607 FOUS30 KWBC 132010 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 410 PM EDT Sun Sep 13 2026 Day 1 Valid 20Z Sun Sep 13 2026 - 12Z Mon Sep 14 2026 ...THERE ARE SLIGHT RISK AREAS FOR EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHEAST & CENTRAL PLAINS... 20Z Discussion... Based on latest radar trends...expanded the Southwestern US Marginal Risk area where convection had developed in a high CAPE environment outside the previously-issued Marginal area. Also removed the Marginal and Slight risk areas from the Northeast US. Trends in satellite and radar imagery showed the heaviest rainfall rates have been shunted out to sea and generally light rainfall rates linger. The expectation is that even this area of light rainfall will be moving out to sea shortly. Bann 16Z Discussion Update... Radar showed that moderate to heavy rainfall had all but ended in northern coastal New Jersey as of late this morning but continued from Long Island northeastward into pasts of Massachusetts. Except to trim away the western portion of the outlook area...few changes needed to the orientation or areal coverage of the Slight Risk area. The threat of additional flooding will decrease from west to east throughout the day as the system moves out to sea. The Slight risk over the Plain and the surrounding Marginal Risk area needed only minor nudges given latest HREF/RRFS and radar trends. Expanded the areal coverage of the Marginal Risk area in vicinity of the international border given the RAPs placement of instability during the afternoon and the precipitable water fields from the 12Z upper air cycle. The remainder of the area still tended to fit within the latest chances for heaviest rainfall/rainfall rates as shown by the 12Z HREF/RRFS guidance. Bann Previous Discussion... ...Northeast... The Northeast resides 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 spawn 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. 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. The 00Z HREF also shows moderate chances (40-60%) for 6-hr QPF surpassing 6-hr FFGs. Both the 00Z HREF and REFS show high-chance probabilities (>70%) for rainfall totals over 3" through early afternoon along the southern New England coast. The Slight Risk continues to remain in good standing, with the flash flood threat likely ending throughout southern New England by late afternoon. ...Central Plains... A frontal boundary draped over the Central Plains and a prolonged SWrly LLJ in the Southern High Plains will act as a source of lift and moisture transport throughout much of the day. By evening, an area of low pressure will work in tandem with high pressure over the Great Lakes Sunday night to strengthen the LLJ to as much as 40kt LLJ over the Central Plains. A plume of high theta-e air will continue to be oriented along the frontal boundary, with additional storms likely to develop Sunday night. 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 now showing eastern KS and western MO becoming favored for the locally heavier rainfall totals. 00Z HREF guidance shows low-to- moderate chances (20-40%) for >3" of rainfall over the Kansas City metro area. By Sunday night, the emergence of a strong closed 500mb low over the northern Rockies will direct a rich plume of >99th climatological percentile PWs into the Northern Plains. Given the progressive nature of these storms and modest elevated instability, the flash flood potential is not too concerning at the moment, but the highly saturated profiles do support at least a Marginal Risk through Sunday night. ...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. The footprint of saturated soils has only grown following the active Saturday in AZ, thus making these areas more prone to potential flash flooding. While confidence in higher rainfall rates is lower at the moment, the region's sensitive soils and potential for excessive rainfall rates >1.5"/hr continues to support the Marginal Risk in place. Mullinax Day 2 Valid 12Z Mon Sep 14 2026 - 12Z Tue Sep 15 2026 ...THERE ARE SLIGHT RISKS FOR EXCESSIVE RAINFALL OVER PORTIONS OF THE MIDWEST & SOUTHWEST... 2030Z Excessive Rainfall Discussion... In the Upper Midwest...models tended to agree on the orientation and placement of the axis of heaviest rainfall extending northeastward from Nebraska/Iowa. However...within that axis there was disagreement as to how far north or south. The CMC/UKMET placed the heaviest rainfall in northeast Iowa or nearby Minnesota/Wisconsin while the ECMWF/NBM and latest WPC deterministic QPF aligned farther southwest. Either camp of solutions fit within the previously-issed Slight risk area so left the area unchanged. The Southwest US still looks to become active once again with the model precipitable water and QPF fields comparable to their previous solutions. Bann Previously issued Excessive Rainfall Discussion... ...Midwest... A strong closed 500mb low, sporting 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.. The ECMWF SATs show the IVT topping 1000 kg/m/s over the MO River Valley and into MN Monday morning, then strengthening to >1,250 kg/ms/s over the Upper MS Valley by Monday evening. These values are approaching levels that could challenge the highest observed IVTs for mid- September in the CFSR database (1979-2009), thus showcasing the remarkable moisture transport at play. Strong theta-e advection and anomalous PWs exceeding 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. PWs will also surpass the 97,5 percentile for much of eastern AZ and western NM. 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 additional scattered showers and storms on Sunday, helping to further saturate soils ahead of Monday. QPF has trended wetter over eastern AZ and western NM, and given soils are more likely to moisten rather than dry out in the coming days, the Slight Risk was expanded to include more of southeast AZ and far western NM. 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 3 Valid 12Z Tue Sep 15 2026 - 12Z Wed Sep 16 2026 ...THERE ARE SLIGHT RISKS FOR EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL PLAINS, MIDWEST, & SOUTHERN ARIZONA... 2030Z Excessive Rainfall Discussion... Across the Central Plains/Midwest...anomalously deep moisture, and strengthening low level forcing on an increasingly unstable airmass was still being advertised by the models. Enough spread in the placement of the heaviest QPF still precluded issuance of more than a Slight Risk...but qualitatively appears to be a high-end Slight risk area. No changes needed in the Southwest with respect to the Slight or Marginal risk areas given the monsoon moisture and the synoptic scale forcing. Bann Previously issued Excessive Rainfall Discussion... ...Central Plains & Midwest... There is increasing concern over a multi-day excessive rainfall across portions of the Central Plains and Midwest. For Tuesday, ensemble means (ECMWF/GEFS/EC-AIFS ENS) all show PWs above the 99th climatological percentile taking shape from the central High Plains and stretching towards the MO River Valley. Rounds of storms are likely to unfold during the day Tuesday as a persistent southerly fetch of 850mb moisture is directed towards a stationary front that is forecast to be oriented WSW-ENE from KS to central IL. By Tuesday night the LLJ looks to strengthen, directing high theta-e air and an increasingly unstable air-mass towards the frontal boundary. Moisture for this setup not only comes from the Gulf, but as far east as the western Atlantic, as the strong ridge over the Southeast U.S. causes an extensive fetch of easterly flow across these bodies of water and directs the moisture into the Nation's Heartland. This meteorological setup will not only feature moisture from tropical origins, but a robust 250mb jet streak over the Great Lakes that will place its divergent right-entrance region over the Central Plains. Excellent upper-level ascent will allow for widespread showers and storms that, given the stagnant SWrly low- level flow into the frontal boundary, will provide an ideal setup for training and back-building thunderstorms. There is a Slight Risk that extend from central KS to as far east as northern IL. The greatest concern is for northeast KS and northern MO, where WPC QPF now shows 2-3" of rainfall, implying localized amounts over 4" are possible. This area is deemed to be a "higher-end" Slight Risk. There does remain some uncertainty in the rainfall axis, which is why no higher threat levels were issued this forecast cycle. ...Eastern AZ & Western NM... The same robust 250mb jet streak over the Great Lakes that will place its divergent right-entrance region over the Central Plains will actually stretch as far south and west as the Four Corners states. PWs remain above the 97.5 climatological percentile on both ECMWF and NAEFS SATs, and soils continue to grow more saturated in wake of the active day on Monday. WPC QPF has increased over eastern AZ, much of NM, and both southern and western CO. There is some uncertainty in how much instability will be available following what appears to be an active Monday for much of the region. But given the anomalous PWs, excellent synoptic scale forcing, and at least some minor levels of instability, a Slight Risk was introduced to highlight the potential for scattered instances of flash flooding in portions of eastern AZ and western NM on Tuesday. Mullinax Day 4 and Day 5 Valid 12Z Wed Sep 16 2026 - 12Z Fri Sep 18 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHWEST U.S, CENTRAL PLAINS, AND MISSOURI VALLEY INTO PARTS OF THE CORN BELT AND UPPER MIDWEST... 20z Update: Main adjustment was with the day 5 (Thu) Slight risk across the Plains/MS Valley. Latest model trends support an area of high pressure moving across the Northern Plains, which should help push the front southward over the central Plains and Upper MS Valley and temporarily usher in some drier air. This results in decreased QPF over the inherited Slight risk area. Still a good model signal for organized convection Thursday night as the low level jet ramps up over the Plains and shortwave energy ejects out of the Southwest. The most favored axis for this activity appears to be from far northeast CO into NE and southern SD, and possibly western IA. The inherited Slight risk was thus adjusted in this direction. Much of this Slight risk is now within the higher FFG sand hills, and thus considered removing it altogether. However, the setup is favorable for training convection and heavy rainfall rates, and areas adjacent to the sand hills should see some flash flood threat. Also, given we are still 5 days out, some changes in the rainfall axis are likely. Thus opted to maintain Slight risk messaging, with the understanding that the exact area impacted will likely continue to be nudged on future updates. Chenard ...Previous Discussion.... ...Day 4... 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. ML and ENS suites continue to maintain relative continuity in the placement of heavy rain somewhere between the Central Plains into the western Ohio Valley with the current mean QPF max situated over eastern NE through much of IA. 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. The previous SLGT was maintained from the previous issuance, but did expand a bit further north to encompass the outer fringes of the the percentile NBM QPF threshold of 1" to cover for any wavering of the boundary as it oscillates in the general vicinity of the Corn Belt to Missouri River Valley. Guidance continues to remain aggressive on the threat for widespread convection over the Southwestern U.S, namely for eastern AZ through much of NM as the region sits right on the western fringe of the broad upper ridge centered over the Southeast and Mississippi Valley. Model consensus is for local areas of 1-2" in the means situated over a broad expanse covering the above locations, even hinting at some extension further north into western CO. This setup is conducive for scattered flash flood concerns given the PWAT anomalies pushing upwards of +2 standard deviations (95-99th percentile) when assessing the latest NAEFS forecast for the period. Given the stable signal and elevated QPF outputs within the means, a SLGT risk was issued across southeastern AZ into the eastern Mogollon Rim, extending through much of NM with the highest threat centered over local urban zones, burn scars, and flashier arroyos and small streams in the outlined areas. ...Day 5... By the later part of the week, we will finally begin to see some shifts in the longwave pattern across the CONUS with some motion of the upper ridge shifting westward and losing a bit of its vigor as it migrates away from the Southeast U.S. Height fields off the ensemble means tell the story with some ebb and flow of the ridge itself and a touch of flattening on the northern periphery of the ridge axis which could play a role in the boundary positioning as we move into Thursday. Consensus in guidance, both ML and NWP ensembles remain persistent in another round of convective potential over the Midwest/Corn Belt with the highest probs for at least 1" centered right over IA back into the Missouri River area in NE. NBM QPF alignment in the heaviest precip notes the area along I-80 in the above corridor as the current point of emphasis with the 90th percentile output already depicting well over 2" of rainfall in the region. The overall synoptic pattern supports this prospect with the threat of heavy rainfall for potentially a 3rd or 4th day in a row over some areas during this time frame. Usually at range, unless there was something truly prolific being shown within the means, the forecast would maintain a lower end risk to account for uncertainty. Given the variables referenced above, and the fact this would be the 3rd or 4th round of heavy precip in the vicinity of the area in question, a SLGT risk was justified given the prospect of saturated grounds, even potentially ongoing flood concerns when you couple the hydrologic responses likely to occur from the episodic nature of this prolonged event. MRGL's maintain prevalence around the perimeter of the aforementioned SLGT to account for some uncertainty in the latitudinal element of where the heaviest rain could occur. This continues to be a genuinely impactful threat with an opportunity for higher risks if the previous periods impact is appreciable leading into the D5. Across the Southwest U.S, at least one more day of widespread convection is forecast for areas of eastern AZ into NM with the main axis situated over NM. This could very well be a repeat of the previous period which would be an opportunity for perhaps more elevated concerns as certain areas get impacted multiple days in a row. A SLGT risk was added over similar areas to D4 with the emphasis still over NM with the highest threat centered over local urban zones, burn scars, and flashier arroyos and small streams. 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 $$