FOUS30 KWBC 210846 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 446 AM EDT Mon Sep 21 2026 Day 1 Valid 12Z Mon Sep 21 2026 - 12Z Tue Sep 22 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL APPALACHIANS... ...Central Appalachians and the Mid-Atlantic... The Slight risk has been expanded to include a larger portion of WV and southwest VA. The primary driver of heavy rainfall will be embedded shortwave energy interacting with a backdoor cold front and inverted surface trough. Model guidance indicates a moderately to highly unstable airmass will develop near the boundary over WV, with 2000-3000 J/kg of CAPE expected. As a distinct shortwave/MCV currently positioned over IL/IN progresses into the region, robust convective development is anticipated. While most model solutions keep these cells moving steadily across WV, a quick 1-3" of rain over increasingly saturated soils should result in an additional flash flood threat. By this afternoon convection should propagate into southwest VA along/near the backdoor front. In this region more persistent low level convergence should increase the duration of convective elements and locally enhance rainfall totals. While some guidance pushes the front and best convective focus slightly further south into northern NC, current model consensus supports southwest VA as the primary axis for heavy rain. Recent high res models highlight the potential for localized excessive totals. Both the 00z and 06z HRRR depict a small but impressive swath of rainfall as high as 3-6". Given the available instability, mesoscale forcing, and favorable convergence near the front, these elevated totals appear plausible, justifying the expansion of the Slight risk. ...New Mexico and Nearby Southern High Plains... Both the HREF and REFS indicate the potential for rainfall locally exceeding 3" as isolated convection develops this afternoon over eastern NM and west TX. A widely isolated flash flood risk is possible. ...South Florida... A Marginal Risk has been maintained for portions of South Florida as a plume of anomalous moisture will help to fuel heavy downpours and potentially lead to some isolated flooding/ponding in urban areas. ...CA/NV... Convection underneath a closed low may pose a localized flash flood risk today across portions of eastern CA into NV. Instability is forecast to be weak, but with cold air aloft under the low and PWs forecast to increase over the climatological 90th percentile, an isolated flash flood risk could evolve. Chenard Day 2 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 potent, multi-day heavy rainfall event is set to begin across NM and far southwest TX Tuesday. 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, while embedded mid level shortwave energy advances north out of Mexico and into NM by Tuesday night. Low level easterly upslope flow will also enhance lift in this setup. PW values are forecast to approach mid to late Sep. record levels. With sufficient instability expected, this deep moisture profile will support heavy rainfall rates. Scattered flash flooding is expected across much of NM and far southwest TX, with a more concentrated area of slow moving convection possible over southern NM. Guidance is in pretty good agreement regarding high QPF potential over southern NM. The 00z RRFS and 3km NAM depict swaths of 3" rainfall totals, with the GFS/AIGFS/Gem reg depicting higher QPF across a similar axis. With a day 3 Moderate risk already in effect, and the flash flood threat expected to ramp up by Tuesday night, we opted to upgrade the day 2 to a Moderate as well. 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... An approaching mid-level trough and favorable positioning within the right entrance region of an upper level jet will support widespread rainfall across portions of the OH Valley and Mid- Atlantic. At the surface, a backdoor cold front with strong easterly flow will enhance low level convergence, further amplified by a developing inverted surface trough. Well to the north of the frontal boundary, limited instability will keep rainfall primarily stratiform, with rain rates remaining below flash flood thresholds. Considerable model spread remains regarding where the strongest convergence and sufficient instability will overlap, complicated by dry air advecting in from the northeast. While the 12z EC and 00z GFS were previously in agreement with a heavy QPF axis along the coastal plain from eastern VA into the Delmarva, the 00z EC has trended further inland. Meanwhile, the 00z RRFS and REFS favor a further inland and southward axis stretching from south central VA into central NC. Ultimately, the forecast hinges on far south cold air damming forces the instability axis and also how far inland the surface trough gets nudged. A localized sweet spot may emerge where persistent convergence intersects sufficient instability to drive a heavy rainfall and flash flood threat. Where these ingredients overlap, the environment will be conducive for some low topped convective cores and efficient warm rain processes. However given the current uncertainty, we will keep the risk level at Marginal for this update, and we will reevaluate later today when we have the full suite of high res guidance available. ...South Florida... A Marginal Risk has been maintained for portions of South Florida. Locally heavy downpours will remain possible with anomalously high moisture in place, which could lead to some isolated flooding/ponding in urban areas. Chenard Day 3 Valid 12Z Wed Sep 23 2026 - 12Z Thu Sep 24 2026 ..THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF EASTERN NEW MEXICO... ...New Mexico and West Texas... The heavy rainfall threat established on day 2 will continue and potentially peak over NM on day 3 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. 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 probable. Guidance continues to show some spread regarding the placement of this heaviest rainfall axis. The 00z EC remains the furthest west solution, with the GFS, AIGFS, AIFS and RRFS clustered a bit further east. The current forecast favors this eastern consensus, though given we are still three days out, additional spatial adjustments are probable. Based on this setup the inherited Moderate risk remains justified, and it was expanded to encompass a larger portion of eastern NM. 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. ...Mid-Atlantic... A complex and somewhat disjointed synoptic pattern will be present over the Mid-Atlantic on day 3. A mid to upper trough will will drift slowly eastward from the OH Valley into the Mid-Atlantic while a separate coastal low develops offshore. Meanwhile, surface high pressure to the north will advect drier and more stable air into the corridor between these two systems. While some instability may materialize directly beneath the cold core of the mid level low, PW values will have generally decreased by this time. The bulk of the deeper moisture and instability may remain offshore with the coastal low. Despite the intrusion of dry air, there remains enough uncertainty regarding the exact evolution of these features to warrant a continuation of the Marginal risk. An embedded Slight risk may be needed in subsequent updates, contingent upon the exact track of the upper low, position of the moisture/instability axis, and the state of antecedent soil conditions from previous days rainfall. ...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 4 and Day 5 Valid 12Z Thu Sep 24 2026 - 12Z Sat Sep 26 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NEW MEXICO ON DAY 4... ...Southern Rockies/High Plains... The threat of additional flooding lingers into Thursday given a favorable upper level pattern for additional showers.Even though rainfall rates should be diminishing and the bulk of the rainfall will be shifting into the Texas and Oklahoma panhandles, but the concern in New Mexico is that any additional rainfall will be falling on wet to saturated soils. Consequently, introduced a Slight Risk area within the Marginal Risk area. At the same, refrained from expanding the Marginal Risk area too far eastward in order to better assess the potential rainfall amounts/rates that move over considerably drier ground. By 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. Introduced a Marginal Risk area there due to the antecedent conditions and given the low level feed of moisture being drawn northward from the Gulf. At the same time, maintained a Marginal Risk in parts of New Mexico. ...Ohio Valley/Appalachians to Mid-Atlantic... By Thursday, a coastal low will likely be developing in the Atlantic. Depending on the low position, some guidance shows heavy rainfall reaching onshore in the Mid-Atlantic on Thursday then spreading towards southeastern New England on Friday. Models continue to show the heaviest rain could remain offshore, and because that region is not very sensitive to heavy rain given the sandy soils if there are moderate amounts onshore. But this should continue to be monitored. 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 $$