Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Feb 15 2023 20:23:26 FOUS30 KWBC 152023 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 321 PM EST Wed Feb 15 2023 Day 1 Valid 16Z Wed Feb 15 2023 - 12Z Thu Feb 16 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE LOWER MISSISSIPPI VALLEY, WESTERN TENNESSEE VALLEY, AND LOWER OHIO VALLEY... ....16Z Update... One very small change was made to the inherited Marginal Risk area this morning. Based on the new 12Z HREF probabilities, included a small expansion of the Marginal Risk into south central TN to cover the probabilities of exceeding FFG/ARI in the area. By far the most rain with the day's ERO will occur after 06Z. There remains some uncertainty exactly where the heaviest rain will fall, but increasing instability supports the presence of storms that will be capable of producing locally heavy rain. The rainfall event as a whole will cross the 12Z end of the Day 1 ERO, so those on the eastern end of the ERO risk area can expect additional rainfall into Thursday. The details and meteorology discussed in the previous discussion below remain in effect. Wegman ....Previous Discussion... Upper level jet streak and associated shortwave energy will round the base of the longwave trough today, then lift northeast along the eastern periphery Wednesday night as the the trough begins the early stages of phasing with the northern stream shortwave. At the surface, the guidance is fairly well clustered with the lee-side low emerging across the TX Panhandle by midday Wed, along with the track and timing into the mid MS Valley by 12Z Thursday. As the upper level jet streak and associated shortwave energy lift across the Central Plains and Lower MO Valley Wednesday night, the longwave trough will be nudging eastward across the southern High Plains. The increasingly diffluent flow aloft, combined with the right-entrance region forcing from the upper jet streak to the north, will strengthen the low-mid layer moisture transport and deep-layer ascent across the outlook area tonight and early Thursday morning. Per both the GEFS and SREF, 850-700 mb moisture flux anomalies climb between 3-4 standard deviations above normal between 00-12Z Thu, as 850 mb SSW flow increases to 40-50 kts. Increasingly favorable thermodynamic environment in evolving warm sector will also include PWs increasing to 1.5-1.6 inches and MUCAPE values between 500-1000 J/Kg. The dynamic and thermodynamic setup will favor 1-2" hourly rainfall rates Wed night underneath the stronger convective clusters, however at this point it appears there will be enough low-mid layer shear to maintain some forward propagation, at least through 12Z Thu, especially over eastern portions of the outlook area. Because of this and overall later timing of the heavier rain (latter half of the outlook period/after 00Z Thu), will maintain the Marginal Risk from yesterday's Day 2 ERO with only subtle changes, as continue to expect at most isolated/localized short-term runoff issues. Hurley Day 2 Valid 12Z Thu Feb 16 2023 - 12Z Fri Feb 17 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE TENNESSEE VALLEY AND CUMBERLAND PLATEAU... ....2030Z Update... No big changes with this update as far as the area covered under the Slight and Marginal risks, but there was a fairly large increase in 24-hour QPF totals across portions of central TN into southeastern KY. QPF forecast values increased by as much as 1", with a large area increased by over 1/2 inch. Despite this, somewhat dry antecedent conditions and at least some uncertainty as to how the convection that will produce all of the rainfall across the Slight Risk region will evolve has prevented any category upgrades. That said, if there were an "enhanced Slight" category, it would be used for these same areas. Broad southwesterly flow into the Appalachians should result in somewhat higher total rainfall across portions of southeast WV, extending north across the bulk of the state. Another area of higher QPF was also added from about Baton Rouge, LA to the GA/AL border in central AL. However, this area has somewhat higher FFGs than areas further north into TN, so this area was cautiously left in the Marginal Risk. A small expansion of the Slight Risk was made to include the southern Appalachians of far northern GA and far western NC with this update. While QPF should still be higher further west, the combination of mountainous terrain and upslope flow will make for a local maximum in QPF, with the terrain helping to focus the rainwater into the creeks and streams, resulting in higher chances of flash flooding. Wegman ....Previous Discussion... ....Lower MS Valley into much of the TN Valley, southern portions of the OH Valley, and southern-central Appalachians... Aforementioned positively-tilted longwave trough will continue an eastward trek across the Lower-Mid MS Valley and western Great Lakes Region Thu-Thu night. Difluent upper flow early in the period with increasing low-mid layer frontogenesis ahead of the upper trough/surface cold front will maintain favorable forcing and thermodynamics within the warm sector. Favorable depth/degree of anomalous moisture transport will continue downstream on Thursday, with TPW values expected to reach 1.7-1.8" across the central Gulf Coast region. Deep-layer instability, mainly elevated, does not appear to be as robust however per the latest guidance, with MUCAPEs averaging under 600 J/Kg across the TN Valley and points north on Thu. However, the models continue to show the low-level flow becoming better aligned with the 850-300 mb mean wind, which may retard forward propagation somewhat. This would heighten the potential for cell training and/or multiple rounds of pre-frontal convection during Day 2 (Thu-Thu night), especially across northern AL into parts of Middle-Eastern TN including the Cumberland Plateau, where we have maintained the Slight Risk inherited from yesterday's Day 2 ERO. This is far from a "slam dunk" however, given the marginal deep-layer instability and with most of the guidance indicating multiple streaks of heavier rainfall N-S toward the Gulf Coast (while not well clustered with these maximum rainfall axes). Stronger deep-layer forcing over northern portions of the outlook areas and better instability south toward the Gulf Coast would support the multimodal distribution of heavier rainfall per the models; for now will maintain the Slight Risk where on average the 1/3/6 hr FFG values are lower and the 00-06Z HREF indicates higher exceedance probabilities. Hurley Day 3 Valid 12Z Fri Feb 17 2023 - 12Z Sat Feb 18 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-e7cu9rdxKcLsWZvOpIglq4dkSdVN55lCEr81YAmt38V= 9tqWrmsPhChPFYCCzvIw9op0QQPYrFZeXU2FnKqyQ_mZ9II$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-e7cu9rdxKcLsWZvOpIglq4dkSdVN55lCEr81YAmt38V= 9tqWrmsPhChPFYCCzvIw9op0QQPYrFZeXU2FnKqyyhX1xMM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-e7cu9rdxKcLsWZvOpIglq4dkSdVN55lCEr81YAmt38V= 9tqWrmsPhChPFYCCzvIw9op0QQPYrFZeXU2FnKqywEL7Al8$=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .