334 FOUS30 KWBC 201938 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 338 PM EDT Tue Oct 20 2026 Day 1 Valid 16Z Tue Oct 20 2026 - 12Z Wed Oct 21 2026 ...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS IN AND NEAR PORTIONS OF ARIZONA... 1600Z Update: Based on the latest trends with the 12Z guidance, particularly the latest HREF and REFS QPF exceedance probabilities, have shifted the Marginal Risk a tad eastward, while also extending into northern AZ to account for the uptick in 0.50+ in/hr rainfall rate probabilities during the latter portion of the D1 outlook period (after 08Z). Hurley Previous discussion below.. ...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 sunset. The 03z RAP indicates that MU CAPE could broach 1000 J/kg across southern AZ, migrating westward from the AZ/NM border towards Phoenix. When the moisture surge comes in from the Gulf of CA/tropical eastern Pacific, bringing precipitable water values up to 0.75-1.25". There is some concern that shortwaves rounding the incoming upper level low/trough could lead to some convection Tuesday night into very early Wednesday. Raised a Marginal Risk for portions of southern and central AZ given the parameters that come in after sunset, of which there is some, albeit disagreeable, signal for in the 00z REFS/HREF guidance. Roth Day 2 Valid 12Z Wed Oct 21 2026 - 12Z Thu Oct 22 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST & FOUR CORNERS WEDNESDAY/WEDNESDAY NIGHT... 1900Z Update: Based on the 12Z guidance trends, including the latest HREF and REFS QPF and FFG exceedance probabilities, we have expanded the Slight Risk across SoCal and the Central CA Coast, while also expanding the Marginal Risk area through a larger portion of the Central Valley, as often times the guidance (particularly the high-res CAMs) overdo the downslope drying between the coastal and Sierra ranges. Incorporated WWRF-ECMWF QPF guidance, along with the latest (12Z) ECAIFS, when delineating the outlook boundaries. A Moderate Risk within these Slight Risk areas is not out of the question. PW/IVT anomalies are quite high for the time of year (5 to 6 plus standard deviations above normal per the latest NAEFS Ensemble Situational Awareness Table (ESAT), while the latest ECMWF Extreme Forecast Index or EFI is 0.9+ in spots with shift-of-tails >2. Still not convinced however that the degree of deep-layer instability will support excessive short-term rainfall rates, at least with any peristence over more than just isolated areas, given the modest, transient HREF and REFS probabilities of >0.50+ in/hr rates and fairly low probabilities of >1.0+ in/hr rates. Hurley Previous discussion below.. ...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 300-600 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. Per coordination with LOX/the Oxnard CA forecast office, went ahead and upgraded to a Slight Risk for portions of SoCal, including the coastal ranges north of Point Conception and the western Transverse Ranges to its east, into the Los Angeles metropolitan areas. 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 3 Valid 12Z Thu Oct 22 2026 - 12Z Fri Oct 23 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST AND FOUR CORNERS REGION THURSDAY/THURSDAY NIGHT... 1900Z Update: Based on the latest guidance trends, did not make many changes to the Day 3 ERO, outside of expanding the Slight Risk slightly. Latest first-guess fields continue to support a high-end Slight Risk event, however cannot rule out a Moderate Risk at some point along and east of the 4-Corners (including the San Juan Mountains), along with areas east-northeast of the AZ Sun Corridor to the Mogollon Rim (AZ) and Gila Region (NM). Latest QPF trends with the RRFS along with the global models on Day 3 are a bit inconclusive at this point, again predicated on the depth/degree of instability Thurs/Thurs night. Hurley Previous discussion below.. 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. 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+ J/kg in eastern AZ early on. 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. Trends will continue to be monitored and can not rule out the need for embedded Moderate risk area(s) as we get closer to the event. 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 Fri Oct 23 2026 - 12Z Sun Oct 25 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHWEST U.S. AND SOUTH FLORIDA ON SATURDAY/SATURDAY NIGHT... 20Z Update: The current synoptic pattern for both the Southwestern CONUS and the southeast FL coast maintain signals for enhanced convective potential and the prospect for scattered flash flood signals for each respective setup. Both SLGT risks on D5 were maintained, but adjusted to reflect the latest QPF trends. Guidance for FL is keying on the 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 mass of impervious surfaces. Tightened the SLGT risk closer to the coast to reflect the trend of convection likely tied right along the Atlantic coast with the greatest PQPF signature from Port St. Lucie down to northern Miami. For the Southwest U.S, the threat will be tied to the anomalous moisture advection into the area with the best ascent located within the downstream diffluent signature ahead of the broad ULL formation to the west. As of this time, the ensemble mean QPF depiction would note areas along and east of the Continental Divide as the main areas of concern with some signals for locally 2+ inches, even at lead, for the upcoming pattern. Will need to monitor expanse of the heavy rain footprint as guidance differs a bit on the eastern extent of the heaviest rain. This allowed for some adjustment for both the SLGT and MRGL risks over the Southern High Plains of TX where some guidance limits the favorable ascent further west which would negate flash flood concerns overall. Kleebauer ..Previous Discussions... Southwest US... The potential remains for locally heavy rainfall to linger on Thursday into Thursday night as the axis a an upper level trough remains west of the region according to the 20/00Z GFS and ECMWF. That leaves much of eastern and central New Mexico with deep layer southerly fetch of moisture. The potential for excessive rain is in a relative minimum on Thursday before arrival of additional middle- and upper-level shortwave energy reinvigorates the chances of heavy rainfall on Friday across parts of the High Plains to the lee of the southern Rockies. The adjustments made to the Marginal Risk area of Thursday was minimal while a Slight Risk area was introduced in the period ending from Friday and into the early morning hours of Saturday. South Florida... The numerical guidance has been showing increasing moisture with embedded rain approaching the southern Florida peninsula from the southeast. Latest numerical guidance suggests locally heavy rainfall could begin as early as Friday but the signals for greater coverage and higher rainfall amounts are forecast to occur on Saturday as an inverted trough approaches. Given the amount of urbanization in the area combined with locally heavy rainfall in the area during the past week...opted for a Slight Risk at this point on Saturday. 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 $$