Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Sep 18 2023 08:06:49 FOUS30 KWBC 180806 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 405 AM EDT Mon Sep 18 2023 Day 1 Valid 12Z Mon Sep 18 2023 - 12Z Tue Sep 19 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN EFFECT ACROSS PORTIONS OF NEW ENGLAND... ....New England... Surface low across the Carolinas will continue to trek northeast, intensifying under the influence of the right entrance region of a strengthening upper level jet max situated over the Northeast. Upper trough to the west will pivot eastward taking on a slightly negative tilt leading to the surface low being pulled a bit closer to the coast than previously forecast. Increasing meridional flow ahead of the mean trough will advect tropical moisture poleward leading to PWATs running 1-2 standard deviations above normal across much of New England. As the surface reflection reaches the latitude of Long Island and points north, this will be when the low will peak in intensity with a strong east to southeast fetch off the Atlantic. U-vector wind field on both the NAM/GFS deterministic show a anomalous tongue of easterlies across southern New England up through ME and NH with an upslope signature on the eastern slopes of the Whites and Berkshires. 00z HREF blended mean and ensemble bias-corrected QPF indicate a robust area of precip located within the confines of central and northern New England due to both the synoptic scale forcing present as well as the mesoscale components interacting with the terrain across the interior. A general 1.5-2.5" of rainfall is forecast over southeast MA up through northern New England, including areas that are already saturated from Lee. QPF maximum of over 4" are now popping up within the upper quartile of outcomes within the latest hi-res ensemble package. The primary focus is within the terrain and central, Downeast ME where FFG's are already very low after the 2-5" of rainfall from Lee, as well as the priming from the lead wave ahead of the surface low that occurs overnight. In a probabilistic realm, the 00z guidance has also ramped up from previous runs with 3 hr rainfall of greater than 1" and 2" now exceeding 50% and 25% respectively over the targeted area which would create a heightened flash flood risk given the antecedent conditions. Despite the lack of enhanced instability, enough of a signature for ~500 J/kg of MUCAPE over southern MA/ME and Downeast ME would introduce some convective enhancement in banding that occurs within the WCB and deformation axis on the northwest flank of the surface low. Given all of the above and with collaboration agreement from both Gray and Caribou, have introduced a SLGT risk over a large portion of ME and the eastern side of NH with emphasis on the east slopes of the Whites. Kleebauer=20 ....Southern Florida Atlantic Coast... A cold front approaching South Florida will be a catalyst in sparking strong thunderstorms this afternoon. Area averaged soundings within the Marginal Risk show PWs between 1.9-2.2", MLCAPE up to 2,000 J/kg, and falling 500mb heights as a broad upper trough approaches from the northwest. Storms should form through both surface based heating and along the sea breeze front propagating inland, but they could also fire along the cold front to the north as well. This makes for a busy afternoon in South Florida, especially along the more urbanized area of southeast Florida. The 00z HREF shows 50-70% probabilities for 1-hr QPF > 2" within the Marginal Risk, as well as 40-50% probabilities for 3-hr QPF > 3". With such a high concentration of impermeable surfaces along the southeast coast of FL, there is the potential for flash flooding in poor drainage areas. Storms should dissipate by Monday evening with the gradual loss of daytime heating. Mullinax/Kleebauer 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!57j1vInekQ18FPP380fLSgvtXVYGLodwBgpCUIotQWub= tn9obQRjkVABYvnGmkPRNERm2oUlxM9OtvoAea41DoL_jGY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!57j1vInekQ18FPP380fLSgvtXVYGLodwBgpCUIotQWub= tn9obQRjkVABYvnGmkPRNERm2oUlxM9OtvoAea41Mw8xvaE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!57j1vInekQ18FPP380fLSgvtXVYGLodwBgpCUIotQWub= tn9obQRjkVABYvnGmkPRNERm2oUlxM9OtvoAea41OEqZSsk$=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) .