305 FOUS30 KWBC 212023 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 423 PM 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... ...2030Z Update... ...Southwest... Once again no significant changes were made as far as the broad scale forecast. However, a higher-end Moderate Risk was introduced for most of the Moderate Risk area in southwest Colorado, including the San Juan Mountains. In addition to a nominal increase in the forecast rainfall for that area, analysis of the latest 12Z runs of the guidance show there are numerous concerning thresholds that will be threatened with this upcoming event. These thresholds include: 1)an over 90% chance of 5 inches or more of rain in 24 hours to occur 2) similar chances of seeing 100 year Annual-Exceedence- Intervals met 3) 1-2 day rainfall records in areas such as Durango and Telluride broken 4) PWATs of near 1 inch amounting to 4-5 sigma above normal 5) Instability between 500-1,000 J/kg, which for most areas is not a lot, but for this area of the Southwest, amounts to enough to support persistent convection 6)A snowmelt component with much of the early season snowpack on the summits of the San Juans likely melting as snow levels rise to 12,000 to 13,000 ft and 7) A climatological component such that the last time the Durango area saw 4-5 inches of rain in 24 hours, major river flooding resulted, and there was no snowmelt to assist in the last event. All of that is to say that the flooding concerns continue to increase, and there is some potential that a High Risk upgrade may be needed, especially if rainfall forecasts continue to increase and CAMs support continued rounds of convection. Elsewhere across the Southwest, significant rains will also impact western New Mexico, the Mogollon Rim area of Arizona, and much of the National Parks and slot canyons from southern Utah north and east to the Uinta Mountains in the north. A large Slight Risk area remains for much of the rest of the 4 Corners region, with a higher end Slight for the southern tier of Utah as well as the Mogollon Rim of east-central Arizona. Lingering rainfall as the "bowling ball" low in the West continues progressing east into southern California could continue any flooding that occurs this afternoon into tonight. Nearly all of the rain with this event for SoCal will be over by the start of the period, so a Marginal Risk was maintained. ...Southeast Florida... Onshore flow into the urban corridor from West Palm Beach down to Miami may allow robust convection occurring off the coast over the Gulf Stream to try to move into coastal communities. These occurrences will be few and far between. The Marginal Risk was maintained due to the urban influence potentially allowing for an isolated instance of flash flooding. Wegman ...Previous Discussion... 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 conjunction 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 SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE SACRAMENTO MOUNTAINS SOUTH THROUGH THE EL PASO METRO... ...2030Z Update... A Slight Risk update was introduced for the Sacramento Mountains south through the El Paso metro. Persistent rainfall with afternoon instability values between 500 and 1,000 J/kg will support scattered convection capable of supporting up to 1.5 in/hour rainfall rates. The burn scars/topography of the Sacramento Mountains and the urban factor in the El Paso Metro will both lower the thresholds needed to produce flash flooding. This upgrade was coordinated with the EPZ/El Paso, TX forecast office. ...South Florida... Few changes were needed once again highlighting the urban I-95 corridor from West Palm through Miami. Conditions appear a bit more favorable for heavy rainfall in this area on Friday as compared with Thursday, but expected flash flooding is likely to remain isolated. Wegman ...Previous Discussion... 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 Integrated 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... 20Z Update: The general forecast with SLGT risks across the Southwest U.S. and southeast FL on D4 maintain relevant continuity as the overall setup remains consistent within both ensemble means and global deterministic outputs. A minor adjustment west with the SLGT/MRGL risks across the Southwest to Southern High Plains was made as the consensus for a bit of a slower 500mb evolution was evident in the 12z suite which correlated with the heavier precip footprint shifting slightly further west than the original forecast. This will still have limited bearing on the expected flash flood prospects as much of the same areas from prior forecast issuance will be impacted. The one area where this did shift back was over the TX Panhandle where the eastern fringes of the SLGT and MRGL risk were adjusted to reflect the differences in the timing of the ULL ejection. Southeastern FL heavy rain threat remains prevalent with the mean QPF output still favoring the immediate southeast FL coastal plain, overlapping the urban centers from Port St. Lucie to Miami. Totals between 1-3" are forecast in this zone for an areal average, however this setup will be conducive for maxima closer to 4-6", pending the positioning of expected heavier convection anchoring near the coast as easterly flow will allow for enhanced low-level convergence into the coast. This was sufficient for a continuation of the SLGT risk with only minor adjustments on the western edge of the risk. D5 remains consistent for both the Southern and Central High Plains, as well as the southeastern FL coast. The latter is more of a continued approach of heavy rain potential into the coastal areas of southeast FL which is depicted in some of the guidance. That said, some models are less enthusiastic for this time frame comparatively with a smaller footprint for heavy rain potential which could allow for either a smaller SLGT risk in the future, or a downgrade to a standard MRGL risk. This will be something to monitor over time, but with the signal still present in enough guidance and ensemble means, decided to keep the SLGT risk and will assess more in the future. The High Plains will see the next heavy rain distribution from our evolving mid and upper level pattern out west come the second half of the weekend. This signal is pretty locked in among guidance which has produced some respectable QPF totals when assessing the means, and especially the 75/90th percentile outputs of the both the National Blend and ECENS/GEFS, respectively. The consensus for rainfall >2" is positioned over southeast CO down through the TX Panhandle and adjacent northeastern NM High Plains. This setup is textbook for heavy rain prospects in this particular zone as the disturbance over the Four Corners finally opens up more and ejects into the Central Rockies, and eventually the Plains the end of the forecast cycle and beyond. The SLGT was more than sufficient for the D5 period across the area with a potential for a higher risk, pending the eventual evolution of the pattern upstream. Kleebauer 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 $$