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Archives for the SPC Convective Outlook are updated daily (approximately) with a live map at the beginning of each article. Follow the link at the end of the article to check for current updates on the NOAA/NWS Storm Prediction Center website. Also, see Archives for Chicago's hourly weather data on CARDINAL NEWS Magazine.


Friday, August 20, 2021

SPC Aug 20, 2021 0600 UTC Day 1 Convective Outlook

SPC 1200Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1242 AM CDT Fri Aug 20 2021 Valid 201200Z - 211200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE CENTRAL AND NORTHERN PLAINS INTO THE MID MISSOURI AND UPPER MISSISSIPPI VALLEYS... ...SUMMARY... Scattered strong to severe storms are forecast today to spread from eastern parts of the central and northern Plains, into the upper Mississippi Valley, accompanied by a risk for large hail, damaging winds, and a tornado threat. ...Central and Northern Plains/Mid Missouri and Upper Mississippi Valleys... A negatively tilted upper-level trough will move northeastward across the northern and central Plains today. Elevated strong thunderstorms with hail are likely to be ongoing at the start of the period. The severe threat associated with those storms is likely to be located near the northwestern edge of the low-level jet, from eastern South Dakota into southeastern North Dakota. These storms are likely to move northeastward into western Minnesota by midday. Surface-based strong to severe thunderstorms may develop across parts of southwest and central Minnesota during the afternoon. Further to the south from eastern parts of the central Plains into the mid Missouri Valley, moisture advection of surface dewpoints in the lower to mid 70s F will contribute to strong destabilization by afternoon. MLCAPE is forecast to reach the 3000 to 4000 J/kg range from northeast Kansas into western Iowa. A cold front will approach this corridor during the mid to late afternoon, with a trough ahead of the front. Low-level convergence will increase along the pre-frontal trough aiding thunderstorm formation during the late afternoon. Thunderstorms will intensify during the early evening, moving northeastward across northeast Kansas, southeast Nebraska and western Iowa. RAP forecast soundings along this corridor from Topeka northward to Omaha by 00Z/Saturday have MLCAPE peaking near 4000 J/kg. Wind profiles show veering winds with height in the lowest 3 km above ground level. However, 0-6 km shear is only forecast to be in the 20 to 30 knot range. This should be enough for organized multicells and possibly an isolated supercell threat. The potential for supercells will be greatest in areas where the deep-layer shear is maximized. Any supercell should associated with isolated large hail. The more persistent and intense supercells could develop a tornado threat. Wind-damage will also be likely with supercells and could occur along the leading edge of the stronger multicell line segments. The severe threat will likely continue into the late evening and possibly into the overnight period as an MCS moves eastward across the mid Missouri and upper Mississippi Valleys. ..Broyles/Karstens.. 08/20/2021 Read more LIVE:
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