FOUS30 KWBC 100101 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 901 PM EDT Sun Aug 9 2026 Day 1 Valid 01Z Mon Aug 10 2026 - 12Z Mon Aug 10 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER MIDWEST... ...OH Valley to Great Lakes... A locally significant flash flash flood risk is still expected to evolve overnight across portions of IL, IN and possibly far southwest MI. An increasing 30-35kt southwesterly low level jet will result in a very strong corridor of moisture transport tonight. Convective development is likely at the nose of this axis where convergence is maximized. CAPE in excess of 3000 J/kg is expected to coincide with this convective area, with PWs increasing towards 2". Cloud layer mean flow is off the east, but wind profiles are conducive to backbuilding on the west-southwest flank of convection. Upwind corfidi vectors are off to the south at 10-15kts, so we should see an eventual progression of convection in this direction. Nonetheless, we should see enough backbuilding to result in some flash flooding tonight. A bit of uncertainty on the exact axis of heaviest rainfall, and the axis should be rather narrow. Thus will maintain a higher end Slight risk for this area, with an axis of 2-4" of rain and scattered flash flooding likely. While not a guarantee, there is a potential for a narrow axis of rainfall upwards of 5-8" if this training scenario ends up maximized, which could result in locally significant flash flood impacts. Thus trends overnight will need to be closely monitored. ...Southeast... A lingering isolated flash flood threat from northern AL into NC should wind down as the evening progresses. ...NE and KS... Convection moving out of western NE may try to grow upscale into a convective cluster as it pushes southeast into central NE and northwest KS overnight. Definitely a strong enough instability pool to support this...and may see just enough of an uptick in easterly low level flow to support a growing convective cluster capable of producing localized flash flooding. ...AZ... Most of the western U.S. is seeing declining convective coverage and intensity. The exception is over central to southern AZ where convection propagating off to the southwest off the terrain may persist and result in an isolated flash flood risk. Chenard Day 2 Valid 12Z Mon Aug 10 2026 - 12Z Tue Aug 11 2026 ...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MIDWEST, OHIO VALLEY, AS WELL AS SOUTHERN ARIZONA... ...Midwest and Ohio Valley... 20Z Update: The current synoptic pattern defined in the previous forecast remains valid as models are in agreement on a multi-round convective scenario with training potential increasing as we move into the evening hours Monday. Latest CAMs and associated HREF are keen on an axis of heavier convective impacts occurring over northern IN down through western OH with the highest probability for >3" (30-50%) located within the forecast theta_E gradient outlined via the cold pool remnants from the storms that will occur the first half of the period. The trends have favored a cell propagation from west-northwest to east-southeast as directed via the mean flow around the northern periphery of the ridge located over the Southern U.S. and the broad mid-level gyre spinning up over the eastern half of Canada. The combination of convergent flow within the boundary layer and the large scale ascent provided from the jet alignment over southern Ontario should enhance the overall precip component over the Ohio Valley leading to prolonged convective potential over the course of the overnight hours Monday into Tuesday. The likelihood of training/repeated round of thunderstorms in the vicinity of eastern IN and western OH has been increasing over the past series of runs from models leading to an enhanced prospect for flash flooding considering rates between 1-2"/hr are all but certain in the strongest cell cores. Despite antecedent dry soils leading into the period, there will still be bouts of scattered flash flooding, especially within those urbanized zones over the aforementioned areas extending from as far northwest Chicago and the I-65 corridor to as far east as I-75 and north of I-75. This area continues to be outlined within a high-end SLGT forecast with a non-zero opportunity for an upgrade in coming updates. Kleebauer ..Previous Discussion.. The MCS that will be moving across Ohio at the start of the day is likely to persist as it traverses the state and moves into southwestern Pennsylvania and West Virginia. By Monday morning, the MCS should be fast enough moving that flash flooding resulting from it, especially during this most hostile period of the day for MCS sustenance. Nonetheless, the storms will prime the soils in PA and WV for later rounds of rain. During peak heating in the late afternoon, additional storms are likely to form in Indiana and Ohio, which will continue southeastward into West Virginia during the evening hours. Behind this complex of storms, the persistent LLJ advecting additional moisture and instability into the large collective cold pool will lay down a pseudo-cold front from northeastern Indiana into western and southern Ohio. The primary flooding threat will come Monday night as the LLJ running into the cold pool results in numerous additional storms forming along the slowly retreating cold pool boundary. The storms could reform as far back/west as Chicago, but appears most likely from northern Indiana and into western Ohio. The storms will track southeastward across those states, but due to the cold pool setup, the environment will be very favorable for training and backbuilding. Considering the rain in some areas of northern Illinois and northern Indiana could be hitting the same areas for the second consecutive day, a Moderate Risk upgrade can't be ruled out, as well as anywhere the storms could be most persistent into Ohio with a more short-range update. Wegman ...Southern Arizona... 20Z Update: Models continue to indicate an increase in convective coverage and overall magnitude of thunderstorm activity across southern AZ as a stronger mid-level vorticity maxima, coupled with increasing PWATs initiates and maintains a stronger round of thunderstorms from western Cochise county into neighboring Santa Cruz and Pima counties as we move through Monday afternoon and evening. Heavy rain prospects extend well beyond this area, as well, much of AZ likely seeing scattered shower and thunderstorm coverage during the peak diurnal maxima. The area of greatest flash flood concern will continue to lie along and south of I-10 with the core of heavier rains likely situated within that area of I-19 between Tuscon to Nogales. HREF probs for >1" are running between 50-80% for that particular corridor with a secondary max located within the central and western Mogollon Rim. Modest probs exist for >2" across southern AZ leading to a continued high-end SLGT risk over the above area and a broad MRGL expansion through the Desert Southwest up through the Great Basin. Kleebauer ..Previous Discussion.. Monsoonal moisture will peak across southwestern Arizona on Monday. Storms are expected to initiate along the Mogollon Rim, and the numerous ranges of southeastern Arizona as normal during the afternoon hours. Unlike prior days, enough easterly flow, especially in the upper levels, will successfully push those showers and storms off the mountains and towards the lower elevations and valleys of southwestern Arizona. As the storms track westward, they are expected to backbuild given the impressive amounts of moisture and instability they will run into as they move westward. Thus, the bigger cities such as Tucson and possibly Phoenix may see periods of showers and storms that will persist into the evening hours, since the ample moisture and instability in southwestern Arizona will sustain the storms past peak heating. The inherited Slight Risk is little changed, and the I-19 corridor from Nogales to Tucson remains the primary area that could be affected by the storms and their flash flooding. Wegman Day 3 Valid 12Z Tue Aug 11 2026 - 12Z Wed Aug 12 2026 ...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MIDWEST AS WELL AS FOR SOUTHERN ARIZONA... ...Midwest... 20Z Update: Little change necessary from the previous forecast as consensus grows for a period of locally significant heavy rainfall over areas of IN/OH, extending into WV. This setup is favoring significant rainfall on a northwest to southeast alignment as a series of stronger mid-level disturbances migrate quickly through the flow with the height field oriented from the Northern Plains into the Central Appalachians. Both the ECENS and GEFS mean vorticity forecast show 3 distinct vorticity maxima migrating out of the Midwest into the Ohio Valley during the D3, bleeding into the D4 time frame. Multiple MCSs will likely transpire over the area extending from northwest IN down through OH into WV leading to some areas seeing multiple rounds of heavy rain and increasing flash flood prospects as each complex will prime the grounds as we move forward in time. Areas of interest for more significant impacts include those broad urban zones and the terrain influenced areas of WV and eastern OH as flash flood potential is generally higher due to higher run off capabilities and more enhanced riverine issues that couple with the heavy rainfall rates anticipated (1-2"/hr likely, locally higher). The previous high-end SLGT remains in effect, but there is a better than normal chance at an upgrade in future updates based on the proposed setup. Kleebauer ..Previous Discussion.. A rinse-and-repeat pattern will unfold across the Midwest on Tuesday. Any morning MCSs from Monday night will continue towards the southeast into West Virginia from Ohio while the storms generally weaken Tuesday morning. Additional rounds of storms will likely follow in the form of fast moving lines, but still tracking over many of the same areas from prior days, from northern Indiana across much of Ohio, and weakening as they move into West Virginia late in the day. Just as on prior days, the storms will establish a cold pool/pseudo-front across Ohio that will act as a train track for additional storms to form along and track along by Tuesday night. That precise scenario will play out again Tuesday night, as storms form either immediately in the wake of any afternoon lines of storms or along their cold pool. Westerly flow of deep moisture will support the individual storms rather fast southeasterly motion from Indiana across Ohio and eventually into West Virginia. However, training and backbuilding will be common, and along a narrow corridor that could stretch from as far north as Pittsburgh south through the southern tip of West Virginia. It appears this training and repeating scenario will ramp up as compared to previous days on Tuesday, resulting in more common, stronger, and more persistent lines of storms developing and moving southeastward along the front. Given the likelihood that prior days' rains over these same areas will have saturated the soils by Day 3, and the general very flashy nature of the rivers of West Virginia, it appears a Moderate Risk upgrade will be probable in this region in the coming days. Wegman ...Southern Arizona... 20Z Update: Little to no change necessary for the SLGT and MRGL risk placements across the Western U.S. for D3 as the general model consensus maintains a consistent output compared to the previous forecast. Kleebauer ..Previous Discussion.. The strongest storms associated with the monsoon across southern Arizona on Tuesday are expected to form closer to the Mexican border as compared to prior days. Given the likelihood of flooding rains in this same area on Monday, despite the relative lack of rainfall expected on Tuesday as compared with Monday, a small Slight Risk upgrade for along the Mexican border was introduced with this update. Otherwise, expect a return to a "base state" as far as storm coverage, notably a significant decrease as compared to Monday, but similar to today/Sunday. This should allow some areas to dry out due to lack of activity on Tuesday. Wegman Day 4 and Day 5 Valid 12Z Wed Aug 12 2026 - 12Z Fri Aug 14 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE OHIO VALLEY, MIDWEST, & CENTRAL HIGH PLAINS... 20z Update: More uncertainty with the OH valley Slight risk on day 4 (Wednesday), with the GFS, AIGFS and AIFS showing a farther south convective axis by this time. Meanwhile the GEFS mean, ECMWF, UKMET and CMC remain solidly within the inherited Slight risk area. Opted to make little to no change to the Slight given this uncertainty, and will future shifts make adjustments if/when a clearer model trend becomes evident. Actually greater confidence in a heavy rainfall threat from IA/MO southeastward into IL/IN and KY on day 5 (Thursday). Looks like a favorable training convective setup on the periphery of the ridge, with anomalous 2"+ PWs pooling across this corridor. Models have been very consistent in highlighting a swath of heavy rainfall, so confidence is high in there being some flash flooding this day. Confidence is lower on the exact axis, with some run to run latitudinal differences noted. The issued Slight risk swath aligns with the current model consensus. This event seems to have some potential to evolve into a locally more significant flash flood threat across portions of this area. Chenard ...Previous Discussion... ...Portions of the Ohio Valley/Midwest/Corn Belt... Early to mid next week, broad cyclonic flow expected to persist across the northern tier of the Lower 48 from the Northern Plains/Upper MS Valley, eastward through the Midwest/Great Lakes and into the Northeast, to the south of a closed mid-level low in and near Quebec. A frontal boundary will lie along the southern periphery of this broad trough. Embedded shortwaves in this broad cyclonic flow will accentuate lift in the vicinity of this front where PW values are expected to rise to ~2" ahead of each set of shortwaves and low- level confluence is expected. When looking at the ingredients, organized convection looks most likely near portions of the Ohio Valley and the Corn Belt Wednesday into Thursday night, where the better 1000-500 hPa thickness difluence is forecast. Hourly rain amounts to 3" are possible. The Slight Risk on Wednesday/Wednesday night was broadened somewhat from continuity, with another Slight Risk introduced for Thursday/Thursday night near the IA/MO/IL state junction where guidance spread on the heavy rain signal is smaller than average. ..The West/Central & Northern Plains... A broadening upper level trough is forecast across portions of the West and Great Basin Wednesday into Thursday night. Broad Marginal Risk areas exist near and ahead of this upper level trough for portions of the Southwest, Intermountain West, and Northern/Central Plains. Increased upper level troughing opens the door to strengthening low-level level flow and broader convective development/coverage across the West and Plains as the mid-level cap further erodes. Above average precipitable water values slowly shift northward through the region, while AZ's return back towards seasonal averages Thursday night/early Friday. While there continues to be low confidence with the details, locally heavy rains with the potential of 2.5" an hour amounts could be problematic. A Slight Risk was introduced for areas near the CO/WY/NE irregular border intersection where there was a fairly cohesive signal for heavy rainfall within a region of probable antiparallel flow between the low-level upslope flow and organized thunderstorm propagation moving against the low level wind. Urban areas, arroyos/dry washes, box/slot canyons, and burn scars would be most at risk for flash flooding. Roth 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 $$