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Thursday, July 22, 2021

SPC Jul 22, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0744 AM CDT Thu Jul 22 2021 Valid 221300Z - 231200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THIS AFTERNOON INTO EARLY TONIGHT FROM PARTS OF CENTRAL INTO NORTHEAST MT... ...SUMMARY... Thunderstorms capable of severe wind gusts are possible late this afternoon into early tonight from parts of central into northeast Montana. ...Northern High Plains this afternoon into early tonight... Around the northern periphery of the persistent midlevel ridge over the Rockies, and shortwave trough will move over southern AB/SK and glance northern MT. The midlevel trough will be accompanied by a cold front across MT this evening into tonight, and preceded by a lee trough across the northern High Plains. Low-level moisture is a bit limited across the northern Plains with a shallow boundary layer of low-mid 60s dewpoints this morning. The shallow moisture will be redistributed vertically by deep mixing, resulting in largely inverted-v profiles this afternoon across MT and the Dakotas to the south of a weak stalled front that now stretches from northeast MT across northern ND. North of the stalled boundary, moisture will be less prone to mixing, but a little cooler surface temperatures will help maintain some convective inhibition. Two general scenarios are apparent for storm initiation today: 1) with deep heating/mixing along the lee trough this afternoon close to the MT/ND/SD borders, and 2) in the zone of deep mixing and increasing large-scale ascent in closer proximity to the cold front and midlevel height falls across central MT this evening. The former scenario is a bit uncertain in terms of storm coverage, with an environment favoring strong outflow winds along the lee trough, and a somewhat greater threat for large hail with larger buoyancy and stronger vertical shear north of the stalled front (closer to the international border). Storm initiation appears more probable in the latter scenario across central MT, with storms spreading into northeast MT early tonight. Here, damaging winds will be possible with the steep lapse rates and at least some potential for cluster development. Isolated large hail may also occur, primarily with any splitting/supercell storms. ...AZ this evening into early tonight... Another day of scattered thunderstorms is expected, with the more concentrated convection expected to move generally westward this evening into tonight from eastern into central/south central AZ. East and northeasterly mid-upper flow around the Four Corners high will continue to steer convection from the higher terrain toward the lower deserts, while thermodynamic profiles will favor isolated severe outflow winds given typical inverted-v profiles with DCAPE of 1000-1500 J/kg. ..Thompson/Smith.. 07/22/2021 Read more LIVE: spc.noaa.gov
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