Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Nov 15 2025 01:00:04 FOUS30 KWBC 150059 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 759 PM EST Fri Nov 14 2025 Day 1 Valid 01Z Sat Nov 15 2025 - 12Z Sat Nov 15 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN CALIFORNIA... ....0100 UTC Update... ....Southern California... The latest Day 1 ERO update included an expansion of the Slight=20 Risk area along the Transverse Ranges to the Central CA Coast,=20 while a also nudging the Moderate Risk area Slightly westward to=20 include more of the Santa Barbara area. The reasons were two-fold.=20 First, they incorporate the heavier rain that has already fallen=20 (especially areas west of KSBA). Secondly, the slight westward=20 adjustment also aligns with the latest high-res guidance=20 expectations from recent HRRRs along with the 18Z HREF and 12Z RRFS ensemble suite. The Moderate Risk area aligns well with the=20 highest probability of >0.50"/hr rainfall rates later tonight=20 (after 06Z, and especially after 09Z when those probabilities climb to over 60%), along with where the highest probabilities of >3"=20 will fall through 12Z (25-30%)=20 Hurley Day 2 Valid 12Z Sat Nov 15 2025 - 12Z Sun Nov 16 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN CALIFORNIA... 20Z Update: The overall synoptic evolution remains unchanged in the run-to-run variance within guidance today. The prospect for heavy rainfall will carry forward into Saturday with the low progression allowing a protrusion of heavier rainfall to impact the coast to areas inland over the deserts of southern CA/NV into western AZ and southwest UT. Assessment of the 12z HREF blended mean QPF footprint and associated prob fields, both neighborhood and EAS indicate the eastern Transverse and northern Peninsular Ranges of southern CA will be the beneficiary of the heaviest precip threat due to the added orographic enhancement as flow remains mostly orthogonal to the terrain north and east of the LA metro. HREF neighborhood probs between 40-60% for >5" exist over portions of the San Bernardino and San Jacinto Mountains spanning San Bernardino and Riverside counties. As the disturbances moves eastward, IVT plume will shift inland to impact the lower deserts in CA and areas further inland towards NV/AZ and southwest UT generating flash flood concerns with a weak convective pattern anticipated through the storm life cycle. The prospects of significant precip on the order of 1" or more are generally high for the above locations which will be sufficient for at least widely scattered flash flood concerns as we move into Saturday afternoon and evening. Probabilities for >2" a bit more subdued and relegated to the CA/NV border into the Imperial Valley, but that doesn't mean a few areas across southern NV into the Colorado River basin can't approach the 2" threshold in this scenario just given the anomalous airmass involved. The previous MDT risk was expanded to include more of the San Jacinto mountains where the highest probabilities for >5" are located. The SLGT risk was also expanded in the interior given the latest trends. The MDT was not trimmed in any one location, leaving the LA metro firmly within the highest risk forecast for the period. Kleebauer ...Previous Discussion.. The peak of the heavy rainfall event across southern California will occur on Day 2/Saturday. The upper level low off the coast will barrel inland, drawn eastward by the strong shortwave trough leading it. PWATs are expected to increase to around 1.25 inches, which is as much as 4 sigma above normal for this time of year. Meanwhile MUCAPE values could increase to around 250 J/kg near the coast. As the low moves into central California to near the NV/CA border in central CA/southwest NV, the plume of heaviest rain will push inland and slowly weaken with time. Meanwhile, an additional shortwave will round the low and impact much of southern California into Saturday night. Given the expected very wet Friday across much of southern California, and with the heaviest rains expected to be ongoing at the start of the period, a Moderate Risk upgrade was also introduced on Day 2/Saturday. This is in line and for many of the same areas as on Day 1/Friday. However, unlike Friday's rains, these rains will have Friday's rains to build upon, worsening the number, severity, and duration of the heaviest rains across southern California. Thus, certainty with the Moderate Risk on Saturday is much higher than on Friday. As on Friday, as the shortwave and in this case the entire upper low push ashore, expect occasionally heavy rain with embedded thunderstorms to continue north and east to impact the southern Sierras of central California, much of southern Nevada, including Las Vegas, and much of western Arizona. Rainfall rates will remain high as heavy rains impact much of these areas. Thus, in coordination with VEF/Las Vegas, NV forecast office, the inherited Slight Risk was expanded east to include Esmeralda, southern Nye, and Clark counties in southwestern Nevada, as well as Mohave County in Arizona. Urban concerns will be present across the Las Vegas metro area from the heavy rains as well. The desert soils in this area should be rather hydrophobic from recent dry weather, which should support increased runoff, especially early in the day. In coordination with LOX/Oxnard, CA and HNX/Hanford, CA forecast offices, a Moderate Risk area was introduced from Point Conception east through L.A. This especially includes the Palisades fire burn scar near Malibu, as well as others in the area. Debris flows and fast moving water are likely as the rainfall drains into the flashy creeks and streams that drain the Transverse Ranges. Expect the heaviest rains to impact the L.A. Basin through Saturday morning, with diminishing rates down to occasional showers and light rain from Saturday afternoon through the overnight. Wegman Day 3 Valid 12Z Sun Nov 16 2025 - 12Z Mon Nov 17 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF CENTRAL AND NORTHERN CALIFORNIA... 20Z Update: Subtle changes were made to the Day 3 ERO, based on the latest guidance trends. Still some timing differences during day 3 (Sun-Sun night) as the initial closed mid-level low opens upon lifting through the ridge across the Great Basin-Central Rockies, while the next deepening low drops along the northern CA Coast and Bay Area. One notable trend -- the bulk of the guidance (outside of the GEM and 12Z RRFS) keeps the heavier rainfall north of the Central CA Coast and Transverse Ranges. Hurley Previous Discussion Below... As the cutoff low that impacted southern California on Friday and Saturday moves inland, it will rapidly weaken and quickly become a non-factor as far as heavy rain is concerned. Nonetheless, especially early Sunday morning, light rainfall associated therewith may be ongoing. The bigger event will be a cutoff low that follows a similar pattern to the first, but much further north and with less of a tropical connection than the first. The cutoff low will sag southward from Oregon into northern California. Upper level energy associated therewith will use what moisture it can muster to support a progressive cold front that will sweep west to east across the state. Once again there could be some modest elevated instability of 300 J/kg or less with the front. Thus, expect some convection/thunderstorms to be associated with the front. Those periods of heavy rain when combined with recent heavy rains from today could lead to isolated instances of flash flooding in prone areas. Thus, a Marginal Risk was introduced with this update for much of central and northern California, as well as far southwestern Oregon. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8nke49MV169WR97XRcAgTEqLxIr_rVCAKts2bua-L4oC= D0s0zM7P4W7BZnC9r5tVPdDeMhQmsycq5qsKApLbKqVS2qQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8nke49MV169WR97XRcAgTEqLxIr_rVCAKts2bua-L4oC= D0s0zM7P4W7BZnC9r5tVPdDeMhQmsycq5qsKApLbIJ52TT0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8nke49MV169WR97XRcAgTEqLxIr_rVCAKts2bua-L4oC= D0s0zM7P4W7BZnC9r5tVPdDeMhQmsycq5qsKApLbNwBLypQ$=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) .