outlook Images

overview

tornado 2%

wind 15%

hail 15%

Detailed Outlook

SPC AC 181954

Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0254 PM CDT Wed Sep 18 2024

Valid 182000Z - 191200Z

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF NORTHEAST NEBRASKA INTO FAR NORTHWEST IOWA…EASTERN SOUTH DAKOTA…AND WESTERN MINNESOTA

### SUMMARY

Thunderstorms capable of producing severe wind gusts and large hail are possible today across parts of the eastern Dakotas and Minnesota into the mid Missouri Valley. A marginal severe threat will also extend southward into parts of the central and southern Plains.

20Z Update

No changes were made to the ongoing forecast. For mesoscale details concerning eastern South Dakota and vicinity, see MCD 2083. Additional details can be found in the previous discussion below.

..Wendt.. 09/18/2024

.PREV DISCUSSION… /ISSUED 1118 AM CDT Wed Sep 18 2024/

Northern Plains/Upper Midwest to the Southern High Plains

A closed upper low centered over eastern MT will continue northeastward during the period, reaching southeast SK by early Thursday morning. Upper-level ridging extending northward from the northwest Gulf Coast into the upper MS Valley will slowly shift eastward to the Upper Great Lakes. A belt of enhanced southwesterly mid-level flow will extend from southern CA through the central High Plains, while also wrapping cyclonically through the eastern Dakotas and into southern Canada. At the surface, a trough/front extends southward from central ND into SD and western NE, before continuing southwestward over the central/southern High Plains. This boundary may serve as a focus for renewed thunderstorm development later today.

A band of elevated convection has generally moved northward from northeast ND and northwest MN into southern MB. In the wake of this activity, cloud cover remains prevalent across much of the warm sector across the northern Plains/Upper Midwest. Still, filtered daytime heating of an adequately moist boundary layer should aid in the development of around 1500-2000 J/kg of MLCAPE by late afternoon. Isolated to scattered thunderstorms may form along/east of the surface trough/front within a favorable zone of 30-40 kt of effective bulk shear. But, better large-scale forcing associated with the upper low over the northern High Plains will tend to remain displaced north and west (in southern Canada). Accordingly, 12Z high-resolution guidance continues to vary regarding storm coverage across the Slight Risk (isolated versus scattered) given the weaker forcing aloft. Even with this continued uncertainty, various NAM/RAP forecast soundings still suggest a somewhat conditional risk for organized storms (mix of multicells and supercells). Severe hail/wind may accompany the stronger storms from late afternoon through early evening, before this activity eventually weakens.

Further south into the central/southern Plains, low-levels are forecast to be somewhat drier near the surface trough, with surface dewpoints expected to mostly range in the upper 50s to low 60s by late afternoon with pronounced boundary-layer mixing. Warmer mid-level temperatures and limited large-scale ascent across these regions should temper the overall intensity of any updrafts that can form and be sustained, and resultant intensity/coverage of this convection. Still, some guidance shows locally greater thunderstorm coverage and resulting isolated severe wind threat from the eastern TX Panhandle into far northwest OK and southern KS during the late afternoon/early evening, where low-level lapse rates should be maximized with robust daytime heating.

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National Risk Overview

Wednesday, September 18: TORNADO: 2%; HAIL: 15%; WIND: 15%
Thursday, September 19: TORNADO: 5%; HAIL: 15%; WIND: 15%
Friday, September 20: ANY SEVERE: low
Saturday, September 21: ANY SEVERE: low / uncertain
Sunday, September 22: ANY SEVERE: low / uncertain
Monday, September 23: ANY SEVERE: low / uncertain
Tuesday, September 24: ANY SEVERE: low / uncertain
Wednesday, September 25: ANY SEVERE: low / uncertain

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