Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Sep 30 2025 00:58:05 FOUS30 KWBC 300057 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 857 PM EDT Mon Sep 29 2025 Day 1 Valid 01Z Tue Sep 30 2025 - 12Z Tue Sep 30 2025 ....THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN CALIFORNIA, SOUTHWESTERN OREGON, THE GREAT BASIN AND COASTAL NORTH CAROLINA... ....West... Showers and thunderstorms will continue to move across portions of NV and ID tonight. Not a lot of instability to work with, and cells are quick moving...thus it seems unlikely we will get high enough=20 rainfall rates to cause too much of a flash flood concern. However=20 looks like decent coverage of 0.5" to 1" rainfall, with amounts=20 locally exceeding 1". Thus we will maintain the Marginal risk as a=20 very localized flash flood risk can not be ruled out. Also will=20 need to keep an eye on the northern Sacramento Valley north of=20 Redding. Recent HRRR runs indicate the potential for slow moving=20 convection where low level flow locally enhances convergence in the terrain. ....Southeast... A Marginal risk was maintained for coastal areas of NC. Moisture=20 well to the north of Imelda will continue to interact with the=20 frontal boundary near the coast producing areas of showers and=20 locally embedded heavier convection. Still looks like most of the=20 higher rates will remain offshore overnight, but a few heavier=20 cells may continue to move into the immediate coast. Overall the=20 flash flood risk is low, but can not rule out a very localized=20 issue along areas of coastal NC, where upwards of 2-4" of rain has fallen over the past 24 hours. Elsewhere over the Southeast the higher rates are expected to stay offshore, with just some showery conditions continuing which is unlikely to produce flash flooding. Chenard 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 stream with modest remaining instability may result in spots of ..5-1.25" in 1-3 hours, mainly early in the period. Naturally lower FFG values would suggest scattered incident or two of localized flash flooding may result, especially along the upwind edge of the longer overall Marginal area, where backbuilding may be more likely. ....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 Hi- Res guidance members tickle the potential of 5"/24hrs (15-20% 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. Gallina ~~~~Prior Discussion~~~~ The marginal risk area inherited from the previous cycle was mostly maintained with an expansion south into central Utah due to trends in the guidance. The cold front responsible for heavy rainfall potential in northern California on Monday will propagate through the Great Basin and Northern Rockies on Tuesday, focusing additional instability and moisture along its convergence zone. Ensemble exceedance probabilities of 0.5 inches in 24 hours are between 5-25% over parts of northeastern Nevada, northern Utah, southeastern Idaho and western Wyoming. The progressive nature of 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 localized convective cores that will add to saturating soils over the prolonged 3 day event. An additional 2-4" seems likely, but given scattered convective nature, will be interesting to see any localized enhanced totals as we move toward the full Hi-Res CAM window, but end of 12z NAM-Nest and GEM Regional already hint at slightly higher values...pretty good for an Atmospheric River 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 Columbia Coast by Wednesday. This new system will send a cold front into the Pacific Northwest. The proximity of the upper low and associated vorticity lobes should destabilize the environment (100-300 J/Kg MUCAPE) enough to support isolated thunderstorm activity. Favorable RER dynamics and mid- level moisture advection should support efficient rain rates especially over the Olympic Mountains. ECENS 24 hour exceedance probabilities of 2 inches is 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!_eS4alwTQPGfJTYKCe9UlUvtB1Er220Iz0r5jfZhpnJ_= DNStGZglIJc3qHTsDLyjiachyN5L_CJcaWC7rAamV1fwndA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_eS4alwTQPGfJTYKCe9UlUvtB1Er220Iz0r5jfZhpnJ_= DNStGZglIJc3qHTsDLyjiachyN5L_CJcaWC7rAamT4LxE7c$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_eS4alwTQPGfJTYKCe9UlUvtB1Er220Iz0r5jfZhpnJ_= DNStGZglIJc3qHTsDLyjiachyN5L_CJcaWC7rAam8J--Tes$=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) .