901 FOUS30 KWBC 220818 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 418 AM EDT Tue Sep 22 2026 Day 1 Valid 12Z Tue Sep 22 2026 - 12Z Wed Sep 23 2026 ..THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN NEW MEXICO... ...New Mexico and West Texas... A potentially high impact multi-day heavy rainfall event is set to begin across NM and far southwest TX today into tonight. An upper level trough approaching from the west will help strengthen an upper level jet, persistently placing the region in the favorable right entrance region. Deep southerly flow will transport tropical moisture northward, driving PW values toward mid to late September record levels by later today into Wednesday. Lift will be further augmented by low level easterly upslope flow and well defined mid level shortwave energy lifting north out of Mexico by later tonight into Wednesday. Daytime hours today will see scattered thunderstorm activity and at least some flash flood potential. Large scale forcing intensifies overnight allowing for a more concentrated rain shield with embedded heavier convective cores to impact southern NM. Neighborhood probabilities from both the 00z HREF and REFS show a 40-70% chance of rainfall exceeding 3" across NM today, though 5" probabilities drop off significantly. This suggests a reasonable max rainfall footprint of 2-4" through early Wednesday. Minimal changes were made to the inherited Slight and Moderate risk areas, as they still look in good shape. Scattered to numerous instances of flash flooding are anticipated, some of which could be locally significant. This threat includes the sensitive burn scar areas within the Sacramento Mountains. ...Ohio Valley and Mid-Atlantic... A setup for heavy rainfall will unfold across the Mid-Atlantic today into tonight as an approaching mid level trough aligns with the favorable right entrance region of an upper level jet. At the surface, strong high pressure to the north will drive a backdoor front southward, with strong easterly flow providing enhanced low level convergence. The 00z HREF and REFS show generally good agreement on two primary axes of elevated FFG exceedance probabilities. The first axis is from southeast VA into SC. Positioned along a coastal convergence zone and the backdoor front, this corridor will feature greater instability and support efficient warm rain processes. High neighborhood probabilities exist in both ensemble suites for 3" of rain. While 5" exceedance probabilities in the REFS are likely inflated due to its known high convective QPF bias, the HREF still depicts low to medium probabilities, indicating a few localized 5" totals are possible. FFG neighborhood exceedance probabilities of 15-40% justify a Slight risk, though overall coverage may end up on the lower end of Slight risk criteria. The second axis is across WV into southwest VA. Focused along an inverted trough, this area should see lower overall QPF magnitudes than the farther southeast axis mentioned above. However, antecedent conditions are wetter, resulting in much lower FFG thresholds. Strong convergence along the inverted trough combined with sufficient instability will allow for convective development, with FFG neighborhood exceedance probabilities over 25% in both the HREF and REFS. A Slight risk is warranted to cover this threat, with at least a few instances of flash flooding anticipated. Farther north into the Mid-Atlantic, insufficient instability will keep rainfall predominately stratiform, keeping rain rates below flash flood thresholds. Although strong low level forcing over top the backdoor front will still support a broad area of rainfall, with some areas receiving 1-2" of rain. ...South Florida... A Marginal Risk has been maintained for portions of South Florida. PWs remain elevated and well above average, supporting heavy rainfall rates and a localized urban flash flood risk today. Neighborhood probabilities of exceeding 5" of rain in the HREF and REFS are around 20-40%. Chenard Day 2 Valid 12Z Wed Sep 23 2026 - 12Z Thu Sep 24 2026 ..THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF CENTRAL AND EASTERN NEW MEXICO... ...New Mexico and West Texas... The heavy rainfall threat established on day 1 will continue and potentially peak over NM on day 2 as large scale forcing gets even stronger. A well defined mid level shortwave is forecast to lift northward across the state, accompanied by robust upper level divergence and the likely development of a surface low. PW values will remain near record levels for this time of year, with an anomalously deep moisture profile. 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. Thus while some areas may see predominately stratiform rain, a concentrated swath of intense convective rainfall remains likely. As convection attempts to propagate eastward, strong easterly inflow will act to slow this progression, increasing the threat of training convective cells. Model guidance generally favors central to eastern NM for the maximum QPF axis, as supported by the high res 00z RRFS, REFS, FV3 and 3km NAM, and the global AIGFS and AIFS. The 00z ECMWF remains the furthest west solution, and given the expectation of some eastward convective propagation and instability advecting in from the east, its maximum QPF placement is probably too far west. Only minor changes were made to the inherited Moderate risk area. High res guidance supports localized swaths of 3 to 5 inches of rain, which appears plausible given the environmental ingredients in place. Scattered to numerous instances of flash flooding are anticipated, and locally significant, life-threatening impacts are possible...particularly across sensitive terrain, urban areas and burn scars. ...Mid-Atlantic... A complex synoptic pattern will be present over the eastern U.S. as a mid to upper level low slowly drifts from the Ohio Valley towards the Mid-Atlantic, 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. Much of the deepest moisture and instability will remain offshore with the coastal low. The primary threat for heavy rainfall over land should focus closer to the cold core of the upper low from the OH Valley down through western NC and into north GA. While PW values will have lowered across this area, model guidance still indicates CAPE exceeding 1000 J/kg within a trough axis over the region. This localized axis of instability, coupled with a stationary convergence axis, will support the development of slow moving convective cells. Even with the reduction in deep moisture, locally heavy rainfall rates are likely. Given that at least some of this region will have compromised FFG and heightened hydrologic sensitivity from previous rainfall, some flash flood risk is expected to continue. Based on signals from the 00z REFS and HREF members, as well as the GFS and ECMWF, a small Slight risk has been introduced stretching from eastern TN into eastern KY and southern WV, where the focus for slow moving convection is expected to be greatest. ...Florida... A Marginal risk continues across south FL where above normal PWs and instability will continue to pose a localized flash flood risk. Chenard Day 3 Valid 12Z Thu Sep 24 2026 - 12Z Fri Sep 25 2026 ..THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NEW MEXICO AND THE TEXAS PANHANDLE... ...Southwest... The stronger mid level forcing and highest PW anomalies will shift northward out of NM and into Colorado and the central PLains on Thursday. Despite the loss of stronger large scale forcing, daytime destabilization over NM is actually forecast to increase in the wake of the exiting system compared to prior days. While PW values decrease some, they will remain above the climatological 75th percentile. With less concentrated forcing and somewhat reduced moisture, the overall areal coverage of heavy rainfall across NM should diminish relative to days 1 and 2. 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, a Slight risk will be maintained for NM into the TX Panhandle. Locally significant flash flood impacts will remain possible. Isolated flash flooding will also be possible across portions of AZ, southeast UT, and CO as shortwave energy lifts northward across the region. ...Central Plains... The deep moisture plume will maximize over the Central Plains on Thursday, pushing PW values toward late September record levels. The mid to upper pattern coupled with a robust low level moisture transport axis supports a favorable setup for organized convection across portions of KS and NE, with backbuilding/training a possibility. We will maintain a Marginal risk to cover this threat. While confidence is growing in a heavy rainfall setup, model spread regarding the exact convective axis and uncertainty around the degree of instability warrant holding at a Marginal for now. An upgrade to an embedded Slight risk may be needed in subsequent updates as confidence increases. Chenard Day 4 and Day 5 Valid 12Z Fri Sep 25 2026 - 12Z Sun Sep 27 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... Ty Friday...some of the convection will be moving into parts of the central Plains which should also have lowered flash flood guidance due to rainfall in the preceding days. The previously issued Marginal Risk areas in southern Rockies and in the High Plains were maintained with some adjustment to the areal outline on Friday. On Saturday...the concern for the Plains shifts eastward to Iowa. In neither case are the rainfall amounts or rainfall rates expected to be blockbusters but the concern for flooding arises from the antecedent conditions from earlier rainfall. Eastern New England... Model solutions are getting in better agreement that a strong coastal low pressure system will be developing over the Atlantic that moves in a general eastward direction. However, the spread in the track forecast remains large. There was enough of a model signal to keep indicating the potential for heavy rains to impact portions of southeastern New England to keep the Marginal risk in place on Friday and continue it into Saturday...especially if the GFS and ECMWF continue to keep shifting their track closer to the coast as they started to do on their 22/00Z runs. 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 $$