Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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000 FXUS63 KOAX 291133 AFDOAX Area Forecast Discussion National Weather Service Omaha/Valley NE 633 AM CDT Sat Apr 29 2017 .SHORT TERM...(Today through Monday) Issued at 332 AM CDT Sat Apr 29 2017 Rain and snow potential through Monday morning remains the primary forecast concern in the short term period. Widespread rain spread across most of eastern Nebraska and southwest Iowa on Friday, with snow the primary precipitation type in much of northeast Nebraska Friday morning. The snow occurred under shortwave trough rotating north where 850 temperatures were in the 2-3C below range. Likely some dynamic cooling under shortwave helped the conversion to snow. Webcams and pictures from social media indicated some accumulations of up to 2 inches likely occurred on grassy areas by noon Friday, when temperatures were generally between 35 and 38 degrees. But by 3 PM Friday, all traces of snow were gone as relatively warm ground temperatures at or above 40 and air temperatures in the upper 30s melted the snow rapidly. A somewhat similar situation is evolving for this weekend, likely covering a larger part of our CWA, when temperature profiles will lean toward a rain/snow mix, with pockets of dynamic lift helping cool column enough for snow to reach the ground. The cooler temperature profiles will favor our northwest CWA, but dynamic cooling regime will likely be more pronounced over our southeast CWA across southeast Nebraska and southwest Iowa. The driving force responsible for all of this is an upper low currently near the Four Corners region which will swing into the Plains by this evening. Models are much more consistent this morning in tracking core of low from the Texas Panhandle this evening, into northeast Kansas by Sunday evening, then across northwest Missouri and into eastern Iowa by Monday morning. 850 temperatures were below 0C across the northwest half of our CWA according to Friday evening upper air soundings, but warm advection ahead of approaching low should warm that layer a degree or two during the day today, with only northeast Nebraska remaining below 0C. Thus given broad warm/moist advection promoting increasing chances of precipitation, expect liquid to be predominant type, except for a potential mix at times in northeast Nebraska. As upper low swings closer to our area tonight into Sunday, warming of low to mid levels of the atmosphere should again keep predominant precipitation type liquid save for parts of northeast Nebraska where colder air is a bit more stubborn. Slight cooling of the surface layer there will promote a mix with or change to snow from time to time. Sunday night into Monday is a bit more problematic as cooling aloft under cold upper low overspreads more of eastern Nebraska and southwest Iowa. There is consensus in model output in driving 850 temps below 0C for all of our area by 12Z Monday on back side of upper low moving into eastern Iowa. There is also consensus in decreasing critical partial thickness progs into the snow side of precipitation type spectrum, with broad area of 536-540 dam 1000- 500mb thicknesses shown by GFS and ECMWF. Forecast soundings are stubbornly indicating a sufficient near-surface warm layer to melt snow in the southeast half of our area Sunday night, but given above temperature and thickness forecasts, see no reason cooling of the surface layer to allow snow will not occur until later Monday morning when core of upper low moves east. Accumulations of snow is another question. First of all, even where snow occurs in most of eastern Nebraska and southwest Iowa, would only expect a brief accumulation where rates can overcome melting processes with air and surface temperatures remaining above freezing. However in northeast Nebraska where surface temperatures are cooling, and air temperatures will be closer to freezing, 2 to 4 inches of snow is not out of the question Sunday night into Monday morning, especially in areas west of a line from Columbus to Norfolk and Wayne. Am not expecting sub-freezing temperatures there, so impacts are not forecast to be harsh with some limited visibilities and slush on roadways the major hazards. Precipitation will be waning by Monday afternoon as upper low and attendant trailing trowal region lifts northeast. A rebound in temperatures into the upper 40s and middle 50s is possible Monday afternoon, rising from 30s and low 40s which are expected to persist through the weekend. .LONG TERM...(Monday night through Friday) Issued at 332 AM CDT Sat Apr 29 2017 A much more quiet weather period is forecast for the rest of the week, with gradually warming temperatures. A quick shot of zonal mid level flow will follow Monday`s upper low before strong shortwave drops into the Plains Tuesday night. This system could spark scattered showers from Tuesday afternoon into Wednesday, but bulk of expected significant precipitation should remain well south of our area where moisture fields support widespread convection. Mid level heights begin to rise by Thursday and Friday as upper ridging is advertised to develop in the Rockies and possibly the High Plains. Height increases along with more sunshine will help our temperatures rebound into the 60s or lower 70s by week`s end. && .AVIATION...
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(For the 12Z TAFS through 12Z Sunday morning) Issued at 630 AM CDT Sat Apr 29 2017 Conditions area expected to deteriorate through the period as upper low over the southern Rockies moves into the southern plains. MVFR cigs expected to spread into southern parts of the area 15Z to 18Z at KLNK and KOMA becoming IFR after 00Z. KOFK will be on the northern edge of the system with MVFR cigs not expected there until after 00Z.
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&& .OAX WATCHES/WARNINGS/ADVISORIES... NE...None. IA...None. && $$ SHORT TERM...Dergan LONG TERM...Dergan AVIATION...Fobert

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