053 FOUS30 KWBC 191938 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 338 PM 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... The probability of rainfall exceeding flash flood guidance is less than 5 percent. Anomalous moisture will remain in place Tuesday ahead of an upper- low digging southward just off the West Coast. However, instability looks to remain rather low in the period compared to Monday which should limit the potential for more robust storms/locally heavy rainfall rates. The risk for heavy rainfall and flash flooding is non-zero, but appears to be below the 5 percent threshold at the present time. Putnam 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 AND FOUR CORNERS REGION WEDNESDAY/WEDNESDAY NIGHT... ...Southwest and Four Corners Region... A northern stream upper-low will dig southward along the West Coast as stronger flow remains in place in the southern stream across the northeastern Pacific/Baja California. These merging features will shift eastward by Wednesday further into the Southwest/Four Corners region. Meanwhile, already anomalous moisture in place will be augmented by additional moisture from remnant Trudy in the Pacific, bringing PWATs of 0.75-1.5" (upwards of 3-4 standard deviations above mid-October means depending on location). The combination of anomalous moisture and cooling mid- levels will lead to sufficient instability (upwards of 1000 J/KG) for thunderstorms with locally heavy rainfall across portions of the Southwest into the Four Corners region. The risk for at least scattered instances of flash flooding looks likely given the anomalous moisture in place, especially for urban areas as well as terrain sensitive areas such as burn scars, slot/box canyons, and dry washes/arroyos. The influx of warm, moist air will also keep snow levels very high, as much as 12000 to 13000 feet, limiting any snow to only the highest peaks. With rain into higher elevations, there is the potential that the rain/warmer air could lead to snowmelt where snow has already fallen which would raise the prospects of downstream issues as well, most likely in vicinity of the San Juan range in southwest Colorado. ...Southern California... Further west, Pacific moisture will stream northeastward towards southern California as the upper-low drops southward. Showers and storms are likely within stronger low to mid-level flow ahead of an approaching offshore low/cold front. Storm coverage looks to generally remain at least a bit progressive overall as the front/flow pushes eastward, though there is the potential for some locally heavy totals as storms train/backbuild within the mean southerly flow. Expect at least an isolated risk for flash flooding, especially for more terrain sensitive areas such as burn scars. Putnam 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... 1930z Update: The inherited Slight risk on Thursday over the Southwest still looks in good shape. Models remain in agreement that a heavy rainfall event is likely, with the possibility of at least locally significant impacts. The Marginal risk on day 5 was adjusted slightly westward based on model trends lingering instability/moisture over NM. Chenard ...Previous Discussion... 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 $$