SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1241 AM CDT Wed Sep 15 2021 Valid 151200Z - 161200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE CENTRAL APPALACHIANS AND NEW ENGLAND... ...SUMMARY... Severe thunderstorms capable of damaging winds, hail and an isolated tornado threat will be possible today from the central Appalachians into New England. A marginal tornado threat will also exist across parts of the central Gulf Coast. ...Central Appalachians/New England... A shortwave trough will move eastward across the lower Great Lakes and Ohio Valley today. At the surface, a cold front will advance southeastward into New England and the central Appalachians. Ahead of the front, surface dewpoints in the 60s F will contribute to moderate destabilization by afternoon across parts of central and eastern Pennsylvania, southeast New York and New England. The NAM and RAP suggest that MLCAPE will peak in the 2000 to 2500 J/kg range along parts of this corridor. Thunderstorms that develop near and ahead of the front will move east-northeastward across the western edge of the stronger instability during the mid to late afternoon. The magnitude of instability should allow the convection to persist into the early evening. Forecast soundings along the axis of strongest instability by 21Z have supercell wind profiles, with 0-6 km shear in the 35 to 40 kt range and veering winds with height in the lowest 3 km. Supercells will be capable of producing wind damage and isolated large hail. However, most of the convection will likely remain linear suggesting that wind-damage will be the greater of the two threats. As low-level winds strengthen during the late afternoon, the most favorable low-level shear for tornadoes will be located from far eastern New York into central New England. The tornado threat appears conditional upon whether any cells can remain discrete late in the event. Further southwest into the central Appalachians, a marginal severe threat is expected to develop this afternoon. However, deep-layer shear will be considerably less keeping any potential for wind-damage much more isolated. ...Central Gulf Coast... The remnants of Nicholas are forecast to continue moving slowly eastward across the central Gulf Coast today. RAP forecast soundings around midday show the strongest low-level shear from southeast Louisiana into far southern Alabama, where 0-1 km shear could be in the 15 to 20 kt range. This combined with surface heating and moderate destabilization could be enough for a marginal tornado threat with cells that remain semi-discrete. ..Broyles/Bentley.. 09/15/2021
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