FOUS30 KWBC 102250 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 650 PM EDT Thu Sep 10 2026 Day 1 Valid 2225Z Fri Sep 11 2026 - 12Z Fri Sep 11 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE SOUTHERN OZARKS AND ARKANSAS RIVER VALLEY NORTHEAST INTO THE OHIO VALLEY... 2200Z Update: Have expanded (elongated) the Slight Risk area, originally across the Lower Ohio Valley, southwestward across the MO Bootheel and into the AR River Valley and southern Ozarks. This area is along and just south of the frontal boundary, along the axis of maximum TPW (1.8-2.0"), and where mixed-layer CAPEs and 0-6km bulk shear values are most supportive for more organized, multi-cellular convection. Moreover, per the latest HRRR/RAP trends, the alignment of the 1000-850 mb wind vectors with the 850-300 mb mean flow -- and subsequent diminishment in Corfidi Vectors -- will increase the prospects of cell training within this relatively narrow corridor. Based on the latest HREF and REFS QPF and FFG exceedance probabilities, it's more like a 'low-end' Slight Risk for most areas, i.e. closer to a 15% neighborhood probability than >25%. With more rain moving in, we have also "bumped" the Slight Risk area across southern OH and western WV to account for the wetter antecedent soils from the heavy rainfall last night and early this morning. This is an area where the 18Z HREF in particular shows the highest probabilities of 3-hourly rainfall exceeding the current (lower) FFGs. Hurley Previous Discussion.. ...Ohio Valley & Central Appalachians... A 300mb jet streak placing its diffluent right-entrance region over the Ohio Valley and Central Appalachians paired with an approaching cold front will foster a favorable environment for developing thunderstorms. PWs within the Slight Risk will contain anomalous PWs of at least 2.0", which are approaching the 99th climatological percentile. In addition, MUCAPE >1,000 J/kg will also be available within a highly saturated environment to support between 2-3"/hr rainfall rates. Area-averaged soundings show mean 850-300mb winds are generally out of the WSW, and would parallel the frontal boundary as it moves east, indicating there could be potential for training. Soils have been drier of late, evident by NASA SPoRT-LIS 0-10 and 0-40cm soil moisture percentiles. Still, 1-hr FFGs are generally around 2" within the region and given the potential for training storms, scattered instances of flash flooding are possible. ...Mid-Atlantic... Some of these storms in the Central Appalachians and Ohio Valley are likely to spill over the central Appalachians and track across northern VA, northern MD, and the Mid-Atlantic I-95 corridor Thursday evening. Storms should be progressive, but the same anomalous moisture content will move into the region and some training could occur. There is also westerly low-level wind field, which could limit areas flash flood potential given downsloping effects. Localized flash flooding cannot be ruled out, given the potential for training storms and storms tracking into a higher concentration of urban/hydrophobic surfaces. ...South-Central Plains & Mid-Mississippi Valley... A passing cold front and a pool of anomalous PWs for mid-September (1.8-2.0" PWs) will be available for thunderstorms as the develop during the afternoon and linger into Thursday night. Plenty of instability will be available (1,000-2,000 MUCAPE per 00Z HREF) to support thunderstorms producing 2-3"/hr rainfall rates. FFGs are higher in these areas, and given storms will also be generally progressive, the flash flood threat will remain isolated. A Marginal Risk for flash flooding remains in place in these regions. ...Southwest... A Marginal Risk remains in place over portions of AZ/NM and as far west as the Peninsular Range of southern CA. For southeast AZ and much of southern NM (along and west of the Sacramentos), the 500mb ridge axis remains located over central AZ will contain spokes of 500mb vorticity maxima that revolve around its southeast flank, providing some weak upper-level support aloft. PWs remain elevated for mid-September with >90th climatological percentile values along the AZ/NM borders with Sonora and Chihuahua. Farther west, PWs are above the 97.5 climatological percentile in southern CA with strong daytime heating causing diurnally-driven thunderstorms along the higher terrain of the Peninsular Range. Low-chance probabilities (10-30%) remain present on HREF/REFS guidance there, so maintained the Marginal Risk for Thursday where areas of complex terrain have localized flash flood potential. Mullinax Day 2 Valid 12Z Fri Sep 11 2026 - 12Z Sat Sep 12 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN FAR SOUTHERN ARIZONA... 21Z update... The latest deterministic and CAMs guidance show an uptick in QPF further east than previous cycles for eastern portions of Virginia and North Carolina. Given the expected location of the frontal boundary, instability and more than adequate moisture pool convection capable of producing heavier rain will like materialize. The Marginal Risk area was expanded to cover more of southeast Virginia and Northeast North Carolina. Elsewhere across the country remains in good shape and the inherit risk areas remain unchanged. Campbell Previous discussion... ...Southern Arizona... The ridge over southern AZ will be remain in place, although small lobes of 500mb vorticity will move east to west along the US/Mexico border on Friday. Much of AZ, southern UT, and western CO will continue to contend with anomalous moisture content with PWs >1.5" likely throughout much of southern AZ that supports a localized flash flood threat. REFS/HREF probabilistic guidance does show a section of far southern Arizona particularly at-risk for flash flooding. The HREF most notably shows moderate chances (40-50%) for 3-hr QPF surpassing 3-hr FFGs around Santa Cruz county Friday afternoon. These signals in guidance support an upgrade to a Slight Risk in far southern AZ. ...Lower MS Valley, TN Valley into the Southern Appalachians... A 500mb shortwave trough tracking towards the Mid-MS Valley and a sharp 200mb trough axis aloft is set to generate healthy divergence aloft over the Mid-South, while copious amounts of moisture continue to reside over much of the Southeast. A nearby frontal boundary will trigger widespread thunderstorms that are likely to contain >2"/hr rainfall rates within the more intense storms. PWs >2.0" will be common from the central Gulf Coast to the mid-MS Valley, while PWs in the OH/TN Valleys range between 1.75-2.0", which are still above the 90th climatological percentile. Storms are likely to form as far east as the Southern Appalachians and Carolinas, although the flash flood threat is less likely along the Southeast coast. Localized flash flooding is possible, especially in poor drainage areas and within more urbanized communities. ...Upper Midwest... A 500mb shortwave trough and an accompanying frontal boundary will track through the Upper Midwest at the same time a plume of >90th climatological percentile PWs manifest over the region. Storm motions should be progressive, although some training thunderstorm activity closer to the warm front is possible as low pressure heads for the MN Arrowhead. Rainfall totals could range between 1-2" (locally higher) in northern MN, while rainfall rates could approach 1.5"/hr as storms track across the region Friday afternoon and evening. Mullinax Day 3 Valid 12Z Sat Sep 12 2026 - 12Z Sun Sep 13 2026 ...THERE IS A SLIGHT RISK IN PORTIONS OF THE CENTRAL PLAINS... 21Z update... The areas across the country that have been identified as having an elevated threat for excessive rainfall and flash flooding remain in good order as supported by the locations of the highest QPF in the latest suite of guidance. There has been a small trend for a reduction in QPF amounts/coverage across portions of the central Gulf Coast as the trough lifts. There is still some lingering PWs that could help enhance convection over the area hence why the Marginal Risk remained in place. However, there may be the need to trim part of the Gulf Coast out of the Marginal with future updates should the reducing trend persist. Campbell Previous Discussion... ...Central Plains... An approaching 500mb disturbance and a strengthening southerly low-level jet will interact with a frontal boundary that results in scattered thunderstorms from the central High Plains to the Mid-MS Valley. PWs ranging between 1.5-1.75" are generally around the 90th climatological percentile and ample instability will be supportive of strong thunderstorms producing >2"/hr rainfall rates. Uncertainty exists in the placement of the heaviest rainfall, but the signal supports a scattered flash flood threat Saturday afternoon and into Saturday night. ...Southeast & Mid-Atlantic... The broad 500mb troughing setup in the Mid-South will induce healthy upper-level ascent over the Southeast and Mid-Atlantic on Saturday. A warm front is forecast to lift north through the Mid- Atlantic on Saturday that will act as a trigger for storms, while daytime heating and height falls support widespread showers and thunderstorms from the Gulf Coast to the northern Mid-Atlantic. PWs will rise to above 2.0" as far north as the DC/Baltimore area by Saturday evening with no shortage of instability as the warm sector drifts north. QPF has been increasing across most guidance, although the location of the heaviest rainfall is still unclear. This setup is ideal for a localized flash flood threat, particularly in the more urbanized sections of the Mid-South and Mid-Atlantic. ...Florida... Much of the Sunshine State will remain mired in an environment containing copious amounts of moisture that will continue to support scattered thunderstorms containing torrential downpours. PWs between 2.0-2.25" will be common and storms will be capable of causing localized flash flooding, especially in poor drainage areas and in the more urbanized sections of Florida. ...Rockies & Southwest... A strong 500mb closed-low with heights below the 10th climatological percentile will work with an increasingly anomalous PW field to produce scattered thunderstorms with efficient rainfall over the Northern Rockies. Farther south, the pool of anomalous PWs that supported daily rounds of excessive rainfall in the Southwest through the week will stick around and cause additional thunderstorms that produce locally excessive rainfall rates. Instability will be lacking over the northern Rockies but soils are more saturated following recent rainfall and may provide a slightly more favorable environment for localized flash flooding. Mullinax Day 4 and Day 5 Valid 12Z Sun Sep 13 2026 - 12Z Tue Sep 15 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR EASTERN PORTIONS OF THE MISSOURI VALLEY INTO PARTS OF THE CORN BELT AND UPPER MIDWEST... ...Eastern MO Valley...Corn Belt...Upper Midwest/Upper Great Lakes... 2000Z Update: Removed the outlook areas across MT and the Northern Intermountain West, based on the latest guidance consensus with the frontal positions and ensuing post-frontal stability. Elsewhere, have shifted the orientation of the Slight Risk area on Day 4 (12Z Sun-12Z Mon), i.e. now oriented more NW-SE along the deep-layer thickness and PW gradients. This also correlates with the 12Z GFS/ECMWF/UKMET QPF trends, as well as latest trends with the GEFS/ECENS QPF exceedance probabilities. On Day 5 (12Z Mon-12Z Tue), the Slight Risk area was narrowed a bit (especially on the east side, now no longer affecting Lower MI), as too was the Marginal Risk. This given the slower eastward progression of the of the upper trough and surface front, per the 12Z operational GFS/ECMWF/UKMET along with the AIGFS and EC AIFS. Hurley Previous discussion.. Slight risks of excessive rainfall are in effect for both Sunday and Monday for portions of the upper Midwest. On Sunday, the Slight covers from eastern South Dakota and Nebraska east through much of Wisconsin. On Monday, the Slight continues across much of Wisconsin, then continues over all of northern Michigan. Abundant moisture from the Gulf (and the Caribbean before that) will be in place along a well-established low level jet extending from eastern Texas straight north through eastern Nebraska. A polar high to the north will act as an effective barrier to that heat and moisture from the Gulf pushing any further north. Between the high to the north and the low level jet, an effective front will act as the source of lift for all of the moisture running into it. As the high moves south and east across the Great Lakes, the southerly low level jet will get to continue north and east behind the high. Meanwhile a pair of leeside lows developing over the northern High Plains will allow for further convective development Sunday night. Given the aforementioned pattern, expect periods of showers and thunderstorms across the Slight Risk area through the day on Sunday, but the much heavier rains will be overnight Sunday in the Slight Risk area as the nocturnal LLJ both naturally strengthens, and intensifies further due to the developing pressure gradient between the lows to the west and high to the east. On Monday, the lows to the west consolidate into one, which tracks northeast into Ontario. The trailing front will quickly stall out from the central Plains (KS) northeast through Wisconsin and Michigan. This will make for an extended and expansive corridor of training thunderstorms, especially over Wisconsin and into Michigan where the best forcing will be at the nose of the low level jet, which will have turned towards the northeast, resulting in unidirectional flow favorable for training and backbuilding storms. Wegman 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 $$