Area Forecast Discussion
Issued by NWS Omaha/Valley, NE
Issued by NWS Omaha/Valley, NE
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833 FXUS63 KOAX 221138 AFDOAX Area Forecast Discussion National Weather Service Omaha/Valley NE 638 AM CDT Sun Sep 22 2024 .KEY MESSAGES... - Cooler and breezy with widespread (50-75%) showers for the first day of Fall. Diminishing by Sunday afternoon and evening. - More seasonable temperatures for the work/school week with very low chance (10-15%) chance of showers Tuesday; otherwise mostly dry conditions through Thursday. - Complex weather pattern late in the week, as upper low interacts with tropical system, could affect longer range precipitation chances. && .DISCUSSION... Issued at 357 AM CDT Sun Sep 22 2024 Today: Elongated area of showers is stretched across portions of the forecast area this morning due to moist convergence 305-310K and forcing for ascent associated with right entrance area of 70-80kt 250dm upper level jet and PIVA from southwest U.S. 500dm trough. Forecast models are in consensus in slowly moving trough eastward, and shearing the system as it syncs with another 500 dm trough that will move into Manitoba and Ontario. Southern portions of the forecast area remain on the cyclonic shear side of U.S. trough through 18z which will provide modest dynamic forcing for ascent; however, moist convergence 305-310K surfaces moves south of the area, with isentropic downglide occurring during the afternoon. Thus expect showers, and possible isolated storms to vary between scattered to numerous coverage through midday, then wane during the afternoon hours. There remains 5-15% potential for heavier rainfall, as HREF precipitable water values 1.4-1.8 (75-99 percentile)persist through late morning; however, average rainfall will range from trace to 0.75 inches. Breaks in cloud cover are expected across northeast Nebraska and northwest Iowa, with skies remaining mostly cloudy further south. Low level cold advection will keep temperatures 15-20 degrees cooler than Saturday, with highs ranging from the lower 60s along the Kansas and Missouri borders, to to around 70 along the South Dakota border. Tonight: A few showers may develop this evening as upper level low moves across the area. With isentropic fields moist convergence again developing 305-310K. Coverage at best will be isolated so kept PoPs < 20%. Cloud cover is expected to linger overnight, which will mitigate potential cooling, and NBM temperatures appear reasonable with a small adjustment above mean. Monday: Upper low slowly moves generally along KS-NE border during the day and weak, moist isentropic convergence 305-315 persists, so isolated showers possible through mid-day southeast Nebraska and southwest Iowa. By afternoon, forecast area on anticyclonic shear side of upper trough, along with large scale isentropic descent, thus any showers will come to an end. 800dm thermal trough will remain over area through midday, resulting in continued low level cold advection. By mid- afternoon, southerly flow returns. Net result is that temperatures may be a little warmer on Monday, but still near seasonal norms in the 70s. Clearing skies and radiational cooling may allow patchy fog to develop overnight, especially where ground is moist from rainfall. Tuesday: A vigorous upper trough dives southward from Alberta into the central plains on Tuesday. The deterministic models have been somewhat inconsistent in development of upper low centers, but are in better agreement each successive run. Passage of trough and any associated upper low will result in very low chance (10-15%) of very light rain or sprinkles, but that`s about it given moisture is limited to mid-levels of the atmosphere. Downslope winds will allow for warming max temperatures in the middle to upper 70s. Wednesday through Saturday: Generally dry conditions are expected. Friday into Saturday somewhat in question. EC and CMC now depicting a similar evolution of the large scale pattern similar to earlier runs of GFS. Both develop closed low near ArklaTex, with fujiwhara effect between upper low and a tropical system. LREF also depicting a similar scenario. Not sure how much it will affect the area, but may result in isolated to scattered tropical-like convection advecting into southern parts of the forecast area. Temperatures remain seasonable with highs in the 70s and lows in the upper 40s to middle 50s. && .AVIATION /12Z TAFS THROUGH 12Z MONDAY/... Issued at 636 AM CDT Sun Sep 22 2024 The main story of the TAF period continues to be the light shower coverage that has started redeveloping near the KOMA/KLNK terminals, and should gradually dissipate by 14z or so. No lightning is expected with any of the showers, and northerly winds will carry us into the afternoon and overnight hours, with winds dropping to 5 kts or lower after 01z or so. && .OAX WATCHES/WARNINGS/ADVISORIES... NE...None. IA...None. && $$ DISCUSSION...Fortin AVIATION...Petersen