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Monday, July 12, 2021

SPC Jul 12, 2021 1730 UTC Day 2 Convective Outlook

SPC 1730Z Day 2 Outlook Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1230 PM CDT Mon Jul 12 2021 Valid 131200Z - 141200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF SOUTHERN MT AND NORTHERN WY... ...SUMMARY... Thunderstorms capable of severe wind and isolated hail will be possible across southern Montana, northern Wyoming, and the Black Hills area of South Dakota Tuesday afternoon and evening. Thunderstorms capable of locally damaging wind will also be possible across portions of the Northeast and Southwest. ...MT/WY into the northern High Plains... A shortwave mid/upper-level trough is forecast to be well-timed for the development of scattered thunderstorms across southern MT/northern WY Tuesday afternoon. Despite relatively limited low-level moisture, steep low/midlevel lapse rates will support weak to moderate buoyancy, while effective shear will increase into the 40-50+ kt range as the shortwave approaches from the west. A couple of supercells are possible initially, but a rather quick transition to an outflow-dominant cluster is expected as convection spreads east-southeastward toward the northern High Plains. Severe wind gusts will likely be the primary hazard, though isolated hail will also be possible with any semi-discrete activity. Isolated supercell development also cannot be ruled out across northeast CO into western NE by late afternoon, which would pose a conditional threat of severe hail and wind. ...Northeast... The primary upper trough is forecast to move northeastward toward New England by Tuesday evening. The timing of the trough ejection may be a little later than optimal, but scattered strong thunderstorm development still appears possible by late afternoon across portions of NY/PA, within an environment characterized by moderate buoyancy, rich low-level moisture, marginally favorable effective shear, and poor midlevel lapse rates. Organized cells/clusters will be possible, with a primary threat of locally damaging wind, and perhaps a brief tornado if any supercell can be sustained. The greatest relative threat appears to be over portions of eastern PA, where somewhat greater surface heating is expected prior to storm arrival, and a Slight Risk has been included in this area. Higher wind probabilities may eventually need to be extended into NY, depending on observational and model trends as the event approaches. ...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 with time. ...Upper MS Valley region... A few strong thunderstorms will be possible Tuesday afternoon/evening across portions of MN and western WI. Moderate instability is expected to develop along/ahead of a weak cold front, with weak northwesterly midlevel flow providing marginally supportive deep-layer shear for briefly organized updrafts capable of localized hail and damaging wind. ..Dean.. 07/12/2021 Read more LIVE: spc.noaa.gov
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