SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1246 AM CDT Sat Jul 17 2021 Valid 171200Z - 181200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE NORTHEAST... ...SUMMARY... Thunderstorms associated with wind damage will be possible in parts of the Northeast today. Marginally severe storms with hail and strong gusty winds will also be possible in parts of the central and northern Plains. ...Northeast... An upper-level trough will move across southwestern Ontario today as southwest mid-level flow remains over much of the Northeast. At the surface, a cold front will advance southeastward across the lower Great Lakes region with a moist airmass in place from the Mid-Atlantic northeastward into southern New England. Scattered thunderstorm development should take place this morning across the central Appalachians. The potential for surface-based thunderstorm development will gradually increase as surface temperatures warm during the day. Moderate to strong instability is forecast to develop by early afternoon from eastern Virginia northward into southeastern Pennsylvania. RAP forecast soundings at 21Z near Philadelphia have MLCAPE near 2600 J/kg with 0-6 km shear of 20-25 kt. 0-3 km lapse rates are forecast to be very steep reaching 8.0 C/km. This should be enough for severe wind gusts with the stronger multicells that form along the eastern edge of a large convective cluster. The more organized severe threat should gradually move toward the coastal areas during the late afternoon. Further south across the Mid-Atlantic, moderate to strong instability is also forecast to develop by early afternoon. However, deep-layer shear should be too weak for an organized severe threat. A few marginally severe wind gusts will be possible near or just after peak heating when low-level lapse rates become the steepest. ...Central and Northern Plains... An upper-level ridge will move and build northeastward across the northern High Plains today. At the surface, a moist airmass will be in place across much of the central and northern Plains, with surface dewpoints generally in the 60s F. Scattered thunderstorms will likely develop during the afternoon along the western edge of the stronger instability, with this convection moving eastward across western Nebraska and the western Dakotas. In addition to the instability, moderate deep-layer shear will be in place across much of the High Plains. This should be enough for a marginal wind damage and hail threat. The main limiting factor will be the upper-level ridge which should keep any severe threat more isolated. ..Broyles/Squitieri.. 07/17/2021 Read more LIVE: spc.noaa.gov
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