FOUS30 KWBC 092010 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 410 PM EDT Sun Aug 9 2026 Day 1 Valid 16Z Sun Aug 09 2026 - 12Z Mon Aug 10 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER MIDWEST... ...Upper Midwest... 16Z Update: Initial wave of convection is currently pushing through IA/WI at this time with heavy rain focused over southern WI with a primary convective cluster leading to bouts of heavy rainfall between 1-2"/hr based on current obs. The forward propagation of the storms should limit the threat for flash flooding, but some isolated instances of flash flooding will plausible between Madison to Milwaukee with emphasis on the area between I-94 and I-41 (For more on this evolving setup, please see MPD #0984). This shortwave associated with the current convection will migrate east-southeast with a steady motion into the southern Great Lakes by the afternoon, kicking out of the area by the time the evening rolls around. Still some risk of thunderstorms across southern MI into the northern Ohio Valley with the disturbance, but flash flood threat will remain isolated overall. The coming evening is when the next round of thunderstorms will really cause issues as we see the approach of a warm front moving up from the south with the aid of a budding nocturnal LLJ and some backing flow centered over northern IL/IN up into southern WI and MI. The backed flow along with increasing surface convergence within the proximity of the warm front and any remnant cold pool(s) generated via the storms early in the period will provide the opportunity for a secondary pulse of convection in and around the area focused along and east of I-39/55 in IL over into northwestern IN and southwest MI after 00z. Hi-res suite continues to pin some significant rainfall totals between 2-4", locally higher near or within the Chicago metro and surround suburbs over into those areas downstream around the shores of Lake Michigan across IN/MI. 12z HREF neighborhood probabilities for >3" are actually a touch lower than the previous forecasts, but still a respectable 40-60% centered over the above area with the max positioned over southwestern MI. The urbanization factors along with rates between 1-2"/hr all within the expected environment will be sufficient for at least scattered flash flood instances with the highest prospects in those heavily urbanized areas within the Chicago metro to along the I-94 corridor bordering the lake. The northern inflection of the threat seems to be near Holland, MI with the primary core just south. There is some prospects of the heavy rainfall axis focusing a bit further south with more emphasis on areas between I-80 and I-74 in IL into I-65 in IN. These outputs are generally more aggressive with the cold pool positioning being further south than the consensus leading to the convergence pattern displaced away from the southern portion of Lake Michigan. Something to monitor when assessing any potential deviations from the current forecast, but still a lower probability comparatively. As a result, maintained a broad SLGT risk across the Midwest into the southern portion of the Great Lakes and northern Ohio Valley with the axis of the risk genuinely encompassing the forecast positioning of the warm front by tonight into early tomorrow morning. Kleebauer ...Southeast... 16Z Update: Changes were minor from the inherited MRGL risk across the Southeast and southern Mid Atlantic. The main area of concern will continue to be the Cumberland Plateau and points east through the southern Appalachians where slow cell motions and rainfall rates between 1-2"/hr could spur some localized flash flood impacts in the terrain over Tennessee into the Carolina mountains. Totals between 2-4" in the hardest hit areas seems to the consensus within the HREF prob fields indicating modest >3" probs between 30-50% across southwest NC into the Smokey Mountains. Heavy rain prospects extend southwest into AL/MS/GA with local maxima >3" plausible in the mountains of north GA and over the Central Gulf coast in southern AL. Broad MRGL risk remains for all areas. Kleebauer ..Previous Discussion.. Abundant deep tropical moisture will remain in place across much of the Southeast, as it has been for several days already, through the day today. Unlike further north, at these latitudes, a comprehensive forcing mechanism, especially from the upper levels will be lacking. Thus, with plentiful daytime heating, widespread disorganized convection is likely once again this afternoon over a large swath of country. The inherited Marginal was expanded down to the Gulf coast, as storms in the vicinity of New Orleans and Houston could cause localized and isolated flash flooding since any storms in this environment will be capable of short periods of multiple inch per hour rainfall rates given the abundance of moisture for the storms to work with. If flash flooding were to occur, it appears most likely in this region in the southern Appalachian region of far north Georgia and western North Carolina, where terrain effects could locally increase the duration of heavy rain from any storms that form. Wegman ...Southwest... 16Z Update: No changes to the previous MRGL risk across the Southwestern CONUS as the forecast remains on track with solid run-to-run continuity on the expected thunderstorm coverage this afternoon into the early evening. One area of interest not within any risk is across the Peninsular Range in Southern California where thunderstorm genesis is anticipated after 18z today. Environment doesn't appear to be as robust as the coming days with more of a modest threat for locally heavy rainfall this afternoon. Considering the deterministic outputs running below what we would want to see for a more formidable flash flood setup, decided to maintain the nil in the area, but will make mention of a non-zero opportunity in that corridor of terrain for the period. Kleebauer ..Previous Discussion.. Of the next 3 days, the monsoonal storms impacting the Southwest will be at a minimum today, generally focused on the larger terrain features such as the Mogollon Rim, and portions of Utah. The greatest moisture, as usual, will be closest to the Gulf of California in southwestern Arizona. So any storms that form along the Mexican border will likely have a higher chance of producing flash flooding compared to most other areas of the Southwest. Regardless, with a lack of real forcing, the storms should still be disorganized and therefore only produce isolated flash flooding in their localized areas today. Wegman 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 anti-parallel 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 $$