Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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000 FXUS63 KOAX 211102 AFDOAX Area Forecast Discussion National Weather Service Omaha/Valley NE 602 AM CDT Mon Aug 21 2017 .SHORT TERM...(Today through Wednesday) Issued at 225 AM CDT Mon Aug 21 2017 Rapid MCS formation has occurred early this morning over western IA near the intersection of a synoptic surface front and residual outflow boundary, per mesoanalysis. The presence of a 40-45 kt southwesterly LLJ will aid in back-building storms across our IA counties through 12z with a net south or southeast displacement of the MCS through that time frame. Elsewhere, IR satellite data indicate a plume of mid to high-level moisture (and associated clouds) advancing northeast through the High Plains in association with a subtropical-branch shortwave trough over NM. --Eclipse forecast-- While it appears that the ongoing storms across IA will shift east of the area by mid morning, model guidance remains consistent in showing the High Plains cloud shield steadily advancing northeast into the mid-MO Valley today. Model cross sections of relative humidity along the path of totality indicate the potential for considerable cloudiness at 1 PM CDT at or above the 18-20 kft level. At present, we remain uncertain of the opacity of these clouds, and this is something visible satellite should clarify in the hours leading up to the event. High temperatures will be generally in the lower to mid 80s with afternoon heat index values rising into the lower 90s across southeast NE and southwest IA. Precipitation chances will remain low through early afternoon. By mid afternoon into this evening, height falls associated with an amplifying polar-branch trough over the ND-MN Red River Valley coupled with the influence of the subtropical-branch system tracking into the central Plains will yield an uptick in convective activity across the forecast area. The most intense storms will likely form along a cold front moving south into northeast NE and west-central Iowa where forecast soundings indicate ample instability and vertical shear for organized severe weather. Damaging winds, hail and heavy rain will be the primary hazards with these storms as they advance southeast across the area this evening and tonight. An isolated tornado is possible, especially with any storm that can favorably interact with an outflow boundary remnant from the MCS currently ongoing across IA. A cooler and drier Canadian air mass will reside across the mid-MO Valley on Tue and Wed with dry conditions expected. .LONG TERM...(Wednesday night through Sunday) Issued at 225 AM CDT Mon Aug 21 2017 A longwave trough will persist across eastern North America from the middle to latter part of this work week before a more substantial upstream trough and lower-latitude perturbations move into the Canadian Prairies and Great Plains this weekend. This large-scale pattern will yield continued mild and dry weather through Friday, before precipitation chances increase this weekend. && .AVIATION...
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(For the 12Z TAFS through 12Z Tuesday morning) Issued at 602 AM CDT Mon Aug 21 2017 A back-building MCS is in progress as of 1045z across western IA with more recent storm development along the western edge of the associated cold pool between KOMA and KLNK. Expect storms to persist for the next two to three hours before dissipating with the cessation of the nocturnal LLJ. We will mention prevailing VCTS at KOMA and KLNK with a TEMPO group for MVFR TSRA at KOMA between 12 and 14z. From mid morning through early afternoon, we expect prevailing VFR conditions across eastern NE with increasing high- level cloudiness. Thunderstorm potential will increase by mid to late afternoon at KOFK and early evening at KOMA and KLNK as a cold front moves south through the area.
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&& .OAX WATCHES/WARNINGS/ADVISORIES... NE...None. IA...None. && $$ SHORT TERM...Mead LONG TERM...Mead AVIATION...Mead

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