FOUS30 KWBC 090830 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 430 AM EDT Thu Jul 9 2026 Day 1 Valid 12Z Thu Jul 09 2026 - 12Z Fri Jul 10 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF THE CENTRAL HIGH PLAINS TO THE MID-ATLANTIC... ...Mid-Atlantic... Moisture and instability will increase today ahead of a mid-level shortwave tracking through the area during peak heating. PW values are forecast to exceed 2" alongside SBCAPE over 2000 J/kg, a favorable profile for heavy rainfall rates. While convection is expected to eventually consolidate into a forward propagating convective line or cluster, scattered thunderstorms should develop initially from northern VA into southeast PA. Cell mergers are possible with this initial activity and as the upstream convective line/clusters arrive. With both the REFS and HREF showing locally heavy rain and 15%+ FFG exceedance probabilities, the Slight risk was expanded westward into more of southern PA and portions of WV. The highest EAS probabilities from the HREF/REFS point to an axis across northeast MD, southeast PA, southern NJ and DE as the most favored corridor for concentrated heavy rainfall. Neighborhood probabilities from both ensemble suites indicate a 50-80% chance of locally exceeding 3" of rain, a 15-30% chance of exceeding 5", and a 30-50% chance of 2"'hr rainfall. Given the impressive CAPE/PW overlap, wherever clusters manage to merge and briefly train, some flash flooding is likely and consider this a higher end Slight risk. Locally significant urban flash flood impacts are possible if heavier cells end up over a sensitive urban location. ...Mid MS Valley into the OH Valley... Morning convection moving across MO is expected to weaken, however robust redevelopment is expected by this afternoon ahead of a shortwave extending from MO into IL. While cells should generally remain progressive as they track eastward, brief cell mergers could locally elevate the flash flood threat. The 00z HREF and REFS are in good agreement that southeast MO, southern IL and western KY will see the most concentrated heavy rainfall axis. As a strengthening low level jet boosts moisture transport this evening, the southern tail of the convective line may hang up, allowing for a period of backbuilding and training. HREF/REFS neighborhood probabilities show a 40-70% chance of rainfall locally exceeding 3" along this axis. Scattered flash flooding appears likely given this setup, although just enough southward propagation of convection may cap rainfall totals and keep the flash flood risk from being even higher. ...Central High Plains... Mid-level shortwave energy combined with upper jet support will produce convection across the High Plains from CO to SD this afternoon. Deep layer shear and instability should be sufficient to support slow moving supercells capable of pockets of intense rainfall and localized flash flooding. With storm motion vectors quite weak, most high res models seem too quick in moving this activity off to the east (a common bias). Convection will eventually shift east by tonight, and a majority of the 00z high res models support some upscale growth as it moves into western and central KS. Portions of eastern CO, southwest NE and western/central KS have been upgraded to a Slight risk with isolated to scattered flash flooding possible. ...Arizona... Convection developing over the higher terrain may propagate southwestward into the lower deserts during the late afternoon and evening hours. Uncertainties persist regarding storm propagation speed (whether cells will move too quickly to produce any flooding) and the exact spatial extent/magnitude of instability. While the overall threat looks more robust later this week into the weekend, we will maintain the inherited Marginal risk. Chenard Day 2 Valid 12Z Fri Jul 10 2026 - 12Z Sat Jul 11 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF THE MID MISSISSIPPI VALLEY INTO THE CENTRAL APPALACHIANS... ...Mid MS Valley to the Central Appalachians... A favorable environment for convective training and backbuilding should take shape across portions of the OH and TN valley on Friday. Multiple shortwaves embedded within the west to northwest flow aloft will interact with a persistent westerly low level moisture transport vector. This wind profile will result in very weak Corfidi vectors, and thus upwind propagation and backbuilding into the low level flow is probable. This setup will overlap an unstable and moist airmass, with PWs forecast around or over 2". Some uncertainty remains regarding convective evolution, especially how morning convection affects afternoon destabilization and exactly where the instability axis recovers and sets up. There are also some differences in the strength/timing of the convectively enhanced shortwave that will be tracking into the area from the west. The current model consensus consisting of the ECENS/GEFS/AIFS/AIGFS favors much of KY into northern TN for the axis of heaviest rain. A higher end SLight risk will cover these areas, with the caveat that the exact axis will likely require some spatial adjustments in future updates. Will need to continue to closely monitor the evolution of this threat, as a setup such as this could result in an eventual MDT risk pending model trends. ...High Plains... Additional shortwave energy ejecting out of the Rockies will trigger another round of convection from the TX Panhandle into CO and KS. The low level jet and overall forcing may be a bit weaker than Thursday, which may limit the degree of organization and upscale growth. Nonetheless, some slow cell motions and high rainfall rates will maintain a threat for localized flash flood impacts. ...Arizona... The synoptic and mesoscale setup on Friday will be very similar to day 1, although instability maybe slightly higher. Both the RRFS and REFS depict more robust convective development capable of producing localized rainfall totals over 2 inches. While this risk should remain isolated, the flash flood threat appears slightly higher than on Thursday. Chenard Day 3 Valid 12Z Sat Jul 11 2026 - 12Z Sun Jul 12 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE OHIO/TENNESSEE VALLEYS AND CENTRAL APPALACHIANS... ...OH and TN valley... The favorable heavy rainfall pattern will persist into Saturday across portions of the OH and TN Valley. Deep layer westerly flow, persistent low level moisture transport, and weak Corfidi vectors will support the potential for training/backbuilding cells. While convective scale details remain uncertain this far out, large scale forcing will be strong, with a well defined mid level shortwave diving southeast into the region beneath the right entrance region of an upper level jet streak. Organized convection within an airmass characterized by PWs around 2" will support heavy rainfall rates. Given the potential for training and backbuilding in this environment, some flash flooding again appears likely. The exact heavy rainfall axis is unclear and will depend on leftover mesoscale boundaries and the exact strength/track of the shortwave, but an area from TN and KY into WV appears most at risk. This is a solid Slight risk, with isolated to scattered flash flooding expected. If this axis ends up overlapping with heavy rainfall from day 2, then a higher risk category may eventually be required. ...Southwest... The terrain driven convective pattern will continue across AZ, with storms rolling off the terrain and into an instability pool over southern AZ. Both instability and PWs should be higher Saturday than prior days, which should drive greater convective coverage and at least isolated flash flooding. Trends will be monitored, with a Slight risk upgrade in future cycles a possibility. Chenard Day 4 and Day 5 Valid 12Z Sun Jul 12 2026 - 12Z Tue Jul 14 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS, MID/LOWER MISSISSIPPI VALLEY, TENNESSEE VALLEY, SOUTHEAST AND THE MID-ATLANTIC...| ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF ARIZONA AND NEW MEXICO... A frontal boundary will be advancing southward through the Midwest, Tennessee Valley and Southeast which will be a focus for scattered convection Sunday and Monday. Any of these thunderstorms have the potential to produce pockets of heavy rainfall that could lead to isolated instances of flash flooding. A Marginal Risk was maintained for the Sunday from eastern Texas and Oklahoma to the Mid-Atlantic and Southeast, while a new Marginal Risk was raised for the Gulf Coast and Southeast for Monday. Increasing southerly flow around the western periphery of the central U.S. ridge will draw monsoonal moisture into the southern Four Corners and Great Basin later this weekend into early next week. This moisture will likely lead to the development of scattered to numerous showers and thunderstorms with the potential to produce heavy rainfall and localized flash flooding. A Marginal Risk is in effect for portions of Arizona and New Mexico for both Day 4 and Day 5. Campbell 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 $$