971 FOUS30 KWBC 140740 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 340 AM EDT Mon Sep 14 2026 Day 1 Valid 12Z Mon Sep 14 2026 - 12Z Tue Sep 15 2026 ...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL OVER PORTIONS OF CENTRAL AND SOUTHWESTERN IOWA... ...Midwest... The beginning of the week will feature a significant shift in the upper pattern across the northern tier of the CONUS with a strong mid-level trough ejecting out of the Northern Rockies into the Northern Plains on Monday and beyond leading to a pronounced area of diffluent flow centered over the Northern High Plains to Midwest corridor. Further south, a strong upper level ridge continues to be a factor in the overall longwave pattern across the CONUS leading to a true stalemate in the progression southward of any cold air advection regime and associated frontal boundary. Expectation has been for a cold front to sweep southeast into the region of the I-80 corridor, perhaps a bit further south, but truly slow in its progression as the front encounters prevailing southerlies in place over the Mississippi Valley back into the High Plains thanks to the strong ridge centered over the Southeastern U.S. The progged synoptic progression, along with the mid and upper level evolution will aid in the initiation of a prolonged heavy rain threat with a bimodal distribution of convective impacts with the time frames of interest occurring in the morning and evening of D1, and eventually in the days following. A deep, moist environment centered over the nations mid-section will provide the necessary element for deeper, stronger convective cores capable of producing heavy rainfall prospects whenever convection can materialize. According to the latest ECENS and NAEFS anomaly fields, PWATs will be on the order of +2 to +4 standard deviations leading to true values of 1.7-2.1" with a sharp gradient on the northern edge of a quasi-stationary front that will bisect the region by later this evening. Multiple mid-level shortwave disturbances will motion out of the Central High Plains, the first of which occurring in the morning hours today, along with an approaching warm front, will lead to an initial wave of convection migrating southwest to northeast, but remaining relatively progressive in its progression. Heavier rainfall will occur in the afternoon time frame further northeast into the Upper Midwest with this particular disturbance, but the forward propagation speed will likely deter a more appreciable rainfall footprint leading to perhaps some isolated flash flood concerns in mainly urbanized zones centered from the Missouri River Valley up through the southern-third of MN. By the evening hours, a shortwave will enter the region from the southwest with the primary shortwave trough migrating through the Dakotas leading to increasing ascent located over the Central Plains over into the Corn Belt. At the surface, the aforementioned cold front will slow its progression to the southeast and fan out with an alignment from southwest to northeast, matching the mean flow between 850-300mb. A nocturnal LLJ enhancement will occur over the Mid-Mississippi Valley, extending into the vicinity of the front leading to enhanced low-level convergence and modest low- level frontogenesis across the Corn Belt back into the Missouri River Valley. Expectation is for a rapid development of convection across the the corridor of KS/NE, expanding to the east with a centroid of the heaviest rainfall and convective coverage over southwestern to central IA. This has been a signature that has grown with time as the event has approached with the latest HREF able to capture the entirety of the D1 within its temporal range. The results of this evenings CAMs output depicted a more prolific heavy rain threat for the above area with the HREF blended mean QPF running between 3-5" over the area encompassing Des Moines and points south and southwest. Associated neighborhood and EAS probabilities are running generally hot for the setup with the former producing a resounding 60-80% probability for >3" across the aforementioned area, even depicting a 25-30% prob for >5" in the same location. The EAS probabilities for >2" are upwards of 60-70% in the area near Des Moines, a signature typically seen during more appreciable setups with strong agreement from the CAMs suite. FFG exceedance probabilities are also elevated with 3hr exceedance probs firmly between 50-70% noting the potential widespread impacts increasingly likely over that area of IA. There is still a bit of uncertainty for how far back west the heaviest rainfall potential extends with some CAMs noting significant 3+ inch totals back into southeastern NE as well, but the EAS probs are not as keen on this area, noting less overlap in the CAMs overall. At this juncture, considering a strong confidence in the CAMs given the EAS signal, as well as FFG's running between 1.5-2.5" for 3-hr exceedance, and in coordination with the Des Moines WFO, a small Moderate Risk was introduced for the D1 ERO to cover for the forecast threat. Locally significant rainfall between 3-6" is forecast over the area outlined with the main concerns being urban footprints and those low-water crossing locations located south of I-80 in IA. A broad SLGT still encompasses much of the Midwest from the Door Peninsula down through eastern NE and points in-between. Expectation is for at least scattered flash flooding to occur with this setup with an expectation for heavy rainfall to linger into the early morning hours Tuesday. ...Southwest... The Southwestern U.S. will remain locked into prolonged southerly flow aloft as the region sits on the western fringes of a broad ridge axis further east. This will maintain a prominent moisture advection pattern over the area with PWAT anomalies firmly in the +2 to +3 standard deviations range, coincident with PWATs encroaching on the 95-99th percentile over eastern AZ up through much of NM into southern CO. Coupled with a suitable diurnal heat flux leading to SBCAPE signatures between 1000-2500 J/kg over the area, scattered to widespread showers and thunderstorms will materialize over the above area leading to scattered flash flood concerns for the afternoon and evening hours. A strong 60-90% neighborhood probability for >1" exists over a large portion of NM into eastern AZ and southwestern CO, coinciding with an array of complex terrain features and remnant burn scars noted for flash flooding in any stronger convective environments. The signal for >2" maintains a modest signature as well, with the greatest signal residing in southeastern AZ within the forecast thermodynamic max (SBCAPE >2000 J/kg). The environmental factors and widespread nature of the convection are suitable for a broad SLGT risk over the region with the primary target currently aimed west of the I-25 corridor. Kleebauer Day 2 Valid 12Z Tue Sep 15 2026 - 12Z Wed Sep 16 2026 ...THERE ARE SLIGHT RISKS FOR EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL PLAINS, MISSOURI VALLEY, AND SOUTHWEST U.S... ...Central U.S... The D2 period will continue to exhibit the threat for heavy rainfall across portions of the Central U.S. with the area located from northeastern KS up through southeastern NE and northwestern MO as the primary area of impact. Cold front will slowly sag southward with the aid of a high pressure building in from the Canadian Prairies, edging southward with the push of cooler, more stable continental air leading to the threat of heavy rain over much of IA to cut off by Tuesday afternoon. The setup is a bit similar to the previous period with increasing convergence in vicinity of the front with the main time frame of interest centered over Tuesday evening with the additional LLJ element. Forecast soundings from various NWP indicate a prevailing wind field aloft that would align parallel to the boundary leading to another round of training convection from KS to points east-northeast. PWAT anomalies will remain around +2 to +3 deviations with true values hovering around that 2" marker. There's a general consensus on a swath of 2-3" of rainfall over that area of northeastern KS and points eastward with much of the global deterministic solidly in the 2-4" range which agrees with the latest ML outputs. This agreement among models is a solid signal for at least scattered flash flood prospects as the environmental conditions favor areas of 1-2"/hr rates as convection motions over the area. A high-end SLGT is forecast for this region of the CONUS with a chance at a higher risk pending future forecasts. We are just outside the full temporal range of the CAMs for entire D2 period, so its plausible to see a higher risk added if the CAMs indicate a higher risk for 3+ inches of rainfall. Until then, maintained continuity from the previous forecast. ...Southwest U.S... The Southwest CONUS will remain active with a multitude of small shortwave perturbations rounding the western periphery of the broad ridge positioned over the Southern U.S, entering into the region with PWAT anomalies firmly +2 or greater across eastern AZ into NM and southern CO. Models continue to pinpoint areas of northern and western NM as the beneficiaries of the setup with scattered 1+ inch totals depicted across much of the global deterministic. This will be the 4th day in a row with this pattern persistence leading to some areas becoming more and more vulnerable for flash flood prospects. Continue to see the 18-06z period as the time range of interest which is coincident with the diurnal instability maximum. A broad SLGT remains positioned over the aforementioned areas with an emphasis on eastern AZ and the western half of NM. Kleebauer Day 3 Valid 12Z Wed Sep 16 2026 - 12Z Thu Sep 17 2026 ...THERE ARE SLIGHT RISKS FOR EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL PLAINS, MIDWEST, & SOUTHWESTERN U.S... ...Central Plains & Midwest... Our multi-day setup across the Central U.S. continues through mid- week with the pattern shifting back north into the Corn Belt and adjacent areas of the Midwest with the synoptic evolution entailing another approach of our quasi-stationary front, although this time from the south as the high pressure to the north slides eastward and brings back return flow to the Plains. Aloft, a strengthening 250mb jet streak will situate itself over the Upper Midwest and Great Lakes providing excellent RER jet dynamics under the influence of a 120kt speed max centered over the northern tier of the CONUS. The next wave of convection will likely generate with the addition of yet another prominent LLJ development, this one likely to be with more vigor as the jet has influence from the return flow regime on the western side of the surface ridge to the northeast. Deep moisture plume will advect poleward leading the convergent pattern to materialize, once again, for the area between northeast KS, eastern NE, through much of IA for Wednesday evening. This area is increasingly likely to see a broad axis of 2-3" as an areal average with embedded maxima likely to breach 4" when looking at the different global deterministic outputs and 90th percentile ensemble means. This setup is favorable for overlap with some areas that will be impacted on D1 in the region of IA which could very well lead to more widespread flash flood concerns for the period. As of now, elected to maintain the broad SLGT risk over the Midwest back into portions of the Central Plains, however a high-end SLGT is now forecast for much of IA, especially areas along and south of I-80 between Council Bluff to Davenport. There is a potential for an upgrade in future updates, so stay tuned for the latest forecast over the next 48hrs. ...Southwest U.S... Another period of scattered to widespread convective activity is forecast for areas of the Southwestern U.S. with the general axis aligned over eastern AZ into NM and southern CO. Wednesday is leaning towards the peak of impacts, however, as a more robust shortwave ejection out of Mexico will likely provide further mid and upper level ascent that can exacerbate an already formidable environment. PWAT anomalies will remain in the +2 to +3 standard deviation range which is already an asset to more appreciable rainfall potential when you couple with a primed thermodynamic environment. Ensemble means are aggressive for Wednesday afternoon into early Thursday morning with some models printing out 2-3" in spots across western and northern NM, areas where flash flood concerns will be elevated from the repeated nature of this pattern, as well as the location of the more sensitive burn scar locales within the state. This threat has led to an introduction of higher end SLGT risk wording with a chance for even an upgrade further if the models show more appreciable rainfall forecasts in the coming days. Unlike previous periods, there is more of a threat further west with this evolution leading to an extension of a MRGL back into southwestern and southern UT to cover for the threat of flash flooding within the slot canyon regions of the west. SLGT risk expands west through the Mogollon Rim, as well, thanks to the surge in PWATs and added forcing. Kleebauer Day 4 and Day 5 Valid 12Z Thu Sep 17 2026 - 12Z Sat Sep 19 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHWEST U.S, CENTRAL PLAINS, MISSOURI VALLEY, AND INTO THE UPPER MIDWEST... The Days 4-5 period (Thu. Sept 17 - Fri. Sept 18) can be summarized by an expansive and highly anomalous upper-level ridge of high pressure taking hold over much of the Southern U.S., while an upper-level trough in the West makes its way from the West Coast towards the Northern Rockies. There is also a deep upper-trough tracking through eastern Canada, which in contrast with the upper- ridge to the south will create tight thermal and moisture gradient from the Great Lakes to the Northeast. The 850mb ridge will remain positioned over the Mid-South and is ideally located to produce an extended fetch of southerly flow over the Southern Plains. This results in unusually strong IVTs over the Great Plains both Days 4 and 5. The ECMWF SATs show >90th climatological percentile IVTs aimed at the Great Lakes and Northeast on Day 4, which combined with increasing instability and a nearby frontal boundary present, could lead to scattered thunderstorms capable of producing localized flash flooding. By Day 5, the Midwest becomes the focus for flash flood potential as a pair of jet streaks; one over the Great Lakes and another emerging from the Rockies, place their divergent jet streak quadrants overhead. Combined with PWs >1.75", which is above the 99th climatological percentile, this serves as an ideal setup for additional rounds of excessive rainfall. The Midwest will have also dealt with heavy rainfall the first half of the week, making overly saturated soils a key player in potential flash flooding late week. A pair of Slight Risks are in place over parts of the North Central U.S. for both Thursday and Friday to cover this prolonged window of excessive rainfall. Lastly, the Southwest remains mired in a steadfast monsoonal flow regime that is pumping copious amounts of moisture into the region. Day after day, scattered thunderstorms will produce locally excessive rainfall rates in excess of 1"/hr, keeping the threat for flash flooding in place through the end of the week. A Slight Risk remains in place for Day 4 (Thursday) while a large Marginal encompasses an area that extends from southern NM and western TX, to northern AZ and the Central Rockies. Mullinax 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 $$