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Monday, August 9, 2021

SPC Aug 9, 2021 0600 UTC Day 1 Convective Outlook

SPC 1200Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1243 AM CDT Mon Aug 09 2021 Valid 091200Z - 101200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF THE NORTHERN PLAINS...AND OVER PARTS OF THE NORTHERN ILLINOIS/NORTHERN INDIANA VICINITY... ...SUMMARY... Scattered severe storms are possible this afternoon and evening over parts of the northern Plains, accompanied by potential for damaging wind gusts and hail. Other strong/locally severe storms accompanied by hail/wind risk, are expected from portions of the Upper Great Lakes southward to the Mid Mississippi/Lower and Mid Ohio/Tennessee Valleys. ...Synopsis... A slowly weakening mid-level short-wave trough is forecast to shift east-northeastward out of the mid and upper Mississippi Valley, across the Upper Great Lakes and Midwest region today, in advance of a second/stronger trough. This second trough -- initially crossing Montana and the adjacent Intermountain West, will shift steadily eastward into the northern Plains through the end of the period. At the surface, high pressure will prevail in the East, while a cold front advances into/across the northern and central Plains. By the end of the period, this front should extend from Minnesota southwestward to western Kansas and the central High Plains region. ...Northern and central Plains... Diurnal heating/mixing across the central and northern Plains ahead of the advancing short-wave trough will result in modest CAPE above a deep surface-based mixed layer. As the upper system -- and associated cold front -- shift steadily eastward, scattered/high-based storm development is expected -- initially across central North Dakota. Farther south, more isolated storms may develop from eastern South Dakota into portions of Nebraska/northern Kansas. With sub-cloud evaporation aiding downdraft strength, damaging winds will likely be the primary severe risk, as convection spreads eastward toward/into Minnesota. However, ample CAPE may be present to support some hail as well. Farther south, where storms should be even higher-based and CAPE weaker, locally damaging gusts will likely be the primary risk, with a few of the stronger storms. While a gradual decrease in severe risk is expected after sunset, as the boundary layer cools/stabilizes, storms will likely continue into the overnight hours. As such, limited/local risk for gusty outflow winds with a few stronger storms may likewise linger overnight. ...Upper Great Lakes region including northern Illinois/Indiana... As short-wave troughing lifts northeastward across the upper Mississippi Valley and into the Upper Great Lakes region through the first half of the period, ongoing early-day convection is expected to spread from southern Wisconsin/northern Illinois across Lake Michigan and into Lower Michigan. Some severe risk may occur with this convection -- especially during the afternoon across Lower Michigan as modest destabilization occurs across this area. Meanwhile, though weak height rises are expected across Illinois/Indiana through the afternoon as the trough shifts northeastward, a favorably unstable airmass is expected to evolve. Weak ascent in the presence of remnant outflows from earlier convection may be sufficient to initiate new storms -- which is hinted at in some CAM runs across the northern Illinois vicinity. Should this occur, and aided by favorably veering flow with height through the lower half of the troposphere, storms would likely become severe -- posing risk for damaging winds and hail, along with possibly a tornado. While this risk is conditional upon focused convective development within a weakly forced environment, potential appears sufficient -- given the favorable environment -- to introduce SLGT risk across the northern Illinois/northwestern Indiana vicinity. Storms/severe potential should shift southeastward with time, and may continue through the evening before a gradual weakening overnight. ...Mid Mississippi/Lower and Mid Ohio/Tennessee Valleys... A rather complex scenario is evident from the Ozark Plateau eastward across the Mid Mississippi Valley into southern Indiana/Kentucky/northern Tennessee, with storms ongoing early in the period across parts of the area and likely to increase in coverage along various convective boundaries through the afternoon. This will likely result in widespread convection -- and multiple episodes of storms in some areas, through afternoon/evening and into the overnight hours. The general/widespread nature of the convection that is expected to evolve renders highlighting specific/possible areas of more concentrated risk difficult. But with a moist/unstable environment expected, and moderate westerly flow aloft across the area, occasionally stronger/potential severe storms are likely to occur locally. Damaging wind gusts and marginal hail will both be possible with stronger storms, with risk likely continuing across some portions of the area into the overnight hours. ..Goss/Lyons.. 08/09/2021 Read more LIVE: spc.noaa.gov
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