FOUS30 KWBC 191556 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1156 AM EDT Mon Oct 19 2026 Day 1 Valid 16Z Mon Oct 19 2026 - 12Z Tue Oct 20 2026 ..THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHWEST... Afternoon heating combined with unseasonably high moisture for mid-October (PWATs 0.75-1.0", 2-2.5 standard deviations above the mean) will lead to sufficient instability (~1000 J/KG CAPE) over southeastern Arizona/southwestern New Mexico today to support some more robust thunderstorms capable of locally heavy downpours. The hi-res guidance shows the potential for hourly rates of 1"+ which could lead to some isolated instances of flash flooding, especially for urban areas as well as terrain sensitive areas such as burn scars, box/slot canyons, and dry washes/arroyos. Putnam Day 2 Valid 12Z Tue Oct 20 2026 - 12Z Wed Oct 21 2026 ...Southwest... Anomalous moisture will remain in place Tuesday ahead of the deep layer cyclone digging southward offshore CA. Integrated Vapor Transport broaches 500 kg/ms by early Wednesday morning, with the main increase after daytime heating. However, overall instability remains fairly low with splotches of 500 J/kg of MU CAPE available. The risk for heavy rainfall and flash flooding is non-zero, but appears to be below the 5 percent threshold at the present time. The probability of rainfall exceeding flash flood guidance is less than 5 percent. Roth Day 3 Valid 12Z Wed Oct 21 2026 - 12Z Thu Oct 22 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST WEDNESDAY/WEDNESDAY NIGHT... ...Southwest... On Wednesday/Wednesday night, a deep layer cyclone moves into southwest CA and scoops up moisture from a dying Trudy, flinging it into the Southwest. A broadening zone of 300-600 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. There are enough ingredients available/heavy rain signal for the existing Marginal Risk for the Transverse and Peninsular Ranges, which is there to help cover burn scar concerns. Across portions of the Southwest, MU/ML CAPE is forecast to rise to 1000-1500 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. 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 4 and Day 5 Valid 12Z Thu Oct 22 2026 - 12Z Sat Oct 24 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST THURSDAY... Maintained the Slight Risk area in parts of the Southwest on Thursday as a deep mid/upper level low moves across the region. Heavy to potentially excessive rainfall remains a concern given the magnitude of the tropical moisture being advected into the region. Deep southerly flow ahead of the approaching trough will advect an airmass with precipitable water values that approach record values for the latter part of October. There is still spread in the amount of instability available by then but the overall uptick in model QPF over the past couple of runs is indicative of the models are doing better at capturing the steepening lapse rates with the upper low on Thursday should also produce some destabilization. By Friday, the trough axis has have made its way across the southern Rockies and emerged out over the adjacent High Plains. Some of the moisture will be lost in the complex terrain as the system moves eastward...but additional low level moisture should start being drawn out of the Gulf to support a lower-end risk of excessive rainfall lingering over portions of New Mexico and portions of adjoining states. Even though confidence on the exact details remain low given we are 4 to 5 days out, there is growing confidence in a heavy rain and flash flood threat over the Southwest region beginning on Day 3 which continues into Thursday. The ingredients in place still suggest that at least locally significant impacts are a possibility. Trends will continue to be monitored and can not rule out the need for embedded MDT risk area(s) if/where enough instability materializes. 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 $$