Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Sep 27 2025 19:25:11 FOUS30 KWBC 271924 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 324 PM EDT Sat Sep 27 2025 Day 1 Valid 16Z Sat Sep 27 2025 - 12Z Sun Sep 28 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHWEST AND MID-ATLANTIC... ....Desert Southwest, Four Corners, and Southern Rockies... An anomalous upper low (700-500mb heights falling beneath the 10th percentile from the CFSR climatology) will slowly fill today as it pivots slowly over the low deserts of CA/AZ. Downstream of this feature, confluent low and mid level flow will push northward out of the Baja region of Mexico, leading to a surge of PWs reaching as high as 1.25 inches in southern CA/AZ, and as high as 0.75 inches into UT, around the 90th percentile for the date according to the SPC sounding climatology. Farther east, and somewhat displaced from the upper low, a secondary surge in moisture will lift out of the Rio Grande Valley, surging PWs to around the 90th percentile as well into southern NM. This moisture will overlap with increasing instability (MUCAPE 500-1000 J/kg) to result in widespread showers and thunderstorms today, first beneath the upper low through late morning, and then expanding across a much larger region during peak heating this aftn/eve. In general, convection that develops today will move steadily north/northwest in the vicinity of the upper low, with 0-6km mean winds progged to be 10-15kts. This suggests that storms should be generally progressive outside of terrain influences during development, but aligned propagation vectors in the cyclonic regime will allow for repeating rounds of cells in some areas. With rain rates progged by both the HREF neighborhood probabilities and the UA HRRR-forced WRF to potentially reach 0.5-1.0"/hr, this could result in stripes of 1 to as much as 3 inches of rainfall, leading to scattered instances of flash flooding. The inherited risk areas were still well supported by the latest guidance suite, so only minor adjustments were made to the previous outlook. ....Mid-Atlantic States and Coastal Southeast... A mid-level longwave trough anchored just inland from the Southeast Coast and aligned into the Ohio Valley will maintain intensity today, shedding vorticity lobes and accompanying weak impulses northeast through the region. At the surface, a wavy front will meander in the vicinity, with multiple low-pressures rippling along it in response to the mid-level shortwaves. The entire area will embedded within favorable thermodynamics for convection (PWS 1.5-2.0 inches, locally above the 97th percentile according to NAEFS, overlapped with MUCAPE around 750 J/kg), with freezing levels of 13,000-14,000 ft supporting efficient warm rain processes this aftn. It is likely that thunderstorms will become widespread, especially during the peak diurnal cycle, and the HREF neighborhood probabilities for 2"/hr reach as high as 30-40%. While in general FFG exceedance probabilities are modest at 10-20%, there are pockets of much higher probabilities (up to 40%) across the Tidewater region of VA where a wave of low pressure and enhanced bulk shear will help organize thunderstorms a bit more significantly (and some of this is already ongoing, please see WPC Metwatch #1145 for more information). Here, FFG is also compromised below most adjacent areas, and the CSU First Guess Field suggests a higher- end SLGT risk today. So while in general flash flood instances should be pretty widely scattered, some locally more numerous impacts are possible in southeast VA through this evening. Weiss 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.=20 This suggests that precipitation may be a bit slower to move=20 onshore from the FL Peninsula into the Carolinas, but as Nine=20 deepens and the subsequent 850mb winds to the north intensify, the=20 accompanying strengthening cyclonic flow will transport impressive=20 tropical moisture westward and onshore, with PWs likely reaching=20 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=20 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!5SAbkh4zNICpaHd6TpGKcYsey254oPT-WO4_kxPGpkCc= nxSOYBx9Nzlz29keLykPRIfXX_2W9snq7GcHwtsWFyCYFIc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5SAbkh4zNICpaHd6TpGKcYsey254oPT-WO4_kxPGpkCc= nxSOYBx9Nzlz29keLykPRIfXX_2W9snq7GcHwtsWjZnSZyM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5SAbkh4zNICpaHd6TpGKcYsey254oPT-WO4_kxPGpkCc= nxSOYBx9Nzlz29keLykPRIfXX_2W9snq7GcHwtsW1R4SeSw$=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) .