Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jul 12 2023 08:29:05 FOUS30 KWBC 120828 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 428 AM EDT Wed Jul 12 2023 Day 1 Valid 12Z Wed Jul 12 2023 - 12Z Thu Jul 13 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE LOWER MISSISSIPPI VALLEY... ....Lower Mississippi Valley and Gulf Coast States... An impressive vorticity maxima associated with a potent shortwave will be drifting SE from the Arklatex this morning. This feature is progged to drift slowly southeast while weakening through the day, but maintain a modest trough along the central Gulf Coast with weak accompanying height falls. This shortwave will be the primary mechanism for ascent today, but a dissipating surface trough and potentially a lingering outflow boundary from other convection could also serve as a focus for low-level convergence and ascent. The environment will be extremely favorable for heavy rainfall, however, with PWs 2-2.25 inches, +1 to +2 standard deviations above the climo mean according to the NAEFS ensemble tables, with overlapped MUCAPE of 2000 J/kg. These thermodynamics acted upon by the mesoscale features will likely yield scattered to widespread heavy rain producing convection today. The guidance has become more aggressive with thunderstorm potential today, and that led to the (then D2) slight risk upgrade for LA/AR. The area of greatest risk for heavy rainfall will be beneath the aforementioned vorticity max as it will likely manifest as an impressive MCS with slow moving thunderstorms possibly backbuilding along outflow boundaries to prolong rainfall. There is still uncertainty as to how this will evolve through the morning, but it is likely that as the LLJ of 20-25 kts veers it will less efficiently upglide atop outflows and lead to a reduction in coverage and intensity of convection. The timing of this is uncertain, but it is likely there will be several hours of moderate to heavy rainfall with rates 1"/hr or more before waning late morning into early aftn. The remnant MCV that is likely to develop will then serve as an additional focus for convection as it shifts southeast through the day, providing additional rounds of heavy rain as far as the Florida Panhandle. The inherited SLGT was expanded to the Gulf Coast as Corfidi vectors of just 5-10 kts aligned against the mean flow and into the higher instability suggests a good potential for backbuilding storms, and with rain rates of 2"/hr more, this could produce 3-5" of rain in some areas as reflected by HREF neighborhood probabilities. This could result in scattered flash flood instances despite relatively dry antecedent soils, with locally significant flash flooding possible across far southern AR/far northwest LA where event total rainfall (from pre-12Z through the morning) could reach 10 inches. ....Mid Mississippi Valley through the Great Lakes and into the Northeast... A broad but expansive trough centered near the Hudson Bay will maintain lowered heights and cyclonic flow into the eastern 2/3 of the CONUS today and tonight. Within this cyclonic flow, multiple shortwave impulses and accompanying vorticity maxima will rotate west to east in conjunction with a 90-100 kt zonally oriented upper jet streak to produce increasing deep layer ascent. This synoptic lift will impinge into a stationary front that will be draped west to east across the area and waver through the day. This will not only result in locally enhanced convergence for ascent, but may manifest as waves of low pressure in response to the vorticity maxima rotating overhead. While the exact timing of these shortwaves and any low pressure waves are a challenge to resolve even on this time range, it is likely scattered to widespread thunderstorms will be the result. Thermodynamics will be quite impressive across a large portion of this area, suggesting the widespread thunderstorms could all pose a heavy rain threat. PWs south of the stationary front will reach 1.75-2 inches as low-level SW flow reaches 30 kts and impinges into the front. This will supply the high PWs and collocated MUCAPE of 1000-3000 J/kg northward to enhance the environment, while also lifting isentropically atop the front and any residual outflow boundaries. Within the widespread convection, there is a signal for two areas of heaviest rainfall noted by the best convergence of moisture transport vectors along the greatest instability/PW gradients. The first is draped west to east along the stationary front from eastern IA through the thumb of lower Michigan. Here, convergence of the LLJ into the front will help turn the 0-6km mean winds to be parallel to the front, leading to an increased likelihood for training of cells which the HREF indicates will have rainfall rates of 1-2"/hr. This will likely result in waves of convection repeating across the area. The high-res guidance is actually in pretty good agreement and alignment for today, noted by HREF EAS probabilities for 1" and 2" that are quite high from Chicago, IL eastward to Detroit, MI and just north. Other than Chicago, this area has generally been dry the last 2 weeks, which is reflected by high FFG. However, training of echoes to result in locally more than 3" of rain and some exceedance of the FFG, which could result in scattered instances of flash flooding. The other area of concern is around Missouri where the most intense CAPE gradient and best moisture confluence will likely help develop convection in the evening which will then grow upscale into an MCS tonight. Some of the lead convection will likely develop along a residual outflow from prior thunderstorms, so uncertainty in the exact placement remains high. However, once convection begins it should expand rapidly and organize through 30-40kts of effective bulk shear. The guidance again is insistent that an impressive MCS will develop, and as Corfidi vectors collapse to around 5 kts this suggests some training from NW to SE with backbuilding also likely into the higher instability. Much of Missouri has been dry recently which has maintained high FFG, and the HREF exceedance probabilities are modest. However, training of 1-2"/hr rain rates could still produce 3-4" of rain in some areas as noted by the HREF probabilities, and the SLGT risk was adjusted just cosmetically for updated guidance. ....Southwest... The inherited MRGL risk for southeast AZ and far southwest NM was maintained, although the signal for excessive rainfall is modest. The difference today from the prior few days is that the area has received rain recently, so there are pockets of 7-day AHPS rainfall that are 100-150% of normal. FFG is still quite high, but does feature some pockets of 0.75-1"/1hr, and with PWs nearing +1 standard deviation according to NAEFS, the HREF suggests at least a 15-20% for 1"/hr rates. With storm motions progged to be just around 5 kts, and redevelopment probable on subsequent outflow boundaries in the pulse environment, isolated flash flooding is possible, especially in any urban areas. Weiss Day 2 The Day 2 outlook will be updated by 0830Z. Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6tFRccxb4pG_RFovqb4d2B3FZkEZXi_1TOCkg9FRimir= WrdZpLwQDlBvfmTsb8Oj5GaWUelVAGJJJPqRTMKYJSvCWqc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6tFRccxb4pG_RFovqb4d2B3FZkEZXi_1TOCkg9FRimir= WrdZpLwQDlBvfmTsb8Oj5GaWUelVAGJJJPqRTMKYtQusZGg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6tFRccxb4pG_RFovqb4d2B3FZkEZXi_1TOCkg9FRimir= WrdZpLwQDlBvfmTsb8Oj5GaWUelVAGJJJPqRTMKYqaA8wxM$=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) .