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Tuesday, July 13, 2021

SPC Jul 13, 2021 0600 UTC Day 1 Convective Outlook

SPC 1200Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1220 AM CDT Tue Jul 13 2021 Valid 131200Z - 141200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM SOUTHEAST MONTANA AND WYOMING INTO THE NORTHERN HIGH PLAINS...AND OVER PARTS OF PENNSYLVANIA AND WESTERN NEW YORK... ...SUMMARY... Thunderstorms capable of severe wind and hail will be possible from southern Montana across northern and eastern Wyoming and into the Black Hills area of South Dakota from afternoon through evening. Thunderstorms capable of locally damaging wind or a brief tornado will also be possible across portions of the Northeast, with sporadic strong storms over parts of the Southwest. ...Synopsis... An upper ridge will remain over the Northeast during the day, but height falls will occur overnight as a shortwave trough moves across the Great Lakes. To the west, an upper high will weaken over the Southwest as a stronger shortwave trough moves across MT and WY during the day, and into the northern Plains overnight. At the surface, a front over eastern PA and NJ will be nearly stationary during the day but will slowly move north late in the day as a warm front, with low 70s F dewpoints along/south of it. Shear will be stronger than the previous day, supporting a few severe storms during the afternoon and evening. To the west, a lee trough will develop over eastern WY and CO, with southeasterly surface winds maintaining mid 50s F dewpoints into the northern High Plains. Scattered storms, some severe, will be focused over southeast MT and northern/eastern WY. Overnight as the upper trough approaches, a 30-40 kt low-level jet will develop, enhancing lift and supporting a few strong storms into SD and NE. Elsewhere, daytime and evening storms are again likely over the Four Corners states, and persisting over AZ overnight. ...Southeast MT...northern and eastern WY...Black Hills area... Storms are initially expected from south-central MT into northwest WY prior to 21Z, with increasing organization from southeast MT into northern and possibly central WY through 00Z. Damaging winds and hail will both be possible as lapse rates will be steep with increasing deep-layer shear. Storms may proceed rather far east across SD and northern NE overnight with low-level jet support and despite increasing CIN. Damaging winds appear to be the main threat across the region. ...Northeast... Southwesterly low-level flow will increase throughout the day as a wave moves across the lower Great Lakes, resulting in effective SRH over 200 m2/s2 by 00Z from PA into NY. This should favor a few supercells, including the threat of a brief tornado. Marginal hail will also be possible, with cells developing around 18Z over central PA. Other storms will develop from northeast OH into western NY, with damaging gusts also possible through early evening as deep-layer shear increases to 35-40 kt. ..Southwest... Scattered thunderstorms are again expected across portions of the Southwest, especially during the afternoon and evening. While deep-layer shear will likely be somewhat less compared to previous days, wet microbursts and consolidating outflows are still likely to pose a threat of isolated damaging wind. Most guidance suggests greater storm coverage across AZ, with some damaging wind threat potentially spreading into the lower elevations of southern AZ during the evening or overnight. ..Jewell/Wendt.. 07/13/2021 Read more LIVE: spc.noaa.gov
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