Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Apr 11 2023 08:03:46 FOUS30 KWBC 110803 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 402 AM EDT Tue Apr 11 2023 Day 1 Valid 12Z Tue Apr 11 2023 - 12Z Wed Apr 12 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF SOUTHERN LOUISIANA, SOUTH FLORIDA AND THE KEYS... ....Southern Louisiana... Overall model agreement is pretty good with the evolution of the closed 850-250mb closed low near the central Gulf Coast today into tonight. However still seeing some spread with the low level mesoscale features, which will likely be the main driver of how much heavy rain makes it onshore during the day 1 period. The bulk of the most intense convection will stay over the open waters, however it does appear likely that at least some heavier convective activity will impact portions of southern LA today into tonight. 00z HREF probabilities of exceeding 3" get as high as 40-70% along coastal areas, and this seems to align fairly well with the global model consensus as well...and fits with HREF instability forecasts. This area is hard to flood, and this rainfall may come periodically though the period, so even with the potential for 3"+ amounts, think a Marginal risk will suffice. Further north towards New Orleans, probabilities of exceeding 3" are closer to 5-10%, which also seems reasonable. Will keep the Marginal risk this far north to account for this low threat of heavy rainfall, as amounts of this magnitude would likely pose a localized urban flood risk if it materializes.=20 ....Southeast Florida... Areas of heavy rainfall are again likely today across portions of southeast Fl into the FL Keys. The better corridor of instability shifts a bit south compared to Monday, so also expecting the favored corridor of heavy rainfall to shift southward. The environmental ingredients for heavy rainfall look pretty good...with low level easterly convergent flow, instability of ~1000 J/KG, PWs getting towards the 75th-90th percentile, and low level easterly winds with mid/upper level westerly winds favoring slow cell motions. HREF neighborhood probabilities of exceeding 5" are as high as 60-90% over the Keys, with 8"+ probabilities over 20%. Given the ingredients and decent HREF performance Monday over southeast FL, hard to argue much with this output. Now whether these more extreme rainfall amounts end up over a Key or over water remains unclear, but pretty confident in areal averaged rainfall of 2-3" over the Keys, and there will likely be pockets of 5"+ amounts in the vicinity. Localized flash flooding is probable with this activity today into tonight. The Marginal does extend north towards Miami as well...as even though probabilities of heavy amounts are lower here, this urban corridor is more susceptible to flooding. So this extension accounts for this susceptibility and at least some lower probabilities of 3-5" rainfall amounts today. ....Northern ID into western MT... Pockets of moderate rainfall will be ongoing at 12z this morning. However snow levels will start dropping shortly after 12z, and so no longer seeing enough rainfall potential to justify a Marginal risk, as a large percentage of the QPF today/tonight will be snow or a rain/snow mix. Chenard 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!9fQXr_PJkeuWuPC9Y9ocs4mFIcl1AJ6BdQIeEtJTEIxu= 306vBoYbBx39HhxgjvfIycbJT9X2eLf56hUINXLh-gFK5Cw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9fQXr_PJkeuWuPC9Y9ocs4mFIcl1AJ6BdQIeEtJTEIxu= 306vBoYbBx39HhxgjvfIycbJT9X2eLf56hUINXLhz74fu8c$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9fQXr_PJkeuWuPC9Y9ocs4mFIcl1AJ6BdQIeEtJTEIxu= 306vBoYbBx39HhxgjvfIycbJT9X2eLf56hUINXLh9s0Bhpg$=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) .