Area Forecast Discussion
Issued by NWS Omaha/Valley, NE
Issued by NWS Omaha/Valley, NE
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121 FXUS63 KOAX 201045 AFDOAX Area Forecast Discussion National Weather Service Omaha/Valley NE 545 AM CDT Fri Sep 20 2024 .KEY MESSAGES... - Rain and thunderstorm chances return late this evening, particularly for southeast Nebraska and southwest Iowa. through Saturday with it still warm. There is a low chance of a few severe storms early Saturday morning and again Saturday afternoon or evening. - Widespread rain will start Saturday night and persist Sunday, with the heaviest rainfall anticipated from midnight Sunday through noon Sunday (60-90%). Rainfall amounts of 1 to 2 inches are expected, with local amounts up to 3 inches. - Above average temperatures, with highs in the 80s and low 90s, are expected to persist through today. Cooler conditions will arrive over the weekend and into next week, with highs dropping into the 60s and 70s. && .DISCUSSION... Issued at 435 AM CDT Fri Sep 20 2024 Today: Surface cold front moved through the area, and currently is located across the lower Missouri Valley area. Shallowness of frontal zone will be overcome quickly, thus expect return to southerly flow by afternoon. Upper level ridge anchored over Texas, will dominate sensible weather. Perusal of deterministic soundings indicated sufficient dry mixing to allow temperatures to rise toward 75-percentile of NBM, with areas south of I-80 in Nebraska rising into the lower 90s. No precipitation is expected as moisture transport will be limited until later in the evening. Tonight: Slight eastward shift of upper level ridge axis expected. Simultaneously, an upper trough moves Alberta into Saskatchewan, compressing heights and allowing upper level jet streak to develop that will traverse the the northern periphery of upper ridge. Unsure how much forcing for ascent ULJ will provide given forecast area is located on the anticyclonic shear side. Better forcing likely will be due 40-50kt LLJ impinging on a warm front draped across southern Nebraska and northeast Kansas, aided by a diffuse disturbance moving along northern periphery of ridge. LLJ will gradually enhance atmospheric moisture as well, sufficiently to allow ML CAPE to increase into the 1-2K range after 21/06z. There is variance in bulk shear magnitude ranging from 30-50kts, with the upper range indicating the potential for isolated severe storms. The storms should be elevated in nature, thus primary threats hail and wind. CAMS providing different scenarios for initiation and coverage, thus kept highest PoPs in the 30-50% range. Saturday: Any convection that develops during the overnight hours is expected to move east of the area as LLJ veers. The aforementioned upper trough moving across Canadian plains will drive a more substantial cold front across the area as the warm front moves into Iowa and Minnesota. Ahead of the cold front, trended slightly toward 75% NBM, with 50% NBM values appearing sufficient behind the front. Simultaneously, a series of upper level disturbances will move into the central plains ahead of an upper level low that begins to eject out of the desert southwest. The net result is increasing dynamic forcing aloft and low level thermodynamic forcing. HREF indicating SBCAPE around 1500 J/Kg, with MLCAPE up to 2K J/kg. HREF denoting increasing bulk shear values in the 30-40kt range as afternoon progresses. Weak (10-20kt) 0-1km shear producing hodographs with some low level curvature capped by straight-line hodograph above. Thus expect isolated weakly rotating storms to grow upscale into MCS during the late afternoon into the early evening hours. Saturday Night and Sunday: Southwest upper low moves into the central plains Saturday night. Surface boundary likely will move into the southern periphery of forecast area due to convection; however large scale moist isentropic ascent will allow precipitation to persist north of the front and increased PoPs across the region into the 60-90% range across all but extreme northeast Nebraska. Expect some training of convection across extreme southeast Nebraska into southwest Iowa due to persistent H8 jet 25-30kt, where WPC QPF indicates storm total QPF 1.5 to 2 inch range. HREF precipitable water values in the 1.6 to 1.8 inch range are in 90 percentile for mid-September, so wouldn`t be surprised to see localized 3 inch totals. Given soil moisture deficits are in the 1 to 3 inch range, not expecting widespread flash flooding or flooding; however, cannot discount if isolated rain totals exceed 3 inches. Precipitation begins to wane by late Sunday afternoon as upper system lifts into northern plains and jet streak moves from southwest Kansas into western Missouri Sunday evening. Scattered shower will linger Sunday night into early Monday as upper low moves across the area. Temperatures on Sunday will be markedly cooler due to evaporative cooling and low level cold advection, with most areas remaining in the low to middle 60s. Monday through Thursday: Cyclonic flow aloft and low level cold advection will keep temperatures cooler than normal for the early part of the week, ranging from the 60s on Monday to the middle 70s by Wednesday. Currently, the forecast remains dry. There`s a possibility of scattered showers Tuesday night into Wednesday as a short wave drops southward into the mid-Missouri valley. Upper level ridging increases Thursday into Friday which will allow temperatures to increase to climatological normals. && .AVIATION /12Z TAFS THROUGH 12Z SATURDAY/... Issued at 534 AM CDT Fri Sep 20 2024 Weak northerly winds under 5 knots will quickly become southerly at 5 to 10 knots at all TAF sites by 14Z. Isolated showers and thunderstorms will be possible across southeast Nebraska and southwest Iowa after 04Z tonight. Convection should initially develop south of KLNK and KOMA, but will have a tendency to build to the north. While there are chances of rain at these two TAF sites the last few hours of the forecast period, probabilities are too low to include in TAFs at this time. just beyond the TAF period, there will be the potential for scattered thunderstorms, primarily near KOMA and after 18Z tomorrow. The stronger storms will bring the risk for hail up to the size of quarters and wind gusts up to 50 knots. && .OAX WATCHES/WARNINGS/ADVISORIES... NE...None. IA...None. && $$ DISCUSSION...Fortin AVIATION...Darrah