FOUS30 KWBC 311940 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 340 PM EDT Mon Aug 31 2026 Day 1 Valid 16Z Mon Aug 31 2026 - 12Z Tue Sep 01 2026 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF UTAH AND ARIZONA... ...Desert Southwest and Four Corners... 16Z Update...The MDT risk has been expanded southward into northern Arizona, including the Grand Canyon region, due to new model signals for heavy rainfall. Many of the high-res CAMs indicate a focus for convective development across northern AZ today, both due to terrain influence but also likely in response to differential heating boundaries developing around the cloud shield associated with ongoing convection. In general storms across the Four Corners/Southwest will be relatively fast moving today on 20-25 kts on mean flow (except during initiation/organization), but impressive shear, extremely anomalous moisture, and aligned propagation vectors suggests repeating cells with 1+"/hr hourly rainfall is likely. After recent heavy rain and significant flash flooding, the region is extremely vulnerable so any additional heavy rain today will likely lead to flash flood impacts, which locally could be significant. Previous Discussion: A Moderate Risk for excessive rainfall and flash flooding was maintained for portions of Utah for this period as widespread convection is expected. Dual shortwaves are progged to lift northward (one from Mexico, one from California), bringing enhanced ascent to a region already experiencing pronounced synoptic lift through impressive diffluence in the right rear quadrant of a strengthening and meridionally advancing jet streak. This deep layer lift will act upon extreme thermodynamics characterized by PWs above 1" lifting as far north as the Utah Swell, with +2 sigma PWs pooled across much of Arizona, Utah and western Colorado. Once MLCAPE redevelops in the morning it should quickly reach 500-1000 J/kg to support another day of widespread convective activity. Within the extreme thermodynamics, the robust lift will generate thunderstorms which have a 20-30% chance of producing hourly rainfall above 1 inch. This will be despite generally progressive storms on 0-6km mean winds of 15-20 kts, faster over Utah and Colorado, and it is important to distinguish that convection over southern AZ will move much more slowly. However, bulk shear of 30-40 kts will help organize storms, and aligned propagation vectors indicate a strong threat for repeating cells, enhancing the rainfall risk. Impacts will be most significant over sensitive features like dry washes, slot canyons, and burn scars, but intense rain rates repeating anywhere on Monday could cause excessive rainfall related impacts. ...Great Lakes/Northeast... Showers and thunderstorms are expected fire in the proximity of two frontal boundaries that will be draped across the region during this period. Individual storms will have the potential to produce rain rates of 1-2 inches/hour, supported by PWs pooled south of the fronts of 1.75-2.00 inches overlapping MUCAPE above 1000 J/kg. While these rain rates could be sufficient to overwhelm FFG, exceedance probabilities remain low and scattered at 10-15%, likely due to the fast progression expected of any cells today. Some areas may see repeating rounds which could still create local flood impacts, but the MRGL risk remains, and was adjusted only slightly for the new guidance. ...Central Gulf Coast... A mid-level closed low that has been meandering around the northern Gulf the past few days is beginning to organize as reflected by NHC initiating advisories on Potential Tropical Cyclone Five. The latest guidance continues to show this feature drifting westward beneath the mid-level ridge, approaching the Upper Texas Coast by Tuesday morning. The guidance is very well clustered both spatially and temporally, and while the heaviest rain is likely on D2, the accompanying plume of tropical PWs reaching above 2.25" will support heavy rainfall in slow moving thunderstorms along the Central Gulf Coast today. Although focused heavy rain is progged to remain just off the coast, the MRGL risk was maintained and adjusted to account for local exceedance potential due to rainfall rates reaching 2-3"/hr. ...Florida Peninsula... A MRGL risk was re-introduced for southern Florida after coordination with the impacted WFOs. A plume of tropical moisture remains in place across the area reflected by PWs of 2.25" on the 12Z UA sounding out of MFL. Although the TUTT to the east has weakened, a weak impulse moving across the area from east to west will help drive convective development this aftn, with additional storms firing along the sea breeze/gulf breeze and residual outflows. The deeply saturated column with thick warm cloud depths will support 2-3"/hr rain rates, which could overwhelm soils, especially in urban areas. For this reason the MRGL risk was added once again where the HREF and REFS probabilities for 3-5" of rain are highest. Weiss Day 2 Valid 12Z Tue Sep 01 2026 - 12Z Wed Sep 02 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE UPPER TEXAS COAST AND FAR SOUTHWEST LOUISIANA... ...Central Gulf Coast... Tropical Depression Five will be approaching the Upper Texas Coast to start the period /12Z Tuesday/ and move onshore very early during the day. While there is still some uncertainty in the timing, placement, and intensity of this system, the guidance is actually quite well clustered for this time range, with the system moving onshore very close to Galveston Bay before lifting slowly NW through Tuesday and Tuesday night. There has been a subtle shift W/SW with today's 12Z guidance suite, and the QPF/ERO has been updated to reflect this adjustment. The system is expected to remain compact as it organizes and comes onshore, leading to a narrow corridor of heavy rainfall thanks to PWs of over 2.25" in the accompanying tropical moisture plume. While the heaviest rain may occur along the coast and just NE of the center/track, there is potential for some inflow convergence to maintain a corridor of heavy rain even behind the system as it lifts into east-central Texas. Additionally, the forecast movement (using latest NHC forecast positions) is progged to only be around the 20th percentile of Gulf Coast systems, suggesting the potential for locally excessive rainfall where training is most pronounced and prolonged. Both the HREF and REFS ensemble suites indicate a high chance (>70%) of at least 5" in a NW oriented stripe from Galveston Bay through near College Station, with 8"/24hr probabilities exceeding 40% near the coast. This has prompted a small shift in the SLGT risk and internal 25% areas, but after coordination with WFO HGX, opted to hold off on any MDT risk upgrade - both due to uncertainty and concern about the size of the system being too small to support MDT risk impacts. This will continue to be monitored as the storm evolves, but either way an active day with scattered to numerous flash floods is probable at TD5 moves onshore Tuesday. ...Four Corners and Southwest... After several days of heavy rain and substantial impacts across the Four Corners and Southwest, activity is likely to wane considerable Tuesday. However, there will continue to be sufficient forcing and moisture for another day of local impacts, especially atop the vulnerable soils impacted the last few days, and a MRGL risk has been expanded to cover much of the Four Corners. Although PWs do come down, the drier air which has been progged to push east has slowed down, leaving PW anomalies of +0.5 to +1 sigma in place on Tuesday. With significant forcing remaining in place as a trough approaches from the west (leading to height falls and jet level diffluence for synoptic ascent), this will result in another day of at least scattered thunderstorm development during peak heating, or within any enhanced lift beneath weak impulses embedded within the flow. Mean 0-6km winds will remain progressive at around 25 kts, but aligned propagation vectors and continued elevated bulk shear magnitudes above 40 kts will support organization into clusters and potentially repeating cells. The guidance has become more aggressive with coverage Tuesday, including the UA WRF hourly rainfall plots, and it is possible that hourly rainfall of 0.5 to 0.75 inches will occur. Should this rain fall atop sensitive terrain features or recently saturated soils, additional instances of flash flooding could occur, and because of that the MRGL risk was expanded. ...Great Lakes to the Northeast... Another day of mid-level impulses tracking atop the ridge and along a wavering frontal boundary will produce rounds of scattered convection from the Great Lakes through New England and the northern Mid-Atlantic states. As is typical with this convective cluster setups, there is a lot of spread in both timing and placement of the heaviest rain signatures from the available CAMs. Some of this is due to the inherent uncertainty in timing of the impulses, but some of it is also due to expected activity on D1 which will help reinforce boundaries and areas of greatest convergence into D2. While the guidance is in good agreement that many areas will receive periods of heavy rainfall exceeding 1"/hr as storms track steadily WNW to ESE, the duration of any of this rain may be modest due to the progressive nature of cells. However, short duration training is likely at times thanks to aligned propagation vectors/mean winds to the front, which could enhance rainfall amounts locally. In general, rainfall should be 1-2" which will only be sufficient to create isolated impacts related to flash flooding, but if any significant training/repeating could occur, locally higher rainfall is possible. This is reflected by both HREF and REFS probabilities for 3" that peak above 25%, especially across far southern New England, Long Island, and northern PA/southern NY. However, due to uncertainty in timing and placement, combined with generally elevated FFG outside of the Catskills and Tri-State area, the MRGL risk was maintained and no SLGT risk was introduced at this time. ...Florida... Persistent easterly flow across the Florida Peninsula will once again supply tropical PWs around 2.25" to the region to support scattered afternoon convection. Mean flow across the Peninsula will be very weak on Tuesday (0-6km mean winds just around 5-10 kts), which suggests cells will drift west from the sea breeze, but may converge just inland from the Gulf as the gulf breeze also pushes inland. With Corfidi vectors becoming increasingly anti-parallel to the mean flow, some short term training is possible (limited in duration by the pulse nature of the cells) but with rainfall rates expected to be 2-3"/hr, this could create isolated flash flood impacts, especially within urban areas. Weiss Day 3 Valid 12Z Wed Sep 02 2026 - 12Z Thu Sep 03 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE NORTHEAST AND EASTERN TEXAS... ...Northern Mid Atlantic & Southern New England... Small adjustments were made to the inherited SLGT risk to account for the latest guidance. While the setup Wednesday is at least synoptically similar to both Monday and Tuesday, a more impressive shortwave tracking atop the region to interact with a slowly northeast advancing warm front will result in scattered to widespread convection. There is still some uncertainty as to the placement of the heaviest rainfall, but an improving warm sector with favorable thermodynamics along a sharp gradient should support training of clusters of thunderstorms with 1-2"/hr rain rates. The guidance focuses this heaviest rainfall across portions of Upstate New York, and while soil moisture is generally below normal according to NASA SPoRT, this should support up to 3" of rain in some areas leading to scattered instances of flash flooding, with more isolated responses elsewhere. ...Eastern Texas and Southwest Louisiana... Tropical Depression 5 will be well inland by the start of the forecast period, but with slow movement expected to continue, additional heavy rainfall is likely even as it begins to dissipate. PWs of 2-2.25" will support continued heavy rain, and as 850mb inflow only slowly weakens it will drive enhanced convergence to support 2-3"/hr rain rates. While most of the rain should be around the center of the system which will be lifting slowly northward towards north Texas, there is potential for a locally enhanced inflow band on its south side which could prolong and enhance the rain across areas that receive heavy rain on Tuesday as well. The small size of the storm should limit the footprint and potential impacts, but ECENS/GEFS probabilities still indicate that at least an additional 3" of rain may fall, with locally higher amounts, leading to instances of flash flooding. Some further refinements in the SLGT risk area are expected with future updates, both to capture the latest guidance track, but also better resolve where any additional rainfall will occur on top of heavy rain from Tuesday. ...Northern Plains and Midwest... Lingering MCS activity possible early in the period across the Upper Midwest. PWAT anomalies remain elevated at times mid- week, with values of 1.5-2" forecast, however the spread in the deterministic QPF is higher than average. A broad Marginal Risk spans from Minnesota/Iowa eastward to the Great Lakes. ...Florida... Another day where the tropical moisture plume remains entrenched across Florida, reflected by PW anomalies of +1.5 sigma stretching across most of the state. The biggest change from prior days, at least as modeled in guidance, is weaker overall flow leading to slower storm motions, and a better chance for both coasts to experience slow moving storms with 2+"/hr rain rates, as well as across the inland portions of the peninsula where storm mergers/collisions can occur. After coordination, the inherited MRGL risk was expanded to cover the entire Peninsula south of Jacksonville, with the greatest risk for impacts occurring atop urban areas. Weiss/Campbell Day 4 and Day 5 Valid 12Z Thu Sep 03 2026 - 12Z Sat Sep 05 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL THURSDAY AND FRIDAY ACROSS PORTIONS OF ARIZONA... ...2030Z Update... The signal for the return of heavy rain into portions of Arizona with the monsoon remains strong on both Thursday and Friday. As before, the signal on Thursday remains lower than on Friday, but enough moisture, forcing, and instability move into southern Arizona from off the Gulf of California to allow for scattered thunderstorms to develop once again Thursday afternoon. Despite the "break" of the prior days, soils may remain a bit more saturated than normal by Thursday afternoon, and the area around I-19 remains highly susceptible to flooding caused by heavy rains. Thus, have opted to upgrade to a Slight from Tucson south. Moisture off the Gulf of California will continue to spread northward across southern Arizona on Friday, while a nearly stationary positively tilted trough remains parked over the Pacific Northwest. This will allow for some upper level energy, combined with the thermal trough, to allow storms to blossom over much of the area. The Slight Risk area was expanded north and west a bit up the Mogollon Rim on Friday. Marginal Risk areas were added in portions of the Northwest, particularly the area surrounding the northern Idaho Panhandle. This area will be much closer to the upper level trough, so forcing should be much better than in the Southwest. However, moisture and instability will be much lower, resulting in more likelihood of stratiform rainfall. Despite this, the rainfall should still have modest amounts of instability to work with in a generally dry, and therefore flash flood prone, area of the country. Wegman ...Previous Discussion... ...Southwest/Four Corners/Central-Southern Rockies... After a brief respite on Day 3 (Wed-Wed night), moisture returns Thursday and especially by Friday and into next weekend per the guidance. The 00Z GFS was not utilized during this forecast cycle, given the the deeper, more neutrally-tilted full-latitude trough along the West Coast, which draws any potential tropical moisture farther east-northeast closer to northern Baja and the Southwest. Nevertheless, the ECMWF/CMC/UKmet show deep-layer moisture returning by Friday and especially by the weekend, and the non-GFS consensus QPF (including the NBM) has come up. Per collaboration with WFO Tucson, Phoenix, and Flagstaff, have hoisted a Slight Risk on Day 5 across much of southern AZ. ...Middle-Upper TX Coast and East-Central TX to Southwest LA... Maintained the Marginal Risk on Day 4 (Thu-Thu night), which would be dependent on the track and evolution of the current low pressure system over the northern Gulf. NHC currently has a high (70%) chance that this feature would develop into a short-lived tropical depression or tropical storm before reaching the Upper TX or southwestern LA Coast by Tuesday morning. However, the big question would be how long this feature will linger as a rainfall threat inland before dissipating. The 00Z ECMWF and especially CMC are notably slower with the inland progression and dissipation, and as such have maintained the Marginal Risk on Day 4, with the emphasis of it being early in the period (i.e. Thursday morning). ...Upper Midwest/Ohio Valley/Appalachians/Parts of the Northeast... Mid/upper level trough (initially closed H5 low) pivoting through eastern Ontario and Quebec late this week will enhance the upper level confluence south of this feature towards the upper ridge. This will result in at least a couple of 90-100kt jet streaks traversing the along/near the U.S.-Canadian border, providing the necessary dynamical support (deep-layer ascent) for more organized convection (potentially growing upscale into an MCS), again tracking along the outer periphery of the flattened upper ridge, ahead of the surface frontal boundary that will be dipping southward Thursday through early Saturday across the OH Valley and central Appalachians. For now, we have maintained the Marginal Risk on Day 4 (inherited from yesterday's Day 5 ERO), while shifting the outlook area a bit farther south on the new Day 5 ERO (12Z Fri-12Z Sat). Deep-layer moisture anomalies ahead of the front remain high, with with PWATs around 2 standard deviations above normal per the 00Z GEFS and ECENS. ...Central Gulf Coast and Florida... A combination of onshore flow from a warm and unstable subtropical Atlantic and Gulf and Dolly's remnants moving into the region are a recipe for heavy rainfall. However, any heavy rain signal is spotty and not showing any real run to run model continuity at the present time. The Marginal Risk of excessive rainfall from yesterday's Day 5 ERO was maintained on the current Day 4 outlook, but if Dolly ends up more organized than currently expected, a higher risk threshold could be necessary. ...Southwest/Southern Rockies... The combination of a nearby front at the leading edge of a strong western trough for this time of the year and a monsoon surge up the Gulf of CA caused by a future tropical cyclone is expected to cause a substantial rise in the precipitable water values across Thursday and Thursday night, ending up near 2" by Thursday morning. Any shortwave moving through the base of this trough could cause a convective uptick, so the real question is whether or not any outflow cirrus from the as-yet-to-form tropical cyclone moves into the area to inhibit daytime heating. There are more than enough ingredients available for a Marginal Risk area, which was added in this update. Hourly rainfall up to 2.5" would be problematic in urban areas, burn scars, dry washes/arroyos, and box/slot canyons. Hurley/Roth 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 $$