Severe Weather Outlook

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Outlook for Tuesday, September 18

Outlook Summary

At least a marginal risk for mainly hail is expected from a portion of the central and northern Plains into the upper Mississippi Valley today and tonight.

Outlook Images

overview

tornado 2%

wind low

hail 5%

Detailed Outlook

SPC AC 180553

Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1253 AM CDT Tue Sep 18 2018

Valid 181200Z - 191200Z

THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM A PORTION OF THE CENTRAL AND NORTHERN PLAINS INTO THE UPPER MISSISSIPPI VALLEY REGION

### SUMMARY

At least a marginal risk for mainly hail is expected from a portion of the central and northern Plains into the upper Mississippi Valley today and tonight.

Synopsis

The synoptic upper pattern will be characterized by a positive-tilt trough moving slowly east through the western states, a low amplitude ridge across the central U.S. and a weak upper trough advancing through the Middle Atlantic region. Embedded within the eastern trough, remnants of Florence will accelerate through southern New England early in the day. At the surface a quasi-stationary front will persist from the central Plains into the Upper MS Valley region.

Central and northern Plains through the upper Mississippi Valley region

Water vapor imagery and RAP analysis indicate a low-amplitude shortwave trough over the High Plains of southeast WY into western NE moving through the northern periphery of the upper ridge. This impulse will continue east, reaching the upper MS Valley region later this morning. An increase in isentropic ascent in response to this feature has contributed to the initiation of thunderstorms north of the front from western NE into southwest SD, and additional development will occur through the morning farther downstream as this zone of ascent spreads east. Given 7-8 C/km mid-level lapse rates, 1500-2000 J/kg MUCAPE, and 35-45 kt effective bulk shear, a few updrafts may become capable of producing hail. A temporary decrease in thunderstorm coverage/intensity may occur in wake of the initial impulse, and a capping inversion may inhibit surface-based development near the front this afternoon. However, redevelopment of storms may occur overnight as a strengthening low-level jet augments isentropic ascent north of the front from SD, northeast NE into southern MN and IA. Some threat for hail is also expected with this activity.

Southern New England

A 30-40 kt southwesterly low-level jet associated with the remnants of Florence will spread through southern New England later this morning contributing to an increase in low-level hodograph size. While a band of storms will accompany this conveyor belt, limited potential for boundary-layer destabilization will exist before the system moves offshore, therefore any tornado threat is expected to remain low.

..Dial/Wendt.. 09/18/2018

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NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1300Z

National Risk Overview

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

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