Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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000 FXUS63 KOAX 280447 AFDOAX Area Forecast Discussion National Weather Service Omaha/Valley NE 1147 PM CDT Thu Apr 27 2017 .SHORT TERM...(Tonight through Sunday) Issued at 324 PM CDT Thu Apr 27 2017 Rain and potential for snow this weekend are the primary forecast concerns. Broad mid level trough covered most of the western two-thirds of the CONUS this afternoon, with strong low noted over the Northern Plains into the Great Lakes. More of note for our area was shortwave extending from southern Wyoming into western Kansas that was already triggering scattered showers into western Nebraska and central Kansas. This is the feature that will bring most of our weather over the weekend. Shortwave is forecast to lift northeast into Nebraska and deepen, closing off a low in southwest Nebraska for Friday. Low then migrates southwest to the Four Corners region by Saturday morning, and eventually swings into the Plains for Sunday. There is fairly decent agreement between models on this scenario, with some divergence coming on Sunday when GFS brings core of closed circulation into southwest Iowa by the afternoon, and ECMWF/Canadian solutions place low in south central Kansas. NAM is more closely aligned with faster GFS solution. This difference in placement of low and associated cold air aloft will play a big role in regards to precipitation type in our area as NAM/GFS would favor more snow in a more dynamic cooling regime, while ECMWF/Canadian would favor a rain or rain/snow mix. Until Sunday, low level temperature profiles suggest rain will be the most likely precipitation type, save for parts of northeast Nebraska in the late night and early mornings when low level atmosphere cools enough to maintain snowflakes to the ground. Little if any accumulation is expected before Sunday as ground temperatures and near-surface warm layer melt falling snow. As closed low moves closer Sunday, forecast soundings suggest a good chance for a complete changeover to snow in northeast Nebraska, and at least a mix in all of eastern Nebraska and southwest Iowa. If GFS and NAM position wins out, dynamics would tend to cool to below freezing deeper column of air just above the surface, leading to all snow in more of our area. Will have to see model trends the next couple of runs to really pin this down, but given widespread QPF totaling over an inch in eastern Nebraska, if a change to snow does occur over a wider area, expected duration and rates will lead to some significant accumulations of heavy and wet snow. For now, the uncertainty in all of the above factors leads us to maintain only minor accumulations of snow in northeast Nebraska, with a mention of rain snow mix over the rest of the area on Sunday. The evolution of precipitation is expected to begin later this evening as shortwave rotates into Nebraska and western Iowa overnight. Wave will gradually work north of our area as it dissipates Friday afternoon, at the same time low begins its southwest migration. Will have fairly high rain chances later tonight into Friday before support wanes. We should likely see a dry period Friday night into Saturday before broad area of upper diffluence overspreads the Plains Saturday afternoon ahead of ejecting Four Corners upper low. Chances increase Saturday night as low moves closer, and continues all day Sunday with expected change or mix with snow as outlined above. Total precipitation amounts in the inch and a half to two and a half inch range are likely across a good part of our area from tonight through Sunday night. .LONG TERM...(Sunday night through Thursday) Issued at 324 PM CDT Thu Apr 27 2017 Progression of upper low out of the Plains will bring an end to the more significant precipitation Sunday night. In fact, models re- converge on low position over Iowa by Monday morning, then lift system into the Great Lakes. Mid level troughing behind this system sharpens into the middle of the week, keeping our area under a cool temperature regime. A better chance of some sunshine suggests we will be a bit warmer than the weekend. Precipitation chances will be sporadic, relying on minor impulses riding through the flow. A more significant wave should remain to our south, keeping precipitation there for Thursday. && .AVIATION...
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(For the 06Z TAFS through 06Z Saturday morning) Issued at 1150 PM CDT Thu Apr 27 2017 VFR conditions initially, but becoming MVFR and eventually IFR through the forecast. Showers becoming prevailing by 08z, prevailing at KOMA by 10z and by 12z at KOFK. Rain may end from south to north by 18-22z at KLNK/KOFK, but IFR conditions remain. Light winds initially, becoming east northeast through time and increasing to 13 to 23 knots. Could be a rain/snow mix at KOFK 14-17z.
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&& .OAX WATCHES/WARNINGS/ADVISORIES... NE...None. IA...None. && $$ SHORT TERM...Dergan LONG TERM...Dergan AVIATION...DeWald is the U.S. government's official web portal to all federal, state and local government web resources and services.