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Area Forecast Discussion
Issued by NWS Omaha/Valley, NE
Issued by NWS Omaha/Valley, NE
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846 FXUS63 KOAX 130830 AFDOAX Area Forecast Discussion National Weather Service Omaha/Valley NE 330 AM CDT Tue Aug 13 2024 .KEY MESSAGES... - There is a 60-80% chance of thunderstorms tonight into early Wednesday. A few could be strong to severe in southeast Nebraska and southwest Iowa (5-15% chance) with localized flooding, hail, and gusty winds the primary hazards. - Additional strong to severe storms are possible Wednesday afternoon and evening (10-20% chance). Large hail, damaging winds, and localized heavy rain and flooding will be the primary hazards. - Continued small shower and storm chances will persist into the weekend (20% chance), but the overall trend will be toward drier weather. - Today will be the last day of widespread below average temperatures (highs in the 70s). Highs in the 80s return for most on Wednesday and continue area-wide through the weekend into early next week. && .DISCUSSION... Issued at 330 AM CDT Tue Aug 13 2024 Early morning analysis showed mean upper level ridging pushing into the area from the west, though a fairly potent shortwave trough was sliding east-northeast through KS leading to a complex of strong thunderstorms. The northern edge of these storms is progged to cross into southeast NE by around 6-7 AM, though guidance is in pretty good agreement that anything strong should stay just south of the NE/KS border. Otherwise, fog and stratus was building into the area with patchy visibility under 1 mile as of 3 AM. Will keep an eye on trends for a potential dense fog advisory to match neighbors to our southeast, but latest HREF guidance suggests generally a 20-40% chance for sub-1/4-mile visibility. Clouds will linger through much of the day with perhaps a few sprinkles at times, though could see a few breaks by this afternoon. Temperatures are expected to top out in the mid 70s to perhaps right around 80 where we get any said breaks. Attention then turns to storm chances this evening and overnight as another shortwave trough (currently over the Four Corners region) approaches. At the surface, a low will eject eastward into KS while an attendant warm front will edge northward, but should stay in KS. However, southerly low level flow will strengthen, with strong moisture transport riding over the surface front and pointing into southeast NE. As a result, expect storm development in the area this evening, as early as 10 PM, but more likely just after. Questions still remain on just how far north/south they develop, but wherever it happens, instability should be sufficient for a few stronger storms, though shear will be fairly limited suggesting the threat of large hail and damaging winds will remain somewhat low (5-10%). Perhaps more concerning will be the threat of heavy rain and flooding, especially given trends in some CAMs which depict some potential for training thunderstorms. In addition, guidance suggests precipitable water values approaching 2" with warm cloud depths over 4 km, meaning any storms that do develop will be efficient rain- producers, with nearly every HREF member producing a band of 1-2" somewhere in the area. Particularly notable are the 13.00Z and 13.06Z runs of the HRRR which place several inches of rain very near or right over the 1-2" band of rain we received Sunday night into Monday (stretched from around Omaha/Fremont toward Red Oak and Villisca). At this point, these solutions are outliers and seem unlikely. Still, will definitely want to watch trends...if something like that were to pan out, impactful flooding would be likely. Most of this precip should exit by 8-9 AM on Wednesday, though cloud cover and perhaps a few light showers could linger through a good chunk of the day. Meanwhile, the surface low and warm front in KS look to eventually push northeastward into our area with model consensus suggesting the warm sector reaches southeast NE and southwest IA with clouds clearing. This could lead to a severe weather setup in that area, though there are still differences in exact track of the low (and any clearing), which will play a large role in said severe weather threat. Within the warm sector, expect ample moisture and instability with dewpoints in the 70s and MUCAPE values approaching 3000 J/kg. Deep layer shear looks somewhat limited (25-30 kts in the 0-6 km layer), but a strengthening low level jet and being in vicinity of the surface low could yield enough low-level hodograph curvature for at least a low-end tornado threat. Otherwise the main threats will be large hail and damaging winds, along with heavy rain and localized flooding, especially should these storms overlap with any areas that received heavy rain the previous night(s). Some of these showers and storms are expected to linger into Thursday morning before yet another shortwave trough approaches along with a surface "cold" front. Depending on exact timing of the front, there could be sufficient instability in place ahead of it for another strong to severe storm threat Thursday afternoon and evening, mainly near and east of the Missouri River. That said, it`s a fairly low confidence forecast and there`s a lot to work out with the storm chances before then. By Friday, a cutoff low is progged to develop somewhere over MN/WI/the Great Lakes, with associated precip on the back side remaining just to our east. However, guidance is in decent agreement that we`ll see continued weak bits of shortwave energy sliding southeast through the area into the weekend, bringing low-end precip chances, though expect more dry time than wet. Finally, temperature-wise, today should be the final cooler- than- average day for most of the area. By Wednesday, expect widespread 80s to perhaps 90 degrees in areas that clear out (though northeast NE could stick in the upper 70s). Beyond that, expect area-wide highs in the 80s through the weekend and into early next week. && .AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/... Issued at 1205 AM CDT Tue Aug 13 2024 Current satellite trends indicate an expansion of IFR and MVFR ceilings westward, as low level moist isentropic ascent increases across the area. Deterioration of ceilings is expected sooner than previous forecast based on satellite and observational data trends, with LIFR ceilings FL002-005 by 07-08 UTC. In tandem visibilities are expected to diminish to between 1/2 to 3/4SM by 10z and persist through 13z before diurnal mixing allows visibilities to improve into MVFR range by mid-morning. As for ceilings, deterministic and ensembles keep LIFR ceilings at all three TAF sites through 14-15 utc. Improving ceilings expected by midday as a drier air 850-700mb may erode depth of cloud layer, and for now have improved ceilings into low VFR category between 19-20 utc. There are low chances (15-30%) of showers affecting KLNK 11-13 UTC that could further diminish visibilities to below 1/2SM. && .OAX WATCHES/WARNINGS/ADVISORIES... NE...None. IA...None. && $$ DISCUSSION...CA AVIATION...Fortin