Area Forecast Discussion
Issued by NWS Omaha/Valley, NE
Issued by NWS Omaha/Valley, NE
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988 FXUS63 KOAX 210859 AFDOAX Area Forecast Discussion National Weather Service Omaha/Valley NE 359 AM CDT Sat Sep 21 2024 .KEY MESSAGES... - Rain and thunderstorm chances increase this afternoon, particularly for southeast Nebraska and southwest Iowa. There is a 5-15% chance of severe storms Saturday afternoon or evening. - Widespread rain is expected Saturday night and persist Sunday, with the heaviest rainfall anticipated from midnight Sunday through noon Sunday (50-90%). Rainfall totals through Sunday of 1 to 2 inches are expected south of Harlan Iowa to Omaha/Council Bluffs to Fairbury Nebraska line. Isolated totals of up to 3 inches are possible across extreme southeast Nebraska into southwest Iowa. - One more warm day, with 90 degree readings along and south of I-80.Cooler conditions will arrive Sunday and persist through the following week, with highs dropping into the 60s and 70s. && .DISCUSSION... Issued at 400 AM CDT Sat Sep 21 2024 Today: Early this morning isolated showers persist across portions of the forecast area, with coverage expected to increase slightly as 25-45kt LLJ increases moisture transport into the region,with isentropic ascent and weak disturbance moving through periphery of H5 ridge axis providing kinematic forcing. There`s a a slim chance (5%) of a stronger storm developing with MUCAPE 1-1.5K J/kg and bulk shear values 35-40kt, but relative warm H7 and H5 temperatures will limit hail production, thus wind primary threat. Convection will wane in coverage during the morning and early afternoon hours as LLJ veers and forcing diminishes. But, expected uptic in convective development by 19-21z. By then dynamic and kinematic forcing increases due to interaction of upper level trough moving across central Canadian provinces and upper low moving out of southwest U.S. A 80-100kt ULJ moves along periphery of H5 ridge as several disturbances advect into central plains in advance of southwest U.S. upper low. H5 trough moving across Canada drives cold front through the forecast area, and will be located generally along a Logan IA to Hebron NE line by 18-19z. Convective temperatures met or exceeded by 19-21z, with deterministic soundings indicating MU CAPE 2-3K. Limiting factor will weaker bulk shear values in the 25-35kt range, but there`s enough to provide some storm organization. Nudged temperatures toward 75% NBM, that are closer to deterministic convective temperatures in the lower 90s across southeast Nebraska and southwest Iowa. Tonight: Severe weather threat expected to end by early evening as front moves south of the forecast area, but risk of moderate to heavy rainfall will continue. However, southwest U.S. upper trough slow to lift into the plains, thus large scale moist ascent will continue across the region through the overnight hours. Isentropic convergence along the 305-310K surfaces will allow convection to train across southeast Nebraska and southwest Iowa overnight, tapping into precipitable water values up to 2 inches. Evaporative cooling and low level cold advection behind the front will allow overnight temperatures to fall into the 50s. Sunday: Rain with embedded thunderstorms will continue into Sunday as upper trough moves across the region. Each successive model run places band of heavier rainfall to the south of forecast area, however, the past couple issuances of WPC QPF has been consistent in broadbrushing 1 to 2 inch along and south of I-80, which will help reduce current soil moisture deficits of 1 to 3 inches. The likelihood of any flash flooding or flooding appears low based on current rainfall forecast. Lack of any appreciable sun, evaporative cooling and sustained low level cold advection will keep temperatures fall-like, with highs in the 60s. && .AVIATION /06Z TAFS THROUGH 06Z SUNDAY/... Issued at 1124 PM CDT Fri Sep 20 2024 VFR conditions are favored with the caveat of potential scattered showers and storms tonight into tomorrow bringing periods of reduced visibility and ceilings. Due to the low confidence in coverage of these storms (20% chance), have opted to withhold them from the TAFs at this time. Amendments will be made if storms develop within the vicinity of a terminal. Winds will remain southerly and below 12 kts before a cold front shifts winds clockwise to northerly. The front is expected to pass KOFK 14-16Z, KOMA 19-20Z and KLNK 18-20Z. Gusts above 12 kts will be possible after the frontal passage. More widespread showers and thunderstorms will move in from south to north tomorrow afternoon and continue into Sunday. This will likely impact KLNK by 18-19Z and KOMA by 19-20Z. Confidence in widespread showers reaching KOFK is low at this time. && .OAX WATCHES/WARNINGS/ADVISORIES... NE...None. IA...None. && $$ DISCUSSION...Fortin AVIATION...Wood