FOUS30 KWBC 201958 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 358 PM EDT Sun Sep 20 2026 Day 1 Valid 16Z Sun Sep 20 2026 - 12Z Mon Sep 21 2026 ...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MIDWEST/OHIO VALLEY AS WELL AS THE NORTHERN HIGH PLAINS... ...16Z Update... Northern portions of the inherited Slight Risk over the Midwest (northern Illinois, southern Iowa, northern Missouri) were removed based on the updated 12Z guidance. Expect that the front will remain further southward keeping forcing and instability more focused across central Illinois into east-central Missouri. An approaching upper-wave will help to trigger a renewed round of convection later this evening and into the overnight hours. There is the potential for some training/backbuilding with flow parallel to the boundary which could lead to some locally heavy totals of 2-4". Confidence in heavy rainfall remains higher further to the east into the Upper Ohio Valley where pooling moisture and higher instability will help lead to more robust rainfall rates. Storms yesterday that continue into this morning have also left wetter antecedent conditions with 24-hour areal average totals in the 1-2" range, with localized totals of 3-4". Both the latest 12Z HREF and RRFS show moderate probabilities for additional totals of 3"+. Otherwise, the prior forecast remains on track. Expect another round of storms to develop and meander around the Black Hills vicinity in the northern High Plains as an upper-low over the Rockies progresses eastward over the region. Anomalously moist upslope flow remains in place to support locally heavy rainfall. Similar to further east conditions are also more sensitive given rainfall on Saturday. Putnam ...Previous Discussion... ...Midwest and Ohio Valley... Overnight convection leading into the start of the Day 1 period will keep the threat of excessive rainfall lingering from southeastern Iowa and northern Missouri eastward into portions of the Ohio Valley.. The threat will slowly wane as the back edge of the rainfall makes its way from west to east this morning into the early afternoon. The risk is due to some locally intense rainfall rates overlapping with compromised Flash Flood Guidance values. Farther to the east...a separate area of overnight convection had been moving eastward in proximity to the Ohio River main-stem towards the higher terrain in West Virginia which also was likely lowering Flash Flood Guidance leading into the start of the Day 1 period. The HRRR tapers off the convection here during the late morning and then redevelops the threat of moderate to heavy rainfall again. Given the precipitable water plume in the area...there is higher confidence in both the placement and potential for heavier rainfall/flash flooding due to antecedent conditions. A bit higher/anomalous moisture pooling in the valley will also bring a greater chance of heftier totals in the 2-5" range. ...Northern High Plains... Maintained a Slight Risk area mainly focused over southwest South Dakota as a slow moving upper-low over the Rockies will begin to progress eastward as an open wave later today. Moisture remains anomalously high and low level flow remains upslope flow. That will continue to promote efficient rainfall with rain rates of 1-1.5"+ per hour. The region will also be more prone to flooding and runoff problems where there is overlap with rain that fell on Saturday. Rockies to the Central Plains... There continues to be a Marginal Risk of excessive rainfall from the Southern High Plains northward to the Western High Plains adjacent to the Northern Rockies as deep moisture remains in place with weak shortwave energy embedded helping to fuel scattered convection. ...Elsewhere... A large Marginal Risk area was maintained from the northern Rockies through much of the Plains and into western Texas. Scattered thunderstorms embedded within a large plume of anomalously high moisture will pose the threat for localized heavy rainfall and high instantaneous rain rates. However, confidence is low on placement, so a Marginal Risk seems appropriate. Bann Day 2 Valid 12Z Mon Sep 21 2026 - 12Z Tue Sep 22 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL APPALACHIANS... ...Central Appalachians and the southern Mid-Atlantic... A Slight Risk has been introduced for portions of the central Appalachians covering eastern West Virginia and adjacent southwestern Virginia. Storms overnight Sunday may continue into part of the day 2 period Monday morning. In addition, moisture and instability will remain in place through the day as the cold front hangs further north along the mountains. Forcing from the front as it pushes southward to the east along the mountains and also approaches from the northwest will lead to additional storms during the day and into the overnight hours. The now available CAMs support rainfall totals of 2-5" with low to moderate HREF probabilities of this rainfall exceeding FFGs. Antecedent conditions remain wet with 24-hour areal average rainfall of 1-2" (locally as much as 3-4") over central West Virginia according to MRMS estimates. Additional storms will be possible further east along the front into southern Virginia and northern North Carolina. However, FFGs are comparably much higher across this region. ...New Mexico and Nearby Southern High Plains... There is a low-end risk of excessive rainfall developing across New Mexico and western Texas as low-level flow of Gulf moisture increases the amount of moisture available for late day convection. Some locally heavy totals in the 1-3" range will be possible. ...South Florida... A Marginal Risk has been maintained for portions of South Florida as a plume of anomalous moisture will help to fuel heavy downpours and potentially lead to some isolated flooding/ponding in urban areas. Putnam/Bann Day 3 Valid 12Z Tue Sep 22 2026 - 12Z Wed Sep 23 2026 ..THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF NEW MEXICO AND WEST TEXAS... ...New Mexico and West Texas... Models maintained a favorable setup for heavy rain and scattered to numerous instances of flash flooding setting up across much of New Mexico Tuesday and continuing into Wednesday. Broad troughing over the western US will place New Mexico within the right entrance region of a strong upper jet..with one stronger wave moving to the north of Tuesday. At the low levels...easterly upslope flow will feed deep moisture and instability into the region. This combination of forcing, instability and well over 90th percentile PWs for September, suggest heavy rainfall and a growing flash flood risk. The latest guidance shows the potential for locally heavy totals as much as 2-4 inches particularly from the Rio Grande Valley east through the Sacramento Mountains and into the adjacent Plains. This would be more than enough to lead to at least scattered instances of flash flooding. The potential for a Moderate Risk remains especially if the CAM guidance agrees on similar if not higher totals. For now, there is more confidence in more widespread and significant impacts into Wednesday following the rainfall on Tuesday. ...Ohio Valley and Mid-Atlantic... Mid level troughing with multiple embedded shortwaves will slowly track eastward into the OH Valley and Mid-Atlantic where plenty of moisture should already be in place as a backdoor cold front approaches. Quite a bit of uncertainty with the details, as a coastal low should eventually develop offshore either in the Day 3 period or just beyond in the Day 4 period. This would help advect drier air in from the north over portions of the Mid- Atlantic. Also the amount of instability, with the backdoor front likely making a push all the way into the Carolinas (leaving a more stable airmass to its north and what that implies for rainfall rates) works against introduction of a Slight Risk area. Although QPF has trended up quite a bit particularly along a frontal wave/inverted trough in vicinity of Chesapeake Bay, expect that the lack of instability and cooler air in place will lead to a mostly steady, long-duration rainfall. The Marginal risk area was maintained given the possibility that a bit of CAPE could drive some low topped convection and locally higher rates. A targeted Slight Risk over the central Appalachians was removed. Despite more sensitive antecedent conditions, expect the environment to be much cooler and more stable as the front finally pushes further southward after hanging up along the Appalachians, which should keep rates down and any additional flooding concerns isolated. ...South Florida... A Marginal Risk has been maintained for portions of South Florida. Locally heavy downpours will remain possible with anomolously high moisture in place, which could lead to some isolated flooding/ponding in urban areas. Putnam/Bann Day 4 and Day 5 Valid 12Z Wed Sep 23 2026 - 12Z Fri Sep 25 2026 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NEW MEXICO ON DAY 4... 20Z Update... As anticipated in the previous forecast cycle, confidence has continued to increase in the likelihood of successive days of heavy rainfall and significant flash flooding impacting portions of the southern Rockies and High Plains early to midweek. Model signals indicate that Wednesday may especially problematic as well- defined energy lifting from the south interacts with a highly anomalous airmass. Amplifying runoff concerns may be heavy rains impacting portions of the region on Tuesday. Therefore, a Moderate Risk was introduced for portions of east-central to northeastern New Mexico where PW anomalies are greatest by late Wednesday and where the NBM is indicating greater than 50 percent probabilities for 24-hr rainfall totals of 2 inches or greater. It should be noted that there is fair amount of spread in the deterministic guidance, suggesting future adjustments to the Moderate Risk may be required. Also worth noting is that although the Moderate Risk is currently centered over portions of eastern New Mexico, much of southern, central, and the remainder of eastern New Mexico can be categorized as a 'high-end Slight.' Elsewhere, made some smaller-scale adjustments to the previous outlook. Pereira Previous Discussion... ...Southern Rockies/High Plains... Day 4/Wednesday will be the second day of a significantly wet period across much of New Mexico, with reasonably high confidence. PWs are forecast to reach 1.5+ inches, around 2.5-3 standard deviations above normal, with a likely PW connection to an eastern Pacific tropical system. This is coincident with deep southerly flow, as the 250mb jet is basically south to north in response to an eastern Pacific mid/upper low just southwest of California, and favorable right entrance region jet dynamics will be in place. Rain totals for the Tuesday-Wednesday period are explicitly forecast to be 3-4" broadly, exceeding some lower end ARIs, and easily could be locally higher. The risk is likely higher-end in areas of overlap between Tuesday and Wednesday. Unfortunately this may be over the Sacramento Mountains, which tend to be sensitive given burn scars. Still anticipate that Moderate Risks may be needed in future forecast cycles, with locally significant flash flooding possible. By Day 5/Thursday, the pattern remains favorable for rainy conditions, but the bulk of reliable model guidance shows the rain shifting east a bit into the TX/OK Panhandles and vicinity, with the EC-AIFS and AIGFS leading the way with the eastern shift. This is dependent on the evolution of shortwave energy from the eastern Pacific upper low and east of it. The heavier QPF focusing eastward would put the rain atop drier antecedent conditions for less sensitivity for flooding, but any QPF farther west could cause flooding problems with the lingering wet soils. Will start with a Marginal Risk to cover both the lower QPF over wetter soils and the higher QPF over drier soils farther east, but this will need to be reassessed in future forecast cycles. ...Ohio Valley/Appalachians to Mid-Atlantic... An upper low will gradually drift across the Ohio Valley Day 4/Wednesday. At the surface, surface high pressure centered over Quebec will send a cold air damming ridge of high pressure through the Appalachians and Coastal Plain. This makes the forecast for heavy rainfall tricky, as instability will be pushed south of the CAD front. There may be a tongue of instability reaching west of the Appalachians into eastern portions of the Ohio Valley, and then some potentially overrunning rain across the Mid-Atlantic. Meanwhile there is also uncertainty in how far south that front gets in the Mid-Atlantic, and QPF axes thus vary from Pennsylvania to North Carolina. Overall, a Marginal Risk was maintained, with some broadening to include more areas given the frontal uncertainty. By Day 5/Thursday, a coastal low will likely be developing in the Atlantic. Depending on the low position, some guidance shows heavy rainfall reaching onshore in the Mid-Atlantic, especially eastern North Carolina. Considered a Marginal for this, but held off because the heaviest rain could remain offshore, and because that region is not very sensitive to heavy rain given the sandy soils if there are moderate amounts onshore. But this should continue to be monitored. ...South Florida... Above average PWs over 2" will persist across the southern Florida Peninsula and the Keys into Day 4/Wednesday after potentially several wet days in the short range period. Maintained the Marginal Risk for heavy downpours that could cause localized ponding or flash flooding. By Day 5/Thursday the higher PWs look to gradually push south, so went without a Marginal by then. Tate 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 $$