Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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000 FXUS63 KOAX 142338 AFDOAX Area Forecast Discussion National Weather Service Omaha/Valley NE 638 PM CDT Thu Sep 14 2017 .SHORT TERM...(Tonight through Sunday) Issued at 336 PM CDT Thu Sep 14 2017 Cold front sweeping through the area Saturday, with warm temps ahead and cooler temps behind the front one highlight of the forecast. Several sporadic thunderstorms chances through Monday are also a forecast concern. Broad upper trough axis extended from Hudson Bay into the Pacific Northwest this afternoon, with weak ridging noted from eastern Nebraska into Mexico. Significant shortwave was noted near Las Vegas, which is forecast to eject northeast into the Northern Plains Friday afternoon. In its wake, trough will build south across much of the Western U.S. placing our area under southwest flow aloft regime for the weekend. During that time, a large chunk of energy will rotate east and northeast from core of western trough, forcing a cold front through the Central Plains on Saturday. Then persistent west-southwest flow will emerge for early next week. Currently at the surface, a weak frontal boundary was noted from eastern South Dakota into southwest Nebraska. That boundary is not expected to move much overnight as upper flow nearly parallels it. Modest theta-e advection on increasing low level jet ahead front will likely spark scattered elevated convection tonight, favoring areas just to the west of our CWA, and may affect northeast Nebraska after midnight. Then much of Friday should be dry and warm as southerly flow and warm mid level temps suggest highs will top 90 in much of the area. Effects of shortwave moving into the Northern Plains on Saturday will begin Friday night as even stronger low level jet is induced and cold front begins moving into northeast Nebraska. Most convection should be confined to near the front, but broad theta-e advection zone could trigger an isolated thunderstorm just about anywhere in eastern Nebraska and southwest Iowa late Friday night. In fact most synoptic-scale model output shows QPF near and east of the Lincoln and Omaha areas a few hours either side of 12Z Saturday. Any morning convection will likely be short-lived, pushing to the northeast rather quickly, with another potential round setting up in the afternoon as front advances southeast. Timing of that convection appears to favor after 3 pm when convective temperatures are reached, generally along and south of Interstate 80. GFS shows over 2000 J/kg of MLCAPE and 30kt of bulk shear during this period, suggesting some risk for storm organization leading to potential severe weather. That risk should wane overnight as frontal boundary slides southeast of our area, then leaving much of Sunday relatively dry. However frontal boundary is expected to lift back north rather quickly Sunday afternoon as persistent southwest flow aloft lowers surface pressures in the northern High Plains. Thus there is a chance for more thunderstorms later Sunday afternoon in our south, but more likely Sunday night. After warm temperatures near or above 90 on Friday, cooling behind Saturday front will be realized Saturday afternoon in our north where temps will struggle to reach 70 in far northeast Nebraska. Later afternoon arrival of front should allow temps to rebound back into the mid and upper 80s in our south. All of eastern Nebraska and southwest Iowa will see highs in the 70s for Sunday. .LONG TERM...(Sunday night through Thursday) Issued at 336 PM CDT Thu Sep 14 2017 As cold front lifts back north as a warm front Sunday night and Monday, widespread showers and thunderstorms appear likely between midnight and noon Monday. Fairly high consistency between all models suggests more than an inch of rain is possible, with some chance for elevated severe hail storms given 1500 J/kg of MLCAPE. Southwest flow aloft is forecast to continue in our region much of next week, however models begin to diverge by Tuesday night and Wednesday on amount of energy ejecting through the Plains, and thus many aspects of sensible weather for eastern Nebraska and southwest Iowa. ECMWF and Canadian are similar in reforming western U.S. trough and maintaining its general position well to our west through Thursday. GFS has the idea to keep more energy in the northern Rockies and Alberta, then ejecting a strong shortwave into the Northern Plains which slams a cold front across our area on Wednesday. The EC/Canadian solutions keep frontal system to our west through Thursday, thus warmer with much reduced precipitation potential. Our going forecast, and what is going out today, favors the EC/Canadian solutions, keeping our are in warm sector of frontal system through Thursday, and thus lower precip potential and warmer temperatures. However we will still have chances for thunderstorms through the period as theta-e advection persists on lower atmospheric southerly flow regime. Periodic impulses ejecting into the region will enhance chances, most notably Monday night and Tuesday night when low level jet increases help drive convection. Otherwise will keep temperatures generally in the low to mid 80s Tuesday through Thursday. && .AVIATION...
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(For the 00Z TAFS through 00Z Friday evening) Issued at 615 PM CDT Thu Sep 14 2017 VFR conditions through the period. LLWS could develop at all TAF sites 04-14z, with winds up around 2000 35-40 knots. Could also see a few showers in the vicinity of KOFK 07-14z as well. Otherwise, gusty south winds redevelop at all sites 14z and beyond.
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&& .OAX WATCHES/WARNINGS/ADVISORIES... NE...None. IA...None. && $$ SHORT TERM...Dergan LONG TERM...Dergan AVIATION...DeWald

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