Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Feb 16 2023 00:21:59 FOUS30 KWBC 160021 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 720 PM EST Wed Feb 15 2023 Day 1 Valid 01Z Thu Feb 16 2023 - 12Z Thu Feb 16 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF MISSISSIPPI AND TENNESSEE... ....01Z Update... A Slight Risk was introduced this forecast update cycle with the recent guidance including the 18Z HREF and latest hi-res guidance clustering around the idea of a swath of heavy rainfall late this evening through early Thursday across portions of far northern Mississippi into western and middle Tennessee. The synoptic setup remains conducive for convection to initiate over the next several hours with shortwave energy lifting across the central Plains to the Lower MS Valley tonight. The increasingly diffluent flow aloft will help support the convection while in the lower levels, increasing 850 mb flow up to 40-50 kts will help surge very anomalous moisture northward (PWs 1.5-1.7"; +3 to +4 sigma). Convection will develop on the nose of the moisture transport axis and better MUCAPE gradient toward 04-06Z then likely track north/northeast across the outlook area. A favorable setup for training convection with storm motions parallel to the mean flow could allow for several repeating rounds across western to middle TN, particularly between 09Z-12Z Thursday. The 18Z HREF neighborhood probabilities show moderate to high signals for 1" hourly totals with some potential for 2" hourly totals, especially after 09Z. HREF probabilities for 6-hr QPF ending 12Z Thursday exceeding 3" are upwards of 25-40 percent with near 20 percent signal for exceeding 10-year ARI. Overall, the setup and ingredients appear favorable for localized to scattered instances of flash flooding despite the near normal soil moisture percentiles and a result a Slight Risk was introduced for this forecast update. Taylor 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!6gk_QjujMJdKgtRiXKPkVLhi7pKUvXnYI6pOVEnWzP3u= KyO-v_PA85bn0s7nqpSpknPUYdLRqu5vMi4PICFVrU20f2A$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6gk_QjujMJdKgtRiXKPkVLhi7pKUvXnYI6pOVEnWzP3u= KyO-v_PA85bn0s7nqpSpknPUYdLRqu5vMi4PICFVht2_X_Y$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6gk_QjujMJdKgtRiXKPkVLhi7pKUvXnYI6pOVEnWzP3u= KyO-v_PA85bn0s7nqpSpknPUYdLRqu5vMi4PICFVUaLncXo$=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) .