Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Sep 29 2025 19:24:21 FOUS30 KWBC 291924 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 324 PM EDT Mon Sep 29 2025 Day 1 Valid 16Z Mon Sep 29 2025 - 12Z Tue Sep 30 2025 ....THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN CALIFORNIA, SOUTHWESTERN OREGON, THE GREAT BASIN, SOUTH- CENTRAL NEW MEXICO AND THE SOUTHEAST... 16z update: 12z Hi-Res CAMs and HREF probabilities remain on track with only small adjustments made for the West and Southeast. CIRA LPW suite shows deeper layer (850-500 layers) show lingering moisture remains across central NM than initially forecast over the last few days, with some weak intrusion noted from the south. As such, 12z Hi-Res solutions continue to show scattered thunderstorms across along the ranges west of the Rio Grande in New Mexico. Given recent rains, saturation and scattered flooding instances over the past few days; the potential remains for similar if relatively slightly reduced to have inclusion into the Marginal Risk already in place over the Sacramento Range, further east. Gallina ~~~~Prior Discussion~~~~ ....Carolinas... Moisture from tropical storm Imelda is expected to continue streaming into the Southeast Coast today. A surface front draped off the coast should provide forcing for thunderstorm development beginning this morning. Convective bands may work their way into a moisture rich environment in the Carolinas, where PWATs will be between 1.5-2.25in.. MUCAPE between 250-500 J/Kg will spread inland throughout the day, but the greatest convective potential will be within the bands that propagate inland through North Carolina this morning and afternoon. Neighborhood probabilities of exceeding 2 inches of rainfall in 24 hours are between 15-30% over parts of western and central North Carolina today. Despite the presence of deep tropical moisture, much of the convection and heaviest rainfall associated with Imelda should stay offshore closer to Imelda and the draped front. ....Northern California/Great Basin... A deep upper low centered over the Gulf of Alaska will send waves of low pressure into the West Coast over the next couple of days. A strong cold front may bring heavy rainfall to portions of Northern California up into southern Oregon and the Great Basin today. A weak instability gradient will develop ahead of the front this afternoon and during peak heating. The 00z HREF suggest up to 0.25in./hr. rates across the coastal ranges, southern Cascades and northern Sierra, with potential for upslope enhancement across windward facing slopes. EAS 24 hour 1 inch exceedance probabilities are over 45% along the coast and southern Cascades, while a more modest 5-15% chance remains over the Great Basin. There's a steep drop off in probabilities once you get into the 2 inch exceedance range. ....South-central New Mexico... There continues to be a signal for heavy rainfall over much of the Sacramento Mountains, where instability (500-1000 J/Kg MLCAPE) could support thunderstorm development and propagation into the mountains early this evening. Much of the state has already received over a half inch of rainfall in the past 24 hours with pockets of 2 inch amounts present including within the Sacramento Mountains. This may contribute to susceptibility of soils to runoff from additional rain. Burn Scars such as the Ft. Stanton Piles prescribed burn area are especially susceptible to flash flooding. Kebede Day 2 Valid 12Z Tue Sep 30 2025 - 12Z Wed Oct 01 2025 ....THERE'S A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE GREAT BASIN AND NORTHERN ROCKIES & OLYMPIC PENINSULA... 21z update... ....Great Basin... 12z Hi-Res CAMs/HREF and global guidance continue a slightly more progressive solutions with respect to the timing of the cold front and therefore the moisture flux convergence through the Great Basin resulting in a slight southeastward expansion of the Day 2 Marginal Risk across east-central NV and portions of west-central UT. Narrow streets of training convection within this anomalous moisture=20 stream with modest remaining instability may result in spots of=20 ..5-1.25" in 1-3 hours, mainly early in the period. Naturally lower=20 FFG values would suggest scattered incident or two of localized=20 flash flooding may result, especially along the upwind edge of the=20 longer overall Marginal area, where backbuilding may be more=20 likely.=20 ....Olympic Peninsula... Second and slightly stronger warm conveyor belt increases moisture flux out of the northeast Pacific. Deep moisture plume of greater than 1" total PWat, but may tick up to 1.25" will be advected on mainly southerly flow. Given the oblique nature to the Coastal Range, mainly the upslope and highest IVT values (peaking around 600-700 kg/m/s) will intersect the SW facing topography of the Olympic Range. A narrow drying period will likely exist after 00z toward 06-09z, before the eastern edge of the strengthening core to the closed low approaches toward the end of the forecast period, 01.12z. HREF probability over 50% of 3"/24hrs exists while a few=20 Hi- Res guidance members tickle the potential of 5"/24hrs (15-20%=20 for HREF) across the range. Combined with rain on Day 1, and continuing into Day 3, felt it appropriate given totals to have a low potential of exceedance worthy of a Marginal Risk, matching up with the inherited Day 3 Marginal.=20 Gallina ~~~~Prior Discussion~~~~ The marginal risk area inherited from the previous cycle was=20 mostly maintained with an expansion south into central Utah due to=20 trends in the guidance. The cold front responsible for heavy=20 rainfall potential in northern California on Monday will propagate=20 through the Great Basin and Northern Rockies on Tuesday, focusing=20 additional instability and moisture along its convergence zone.=20 Ensemble exceedance probabilities of 0.5 inches in 24 hours are=20 between 5-25% over parts of northeastern Nevada, northern Utah,=20 southeastern Idaho and western Wyoming. The progressive nature of=20 the front should mitigate flash flooding concerns. Kebede Day 3 Valid 12Z Wed Oct 01 2025 - 12Z Thu Oct 02 2025 ....THERE'S A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE OLYMPIC PENINSULA... 21z update... No change to the placement of the Day 3 Marginal Risk area, though 12z global guidance is showing a slight uptick in strength of CAA aloft allowing for 250-500 J/kg of elevated CAPE. The overall deep layer moisture reduces toward .75" but with prolonged southerly flow of 30-40kts, IVT values are persistent around 300-450 kg/m/s nearly the entire 24hr period. So consistent light to moderate upslope rains will have occasional, short duration bursts of=20 localized convective cores that will add to saturating soils over=20 the prolonged 3 day event. An additional 2-4" seems likely, but=20 given scattered convective nature, will be interesting to see any=20 localized enhanced totals as we move toward the full Hi-Res CAM=20 window, but end of 12z NAM-Nest and GEM Regional already hint at=20 slightly higher values...pretty good for an Atmospheric River=20 season "opener". Gallina ~~~~Prior Discussion~~~~ A shortwave propagating through the southern periphery of the Gulf of Alaska low will phase with it and move just off of the British=20 Columbia Coast by Wednesday. This new system will send a cold front into the Pacific Northwest. The proximity of the upper low and=20 associated vorticity lobes should destabilize the environment=20 (100-300 J/Kg MUCAPE) enough to support isolated thunderstorm=20 activity. Favorable RER dynamics and mid- level moisture advection=20 should support efficient rain rates especially over the Olympic=20 Mountains. ECENS 24 hour exceedance probabilities of 2 inches is=20 between 20-40% for much of the area. Kebede Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-sQU-92vlWcGu0LZO5wPQPp-uxEY55CuVqAppuYI7cRW= D5zaHHmTkJNZ5Xp4BL5p7vuQ526ZeMjbLH-7F64V6FXWu-A$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-sQU-92vlWcGu0LZO5wPQPp-uxEY55CuVqAppuYI7cRW= D5zaHHmTkJNZ5Xp4BL5p7vuQ526ZeMjbLH-7F64VS1Kq71o$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-sQU-92vlWcGu0LZO5wPQPp-uxEY55CuVqAppuYI7cRW= D5zaHHmTkJNZ5Xp4BL5p7vuQ526ZeMjbLH-7F64VTiK09is$=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. 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