Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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000 FXUS63 KOAX 200435 AFDOAX Area Forecast Discussion National Weather Service Omaha/Valley NE 1135 PM CDT Mon Jun 19 2017 .SHORT TERM...(Tonight through Thursday) Issued at 326 PM CDT Mon Jun 19 2017 The return of thunderstorm chances Wednesday and Thursday is the primary forecast concern. Surface high pressure had settled over the region this afternoon, maintaining a mild and relatively dry atmosphere in the Plains. Upper trough dipping into the Great Lakes was providing northwest mid level flow across the central CONUS, and cyclonic flow in the lower atmosphere aided in cumulus development in our area. These diurnally driven clouds should dissipate this evening, leaving mainly clear skies, light winds and dry air to allow another cool morning Tuesday. Potent mid level wave currently diving southeast through eastern Saskatchewan is forecast into the Minnesota and Iowa Tuesday. Associated showers will remain well northeast of our area, but wave will nudge a backdoor cool front east to west across eastern Nebraska Tuesday afternoon. A secondary shortwave dropping across the High Plains may trigger a few showers or thunderstorms in our far southwest late in the afternoon or evening. The cold front is forecast to retreat back north as a warm front on Wednesday as broad southerly flow returns to the Plains in response to a significant shortwave spreading east through the southern Canadian Plains. Rapid theta-e influx south of warm front and just ahead of trailing cold front will lead to increasingly unstable airmass over much of eastern Nebraska late afternoon and evening. GFS is the most bullish on moisture return, pulling low 70s dew points into northeast Nebraska. But even the more bearish ECMWF and Canadian models show upper 60s surface dew points available for convection, yielding 2500 J/kg of MLCAPE. Sufficient shear profiles will also exist with 35-40kt of bulk shear vectors perpendicular to convergence axis. Thus if storms can get going, organized convection with initial large hail threat and later wind risk is likely. Tornado risk will likely be confined to intersection of cold and warm fronts which should be just north of our area. However convective inhibition may be a bit much to overcome in a good part of eastern Nebraska as 700 mb temperatures approach 14C much of the southern half of the area, with cooler temps and less inhibition making initial development more likely in our north. Convection will likely continue and expand through the evening as cold front progresses southeast and low level jet strengthens, maintaining a wind and hail threat through midnight. Much of the activity will be moving off to the east by that time. On Thursday, models have been offering varied solutions on how far south the Wednesday cold front will travel. The latest consensus suggests front will hang up in our southern CWA, and perhaps lift north a bit during the day. Pooling of moisture near and south of front will lead to MLCAPEs again topping 2500 J/kg Thursday afternoon, with associated large hail threat. Also, mid level flow nearly parallel to surface boundary and only weak low level jet action suggests training of storms may yield heavy rains across much of the area near stalled frontal boundary. .LONG TERM...(Thursday night through Monday) Issued at 326 PM CDT Mon Jun 19 2017 Convection should be ongoing Thursday evening along stalled cold front, and will likely continue into Friday as upper trough swings across the Dakotas and northern Nebraska, inducing continued theta-e advection over frontal zone. Heavy rain looks likely in this scenario, but exactly where will depend on surface frontal position. And timing may be altered as longer term models move upper trough through the region at different times. GFS says front should be shoved south Friday night on back side of Dakota trough, effectively pushing rain chances out of eastern Nebraska and southwest Iowa. However ECMWF is slower, and keeps rain chances in for much of Saturday. With either scenario, a chilly forecast will follow for the weekend when high temperatures linger in the 70s Saturday through Monday. && .AVIATION...
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(For the 06Z TAFS through 06Z Tuesday night) Issued at 1132 PM CDT Mon Jun 19 2017 VFR conditions are expected through the TAF cycle, with skies clear or just a few cirrus and with unrestricted visibility. Calm to light and variable winds tonight will increase to around 6-10kt in the afternoon out of the west to northwest, with light and variable/shifting winds then expected from sunset through the rest of the TAF cycle.
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&& .OAX WATCHES/WARNINGS/ADVISORIES... NE...None. IA...None. && $$ SHORT TERM...Dergan LONG TERM...Dergan AVIATION...Mayes

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