FOUS30 KWBC 200832 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 432 AM EDT Sun Sep 20 2026 Day 1 Valid 12Z 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... ...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 MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MID ATLANTIC REGION AND OVER NEW MEXICO... ...Southern Mid-Atlantic States... Numerical guidance still has a signal for cold air damming to be in place from New England southward into the northern Mid- Atlantic region on Monday with high pressure sprawled across portions of the Great Lakes into eastern Canada. There could be some elevated above the stable surface/low levels to support precipitation but the threat for heavy to excessive rainfall amounts/rates still looks to be muted. Therefore no significant changes were needed to the inherited outlook. Forcing and moisture convergence along the front along the central Appalachians will keep heavier rain chances and isolated flash flooding a concern there in vicinity of the boundary between airmasses. ...New Mexico and Nearby Southern High Plains... There is a low-end risk of excessive rainfall developing across New Mexico once again as low-level flow of Gulf moisture increases the amount of moisture available for late day convection. Precipitable water values should generally be around 1.75 inches from Texas northward into the Mid- Mississippi Valley with enough effective bulk shear and instability to result in organized convection capable of producing heavy rainfall with the associated risk of flash flooding. 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 THE APPALACHIANS AND NEW MEXICO... ...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. The Marginal risk area was maintained given the possibility that a bit of CAPE could drive some low topped convection and locally higher rates. The Slight risk over Pennsylvania and West Virginia was maintained in deference to an inverted trough which should help focus convection there. Even though fairly weak, as much as 500 to 1000 j/kg of CAPE could develop. In addition, this area has lower FFG than the coastal plain given the rainfall expected in the Day 1 and Day 2 period.,,potentially lowering Flash Flood Guidance further. ...New Mexico... Models maintained a favorable setup for heavy rain and scattered to numerous instances of flash flooding ears to be setting up for New Mexico on Tuesday that appears likely to continue into Wednesday. Of particular concern is that conditions will likely get more sensitive with time given possibility of multiple rounds of convection occurring for over multiple days in a row. There was not an upgrade to a Moderate risk but it remains a possibility. 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 level...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. Bann Day 4 and Day 5 Valid 12Z Wed Sep 23 2026 - 12Z Fri Sep 25 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE SOUTHERN ROCKIES/HIGH PLAINS... ...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 $$