FOUS30 KWBC 122011 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 411 PM EDT Sat Sep 12 2026 Day 1 Valid 1948Z Sat Sep 12 2026 - 12Z Sun Sep 13 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE OHIO VALLEY, MID-ATLANTIC AND SOUTHERN ARIZONA... 20z Update: A Slight risk was added to southern AZ where convection is developing within an environment of overlapping high CAPE and PWs. Observational trends and HREF/REFS FFG exceedance probabilities support a scattered flash flood risk over this area. Chenard 1630Z Excessive Rainfall Discussion... Maintained the Slight Risk area in the Ohio Valley and adjacent portions of the Mid-Atlantic region given the ingredients in place to focus and support convection capable of producing downpours. 12Z upper air analysis showed precipitable water values around 1.75 inches in place with the 12Z NAM and HREF both supporting a surge of precipitable water values of 2 inches or more reaching the southeast Pennsylvania by early this evening and reach northern New Jersey and the Tri-State area in the predawn hours Sunday. The previous forecast had a good handle on this so the changes tended to be pretty minor. The heaviest rainfall still looks to be focused farther west through central PA and into the Upper OH Valley, a convergence of elevated low-level theta-e within the strengthening area of upper level divergence associated with the approach of a surface cold front will support convection capable of being efficient rainfall producers. The 12Z HREF/REFS probabilities for 2 inch amounts in 3 hours remained in the 50 to 60 percent range from late this afternoon into the evening. Given the lowered Flash Flood Guidance from recent rainfall in the area going back a few days...still think this is a "higher-end" Slight Risk for the Upper OH Valley, with potential for these rates. Elsewhere...the Marginal Risk areas looked to be in good shape given the anomalous moisture in the Central Plains as a wave lifts northeastward later today and overnight while the monsoon-fueled convection ramps up this afternoon with daytime heating from the Southwest US into portions of the Rockies. Bann 0830Z Excessive Rainfall Discussion... ...Ohio Valley & Mid-Atlantic... The broad 500mb trough in the Mid-South will induce healthy upper- level ascent over the Ohio Valley and Mid-Atlantic through Saturday night. A warm front lifting north through the Mid-Atlantic will act as a trigger for storms, while daytime heating and height falls support widespread showers and thunderstorms from the Gulf Coast to the northern Mid-Atlantic. PWs between 2.0-2.25" will stream north along the I-95 corridor from Richmond and D.C. to Baltimore and Philly by Saturday evening, then into the NYC/Tri-State areas in the pre-dawn hours Sunday. With instability increasing as the warm sector drifts north and warm cloud layers likely to be at least 12,000ft, highly efficient warm rain processes within developing thunderstorms are likely. The 00Z HREF shows low-to- moderate chances (20-40%) for 3-hr QPF >2" from the Richmond, VA area late Saturday afternoon, then from northeast MD into the Delaware Valley and southern NJ Saturday night. The Slight Risk was extended north through eastern PA, northern NJ, and into the NYC metro and lower Long Island. It is important to note for the northern NJ suburbs, NYC, and Long Island that the Slight Risk is primarily for late Saturday night with the excessive rainfall threat lingering into Sunday (Day 2). Farther west through central PA and into the Upper OH Valley, a convergence of elevated low-level theta-e within the strengthening area of upper level divergence in the 200-400mb layer will generate bands of thunderstorms that track parallel to the mean 850-300mb wind flow. Soundings are highly saturated from just above the surface to as high as 300mb (>90% RH). Similar to areas farther east, warm cloud layers are at least 12,000ft deep and FFGs are less than 2" in northern WV and western PA. The 00Z HREF shows worrying signals with moderate-to-high chances (50-80%) for 3-hr and 6-hr FFG exceedance within the Upper OH Valley, and moderate chances (40-60%) for 2" of rainfall within 3-hours Saturday afternoon. This is a "higher-end" Slight Risk for the Upper OH Valley, with potential for these rates to spills over into north- central PA early Saturday evening. ...Central Plains... An approaching 500mb disturbance and a strengthening southerly low-level jet will interact with a frontal boundary that results in scattered thunderstorms from the central High Plains to the Mid-MS Valley. PWs ranging between 1.5-1.75" are generally around the 90th climatological percentile and ample instability will be supportive of strong thunderstorms producing >2"/hr rainfall rates. The latest 00Z HREF guidance shows 24-hr QPF >3" probabilities remain on the low end (10-30%), and are isolated across eastern KS and western MO. For those reasons, decided to hold on with a Marginal Risk this forecast cycle. ...Florida... Much of the Sunshine State will remain mired in an environment containing copious amounts of moisture that will continue to support scattered thunderstorms containing torrential downpours. PWs between 2.0-2.25" will be common and storms will be capable of causing localized flash flooding, especially in poor drainage areas and in the more urbanized sections of Florida. ...Rockies & Southwest... A strong 500mb closed-low with heights below the 10th climatological percentile will work with an increasingly anomalous PW field to produce scattered thunderstorms with efficient rainfall over the Northern Rockies. Farther south, the pool of anomalous PWs that supported daily rounds of excessive rainfall in the Southwest through the week will stick around and cause additional thunderstorms that produce locally excessive rainfall rates. Instability will be lacking over the Central Rockies, but soils are fairly saturated in parts of AZ and NM following recent rainfall and provide a slightly more favorable environment for localized flash flooding. Mullinax Day 2 Valid 12Z Sun Sep 13 2026 - 12Z Mon Sep 14 2026 ...THERE ARE SLIGHT RISK AREAS FOR EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHEAST & MIDWEST... 2030Z Excessive Rainfall Discussion Update... Northeast... Rainfall should be on-going as the Day 2 period begins at 13/12Z over portions of northern New Jersey eastward into southern New England as mid-level height falls induces surface low pressure...albeit weak...in proximity to the Southern New England coastline. This is a bit delayed from the previous suite of model guidance. As a result...there may be a bit more time for moderate to heavy rainfall and the Slight Risk was expanded a bit northward and westward...including more of Long Island and adjacent coastal areas in addition to the previously mentioned portions of Connecticut and Rhode Island into Massachusetts. The 12Z HREF probabilities depicted 45-60%) for 3-hr QPF >3" across the area during the mid- to late morning hours on Sunday. Midwest... The 12Z suite of model guidance tended to show somewhat better clustering in the QPF guidance as flow over the Upper Midwest backs in response to height falls coming out of the Rockies, As a result...the Slight Risk and surrounding Marginal Rusk areas were adjusted. There may be room for additional adjustment but kept the changes modest at this point in case the model opinion swings back. ...Southern Rockies & Four Corners... Monsoonal moisture remains in place as flow aloft strengthens in response to the mid-level system approaching the international border late in the period. Current outlook still appears to capture the area with the greatest chances of heaviest rain based on model spaghetti plots. Bann 0830Z Excessive Rainfall Discussion ...Northeast... The region will reside beneath the right-entrance region of a strengthening 250mb jet streak, creating strong 200-400mb layer Q-vector divergence over the Northeast. This is allowing for unusually strong upper-level frontogenesis atop southern New England, in addition to the surface warm front that is forecast to span a strengthening surface low over Long Island early Sunday morning. This is a continuation of the bands of strong thunderstorms containing highly efficient rainfall rates from Saturday night. PWs up to 2.25" are expected and low-level theta-e advection into the warm front will direct MUCAPE between 500-1,000 J/kg into southern New England.Area averaged soundings across multiple CAMs members paint a concerning picture; highly saturated profiles within a sheared environment that could support enhanced mesoscale rainfall rates >2"/hr within an urbanized corridor. The rainfall threat moves into RI and southeast MA late morning and into the afternoon before exiting off the coast late afternoon. This setup led to the issuance of a Marginal Risk that extends from northern NJ to as far north and east as southern NH/ME, and a Slight Risk from the NYC metro on east to the Boston metro and MA Capes. There is greatest concern for flash flooding impacts over Long Island and the coastal portions of CT and RI where 00Z HREF probabilities should moderate chances (40-60%) for 3-hr QPF >3" across the mid-late morning hours on Sunday. ...Midwest & Central Plains... A frontal passage and a fast moving 500mb shortwave trough heading for the Great Lakes will trigger rounds of showers and thunderstorms from South Dakota and the Nebraska Sand Hills to as far south as northern KS/MO and as far east as the MS River. As the first shortwave trough exits east, a strong 500mb closed low will head through the Northern Rockies, prompting the development of low pressure in the northern High Plains. This area of low pressure will work in tandem with high pressure over the Great Lakes Sunday night to generate a 30-40kt LLJ over the Central Plains. The LLJ will advect rich moisture and warmer theta-e air into the Central Plains with the frontal boundary that passed through early Sunday morning lifting slowly as a warm front Sunday night and into early Monday morning. Latest ECMWF guidance shows a pool of anomalous PWs (above the 97.5 climatological percentile) sitting over the Central Plains on Sunday that lifts north into the Midwest by early Monday morning. With the LLJ intersecting the frontal boundary, and elevated instability available, additional strong thunderstorms are likely to form late Sunday night and into early Monday. While exact placement of the heaviest rainfall still differs, most guidance is in agreement that the Midwest will be the focus for potential flash flooding on Sunday and into Sunday night. The latest WPC QPF has shifted the heaviest rainfall into the MO River Valley with 1-2" rainfall amounts (locally higher possible) in northeast KS, northwest MO, eastern NE, and southwest IA. ...Southern Rockies & Four Corners... The troughing pattern in the Northwest and the mid-upper level ridge axis over the Southern Plains will direct additional monsoonal moisture into the Southwest and Southern Rockies. ECMWF percentiles suggest PWs will rise to above the 97.5 climatological percentile Sunday evening as IVT values >300 kg/m/s direct moisture into the regions. Some locations continue to sport saturated soils, thus making these areas more prone to potential flash flooding. While confidence in higher rainfall rates is lower at the moment, the rgion's sensitive soils and potential for excessive rainfall rates >1.0"/hr continues to support the Marginal Risk in place. Mullinax Day 3 Valid 12Z Mon Sep 14 2026 - 12Z Tue Sep 15 2026 ...THERE ARE SLIGHT RISKS FOR EXCESSIVE RAINFALL OVER PORTIONS OF THE MIDWEST & SOUTHERN ARIZONA... 2030Z Excessive Rainfall Discussion Adjusted the Slight Risk area a bit as a surface cold front associated with a deepening upper trough approaches the region. Expanded the southern boundary of the Slight risk from the placement shown by the deterministic models given some members of the ECMWF and GEFS spaghetti plots. The alignment of the upper and lower level flow becomes favorable for the training of cells and repeat convection later in the period. In the Southwest...another round of late-day and evening convection is expected with monsoon moisture remaining pretty much in place. Given the PW anomalies...saw little reason to make many changes here. Bann 0830Z Excessive Rainfall Discussion ...Midwest... A strong closed 500mb low, sporing 500mb heights below the 10th climatological percentile per ECMWF, will work in tandem with a strong 500mb ridge over the Mid-South to produce an exceptional IVT throughout much of the North Central U.S.. NAEFS shows the IVT topping 750 kg/m/s over the MO River Valley and into MN Monday morning, then strengthening to >1,000 kg/ms/s over the UppeR MS Valley by Monday afternoon. Strong theta-e advection and anomalous PWs that surpass the 99th climatological percentile will set the stage for an active day, consisting of widespread showers and thunderstorms that contain highly efficient warm rain processes. The area of greatest concern is along the warm front, closest to the surface low track, as veering low-level wind profiles ahead of the approaching cold front support clusters of organized thunderstorms. Latest WPC QPF shows widespread 1-2" of rainfall from eastern NE to as far north as the Upper Great Lakes with localized totals surpassing 3" possible. The inherited Slight and Marginal Risks remain in good shape, but extended the Slight Risk into southern NE to account for recent south and west trends in QPF. ...Southwest... Broad upper-level troughing over the West and upper-level ridging in the Southeast will continue to support a healthy monsoonal flow into AZ/NM and as far north as the Central Rockies. NAEFS shows IVT values still topping the 90th climatological percentile for mid- September. In addition, the Four Corners region is ideally placed beneath the right-entrance region of a 250mb jet streak that favors healthy upper-level ascent. The region has remained fairly active in recent days and will continue to see daily rounds of scattered showers and storms over the weekend, helping to further saturate soils ahead of Monday. QPF is heaviest in southeast AZ at the moment, and given soils are more likely to moisten rather than dry out in the coming days, a Slight Risk was issued in ands around the Tucson area for Monday. A broad Marginal Risk accounts for the potential of localized flash flooding from central AZ on east through much of NM and northward into the CO Rockies. Mullinax Day 4 and Day 5 Valid 12Z Tue Sep 15 2026 - 12Z Thu Sep 17 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL PLAINS, MISSOURI VALLEY INTO PARTS OF THE CORN BELT AND UPPER MIDWEST... 20z Update: Still on track for a favorable axis of training/backbuilding convection on Tuesday near a stalling front over the Plains/MS Valley. The relatively consistent AI models and overall guidance trends support a bit of a southwest shift in the Slight risk axis with this update, with the risk now extending from central KS into central IL. The stalling front, persistent moisture transport into the boundary, sufficient instability, and robust model QPF output, all point towards at least a locally higher end flash flood risk. The greatest threat at this time appears to be from northeast KS, into far southeast NE, far southwest IA and into northwest MO...where this is considered a higher end Slight risk. Pending model trends, can not rule out a focused MDT risk at the event nears. Not much change was needed to the day 5 (Wed) Slight risk that stretches from KS into IN. A bit more model spread by this time, but the overall setup will remain favorable for periods of training convection. Depending on antecedent rainfall, an embedded MDT risk is possible as the event nears. The Southwest will see an uptick in convective coverage Tuesday into Wednesday as well. At the moment, day 5 (Wed) seems to be the most active, and a Slight risk may eventually be needed over NM and eastern AZ. Chenard ...Previous Discussion... ...Day 4... The longwave pattern will remain relatively stable east of the Mississippi with a strong mid and upper ridge axis located across the southeastern quarter of the U.S. with a formidable area of surface high pressure dominating the weather pattern for these areas. Across the Western CONUS, multiple shortwave troughs will lead to a general trough axis and prevailing southwesterly flow extending from the Southwest U.S. through the center of the nation with an expected quasi-stationary frontal placement bisecting areas from the Central High Plains to points east-northeast through the Missouri Valley and Corn Belt with the eastern inflection back into the Lower Great Lakes. This setup will be coincident with a bit of a Maddox signature which typically precludes more appreciable heavy rain prospects given the limited forward propagation expected of a surface front with mean flow generally parallel to the surface feature leading to training and back- building scenarios within proximity of the front. Global deterministic continues to imply multiple smaller shortwave perturbations ejecting northeast through the main axis of diffluence situated over the above regions. This will only act as additional regional ascent to aid in convective genesis, and creating a repeated pattern of heavy rain prospects as the setup evolves. Ensembles remain consistent with a mean axis of heavier QPF situated within the confines of the stationary front through the course of the forecast period. A textbook bimodal distribution of the heavier rainfall is forecast with the first enhancement occurring in the early morning hours Tuesday, then once again in the evening as the formation of the nocturnal LLJ will lead to enhanced convergence near the front. Bias corrected QPF aligns a bit further to the north than the ECENS output, as well as the ML guidance which has the frontal alignment a bit further south as noted when assessing the theta_E gradient on each respective model. Heavy precip is likely given the setup with a pretty stout signature regardless of which model depiction you assess. Mean QPF of 2-3" is already forecast within the area of northeast KS, southeastern NE through northern MO and southern IA. The general max will lie within 50 miles of either side of the frontal placement, so finding the exact location of the front will be the key to figuring out the specifics of where the highest flash flood threat would occur. For now, the setup is more than sufficient to maintain a broad SLGT risk over the areas referenced, but did expand more to the southwest to include Central KS, as well as northeastern extension up through the southern fringes of the western Great Lakes. This takes into account some of the known biases of models within the medium range that trend more in these general directions in setups like these with a well-defined front and healthy thermodynamic environments on the flank of the front. Another period of scattered convective potential across the Southwest U.S. will lead to yet another MRGL risk in place for the D4, but will also have to keep an eye for any short term upgrades given the propensity for repeated action, as well as the introduction of better kinematics as these smaller shortwaves make their way around the western periphery of the ridge. ...Day 5... General persistence for the pattern across the CONUS will lead to another period of heavy rainfall across the center of the nation with scattered heavy convective potential lingering over portions of the Southwest U.S. Consensus is a maintenance of a relatively stable upper level setup with a lingering quasi-stationary front bisecting the same areas as the previous period(s). The front will struggle to lose much latitude unless propelled by stronger cold pools emanating from the convective threat that occurs over the previous 48 hrs, continuing into the new D5. There's no real deviation in the ML and ENS suites in the placement of heavy rain somewhere between the Central Plains into the western Ohio Valley with the max crossing through that area in southern IA and northern MO which would make at least 2 consecutive days of heavy rain in those zones. Multi-day totals between 3-5" are forecast at range which is typically coincident with a potential for a significant flash and hydrologic flood threat when looking back at previous events in this region of the CONUS. A broad SLGT was maintained over very similar areas as the previous period with a touch broader expanse due to some uncertainty in exact frontal placement. The risk area is expanded a bit southward compared to the previous forecast to account for some southern frontal motion given the expected cold pool propagation from previous convection. This time frame will be one to monitor, once again, as the threat for more appreciable flooding is at play with this setup. Scattered thunderstorms will remain forecast across the Southwest into the Central Rockies thanks to the stagnant upper pattern in place. Pending more notable shortwave advancements as we move into the short term, an upgrade is plausible for the period considering the incessant nature of the heavier precip occurring for several consecutive periods. For now, the MRGL risk was maintained across the aforementioned areas with the highest threat centered over local urban zones, burn scars, and flashier arroyos and small streams situated over NM/CO/AZ. 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 $$