Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0251 PM CDT Mon Aug 21 2017 Valid 212000Z - 221200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS EASTERN NEBRASKA AND MUCH OF IOWA... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM PORTIONS OF THE CENTRAL PLAINS EASTWARD TO THE WESTERN/CENTRAL GREAT LAKES AND PORTIONS OF THE MID-ATLANTIC... ...SUMMARY... Scattered severe hail and wind is anticipated across parts of the Mid-Missouri Valley and Upper Midwest this afternoon and evening. Isolated severe storms are also possible across a broad region from the central Great Plains to the Mid-Atlantic States. ...20Z Outlook Update... Minimal spatial changes were made to the ongoing outlook primarily to decrease probabilities upstream of a mature MCS over far southeastern South Dakota/northwestern Iowa. This MCS will continue to migrate southeastward primarily across the Slight Risk area over the next several hours and pose a damaging wind/isolated tornado threat before expected weakening due to lesser instability over eastern Iowa. Severe probabilities were also decreased in southern Minnesota, where widespread stratiform precipitation was limiting surface-based destabilization. Reference MCD 1543 for more information. Elsewhere, scattered severe storms will pose a risk for isolated instances of hail and damaging wind gusts across the central Plains eastward into the Midwest and in portions of the Mid-Atlantic. MCD 1542 has more information for the severe threat in portions of the Mid-Atlantic. ..Cook.. 08/21/2017 .PREV DISCUSSION... /ISSUED 1119 AM CDT Mon Aug 21 2017/ ...Mid-MO Valley and Upper Midwest... A small forward-propagating MCS over south-central SD is the primary concern for severe potential this afternoon. Robust insolation is occurring downstream across eastern NE into western IA. This will likely result in a strongly buoyant air mass developing by peak heating amid very steep mid-level lapse rates near 8.5 C/km from 700-500 mb per 12Z Rapid City and North Platte soundings. On the fringe of faster mid-level westerlies over the Upper Midwest, this MCS should remain organized with a threat for severe hail/wind and expand in coverage as it likely evolves east-southeast. The overall threat with this MCS should diminish during the evening over parts of IA given a large swath of ongoing non-severe convection across central IA to northern IL hindering diabatic heating. ...Midwest to Mid-Atlantic States... Outside of the aforementioned scenario, confidence is low in highlighting probabilities beyond a Marginal risk. Pockets of moderate to large buoyancy should develop across parts of southern IL into IN and northern Lower MI, as well as east of the lee trough in the Mid-Atlantic States. A broad belt of moderate westerlies southeast of the amplifying short wave trough over the southern Canadian Prairies will support multicells and a few transient supercell structures with risks for isolated damaging winds and hail.Read more
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