FOUS30 KWBC 241603 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1203 PM EDT Thu Sep 24 2026 Day 1 Valid 16Z Thu Sep 24 2026 - 12Z Fri Sep 25 2026 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF SOUTHEAST NEW MEXICO... ...16Z Update... In collaboration with WFOs ABQ and EPZ, decided to upgrade the existing Day 1 Slight Risk to a Moderate Risk for portions of southeast New Mexico given a favorable parameter space for slow- moving thunderstorms that are more than capable of producing heavy rainfall, in addition to highly sensitive hydrological conditions with very saturated soils given recent heavy rainfall over the past few days. The latest trends in available hi-res guidance suggest a focused potential for qausi-stationary convection over parts of Lincoln, Chaves, and Otero counties this afternoon, with the HREF highlighting neighborhood probabilities over 70% for both exceeding 3" of rain and 10-year ARIs. Combined with NASA SPoRT soil moisture percentiles exceeding the 98th percentile, a targeted Moderate Risk upgrade felt like the prudent move to make in order to bring attention to this higher end potential. Elsewhere, the existing Marginal/Slight Risk across portions of the central/northern Plains were trimmed southward given a lack of appreciable instability, and generally stratiform rainfall with only modest rates/intensity. Other changes to the inherited Marginal and Slight Risk were generally minor and cosmetic only. Miller/Wegman ...Previous Discussion... Heavy rain will remain in the picture for much of the southwest, as mid-level shortwave troughing moves northeast from the Baja Peninsula, eventually ejecting into the Southern to Central Plains. Notable low-level southerly-to-southeasterly flow, around 20kts, will continue to supply abundant moisture from the Gulf into the Plains and Southwest. PW values are expected to remain around 1.5", which still remains above the 95th percentile for the area climatologically, although the highest moisture flux appears more localized over the southeast parts of New Mexico. Sufficient instability (MUCAPE ~1300J/kg) will also be present to support thunderstorms capable of producing locally heavy rain rates. In New Mexico, per the 00Z runs of the HREF and RRFS, rainfall rates appear most favorable to exceed 1"/hr, with medium-to-high chances of exceedance (~50% and ~65% respectively). 3-hour FFG exceedance probabilities among the two also suggest higher-end chances of exceedance; around a 65-70%. Antecedent conditions are also very favorable for continued instances of flash flooding. Given multiple days of Moderate EROs over New Mexico and precipitation continuing last night, soils remain saturated, contributing to very low FFGs. Even modest rainfall amounts at this point will have the potential to produce flash flooding. As such a high-end Slight Risk remains in place for southeast New Mexico, with the surrounding area over the Southwest maintained in the inherited Slight. Minor expansions were made to the inherited Slight Risk area to include northern Kansas. The aforementioned mid-level shortwave will eventually reach the Central Plains, and with a long fetch of southerly flow bringing moisture originating from the Gulf, a broad area of precipitation is expected through the day. While global models remain bullish with QPF, including localized totals of 2-3 inches, limited instability should largely favor a more stratiform and longer-duration rainfall regime. As a result, hourly rainfall rates are expected to remain modest despite notable 24hr QPF, particularly in Nebraska. This is supported by the relatively muted HREF and RRFS FFG exceedance probabilities. As such, the Slight Risk was only expanded to include more of Kansas, where HREF and RRFS FFG exceedance probabilities appear more favorable. Blanco-Alcala Day 2 Valid 12Z Fri Sep 25 2026 - 12Z Sat Sep 26 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM NEW MEXICO INTO THE PLAINS, AND OVER SOUTHEAST NEW ENGLAND... ...Southwest... A lingering mid-level disturbance, along with sufficient instability and moisture influx from the Gulf, will support another day of showers and thunderstorms across portions of the Southwest. Antecedent conditions will remain a concern following multiple rounds of rainfall, particularly across portions of New Mexico where soils remain saturated and FFGs remain low. While the overall coverage and intensity of convection may be reduced compared to previous days, locally heavy rainfall rates will remain possible where stronger convection develops. Given the increasingly sensitive hydrologic conditions, even modest additional rainfall may result in localized flash flooding. The inherited Marginal was maintained with only minor adjustments. ...Central Plains... The mid-level shortwave will continue to progress across the Central Plains, maintaining broad ascent and precipitation from portions of Kansas northeastward through Nebraska and Iowa. Locally appreciable rainfall total remain possible, though limited instability may again favor a substantial stratiform component and temper rainfall rates. A Slight Risk was considered across portions of Nebraska into Iowa. However, uncertainty regarding the coverage and magnitude of heavier rainfall precludes an upgrade at this time, and the Marginal Risk was maintained. An upgrade may become necessary in later forecast cycles as a consensus hopefully becomes more clear, and confidence in mesoscale details increase. ...Northeast... A Marginal Risk has been maintained for parts of the Northeast as the details of a potent coastal low continue to be ironed out. There is still uncertainty pertaining to the time and location of the low track, but there is a consensus forming in heavy rainfall occurring along coastal Massachusetts, Rhode Island, and Connecticut. However, placement of the low along with its central pressure vary. With global guidance remaining in disagreement, enough uncertainty was raised to preclude any upgrade at this time, with continuity of the previous forecast being favored. Blanco-Alcala Day 3 Valid 12Z Sat Sep 26 2026 - 12Z Sun Sep 27 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN THE NORTHEAST... The potential for heavy rainfall increases substantially in the Northeast in the Day 3 period. The coastal low will bring a generally prevailing southeast to easterly flow around a low centered around Long Island, aiding in a substantial influx of moisture onshore into the Northeast and parts of the Mid-Atlantic. Current QPF guidance suggest widespread totals of 2-4" with isolated totals as high as 5" in the 24-hour period. The significant amount of expected rainfall over the long period into the urban corridor of eastern MA will likely cause widespread hydrologic concerns, including flooding in urban areas including Boston, and poor drainage locations. Additionally, given increased confidence regarding the heavy rain area, the now "higher-end" Slight Risk area was maintained as inherited. Further west the focus of the possible heavy to excessive rainfall remains over the Midwest/Central Plains on Saturday as a shortwave energy ejects northeastward from the Rockies. The envelope of solutions span the EC on the north side and the GFS favoring a more southerly solution corresponding to Iowa and Missouri/Kansas respectively. A few of the globals remain rather bullish on the amount of expected rainfall, particularly the ECMWF, while ensembles continue to focus on this area as well. As such, the Marginal Risk in Iowa was maintained where flash flood guidance is lowest after the recent rainfall. Blanco-Alcala Day 4 and Day 5 Valid 12Z Sun Sep 27 2026 - 12Z Tue Sep 29 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF NEW ENGLAND ON SATURDAY... New England... There is a Slight risk of excessive rainfall in proximity to a deep coastal low that slows its forward speed near southeast New England. Heavy rainfall will begin to taper off during the period in terms of areal coverage, rainfall rates and daily rainfall amounts as the upper system fills. Model consensus has been building around the idea that the surface system heads inland compared with pivoting and heading back out to see. The cumulative effect of rainfall over the long period into the urban corridor of eastern Massachusetts will cause hydrologic concerns, including some flooding in poor drainage locations. For that reason, opted to keep the Slight risk on Day 4. Opted for a Marginal risk given the additional amounts shown by the models on Monday...but that may ultimately depend on the hydrologic state by then. ...Southwest US... After a relative lull on Sunday with only a fairly focused Marginal Risk area confined to parts of the Intermountain region, the infusion of mid- and upper-level moisture with a tie to Polo in the eastern North Pacific basin is expected to lead to blossoming coverage of rainfall and increased risk of intense downpours on Monday across the Southwestern states. With uncertainty about the time of arrival...started with a Marginal Risk area. ...Northern Plains/Upper Midwest... A trough in the northern stream will push a cold front across the northern tier of states in the Upper Midwest on Monday. Moisture anomalies are modest and the upper system appears to be progressive...both of which will limit the concern for excessive rainfall. But there are enough ensemble members...especially within the EC ensemble suite...to make a low- end Marginal warranted. 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 $$