Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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000 FXUS63 KOAX 150919 AFDOAX Area Forecast Discussion National Weather Service Omaha/Valley NE 419 AM CDT Fri Sep 15 2017 .SHORT TERM...
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(Today through Saturday) Issued at 351 AM CDT Fri Sep 15 2017 Early Friday morning, a complex of thunderstorms over north central Nebraska was associated with a well-defined mesoscale circulation. This was one of several small-scale embedded short wave circulations in the region with others in far north Texas, eastern Colorado, and a compact but well-organized short wave trough spinning into southern Wyoming this morning and helping to steer the other weaker impulses. Additionally this morning, a zone of broad isentropic ascent was focusing vertical motion into a weakly unstable elevated layer over western Iowa, supporting showers and a few thunderstorms early this morning. The focus for the western Iowa precipitation is expected to shift northeast early this morning while a more focused zone of isentropic ascent and convergence along the nose of the LLJ is expected to shift into eastern Nebraska in association with the aforementioned short wave trough by sunrise. This feature should be supportive of scattered shower and thunderstorm development mainly between 4-10 AM, although some convective precip could linger into the early afternoon hours. There is also some potential for the energy coming out of north Texas to provide a zone of focused ascent as it moves across the forecast area early this afternoon. It seems any precip in this time frame would also be rooted aloft with minimal effective instability to work with. By mid to late afternoon, diurnal heating in advance of a cold front over far northern Nebraska will contribute to a moderately unstable airmass with effective wind shear in the 25-35 kt range. Model guidance also shows (and the environment would support) a surge of deep mixing from southwest to northeast just ahead of the front by late afternoon and expect thunderstorms to develop on the interface of enhanced low level convergence, strong heating, and deep mixing probably in the 3-5 PM time window. The combination of shear and instability may be sufficient to support some storm organization, which in combination with moderate lapse rates could support a large hail threat. Perhaps a bit more interesting is the potential for a few locally strong downbursts given the expectation for a rather dry sub-cloud layer with high LCL/LFC combinations, steep low-level lapse rates, and a rather moist profile above the LFC. In the end, the overall severe threat will be low, but have at least some potential for a few severe storms mainly in northeast Nebraska. Storms may gradually spread southeast but will probably struggle until another weak short wave trough tracks into southeast Nebraska overnight, probably providing enough additional lift to increase storm coverage. Aside from storm potential, it will be hot and windy today. Temperatures will depend somewhat on the extent of elevated convective cloud cover during the day, and expect to be a few degrees cooler than Thursday, but still in the upper 80s to low 90s for most of the area. The cold front will likely stall overnight in northeast Nebraska, keeping a majority of the forecast area warm and breezy overnight. On Saturday, the front will again begin making its way across the area and expect another round of strong storm development mainly over southeast Nebraska and southwest Iowa by peak heating on Saturday afternoon. Shear will once again be on the weak side but storms should have more access to better boundary layer moisture, and could again see a few strong to severe storms on Saturday afternoon and early evening before the front pushes all the way through. .LONG TERM...(Saturday night through Thursday) Issued at 351 AM CDT Fri Sep 15 2017 Sunday will be mostly dry and pleasant in the post-frontal airmass, but the active weather pattern will remain in place through the rest of the forecast period with fairly fast southwest flow aloft and occasional short wave energy crossing the Plains. There is strong model agreement that one particular short wave trough will interact with the retreating surface front Sunday night into early Monday, bringing a good chance for widespread storms to the area. Model guidance diverges a bit beyond Tuesday, mainly lending uncertainty to the timing of specific precipitation chances, but at times with indications neutral to negatively tilted trough passages suggestive of thunderstorms with potential for a round or two of strong thunderstorms through next week. Expect fluctuating temperatures with a pair of frontal passages, while generally windier conditions can be expected as well given the likelihood of a persistent surface pressure gradient across the central Plains.
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&& .AVIATION...(For the 06Z TAFS through 06Z Friday evening) Issued at 1158 PM CDT Thu Sep 14 2017 VFR conditions through the period. LLWS at all TAF sites 04-14z, with winds up around 2000` around 35-40 knots. Could also see a few thundershowers in the vicinity of KOFK 09-14z as well. Otherwise, gusty south winds redevelop at all sites 14z and beyond. Could see additional convection at all TAF sites later in the TAF period, but too far out to predict with certainty. && .OAX WATCHES/WARNINGS/ADVISORIES... NE...None. IA...None. && $$ SHORT TERM...Barjenbruch LONG TERM...Barjenbruch AVIATION...DeWald

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