SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0240 PM CDT Thu Oct 21 2021 Valid 212000Z - 221200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE SOUTHERN APPALACHIANS TO THE UPPER OHIO VALLEY AND ACROSS PARTS OF WEST TEXAS... ...SUMMARY... Thunderstorms associated with isolated strong wind gusts will be possible this afternoon across parts of the southern and central Appalachians into the Upper Ohio River Valley. Isolated storms capable of marginally severe hail and strong gusts may also occur over far West Texas. ...20z Update... No appreciable changes are warranted to the 1630z outlook; however, a small 2 percent tornado probability has been introduced over the upper Ohio Valley region. Latest satellite imagery suggests large-scale forcing is spreading across OH toward western PA. Convection is gradually responding to the approaching short-wave trough with a strengthening band of convection along the surging cold front. Additionally, a narrow corridor of scattered convection extends along the OH River into western PA. Warm advection appears to be partly contributing to this activity and wind profiles do support organized rotating updrafts. While the tornado threat is low with these storms, it is non zero and have opted to introduce a 2 percent probability to reflect this potential. ..Darrow.. 10/21/2021 .PREV DISCUSSION... /ISSUED 1125 AM CDT Thu Oct 21 2021/ ...Synopsis... Upper-level low pressure near the southern tip of Lake MI will move east today while gradually evolving into an open wave. An associated surface low and cold front will move east in tandem with the upper low across the Ohio Valley, with the trailing portion of the cold front moving east through the central/southern Appalachians. An upper-level ridge will be in place over much of the western U.S. as a strong upper trough approaches the Pacific Northwest. ...Upper Ohio Valley and central and southern Appalachians... Buoyancy will remain modest across the OH Valley portion of the Marginal Risk area today owing to 50s dew points and generally poor lapse rates, with afternoon MLCAPE values averaging 500 to locally 1000 J/kg. Moderately strong southwest mid-level flow will result in 45-55 kts of effective shear, more than sufficient to support fast-moving clusters of storms and perhaps supercell structures capable of producing strong/damaging gusts. Expect an increase in thunderstorm coverage this afternoon near the eastward-moving cold front as ascent with the upper trough develops over the region. Farther south from southeast KY into northeast AL buoyancy will be greater, averaging 1000 to 2000 J/kg, while effective shear will only average 25-30 kts. Nevertheless, isolated stronger storms within this environment along and in advance of the cold front will be capable of producing strong wind gusts this afternoon and early evening. ...Far West Texas... A relatively moist air mass in conjunction with orographic influences should contribute to at least isolated thunderstorm development in vicinity of the Davis Mountains this afternoon. Relatively steep mid-level lapse rates, moderate buoyancy, and effective shear on the order of 25-30 kt may support some locally severe storms capable of hail and strong wind gusts through early evening. Read more LIVE: spc.noaa.gov
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