Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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000 FXUS63 KOAX 150439 AFDOAX Area Forecast Discussion National Weather Service Omaha/Valley NE 1139 PM CDT Mon Aug 14 2017 .UPDATE... Issued at 951 PM CDT Mon Aug 14 2017 Expect storms to increase after midnight with coverage mainly across the northern 1/2 to 2/3 of the forecast area. Made some adjustments to clouds and POPs overnight into Tuesday mid morning. Otherwise any changes were pretty minor for the overnight. && .SHORT TERM...(Tonight through Thursday) Issued at 314 PM CDT Mon Aug 14 2017 Active pattern persists across the area. Subjective 12Z upper-air analysis indicates a weakly wavy upper-level pattern, with troughs over the northern Great Lakes and the northern Rockies, as well as in the southern Plains, and a ridge from Co through the northern High Plains. Low-level moisture was increasing in the central US, with 12-16C+ dewpoints at 850mb nosing into the central Plains. The mid-level pattern across the central US was fairly ripply but also fairly weak, with ridging from central SD into central KS and a small low centered in southwest MN. Surface low at 19Z was centered in eastern WY, with a mesolow in northern IA/southern MN and a surface high in northeast ND/northwest MN. A surface boundary was noted across the KS-NE border toward northern MO, and a new round of showers/storms were initiating in the KS- NE-MO mutual border area this hour. Main forecast concern is convective potential this afternoon/tonight and again Tuesday evening into Wednesday morning. Have limited POP mention to areas of southeast NE to southwest IA this evening, with CAMs suggesting convective development would be limited to areas near the surface boundary. Scattered showers/storms may expand across more of eastern NE/western IA tonight as the boundary retreats northward, and have generally narrowed the axis of precipitation to follow the boundary as it progresses northward toward the SD border and northwest IA. As an upper-level trough digs into the Rockies on Tuesday, a surface low also will take shape in northeast CO, with aforementioned boundary sharpening from southwest NE to northeast NE and toward northwest IA. Thunderstorms should develop along that boundary by mid to late Tuesday afternoon in central to northeast NE and into southeast SD. Guidance continues to suggest quick upscale growth into a line, with abundant rainfall associated with the convection. The rain would be welcome in many areas, but it still could fall quickly enough to cause short-lived concerns, and will need to keep an eye on the heavy rain potential for Tuesday night into Wednesday. Convective complex should move across NE through the evening and night, with some weakening toward Wednesday morning. With the CWA getting more of the complex than the initial development, think that threat mode will have moved more toward wind and perhaps some hail, with decreasing severe thunderstorm risk through the night. Boundary will continue to push southward slowly on Wednesday, but it may still be laying across the southern CWA by Wednesday afternoon, when convection is likely to redevelop as a reinforcing shortwave slides across the central Plains. Have less confidence in severe weather potential, as it is likely that the Tuesday night convection will stabilize the airmass, but certainly some areas could see additional rainfall. After precipitation shunts southward on Wednesday afternoon/evening, the remainder of Wednesday night through Thursday should remain dry as a surface to mid-level high moves across the region. .LONG TERM...(Thursday night through Monday) Issued at 314 PM CDT Mon Aug 14 2017 Active weather will continue through the weekend as a series of shortwaves brings potential for at least some areas to see at least isolated to scattered showers/storms. That said, model solutions differ widely from each other on the details, so confidence in timing or location of precipitation is low. Slight chances may skirt the western CWA late Thursday night or Friday morning, depending on eastward progress of High Plains convection from Thursday evening/night. Additional chances may arise on Saturday afternoon/evening and possibly on Sunday night. ECLIPSE DAY FORECAST (MONDAY)... For a few runs now, models have indicated some area of showers/storms in eastern NE and/or western IA on Sunday night into Monday morning. They have differed slightly in the placement of the convection, the speed of its exit, and the amount of cloudiness behind it. For now, it does at least seem to be a reasonable scenario that there could be chances for showers/storms at least on some part of Monday. At the very least, the pattern does not look like a high and dry ridge. Have shaved a couple of degrees off of model guidance and included temperatures dipping downward in the couple of hours on either side of the eclipse peak, consistent with research on the effect of an eclipse. If the day is sunny, the amount of dip (and lowering of temperatures away from guidance) would be several degrees more; for now, this is a conservative estimate. Also, based on this research, have dropped a kt or two from winds near the peak of the eclipse. && .AVIATION...
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(For the 06Z TAFS through 06Z Tuesday night) Issued at 1140 PM CDT Mon Aug 14 2017 Added mention of TSRA at KOFK for the overnight. We could also see some SHRA or TSRA at KOMA, mainly after 10Z. Although surface based instability is limited, coverage of storms should increase after midnight, and may last into mid morning in western IA. For now, left SHRA and TSRA out of the KLNK TAF, but would amend as needed. MVFR ceilings possible late tonight and into mid morning Tuesday, especially in precipitation.
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&& .OAX WATCHES/WARNINGS/ADVISORIES... NE...None. IA...None. && $$ UPDATE...Miller SHORT TERM...Mayes LONG TERM...Mayes AVIATION...Miller

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