000 FOUS30 KWBC 150054 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 854 PM EDT Mon Sep 14 2026 Day 1 Valid 01Z Tue Sep 15 2026 - 12Z Tue Sep 15 2026 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF IOWA... 01Z Update... ...Midwest... In the Midwest, the biggest adjustment from the previous issuance was the removal of the northern extent of the Marginal Risk area from the upper Mississippi Valley and northern Great Lakes area, where either the rainfall threat has ended, storms are progressive, or where limited instability is likely to keep rainfall amounts and the potential for flash flooding in check. It still appears the greatest threat for heavy rainfall and flash flooding through the remainder of the evening into the overnight will center from southwestern to central Iowa. Heavy showers and thunderstorms will continue to expand, with cell-training supporting the potential for heavy amounts and flash flooding. A Moderate Risk remains in place -- largely centered where recent runs of the HRRR and the 18Z HREF suggest the greatest threat. HREF probs indicate localized amounts exceeding 3 inches are possible overnight within this area. Refer to WPC MPD #1284 for additional information regarding the near-term heavy rain and flash flooding threat in this area. ...Southwest... In the Southwest, removed much of central to northern Arizona and Utah from the outlook. However, a broad Slight Risk stretching from southeastern Arizona across much of western New Mexico into southwestern Colorado remains. Widespread convection continues to fire across this area, producing areas of flash flooding. Storms are expected to continue for at least a few more hours, posing additional concerns. Refer to WPC MPD #1283 for further information. Pereira 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 heavy rain focus over the central states will align with the slow moving cold front over Kansas, northern Missouri, and western Illinois where a Slight Risk persists. The Slight was trimmed from the north, but kept for far southeast Nebraska and southern Iowa given wet antecedent conditions despite less QPF there. Deep layer SWly flow will remain parallel to the cold front, promoting training convection. 12Z guidance favors this repeating rain threat area to continue farther southwest to the Texas Panhandle where the Slight Risk was expanded into. PW anomalies of 2 to 3 deviations above normal can be expected with a swath of actual values around 2". QPF in the 2-4" range are in both the 12Z REFS and HREF with the HREF favoring northeast KS while the REFS is broader across northern MO. This area can be considered to have a more enhanced risk than a general Slight. Scattered flash flooding is considered likely, especially for overlap from the several inches of rain that fell north of the KC metro this morning. The overall risk area continues east along the front across northern Indiana to Cleveland where the Marginal was expanded. ...Southwest U.S... The Southwest Monsoon continues in earnest at least through midweek as an upper high persists over the Mid-South and low pressure tracks down the West Coast to Northern California. Moisture anomalies of 2 to 2.5 standard deviations are expected Tuesday for southern/central Arizona, much of New Mexico, and over southern Utah through central Colorado. This will be the 4th day in a row with this pattern leading to increasingly vulnerable areas/wet antecedent surface. Expanded the Slight areas to include all of the Slot Canyon Region of southern Utah east to the San Juans in Colorado as well as more of the Mogollon Rim area in central Arizona. PW anomalies are less over northern Arizona for now, but southerly flow is across the region, so further expansion may be eventually warranted. Jackson Day 3 Valid 12Z Wed Sep 16 2026 - 12Z Thu Sep 17 2026 ...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MIDWEST, CENTRAL PLAINS & SOUTHWESTERN U.S... ...Central Plains & Midwest... High pressure in the wake of the low tracking across the Canadian Prairies today shifts over the northern Great Lakes Wednesday. Return flow west of the high brings the frontal zone and moisture back north into vulnerable areas of southeast Nebraska, northeast Kansas, northern Missouri, southern into central Iowa and northern Illinois. Right entrance region jet dynamics are also in play from the Upper Midwest. Expect more convective development near the front on the strong LLJ. The higher-res 12Z RRFS/Canadian Regional are farther south, but the 12Z ECMWF/EC-AIFS are a tick farther north of Iowa. Some trim to the north side of the Slight Risk was made with areas of central/southern Iowa still considered more of an enhanced Slight. Swaths of 2-5" are given in these dynamic models. Depending on what happens tonight and where guidance favors the maximum going forward, further upgrades are possible. ...Southwest U.S... Vigorous Southwest Monsoon activity on tap for Wednesday as the West Coast low reaches far northern California with downstream troughing pumping stronger southwest flow over the Desert Southwest. Moisture looks to peak for the week from southern AZ through much of NM and southern CO with PW anomalies of 3 standard deviations above normal. 12Z RRFS and Canadian Regional (two of the few higher res models for Day 3) are aggressive with 2-4" over much of southeast AZ up the Mogollon Rim and western NM and southern CO. The Slight Risk is expanded farther up the Mogollon Rim and down over the Sacramento Mountains, with a larger area of more enhanced Slight Risk areas over much of western NM. Jackson 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... 20Z Update... ...Southwest... Overall, no large-scale changes were made to the previous outlook. For Day 4, confidence is increasing that parts of the Southwest -- notably southwestern to north-central New Mexico -- may be categorized as a "high-end Slight." Following what is expected to be a very active Wednesday, areal averages are expected to drop off some on Thursday, but the coverage of impactful amounts still looks to be significant. PW anomalies on Thursday are forecast to remain 2-3+ standard deviations above normal across much of New Mexico and eastern Arizona on Thursday. Meanwhile, mid-to-upper level energy embedded within southerly flow between an approaching low/trough and a downstream ridge will help produce areas of enhanced ascent. One limiting factor may cloud cover from the previous day's activity and reduced instability. However, even with reduced rainfall rates, soils are likely to be remain vulnerable to excessive runoff given the previous days' rainfall. ...Upper Midwest... Made mostly minor adjustments here as well. While the 12Z guidance continued to show a good signal for heavy amounts extending east across parts of the northern Plains toward the upper Mississippi Valley on Day 4, and then from the upper Mississippi Valley toward the Great Lakes on Day 5, they also showed notable latitudinal spread regarding its axis. The Slight Risk areas remains near the middle of the envelope of solutions. Pereira Previous Discussion... 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 $$