SPC 0600Z Day 2 Outlook
Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1248 AM CDT Wed Sep 15 2021 Valid 161200Z - 171200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE NORTHERN PLAINS INTO THE UPPER MIDWEST... ...SUMMARY... Isolated to scattered severe storms may occur Thursday into Thursday night across parts of the northern Plains and Upper Midwest, with damaging winds and large hail the main threats. ...Northern Plains into the Upper Midwest... A large-scale upper trough/low will advance slowly eastward across central Canada and the north-central CONUS on Thursday. At the surface, a cold front should move southeastward across parts of the northern Plains and Upper Midwest through the period. Some guidance suggests that a secondary surface low will form along the front over the eastern Dakotas during the day, and subsequently develop northeastward into adjacent parts of Ontario by Thursday evening. Low-level moisture, with surface dewpoints generally in the low to mid 60s, should advect northward across much of MN ahead of the front. Diurnal heating of this moist low-level airmass, along with steepening mid-level lapse rates from an EML emanating from the Rockies and High Plains, will likely result in moderate to strong instability developing in a narrow corridor ahead of the front by peak afternoon heating. This heating should also act to erode the convective inhibition associated with the EML, and current expectations are for isolated to scattered storms to develop along the cold front by late Thursday afternoon or early evening. Rather strong wind fields at both low and mid levels attendant to the upper trough appear likely to overspread the warm sector by the time convective initiation occurs. The strongest effective bulk shear (50+ kt) should be located across northern MN, closer to the upper trough in Canada. Around 30-40 kt of deep-layer should be present farther south into central/southern MN and the northern Plains. While there are still some concerns about overall storm coverage, any convection that forms along or ahead of the cold front appears likely to become severe given the favorable thermodynamic and kinematic environment. Initial supercells may pose an isolated large hail threat, but storms will probably grow upscale fairly quickly along the front by Thursday evening and have more of a severe/damaging wind threat as they move east-southeastward. Given the strength of the low-level flow and related shear, a tornado or two also appears possible, both with any initially discrete supercells or low-level circulations embedded within the line. The eastward extent of the severe threat Thursday evening/night will likely be constrained by rapidly increasing convective inhibition across southeastern MN/western WI. Finally, an isolated hail/wind threat may also exist along the length of the front across parts of southern SD into northern NE, where a strengthening low-level jet should aid storm development by late Thursday evening. ...Southeast... The remnants of tropical cyclone Nicholas should be located over LA at the start of the period Thursday morning per latest NHC advisory. The surface low is expected to be rather weak by this time, and the low-level wind field to the east of the low should gradually diminish throughout the day. The potential for even isolated severe storms along/near the central Gulf Coast still appears too low to include any severe probabilities. ..Gleason.. 09/15/2021
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