FOUS30 KWBC 211547 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1147 AM EDT Wed Oct 21 2026 Day 1 Valid 16Z Wed Oct 21 2026 - 12Z Thu Oct 22 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF SOUTHERN CALIFORNIA AND THE FOUR CORNERS... ...16Z Update... ...Southern California... Little has changed with the overall flash flooding risk across Southern California. An almost nominal southward shift in the guidance did lead to a minor adjustment to the Slight Risk to include a bit more of the southeastern suburbs of L.A. into the Slight. Urban areas, burn scars and flood prone canyons west of L.A. will have the greatest risks for flash flooding. Areal rainfall rates ongoing have generally been staying under a half inch per hour. However, instability should rapidly increase this afternoon to between 500 and 1,000 J/kg of MUCAPE. This will allow for more elevated convection to spread inland, especially as the plume of rain currently out towards Santa Barbara progresses eastward towards Oxnard, Malibu, and the broader Santa Monica Mountains region this afternoon. With few changes to the rainfall forecast, few changes were needed to the flooding outlook. ...Four Corners... The start of a significant rainfall event for the Four Corners region will begin today, especially into Utah and Arizona, with the heaviest rains eventually spreading into southwest Colorado tonight. Instability will be limited into Utah and Colorado tonight as much of it remains south into Arizona, but the mostly stratiform rain that does impact these areas will act to prime the soils in advance of the incoming upper level low-trough that will push east into the region into Day 2/Thursday. MUCAPE values this afternoon to Arizona will exceed 1,000 J/kg across much of the sate, while it reduces to 500 J/kg or less into southwestern Utah. For this region, even instability values at 500 J/kg could sustain elevated instability capable of allowing thunderstorms that could approach 1 inch/hour rainfall rates. Portions of southern Utah have FFG values at 1/2 inch per hour. For Arizona, FFG values are around 1 inch per hour. Widely scattered instances of flash flooding are likely today through tonight. Wegman ...Previous Discussion... ...Southwest... On Wednesday/Wednesday night, a deep layer cyclone moves to Point Conception and scoops up moisture from the tropics, flinging it into the Southwest. A broadening zone of 300-700 kg/ms of Integrated Vapor Transport envelops the Southwest. Near the cold low itself, MU CAPE of ~500 J/kg and precipitable water values of 1-1.3" with up to 30 kts of onshore flow at 850 hPa is forecast, which would support hourly amounts to 2" where cells manage to train or backbuild within the terrain. The elevated Slight Risk was maintained for portions of SoCal, including the coastal ranges north of Point Conception and the Transverse Ranges. Across portions of the Southwest, MU/ML CAPE is forecast to rise to 1000-3000 J/kg with lesser amounts to the north across the Great Basin/Rockies, with precipitable water values rising into 0.75-1.5" range. The ingredients available along with deterministic guidance QPF output appear sufficient to maintain the existing Slight Risk. Hourly rain amounts to 2.5" would be possible across portions of AZ, NM, and the Four Corners in widely scattered to scattered spots due to cell training or where cells backbuild or merge. Urban areas, burn scars, slot/box canyons, and dry washes/arroyos would be the main areas of concern with the heavy rainfall anticipated. Roth Day 2 Valid 12Z Thu Oct 22 2026 - 12Z Fri Oct 23 2026 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF SOUTHWEST COLORADO THURSDAY/THURSDAY NIGHT... Southwest/Four Corners... The Slight Risk area remains for parts of the Southwest/Four Corners on Thursday as a deep mid/upper level low and accompanying amplified upper trough moves from CA into the Great Basin. A new Moderate Risk was raised in conjuntion with the Grand Junction forecast office. Heavy to potentially excessive rainfall remains a concern given the magnitude of the tropical moisture being advected into the region, with Integrated Vapor Transport of up to 600 kg/ms. Deep southerly flow ahead of the approaching trough will advect an airmass with precipitable water values of 1-1.5" into the region, very high by October standards. Freezing levels will be quite high in southwest CO, close to the mountain peaks. MU CAPE is forecast to be 1000-2000 J/kg near the AZ/NM border during daytime heating. The ingredients in place still suggest that at least locally significant impacts are a possibility. Hourly rain amounts to 2.5" are possible given the ingredients available. This is a higher end Slight Risk for portions of southeast AZ and southwest CO. Urban areas, burn scars, slot/box canyons, and dry washes/arroyos would be the main areas of concern with the heavy rainfall anticipated. South Florida... The 00z HREF/REFS are either very close, or just ashore, with a heavy rain signal across portions of the Gold Coast as moisture returns from the Atlantic/Bahamas with instability from the Gulf Stream. The concern would be any convective banding that tries to develop in the antiparallel flow with height seen in that area. The threat is conditional, but hourly amounts to 2" are possible should any bands set up in this environment, which would be problematic in urban areas. Roth Day 3 Valid 12Z Fri Oct 23 2026 - 12Z Sat Oct 24 2026 ...THERE IS A MARGINAl RISK OF EXCESSIVE RAINFALL IN AND NEAR THE SOUTHERN HIGH PLAINS... Southern High Plains... The guidance shows a modest heavy rain signal in and around the Southern High Plains and far west TX as Gulf inflow moves upslope into the area and Pacific moisture flows in aloft ahead of an incoming upper level low/trough and Intergrated Vapor Transport has values up to 600 kg/ms. Precipitable water values of 1-1.25" are forecast there, which are high for the region and for October. MU CAPE rises to ~1000 J/kg. The flow should be fairly unidirectional from the south, so cell training is possible should enough convective coverage develop. The guidance QPF remains modest, so left the Marginal Risk as is. South Florida... The global guidance is either very close, or just ashore, with a heavy rain signal across portions of the Gold Coast as moisture returns from the Atlantic/Bahamas with instability from the Gulf Stream. The concern would be any convective banding that tries to develop in the antiparallel flow with height seen in that area. The threat is conditional, but hourly amounts to 2" are possible should any bands set up in this environment, which would be problematic in urban areas. Roth Day 4 and Day 5 Valid 12Z Sat Oct 24 2026 - 12Z Mon Oct 26 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE WESTERN/SOUTHWEST U.S. AND SOUTH FLORIDA... The current synoptic pattern for both the Wester CONUS and for southeast FL coast maintain signals for heavy to potentially excessive rainfall this weekend. In each outlook area...only small adjustments were made based on latest deterministic and ensemble guidance on Saturday. Guidance for Florida remained focused on immediate coastal areas as the primary point of emphasis, correlating with the urban corridor that tends to be more prone to flash flooding given the amount of impervious surfaces. The potential for excessive rainfall farther west takes shape in the Southwest U.S. generally to the lee of the southern Rockies on Saturday that propagates north and east-ward on Sunday. The threat will be tied to the anomalous moisture advection into the area with the best ascent located within the downstream diffluent flow aloft. Spaghetti plots of QPF from the EC Ensembles and the GEFS on Saturday still primarily depict areas along and east of the Continental Divide as the main areas of concern with some signals for locally 2+ inches. Conditions supporting heavy to excessive rainfall persist over portions of the western High Plains but the northern extent of the precipitation shield will be fighting a feed of cooler and more stable airmass being fed into the region by building surface high pressure. Bann Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt $$