Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Sep 28 2025 15:57:05 FOUS30 KWBC 281556 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1156 AM EDT Sun Sep 28 2025 Day 1 Valid 12Z Sun Sep 28 2025 - 12Z Mon Sep 29 2025 ....THERE'S A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF NEW MEXICO AND PORTIONS OF FAR WESTERN TEXAS/SOUTHERN COLORADO, AS WELL AS, PORTIONS OF NORTHWEST ARIZONA... 16z update: New Mexico/West TX Panhandle: Mid-level sub-tropical moisture=20 continues to stream northward through the leading height-=20 falls/outer edge of deep-layer cyclone across Western TX Panhandle=20 through the Sacramento Mountains and points northward. Reinforcing=20 surface to boundary layer moisture return through up- slope process will maintain 1-1.25" total PWats. Embedded mid-level impulses=20 through the large base of the cyclone will allow for multiple=20 rounds across S NM/W TX Panhandle to maintain the Slight Risk.=20 Southwest (N AZ): Near the northwest edge of the inner core of the upper- low in NW AZ, there remains a slightly enhanced signal for=20 increased rainfall rates/totals per HREF probs and 12z Hi-Res CAMs. Enough so to be highlighted with 20-40% of 2"+ through 12z . Given slow cell motions and position relative to the deeper cyclone and=20 fairly saturated soils over the last few days is sufficient to=20 highlight a small separate Slight Risk area.=20 Further North (ID/WY): A subtle vorticity center is lifting north=20 through the northern Great Basin toward the Snake River Plain; Hi- Res CAMs indicate slightly better instability from 250-500 J/kg may allow for scattered mountain anchored convection through the afternoon/evening trending northward toward Tetons and Ranges=20 across SE ID/far W WY. Enough so, to expand the Marginal Risk=20 northward to encompass the increase in potential.=20 Southeast: Forecast remains on track, solid onshore flow and enhanced deep layer moisture values over 2-2.25 and some weak 500-1000 J/kg of CAPE should allow for scattered thunderstorm activity capable of locally intense rates. Only minor adjustments were made to the Marginal Risk line.=20 Gallina ~~~~Prior Discussion~~~~~ ....Four Corners/Southwest... An upper trough centered over the Southwest will continue directing mid-level vortices northward through the Four Corners region today. Moisture from Tropical Cyclone Narda in the East Pacific will continue surging north into the Four Corners, where scattered showers and thunderstorms are likely to proliferate throughout the day. PWATs between 1-1.5 inches within the right entrance region of the upper jet and modest instability around 500-1000+ J/Kg (MLCAPE) could yield rain rates between 0.5-1 in./hr., especially where the slight risk is currently drawn. Burn scars, scars slot canyons and upslope areas within the risk areas are especially vulnerable to flash flooding. HREF's neighborhood exceedance probabilities of 2 in. in 24 hours are between 25-50% while EAS probabilities of exceeding 0.5 in. hover around 20-40% within the depicted slight risk area. ....Southeast... An upper trough will pivot eastward over the Southeast while moisture from T.C. Nine continues surging northward toward the Southeast Coast. Current guidance keeps the axis of heaviest QPF offshore, but enough moisture may propagate far enough north and west to interact with a surface front draped along the coast and produce localized excessive rainfall. HREF EAS exceedance probabilities of 1 inch in 24 hours are between 5-15% along much of the Carolina Coast today, which supports the marginal risk area depicted. Kebede Day 2 Valid 12Z Mon Sep 29 2025 - 12Z Tue Sep 30 2025 ....THERE'S A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CAROLINA COAST... ....Southeast... Guidance continue to depict T.C. Nine propagating north toward the Southeast Coast on Monday, while a surface front draped along the coast acts as the focus for heavy rainfall. QPF has trended downward given slowing trends in the track of T.C. Nine on Monday. The GFS suite in particular diverges most from the Canadian and Euro suites when depicting the system's slower forward motion. Despite this, enough moisture should work its way into the coast and interact with the surface front to generate scattered thunderstorms. There'll be enough moisture (1.5-2.5in PWATs) and instability (1000+ J/KG MUCAPE) present to support rainfall rates in excess of 1 in./hr.. ....Northern California/Great Basin/Southern Cascades... An upper low centered over the East Pacific will send mid-level vortices into the West Coast on Monday. Pacific moisture may collide with the remnant tropical moisture from Narda to produce heavy rainfall from northern California into the Great Basin. Synoptic forcing provided by a strong surface cold front should focus PWATs in the range of 0.75-1.25 in. to produce quarter inch rain rates along the immediate coast. More robust instability over the Great Basin should yield higher rain rates between 0.5-1 in./hr. in some areas. Upslope enhancement may generate higher totals in parts of the terrain. Kebede Day 3 Valid 12Z Tue Sep 30 2025 - 12Z Wed Oct 01 2025 ....THERE'S A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CAROLINA COAST... ....Southeast... The inherited marginal and slight risk areas along the Southeast Coast were downsized based on trends in the guidance suggesting T.C. Nine's hard turn out to sea on Tuesday. Despite the resulting smaller QPF footprint than previous cycles, the flash flood risk remains. Instability will be negligible (less than 500 J/Kg MUCAPE), which supports the possibility that the flood threat will be due to runoff over saturated soils from Sunday and Monday's rainfall. Exceedance probabilities of over 2 inches is over 10% in the 00z ECENS/CMCE, while below 10% in the GEFS along the immediate Carolina Coast. Kebede Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6uFAzqYdJ_L88XfT5_yzene3C0vAwFQe-dS9asUoHY2O= tb-02gXBFHsvM_b7AEC68SWxmlR637fniM7cMsFfXBYWidY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6uFAzqYdJ_L88XfT5_yzene3C0vAwFQe-dS9asUoHY2O= tb-02gXBFHsvM_b7AEC68SWxmlR637fniM7cMsFfs82RwyM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6uFAzqYdJ_L88XfT5_yzene3C0vAwFQe-dS9asUoHY2O= tb-02gXBFHsvM_b7AEC68SWxmlR637fniM7cMsFfIs1Rpx0$=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) .