Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Aug 30 2023 08:27:52 FOUS30 KWBC 300827 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 426 AM EDT Wed Aug 30 2023 Day 1 Valid 12Z Wed Aug 30 2023 - 12Z Thu Aug 31 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM THE EASTERN FLORIDA PANHANDLE NORTHEASTWARD INTO SOUTHERN NORTH CAROLINA AS IDALIA TRACKS ACROSS THE REGION... ....Florida to the Carolinas... Hurricane Idalia continues to press towards the FL Big Bend coastline this morning with increasing forward speed as of the last succession of updates from the National Hurricane Center. Despite the forward propagation speed, very heavy rainfall will occur across the northern and western periphery of the circulation with continued banding on the eastern quadrant of the storm as it moves through the FL Panhandle, southern GA, and eventually hooking east near the SC/NC borders by the end of D1. Consensus on the 00z guidance was for the main precip shield to be slightly east of the previous forecast which aligns well with the trends in propagation of the storms center of circulation. Deep moist conveyor of tropical air has funneled up through the southeast, mainly along and east of a stalled frontal boundary bisecting the interior portion of the Southeast CONUS. PWATs will run a general 2-4 standard deviations above normal across much of the southeast U.S with a typical enhancement near the storms core. One of the main components that has been well forecast is the primary shield of heaviest precip with Idalia continuing to lie on the north and northwest quadrants of the cyclone which will define the precip max associated with the storm itself. The latest 00z HREF and hurricane models all shifted about 20-30 miles to the east with the primary axis of heavy precipitation leading to some changes in the regional maximum(s) being depicted a bit east of the previous forecast with a stripe of 6-9" focused just east of a line from Tallahassee-Augusta-Columbia up into southeastern NC. HREF probabilities of greater than 6" over 24 hrs spanning the period indicate a solid 30-45% chance of exceedance of the 100 year ARI with a bullseye centered along the SC/GA line southeast of Augusta, nearing 60%. The saving grace to mitigate the worst possible impacts will be the forward speed of Idalia as she gets steered northeast, then east around the western periphery of the ridge to the west, then shoved eastward thanks to a strong confluent area to the north from a digging longwave trough across New England and adjacent Quebec. Regardless, significant impacts across the southeast U.S with regards to flash flooding will be prominent and will likely cover a long stretch of land that has seen significant rainfall already in the past few days. Antecedent soil moisture coupled with intense rainfall over a short time will lend credence to a continuation of the MOD risk ERO with a broad SLGT extending the bounds of the moderate with a southern reach down the west coast of FL as feeder bands plague the area through at least the first half of Wednesday. The heaviest rainfall the end of the D1 period will be co-located within the zone of the deepest 850-700mb RH layer which will be generally across eastern NC down towards Wilmington. ....New England... Locally heavy rainfall is forecast over New England on Wednesday thanks to a potent upper trough traversing the aforementioned area. Above normal moisture field in place across the region in conjunction with strong ascent will generate periods of heavy rain capable of flash flooding concerns. Forecast 1-2" is in place across western and northern ME, which a lot will occur over a shorter period of time leading to a continuation of the MRGL risk in place. ....Northern ID and Northwest MT... Sharp mid-level trough cutting across the Pacific Northwest will close off and intensify over the northern neck of ID and western MT later this morning and afternoon. Strong ascent focused under the closed low will create plenty of forcing within the terrain to induce a period of scattered to widespread showers and thunderstorms capable of producing some heavier rainfall rates as shown by the latest 00z HREF indicating a fairly stout signal for 24 hr 100 year ARI exceedance within the 60-70th percentile across Northwest MT. 700mb vertical velocity signatures are also quite impressive for the region which gives a good indication of the type of upper environment we're contending with overall. Given the steep terrain and concerns of burn scars and debris flow potential within the axis of heaviest QPF, have maintained the previous SLGT risk inheritance with little changes to the previous bounds as they align with forecast QPF. Kleebauer=20 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!_e8iHvZUqxtHty6f_YaCGkF1S_o_yzN52pWdqdvnel80= FyoPjdXjTUljFQtbr_gRFaHbEsD2YM3pxcAPeUE2gNM4Q_8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_e8iHvZUqxtHty6f_YaCGkF1S_o_yzN52pWdqdvnel80= FyoPjdXjTUljFQtbr_gRFaHbEsD2YM3pxcAPeUE2tJw6vEE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_e8iHvZUqxtHty6f_YaCGkF1S_o_yzN52pWdqdvnel80= FyoPjdXjTUljFQtbr_gRFaHbEsD2YM3pxcAPeUE2WhCufBo$=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) .