Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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000 FXUS63 KOAX 202010 AFDOAX Area Forecast Discussion National Weather Service Omaha/Valley NE 310 PM CDT Sun Aug 20 2017 .SHORT TERM...(Tonight through Wednesday) Issued at 310 PM CDT Sun Aug 20 2017 Main forecast issues are thunderstorm chances through Monday night and eclipse forecast specifics, especially sky cover. Persistent focus of lower level convergence with low level jet and thunderstorm activity in southeast Nebraska into southwest Iowa was decreasing at mid afternoon. Scattered storms continued to develop and push east across parts of western IA. 3 pm surface analysis indicated a cold front curving southwest from near the WI/MN border area into northwest IA, then the boundary was nearly stationary and extended back into eastern CO. A secondary front, a warm front, seemed to stretch east from eastern CO into northeast KS and that was at least partially associated with outflow from storms last night into this morning. South of that boundary, temperatures had warmed well into the 90s across parts of central and western KS. The question becomes where will convection be most prevalent tonight. One other thing of note was the moisture feed from Mexico that extended up into CO and parts of KS, seem in water vapor satellite imagery. Thermal ridge at 700 mb was to our west, with 12Z plot showing +15C at KDEN. Weak height rises were occurring across the region and there did not seem to be any strong impulses tracking toward our area. The strongest signal for convection tonight is from southeast SD, northeast NE and southwest MN into IA. That seems to make sense based on where the nose of the low level jet will provide the strongest forcing, especially if the surface pattern turns out to verify closed to the NAM. Used a blend of the convection allowing models (CAMs) to draw POPs for tonight. Storms that occur will have some potential to be severe. Raised lows slightly from the previous forecast. --Eclipse Outlook for Monday-- Precipitation chances in the totality area (and even for areas farther to the north) look to be mainly 20 percent or less (but more like 25-30 percent at the SD border). We have the sky cover percent forecast mainly 55 to 70 percent. It appears this would be mostly high level moisture (but some mid clouds too). It is often difficult to forecast just how dense the high clouds will be. For now, let`s say that conditions are far from ideal for viewing in our area, but not to the point where the sun would not be visible at all. Highs should reach mainly lower 80s (possibly mid 80s) north, and mid to upper 80s south. If the warmer air is able to mix out a bit more, areas near the KS border could hit lower 90s. Highs should be held down slightly due to less insolation for the day, so reduced highs a bit from MOS guidance. By late afternoon, there could be two areas of showers and storms. One from northern IA back into northern NE and another starting from KS toward southeast NE. Have POPs going up into the 50 to 70 percent range for most of the area Monday night. Strongest focus for excessive rain is from northeast KS into northern MO and southern IA, but we could see some locally heavy rain in parts of our area. This would be shifting out of our area by mid morning Tuesday. Cooler and mainly dry conditions should occur from Tuesday afternoon into Wednesday. There are weak signals that there could be some light precipitation with return flow and warm air advection Wednesday for parts of northeast NE. Look for highs Tuesday 75 to 80 north and 80 to 85 south. .LONG TERM...(Wednesday night through Sunday) Issued at 310 PM CDT Sun Aug 20 2017 At the start of this period, a 500 mb ridge should extend from NM into Alberta and Saskatchewan, with a trough off the coast of southwest Canada and the Pacific Northwest. With time, the pattern deamplifies and then on the weekend a 500 mb anticyclone strengthens from the Four Corners region back into NV and CA. Will have some low POPs Wednesday evening, then look for mainly dry weather later Wednesday night into Friday morning. Pattern looks a bit more unsettled then with slight chance to low end chance type POPs. Temperatures should average a little below normal. && .AVIATION...(For the 18Z TAFS through 18Z Monday afternoon) Issued at 107 PM CDT Sun Aug 20 2017 Morning thunderstorm activity has moved east and south of KOMA and KLNK. Mainly VFR conditions are expected the first part of the TAF period. Mid-level convergence will produce sct-bkn clouds at 80-120 AGL at KOFK. Boundary left over from morning and overnight storms may serve as a focus for isolated aftn/evening convection KLNK and KOMA, but confidence is too low to introduce into TAF. Any storms that develop today should dissipate by early-mid evening, as they will be diurnally driven. LLWS potential will increase during the early morning hours at all three TAF sites, as nocturnal jet increases in strength, with wind shear of 35-40kt expect between surface and 2K AGL. Radiation and advection MVFR fog is expected to develop at KOFK between 09-11z. In additional Scattered convection may impact KOFK during the pre-dawn hours. && .OAX WATCHES/WARNINGS/ADVISORIES... NE...None. IA...None. && $$ SHORT TERM...Miller LONG TERM...Miller AVIATION...Fortin

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