FOUS30 KWBC 231934 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 334 PM EDT Wed Sep 23 2026 Day 1 Valid 16Z Wed Sep 23 2026 - 12Z Thu Sep 24 2026 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE TEXAS AND OKLAHOMA PANHANDLES, AND CENTRAL AND EASTERN NEW MEXICO... 16Z Update... Latest 12Z CAMs guidance was used to make adjustments to the inherited outlooks from night shift. Biggest adjustments were to extend the Moderate Risk into the northwestern TX Panhandle and western OK Panhandle. Both HREF and REFS probabilistic guidance show low chances (20-30%) for localized rainfall totals topping 5" through the remainder of Day 1. The Slight Risk in AZ was extended north and west along the Mogollon Rim given morning satellite shows sufficient surface-based heating beneath a high-cirrus canopy, which in contrast with thicker cloud cover over NM, is likely to cause a CAPE gradient over central AZ. Lastly, the Slight Risk in the OH Valley was extended north into southern WV and to the OH River where ongoing thunderstorms activity has already triggered a handful of flash flood warnings. Storms there are slow moving, and additional instances of flash flooding are possible this afternoon. Mullinax ---Previous Discussion--- ...New Mexico and West Texas... The ongoing heavy rainfall and flash flood threat over NM will continue and is expected to peak today. Large scale forcing will reach its maximum as a well defined mid level shortwave lifts north-northeastward across the state, accompanied by robust upper level divergence and the potential development of a weak surface low. Scattered to numerous instances of flash flooding are expected. Locally significant and life threatening impacts remain possible, particularly across sensitive terrain, urban areas, and burn scars. For the 24 hour window from 00z Wed to 00z Thu both the HREF and REFS depict notable 50-60% probabilities of exceeding the 100 year ARI over portions of southern AZ. With such strong forcing and widespread moisture in place, the primary limiting factor for heavy convective rates will be the extent of cloud cover and its impact on destabilization. While widespread clouds are a concern, daytime heating should allow at least the eastern edge of the rainfall shield to sufficiently destabilize, aided by broad easterly upslope flow. As convection attempts to propagate eastward into a growing pool of CAPE, easterly inflow should help slow this forward progression, promoting a threat for cell training within the deep southerly flow. Given there will still be some eastward propagation and instability is advecting in from the east, the heaviest convective rates are expected to focus across the central and eastern portion of the state. The 00z HREF and REFS mean are in good agreement on this QPF axis, through the 00z HRRR and RRFS are shifted slightly further east. This eastward evolution is plausible given the instability gradient and convective propagation vectors, and may extend the flash flood risk into portions of far west TX. Across the eastern half of the MDT risk, HREF and REFS neighborhood probabilities for exceeding 3" of rain are 40-70%. A widespread 1-2" of rain is generally expected, with localized swaths of 2-4 inches likely...falling in addition to rainfall received prior to 12z. We were reluctant to trim the western extent of the inherited Moderate risk at this time, as part of this area have already received heavy rainfall. While the most significant flash flood risk may ultimately focus over the eastern half of the MDT area, these western locales will still experience a broad shield of rain with embedded heavier convective cores. However, the western extent of the surrounding Slight risk was trimmed back based on expected cloud cover and HREF/REFS consensus. A separate area of higher instability is forecast across portions of AZ. Convective coverage and organization should be lower here, but the instability/PW overlap supports a localized flash flood risk. Southeast AZ may see a bit greater coverage, and with higher FFG exceedance probabilities noted, the Slight risk was extended into this area. ...Mid-Atlantic... A complex synoptic pattern continues as a mid to upper low drifts slowly across the OH Valley, while a separate coastal low develops offshore. Meanwhile, surface high pressure positioned to the north will advect drier and more stable air into the corridor between these two features. The primary threat for heavy rainfall over land should focus closer to the the upper low, extending from OH southward into GA and AL. Although PW values will have lowered across these areas, over 1000 J/kg of CAPE should pool near a north to south oriented trough axis. This instability, co-located with a slow moving surface convergence axis, will support the development of slow moving convective cells. Even with reduced deep moisture, locally heavy rainfall rates are likely. Given that portions of this region feature lowered FFG and heightened hydrologic sensitivity from prior rainfall, a flash flood threat will persist. A Slight risk will be carried across portions of eastern KY, eastern TN, and far western NC, where the HREF and REFS indicate the highest probabilities for FFG exceedance. Peak probabilities are centered over eastern TN, where neighborhood probabilities eclipse 40%, with notable spatial coverage. This aligns with the forecast overlap of maximum instability and low level convergence, supporting a scattered flash flood threat. Further north and east, a Marginal risk covers areas from OH into WV and western VA/NC. While less instability is forecast here, which should keep rainfall rates lower, wet antecedent conditions support a continued localized flash flood threat. ...Florida... A Marginal risk continues across south FL where above normal PWs and instability will continue to pose a localized flash flood risk. A weak low pressure developing off the FL coast may initially help focus heavier rainfall along coastal areas. Thereafter this feature may actually help advect in some drier air behind it, decreasing convective coverage by later today. Chenard Day 2 Valid 12Z Thu Sep 24 2026 - 12Z Fri Sep 25 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF NEW MEXICO INTO SOUTHWEST KANSAS... 20Z Update... Much of the previous forecast rationale remains on track after evaluating the latest WPC QPF and 12Z guidance. There is slightly greater concerns for flash flooding in southeast NM, which includes the Sacramento Mountains, given their soils will continue to saturate in wake of Wednesday's heavy rainfall. Otherwise, the Slight Risk that stretches from southwest NM to as far north and east as southern KS remains in good standing. Mullinax ---Previous Discussion--- ...Southwest and Southern Plains... Heading into Thursday the strongest mid level forcing and highest PW anomalies will shift northward out of NM and into Colorado and the Central Plains. However, in the wake of this exiting system, daytime instability is actually forecast to increase over NM compared to previous days. While PW values decrease some, they will remain above the climatological 75th percentile. Additionally, broad troughing will remain overhead, with embedded shortwave energy continuing to traverse the region. With less concentrated forcing and somewhat reduced moisture, the overall areal coverage of heavy rainfall across NM should diminish relative to prior days. However, the combination of stronger instability and sufficient moisture will still support intense localized convective rainfall rates capable of producing significant rainfall totals. Given the hydrologic sensitivity built up from preceding days of heavy rain, an additional flash flood threat appears likely. While the threat may not be as widespread as previous days, a Slight risk is maintained, as locally significant and impactful flash flooding will still be possible. The Slight risk also extends eastward across the TX and OK Panhandles and into southwest KS. Convection may be ongoing across this corridor at the start of the period. Later in the day, in the wake of the lifting shortwave and surface low, a corridor of enhanced low level convergence and instability may develop, which could result in additional rounds of locally heavy convection. Further west, isolated flash flooding will also be possible across portions of AZ, southern UT and CO as the trough moves across these areas. ...Central Plains... The highest PW anomalies and strongest synoptic forcing will overlap across KS and NE. This forcing aligns well withe a 1-3" consensus QPF swath depicted by a majority of the model guidance. The primary limiting factor for a greater flash flood threat will be the degree of available instability. Thus rainfall may take the form of a broad stratiform rain shield with only embedded heavier convective elements. Therefore, the risk level will be maintained at Marginal for now, with trends continuing to be closely monitored for any needed upgrades in subsequent forecast cycles. Chenard Day 3 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... 20Z Update... Not much change with regards to the Marginal Risk area in the Central U.S.. The Marginal was adjusted a little farther east in the MO River Valley, but otherwise, the Marginal's Central and Southern Plains areas were kept that same from overnight. There remains a fair amount of model spread on the potential placement of the developing coastal storm Friday night. The ECMWF/EC- AIFS suite remains slower and farther west, while GEFS/CMCE ensembles are a little more progressive and farther east. The 12Z GEFS has been continuing a trend over the last 24 hours of its members gradually coming west by 06Z Saturday, which suggests a move towards the ECMWF suite. When accounting for all ensemble members, there has generally been an increase in QPF across southern New England and as far south and west and Long Island. With that being the case on the latest WPC QPF, I did decide to expand the Marginal Risk into eastern CT and eastern LI this cycle. No Slight Risk was proposed for the time being, given the heaviest rainfall arrives late Friday night and any longer duration heavy rainfall would be more likely to unfold by Day 4 (Saturday). Mullinax ---Previous Discussion--- ...NM into the Central Plains... A broad axis of anomalous PWs, low level moisture transport, and CAPE will persist from portions of eastern NM northeastward through NE and IA on Friday. The strongest and most persistent mid to upper level forcing, coupled with the primary moisture transport axis, is forecast to focus across the northern extent of this corridor. Conversely, the most robust instability is expected to pool further south, stretching from eastern NM into southern KS. The spatial offset between the best large scale forcing and greatest instability introduces uncertainty regarding exactly how the flash flood threat will evolve. While it will undoubtedly be a wet period across this corridor, the magnitude and extent of the flash flood threat will depend on whether sufficient rainfall rates can materialize further north, or if convection can organize and become widespread enough further south where the instability is more firmly established. Given these lingering questions, a broad Marginal risk has been maintained across the region. However, the available environmental ingredients certainly support the potential for an embedded Slight risk. Upgrades may be necessary in subsequent forecast cycles as the event draws closer and confidence in mesoscale details increase. ...Northeast... A Marginal risk has been maintained from RI into eastern MA as uncertainty lingers regarding the exact track and evolution of a developing coastal low. While the better chance for more widespread heavy rainfall is likely to hold of until day 4, the northern edge of the rain shield could push into coastal areas by later Friday. Current indications suggest that the best instability and any convective elements should remain offshore through the day 3 period, keeping rainfall rates predominately stratiform over land. Consequently, the overall flash flood threat is expected to remain relatively low. However, with 1-3 inches of rain possible and ongoing uncertainty regarding the precise behavior of the coastal low, continuity with the previous forecast is preferred for now, and the Marginal risk remains in place. Chenard Day 4 and Day 5 Valid 12Z Sat Sep 26 2026 - 12Z Mon Sep 28 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN US/WESTERN PLAINS AS WELL AS EASTERN NEW ENGLAND... ...Southern Rockies to Central and High Plains... 20Z Update: Little change in the previous forecast. The ECMWF remains the more aggressive of the solutions with widespread convective potential from northwest IA down through northern MO. The EC-AIFS continues to pinpoint the area of central IA down into northern and western MO as a target, a bit south of the deterministic ECMWF, and closer to the deterministic GFS solution. In any case, overall magnitude is closer to a higher risk, but with some variations in where the heaviest will occur, went with general continuity and nudged the risk area a touch south to account for ML trends. Kleebauer ..Previous Discussion.. The main focus of attention for 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. Maintained the Marginal in Iowa where flash flood guidance is lowest after the recent rainfall although confidence is waning about the need for one based on the latest QPF. If the ECMWF verifies...the flash flood numbers are higher with a correspondingly reduced threat of excessive rainfall. A broadening mid/upper level trough to the east shunts deepest moisture out of the Midwest by Sunday. ...New England... General consensus is for heavy rainfall to plague much of southern New England with the maxima centered right over eastern MA, including the Boston metro for both D4 and D5 periods. Multi-day heavy rain threat is due to a slow-moving coastal low as the mid and upper height fields note a stacked 850-300mb low presentation as we move into Sunday with a weakening expectation occurring the back half of D4 into the D5 time frame. The prevailing southeast to easterly flow around the low center positioned near LI will aid in copious amounts of moisture being thrown back into the northern Mid Atlantic into southern New England as the main axis of heaviest precip will likely promote widespread 2-4" with locally as high as 5" with the current forecast over the 48-60 time frame of impact. Limited instability over the region will limit the upper threshold of flash flood capabilities, however, the significant amount of rainfall over the long period into the urban corridor of eastern MA will likely cause widespread hydrologic concerns, including some flooding in poor drainage locations. The previous SLGT risks were maintained as a result. 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 $$