Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Sep 28 2025 00:57:52 FOUS30 KWBC 280057 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 857 PM EDT Sat Sep 27 2025 Day 1 Valid 01Z Sun Sep 28 2025 - 12Z Sun Sep 28 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHWEST... ....Southwest... A Slight risk was maintained across portions of southeast CA into western AZ. Isolated to scattered instances of flash flooding continue this evening. Still generally expect to see the coverage and intensity of convection decrease overnight, although with lingering instability and the mid level low overhead we could see at least some localized areas of heavier convection continue. The Slight risk was removed over NM with this update. Still expect to see some convection overnight, and localized instances of flash flooding are still possible. However, activity has generally=20 underperformed so far today, and with instability lower here=20 compared to farther west, generally think the flash flood risk is=20 isolated enough to warrant just the Marginal risk. ....East Coast... A Marginal risk was maintained in the vicinity of the Delmarva where low topped convection will support efficient warm rain processes. Overall not expecting much in the way of a flood=20 threat, but heavy rates could result in a localized risk. Slow moving convection near the mid/upper vort energy will continue to support an isolated flash flood risk over portions of=20 southwest VA and central NC. Overall the expectation is for this=20 activity to decrease in coverage/intensity going forward...but=20 models have not had a great handle on this convection and thus=20 another instance or two of flash flooding can not be ruled out. Increasing easterly flow well north of Tropical Depression Nine=20 may help locally enhance convergence along the east coast of FL=20 tonight into Sunday morning. Low confidence on these details, but=20 if heavier convection is able to develop then a localized urban=20 flash flood risk is a possibility. Chenard Day 2 Valid 12Z Sun Sep 28 2025 - 12Z Mon Sep 29 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF FAR WEST TEXAS AND NEW MEXICO... ....Great Basin through the Southern Rockies... The closed low over the Low Deserts on D1 will gradually open and fill during Sunday while meandering slowly northeast such that the primary trough axis is positioned over the Four Corners by 12Z Monday. Despite this weakening, pronounced height falls and periodic PVA from weak embedded impulses will maintain broad ascent from the Great Basin through the Southern Rockies. At the same time, downstream southerly flow ahead of the trough axis will continue to manifest as a surge of elevated PWs, with widespread PW anomalies from the NAEFS reaching the 97th to 99th percentiles. This broad southerly flow combined with the lowering heights aloft will also produce elevated MUCAPE of 500-750 J/kg across most of this area, with locally enhanced instability above 1000 J/kg over NM. This setup will support scattered to widespread convection as suggested by available CAMs simulated reflectivity, with rain rates potentially reaching 1"/hr at times. Mean 0-6km winds Sunday should again be steady out of the south at 10-15 kts, although aligned propagation vectors could result in some repeating/training, especially across New Mexico. The inherited risk areas were adjusted for new guidance, but changes were primarily cosmetic. ....Southeast Coast... Concern on Sunday turns towards Tropical Depression Nine and its evolution as it starts to lift slowly northward from the Bahamas while strengthening. While there continues to be quite a bit of model uncertainty, the general model trends have been for a slower progression of this system, with very dry air likely positioned just to its west as reflected by 750-500mb RH fields. This suggests that precipitation may be a bit slower to move onshore from the FL Peninsula into the Carolinas, but as Nine deepens and the subsequent 850mb winds to the north intensify, the accompanying strengthening cyclonic flow will transport impressive tropical moisture westward and onshore, with PWs likely reaching above 2" from Cape Fear southward through the Space Coast. As tropical rain showers move onshore, they will be extremely efficient, with rain rates likely reaching 2-3"/hr. These showers will repeatedly generate offshore and then train onshore, so right along the immediate coast where sufficient instability and moistening can occur, total rainfall could exceed 3", especially across Florida, as reflected by HREF probabilities exceeding 40%. Of course, any additional trends west or slower in the track of Nine could enhance this rainfall, and locally higher totals are expected. The inherited MRGL risk was trimmed closer to the coast, especially across the Carolinas, with this update, with additional, but minimal cosmetic adjustments over the Florida Peninsula also made with this update. Weiss Day 3 Valid 12Z Mon Sep 29 2025 - 12Z Tue Sep 30 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE COASTAL PLAIN FROM NORTH CAROLINA THROUGH THE CENTRAL FLORIDA PENINSULA... ....Southeast and Mid-Atlantic... All attention will be on Tropical Depression Nine, which is forecast by NHC to be a strong Tropical Storm or Cat 1 Hurricane on Monday as it lifts slowly northward from the Bahamas. While guidance has been slowly converging on a solution that brings the storm to a stall east of the GA/SC coast by the end of D3, there is still considerable spread in the ensembles. Staying close to the NHC track brings heavy rainfall D3 from the Space Coast of Florida northward into Southeast North Carolina. The exact track and intensity of Nine will determine where the greatest moisture convergence will occur to create the heaviest rainfall amounts, but anywhere along and north of the track will likely receive training tropical showers with 2-3"/hr rain rates. The heaviest rain still appears to be confined to the immediate coast due to dry air to the west, but as the column slowly saturates in response to persistent and increasing moist advection on strengthening onshore 850mb flow, heavy rain will eventually spread inland, especially into SC and NC. While the inherited SLGT risk was trimmed to the south a bit over NC and expanded along the eastern coast of the FL Peninsula with this update, in general, changes were minimal due to continuing track uncertainty. ....Northern California/Southwest Oregon... An impressive trough will push eastward across the Pacific Monday, reaching the coast and angling onshore by 12Z Tuesday. This will drive a frontal system onshore, and the resulting low-level convergence overlapped by a modestly coupled jet structure will provide plentiful ascent for heavy rainfall along the coast of northern CA/southern OR D3. Moisture transport will become impressive ahead of the front as reflected by high probabilities (70-80%) for IVT exceeding 500 kg/m/s from both the ECENS and GEFS ensembles, pushing PWs to above 1 inch on increasing southerly 850mb inflow. Where this moisture overlaps with sufficient instability (progs suggest at least 250 J/kg MUCAPE) it will result in periods of moderate to heavy rainfall with rates of 0.25-0.5"/hr which may be sufficient for at least localized impacts in vulnerable terrain, especially where training or enhanced convergence along terrain features can produce more than 1 inch of rainfall. Weiss Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-6rocvidYNaM1uOSopYxalkmeDx6R8ZniWv4HMxaBGi0= UHiWneK-G6emfOSJa5NE-UMoQlkYTc4XHkMuVF1yUL4CsZQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-6rocvidYNaM1uOSopYxalkmeDx6R8ZniWv4HMxaBGi0= UHiWneK-G6emfOSJa5NE-UMoQlkYTc4XHkMuVF1yoj88jXE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-6rocvidYNaM1uOSopYxalkmeDx6R8ZniWv4HMxaBGi0= UHiWneK-G6emfOSJa5NE-UMoQlkYTc4XHkMuVF1yu09dmxA$=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 3.28-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .