Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Sep 27 2025 15:49:26 FOUS30 KWBC 271549 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1149 AM 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=20 feature, confluent low and mid level flow will push northward out=20 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=20 into UT, around the 90th percentile for the date according to the=20 SPC sounding climatology. Farther east, and somewhat displaced from the upper low, a secondary surge in moisture will lift out of the=20 Rio Grande Valley, surging PWs to around the 90th percentile as=20 well into southern NM. This moisture will overlap with increasing=20 instability (MUCAPE 500-1000 J/kg) to result in widespread showers=20 and thunderstorms today, first beneath the upper low through late=20 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=20 rates progged by both the HREF neighborhood probabilities and the=20 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.=20 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=20 Southeast Coast and aligned into the Ohio Valley will maintain=20 intensity today, shedding vorticity lobes and accompanying weak=20 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=20 will embedded within favorable thermodynamics for convection (PWS=20 1.5-2.0 inches, locally above the 97th percentile according to=20 NAEFS, overlapped with MUCAPE around 750 J/kg), with freezing=20 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=20 compromised below most adjacent areas, and the CSU First Guess=20 Field suggests a higher- end SLGT risk today. So while in general=20 flash flood instances should be pretty widely scattered, some=20 locally more numerous impacts are possible in southeast VA through=20 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, NEW MEXICO AND SOUTHWESTERN COLORADO... Southwest/Four Corners/Southern High Plains/Rockies The upper low in the Southwest is expected to open as it lifts into the Rockies on Sunday. The threat for excessive rainfall thus expands a bit around the trough as the fetch of moisture from the previous day rotates around the low and into parts of the Great Basin and Four Corners. Some elevated instability 500 J/Kg+ MUCAPE could generate 0.5-1.5 in/hr rates, especially over parts of New Mexico, where a slight risk is in effect. Antecedent rainfall from Saturday should lower FFGs, making soils more vulnerable to flash flooding. Slot canyons, dry washes and upslope areas will likely be the most susceptible to runoff. Southeast Guidance continues to trend farther away from the Southeast Coast with the axis of substantial QPF associated with a tropical cyclone moving through the Bahamas at the beginning of the day 2 period (Sunday morning). Given the uncertainty of the system's track, it's difficult to say with any confidence if/where the most impactful rainfall will occur, thus a broad marginal is in place from the Outer Banks down to Miami for now. Kebede Day 3 Valid 12Z Mon Sep 29 2025 - 12Z Tue Sep 30 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST AND MID-ATLANTIC... Southeast/Mid-Atlantic Tropical moisture arrives along the Southeast/Mid-Atlantic Coast on Monday ahead of the associated cyclone, which is currently forecast to be east of Georgia by Tuesday morning. Current deterministic guidance has between 2-4 inches of rainfall occurring along the Carolina Coast with isolated higher amounts. Models are trending away from the coast with the cyclone's track, which is reflected in a decreasing QPF trend with this cycle as well. Northern California/Southwest Oregon A deep East Pacific trough will bring broad diffluent flow aloft over the Northwest Coast beginning Monday morning. Scattered showers and thunderstorms may develop along the leading edge of a surface front, where the presence of weak instability (350 J/Kg MUCAPE) and modest moisture (~1 in. PWAT) could produce localized flash flooding along the northern California/southwest Oregon Coasts and inland into the Southern Cascades and northern Sierra. Kebede Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7GPR1s8hplDuXbIQfWREJlwUHl8acGKQqzJZ0H5Gxvn1= d2mRI00UW5yDCVVoW-9NADub3B_Lb1T17sFZmXmGdWcuS-w$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7GPR1s8hplDuXbIQfWREJlwUHl8acGKQqzJZ0H5Gxvn1= d2mRI00UW5yDCVVoW-9NADub3B_Lb1T17sFZmXmGpt7A6d8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7GPR1s8hplDuXbIQfWREJlwUHl8acGKQqzJZ0H5Gxvn1= d2mRI00UW5yDCVVoW-9NADub3B_Lb1T17sFZmXmGExbtgHo$=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) .