Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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576 FXUS63 KOAX 202238 AFDOAX Area Forecast Discussion National Weather Service Omaha/Valley NE 538 PM CDT Fri Sep 20 2024 .KEY MESSAGES... - Rain and thunderstorm chances return late this evening, particularly for southeast Nebraska and southwest Iowa.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 (50-90%). Rainfall amounts of 1 to 2 inches are expected for southern counties. - End of the 90s? Cooler conditions will arrive over the weekend and into next week, with highs dropping into the 60s and 70s. && .DISCUSSION... Issued at 415 PM CDT Fri Sep 20 2024 The cool air that built into the area last night behind a cold front was quickly mixed out. Most of the area is enjoying sunny skies and high temperatures in the lower 90s. Those 90s don`t come around that often in late September. In fact, over the past 30 years the average last date of 90 degree temps is September 19th. Today or tomorrow may be the final 90 degree day of 2024 for the Corn Belt. RH values are pleasantly low with numbers under 30%. Moisture transport ramps up this evening on the developing LLJ. Within 12 hours, PWAT doubles from less than an inch to nearly 2" which is climatologically in the 99th percentile for September`s second half. CAMs aren`t producing a lot for convection tonight, but with LLJ in place, expect CAMs to under forecast. Elevated convection will have steep mid-level lapse rates available and sufficient instability to produce some stronger storms. The elevated storms won`t have all the shear available to them, so that may be the missing ingredient to produce damaging wind or hail. Still PoPs of 20% seem reasonable as does the marginal severe weather outlook with the best chance of strong storms between 11pm and 3am tonight. .SATURDAY... As the LLJ shifts east, the nose of the warm front pushes northeast toward the Great Lakes and a strong cold front will push through this CWA mid-day. Forcing for ascent along the front will increase as the upper level low spinning across the Four Corners approaches and throws shortwave energy through the Corn Belt. Again, the instability sufficient if not considerable at about 1500 J/kg in areas south of I-80. Shear values of 30-40 knots may produce an isolated thunderstorm after lunch south of the interstate. It won`t be a rain-out for most outdoor activities... not yet... as the convective nature of the precip will leave at least as many locations dry as wet. Heaviest and most widespread rain waits until after midnight as the upper low pushes into Wyoming and increases the isentropic lift along the plunging cold front and produces widespread forcing for ascent. PoPs grow through the evening until peaking at 50-95% for the southern half of the CWA overnight and early Sunday morning. Latest HREF produces a swath of 1" in far southeast Nebraska and southwest Iowa... notably farther south than runs from earlier this week. PMM QPF maps tend to keep the precip bullseyes in central Iowa, western Missouri, and southwest Kansas, keeping the best drought-busters away from our farmers and ranchers. WPC QPF now keeps the 1-2" QPF totals south of I-80. High temps on Saturday may range from near 90 at Omaha, Lincoln, Beatrice, and points south to shy of 80 along the SD state line. .SUNDAY... Much cooler air will be in place with the spinning upper low passing over head and relentless CAA at the surface. Expect highs mostly in the 60s. It`ll be the coolest day since early May. Rain showers - heavy in the morning - will taper and come to an end by Monday morning. .NEXT WEEK... The GFS pushes an upper low through the area on Tuesday and tries to produce some showers but other guidance, including the NBM, aren`t on board. Have kept the forecast dry with temperature moderation forecast over the course of the work week leaving high temperatures close to late September norms of upper 70s by the end of the week. && .AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
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Issued at 530 PM CDT Fri Sep 20 2024 VFR conditions are favored into the evening, with the caveat of potential scattered showers and storms this evening. Scattered showers and storms are possible from the overnight period through tomorrow afternoon. Due to the low confidence in coverage (20% chance), have opted to leave out of the TAFs at this time. This may change with future issuances. If storms do develop, brief periods of MVFR to VFR conditions will be possible. Southerly winds will continue under 12 kts before a cold front sweeps through Saturday (15-20Z) and shifts winds clockwise to northerly. Showers and storms will become more widespread after 18Z. These storms will likely impact KOMA and KLNK by 18-20Z and KOFK around 00Z. This increase in coverage will also decrease ceiling to FL050-070.
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&& .OAX WATCHES/WARNINGS/ADVISORIES... NE...None. IA...None. && $$ DISCUSSION...Nicolaisen AVIATION...Wood