Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Feb 15 2023 00:15:28 FOUS30 KWBC 150015 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 714 PM EST Tue Feb 14 2023 Day 1 Valid 01Z Wed Feb 15 2023 - 12Z Wed Feb 15 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Pereira Day 2 Valid 12Z 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... ....2030Z Update... Very few changes were made with this afternoon's update. The Marginal Risk area was trimmed on the southern end a bit with a focus on northwestern LA across northern MS. The guidance is in good agreement that thunderstorms will develop along an axis from northwestern LA through northern MS. Thus, with increased certainty as to where the convection will develop, the confidence allowed the narrowing of the Marginal Risk area. Meanwhile further north, there were no changes to the previous risk area, and the previous discussion below remains in effect. Wegman ....Previous Discussion... Upper level jet streak and associated shortwave energy will round the base of the longwave trough on Wednesday, then lift northeast along the eastern periphery late Wed-Wed night as the the trough begins to phase 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, and are now in more decent alignment with the track and timing through Wed night (GFS and NAM still a bit faster than the non-NCEP consensus, however both are trending closer to the ECMWF). Wed night into Thu morning, as the upper level jet streak and associated shortwave energy lift across the Central Plains and Lower MO Valley, the longwave trough will be nudging eastward across the southern High Plains. The increasingly difluent 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 Wed night into Thu 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 500-1500 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 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 3 ERO with the anticipation of at most isolated/localized short-term runoff issues.=20 Hurley Day 3 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... ....2030Z Update... Unlike on Day 2, there is unusually low confidence in the forecast for Thursday. Depending on the guidance, an area of convection is expected to develop somewhere in the Marginal Risk area from just north of Lake Pontchartrain (EC solution) to central Ohio (GFS solution). On average, the guidance is focused on an area in between, from northeastern MS through southeastern KY. Thus, there are few changes to the Marginal Risk area, but the Slight Risk area has been shifted a bit to the west, and is now centered on central TN. This shift is largely due to a slowing down of the upper level pattern, with the main low tracking northeast across IL and into the Great Lakes. Thus, the new Slight Risk area is on average the most likely area where convection will train due to a 50 kt southwesterly 850 mb jet streak moving parallel to the low track. Regardless, due to the large uncertainty, there is a greater than normal likelihood that the Slight Risk area will need to be shifted a large distance as the guidance comes into better agreement. 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 guidance trends, 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 3 (Thu-Thu night), especially across eastern portions of the TN Valley and southern Appalachians where the Slight Risk inherited from yesterday's experimental Day 4 ERO continues in today's Day 3 outlook with minor modifications. Hourly rainfall rates of 1+ inch would pose a greater flash flood risk within the Slight Risk area, given the current low 1 hour FFG values of 1-1.5", which are not expected to rise much (if at all) given the anticipated .25-.75 inch of rainfall Wed-Wed night. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9130nXD5CVyRLFz84Gl2QX9OM6vTL4BvY1WYzSAQ-AF6= n2eaJ1x_KH8lvFTTZYU_DgO3dWkFdq1J5qKmaGpiPUxrluo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9130nXD5CVyRLFz84Gl2QX9OM6vTL4BvY1WYzSAQ-AF6= n2eaJ1x_KH8lvFTTZYU_DgO3dWkFdq1J5qKmaGpiZXAzWb0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9130nXD5CVyRLFz84Gl2QX9OM6vTL4BvY1WYzSAQ-AF6= n2eaJ1x_KH8lvFTTZYU_DgO3dWkFdq1J5qKmaGpiXCeOw-8$=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) .