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Sunday, September 19, 2021

SPC Sep 19, 2021 0730 UTC Day 3 Severe Thunderstorm Outlook

SPC 0730Z Day 3 Outlook Day 3 Convective Outlook NWS Storm Prediction Center Norman OK 0209 AM CDT Sun Sep 19 2021 Valid 211200Z - 221200Z ...NO SEVERE THUNDERSTORM AREAS FORECAST... ...SUMMARY... The risk of severe thunderstorms currently appears negligible across the U.S. for Tuesday through Tuesday night. ...Synopsis... While the westerlies are forecast to remain generally progressive across the mid-latitude Pacific into North America, models suggest that blocking may become increasingly prominent across the southern mid- and subtropical Atlantic by early Tuesday. It appears that this blocking will increasingly influence the flow across the U.S. through this period and beyond. Initially, it appears that this will begin to impact the eastward progression of a fairly significant short wave trough, which may be in the process of digging to the lee of the Colorado Rockies at 12Z Tuesday. As it continues southeastward, it may undergo considerable deformation, with larger-scale mid-level troughing elongating north-south across the middle and lower Mississippi Valley vicinity by late Tuesday night, increasingly split from shearing northern branch troughing across northern Ontario. However, spread among and within the output of the various models is substantive concerning this evolution. It does appear that a surface cold front will continue to slowly advance eastward ahead of the larger-scale mid-level troughing. This likely will be preceded by a narrow plume of seasonably high moisture content. A plume of tropical moisture likely will also linger across much of Florida into the Carolinas, around the southwestern periphery of ridging which will maintain much drier and milder air across the northern Atlantic Seaboard. This moisture may contribute to the development of moderately large CAPE by Tuesday afternoon. Largest CAPE probably will be focused beneath the remnants of an increasingly suppressed plume of elevated mixed-layer air, along/above the cold front advancing across central into southeast Texas, northeastward into parts of the Tennessee Valley. This may contribute to scattered vigorous thunderstorm development, but in the presence of rather weak vertical shear, the risk for severe storms currently appears negligible. ..Kerr.. 09/19/2021 Read more LIVE: spc.noaa.gov
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