FOUS30 KWBC 161951 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 351 PM EDT Sun Aug 16 2026 Day 1 Valid 16Z Sun Aug 16 2026 - 12Z Mon Aug 17 2026 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF WEST VIRGINIA... 16Z Update... The biggest adjustment from the overnight is the addition of a Slight Risk covering portions of east-central MO/southern IL, where the CAMs are showing a notable signal for developing storms this evening supported by an MCV moving across Missouri that will be interacting with a deepening pool of moisture (PWs ~2.25 inches) along an axis southwesterly low level inflow. Both the HREF and REFS are showing high probabilities for 2+ inches within the proposed Slight Risk area. Farther east, pulled the Slight Risk west to cover a greater portion of southern Ohio and eastern Kentucky where the models have not been performing overly well, but conditions remain favorable for additional heavy rain and potential flash flooding in the near-term. Refer to WPC MPD #1068 for additional details. Elsewhere, adjustments to the previous outlook are relatively minor. Pereira Previous Discussion... ...OH Valley into the Mid-Atlantic... A complex, multi wave convective scenario will unfold across the OH valley and Mid-Atlantic today, with a Moderate risk in the ERO centered over WV, surrounded by a Slight risk extending eastward into PA, MD and VA, and a Marginal risk extending westward across the OH Valley into MO. At 08z, one MCV accompanied by locally training convection was tracking eastward over southeast OH into WV and southwest PA. This feature will traverse PA, MD and northern VA through the morning and early afternoon hours. As this area of convection outruns the better instability pool, overall convective intensity should gradually lessen...however an anomalously moist airmass with PW values around 2.25" will still support heavy rainfall rates within embedded cores. Although higher FFG east of the Appalachians and weaker instability to the northeast should limit widespread exceedance, a sharp instability gradient setting up along the southern/southwestern flank (from northern VA into MD) could support some backbuilding by late morning into the afternoon. Given the high PW environment, any training or backbuilding will pose a flash flood threat, especially across urban areas. Behind this initial wave, thermodynamic recovery will occur as a secondary MCV shifts into OH closer to an advancing cold front. Additional convective development is expected over OH, WV and western PA...supported by CAPE near 2000 J/kg and PWs approaching 2.2". While the cold front will eventually push the activity southeastward, strong low level westerly moisture transport aligned with the deep layer flow will be supportive of backbuilding and training. A Moderate risk remains centered over WV, where persistent, strong 850mb moisture transport impinging into the complex terrain raises significant hydrologic concern. Multiple rounds of convection are probable...the initial morning round primarily impacting the northern portion of the state, followed by another round of convection dropping south across the state between 00z and 06z. There are some notable differences between the 00z HREF and REFS...with the REFS showing higher QPF and FFG exceedance probabilities. Given the moist upslope flow, high PW values, and robust instability, favor something closer to the wetter guidance...although some uncertainty still exists regarding the exact convective evolution and FFG exceedance coverage today/tonight. Scattered to numerous instances of flash flooding are possible...with considerable flash flood impacts a possibility. The Slight risk extending into PA/MD/VA accounts for the potential of multiple rounds of rain, backbuilding, and localized convergence near a west to east instability gradient that may help focus convection. Although the 00z HREF and REFS explicit FFG exceedance probabilities are muted (probably a reflection of high FFG and QPF spread over multiple rounds), the high PW airmass will promote high rainfall rates capable of producing areas of flash flooding. Also FFG may lower through the day as rounds of rain move across, and thus think the low model FFG exceedance probabilities may be a bit misleading. While widespread flash flooding is not currently expected, would expect this setup to result in an isolated to scattered threat, and locally considerable impacts are a possibility. Further west, a Marginal risk covers the remainder of the OH Valley into MO, where scattered convection along the southward dropping cold front will bring an isolated flash flood threat to areas with saturated soils and lowered FFG. ...Southwest... A Slight risk is maintained from northeast NM into southeast CO, with the 00z high res models supporting minimal changes from continuity. Convection will focus near and north of a front, where enhanced low level convergence and easterly upslope flow will combine with upper level right entrance jet support. While PW values are only slightly above average, ample CAPE of 1000-2000 J/kg will compensate, driving isolated to scattered flash flooding from slow moving storms and cell mergers. Chenard Day 2 Valid 12Z Mon Aug 17 2026 - 12Z Tue Aug 18 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE OHIO VALLEY AND THE CENTRAL APPALACHIANS... 20Z Update... Felt the combination of very moist soils and the potential for additional heavy rains was enough to warrant the upgrade to a Slight Risk across portions of southern Ohio, West Virginia, and southwestern Virginia. A shortwave now moving across Iowa and northern Missouri is forecast to move through the Ohio Valley tomorrow, interacting with a pool of deeper moisture settling south ahead of a cold front. While, widespread additional heavy amounts are not expected, both the HREF and REFS indicate 1-2 inch amounts are likely, with the HREF showing some greater than 50 percent probabilities for amounts over 2 inches within the Slight Risk area. At 12Z, 3-hr FFGs ranged from 0.25-1 inch within much of this area, with the HREF and REFS showing probabilities for rainfall amounts exceeding these values climbing over 50 percent late Monday afternoon and evening across parts of the northern West Virginia and southwestern Virginia mountains. Elsewhere, made only minor changes to the previous outlook. Pereira Previous Discussion... ...OH/TN valley into the Appalachians... A southward dropping cold front will serve as a focus for scattered convection on Monday. While overall storm coverage will be reduced compared to previous days, lingering moisture and instability just behind the boundary will maintain a heavy rainfall threat from KY and TN into the central Appalachians, enhanced by local terrain upslope. A Marginal risk is maintained for isolated flash flooding. Notably, the 00z RRFS and REFS hint at an upstream MCV moving into KY and TN that could increase convective coverage/organization enough to warrant a Slight risk. However, given limited global model support, the risk tier will be held at Marginal pending a full suite of high res guidance in subsequent cycles. ...Southwest... A Marginal risk remains on track across much of NM and adjacent portions of southeast AZ and southern CO. Beneath an upper level ridge strong instability and slightly above average PWs will interact with embedded shortwave energy trapped in the weak flow. Terrain based initiation is expected during the afternoon, with scattered coverage and cell mergers likely as convection drifts off the higher elevations. ...NE to MN... The Marginal risk has been adjusted slightly southwestward, now extending from central NE into far southeast SD and far northwest IA. Frontal forcing combined with a robust instability pool over the Central Plains and a nocturnal low level jet ramp up will support a convective cluster centered across NE by Monday evening/night. Recent high res models, along with the AIFS and AIGFS, show better spatial agreement focusing this activity over NE, supporting the minor shift in the risk area. Chenard Day 3 Valid 12Z Tue Aug 18 2026 - 12Z Wed Aug 19 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL PLAINS AND UPPER MIDWEST... 20Z Update... Following the general trends in the 12Z guidance, made a small southward adjustment to the outlook areas extending from the mid and lower Missouri Valley into the Midwest. Pereira Previous Discussion... A Slight risk is centered over IA and extends into adjacent portions of northeast KS, eastern NE, northern MO, and northwest IL. A stationary frontal boundary interacting with shortwave energy diving southeastward within northwesterly flow aloft will serve as the primary focus for heavy rainfall. Scattered convection during the day should grow upscale into an MCS by late afternoon into the evening as large scale forcing amplifies and the low level jet strengthens. While the MCS is expected to eventually propagate southeastward (limiting the overall duration of the flash flood risk), an impressive instability pool featuring CAPE in excess of 3000 J/kg and elevated PWs near 1.75" will support heavy rainfall rates. Isolated to scattered flash flooding is possible before the systems forward speed caps the threat. Guidance is now in good agreement on the general axis which reflects a southwestward shift from continuity. Given the climatological tendency for these setups to trend further southwest with time, the Slight risk boundary was drawn a bit broader to the southwest to accommodate potential future adjustments. Chenard Day 4 and Day 5 Valid 12Z Wed Aug 19 2026 - 12Z Fri Aug 21 2026 ...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL ON WEDNESDAY ON WEDNESDAY AND THURSDAY ACROSS PORTIONS OF THE SOUTHWEST, CENTRAL PLAINS, OHIO VALLEY, AND MID ATLANTIC... 2030Z Update... There remains two areas of concern for excessive rainfall during the period...with enough moisture and instability across the Southwest US to portions of the Rockies fostering an environment capable of producing locally heavy to excessive rainfall each day while a mid- and upper-level low north of the international border help focus convection from the Great Lakes and eastward. Any storms around over or near the Great Lakes should be progressive enough to keep instances of flooding isolated or perhaps widely scattered. Bann ...Day 4/Wednesday... Broad upper trough migrating across Canada with a stout upper level ridge centered across the Southern Plains will lead to a fanning of a cold front that will press south through the Ohio Valley into the Mid Atlantic with aid from high pressure motioning through the northern tier of the CONUS in wake of a low moving into eastern Canada. The frontal positioning by mid-week will likely have the cold front situated from the Great Lakes down through the Ohio Valley with the western extension likely reaching back into the Low to Mid-Mississippi Valley between MO/AR. Ensemble spread frontal positioning is generally more latitudinal leading to some back and forth with the positioning of convective initiation and eventual impacts for Wednesday afternoon and evening. Consensus will likely shift closer to the southern frontal positioning as surface ridging has been gradually strengthening in the means over the Northern U.S. which would help propel the front further south, matching the ML placement as noted via both the theta_E gradient forecast, as well as the mean surface wind direction off both the AIGFS and EC-AIFS outputs. Magnitude of rainfall will be subject to some variability at the current lead as we will be missing more vigorous mid-level disturbances that could contribute to a more prolific QPF signal. That said, increased surface convergence in proximity to the front, along with sufficient low-level buoyancy positioned south of the boundary will likely provide enough support for scattered heavy rain signals which are already showing up in individual deterministic outputs. Globals have a smattering of elevated QPF clusters within the confines of the frontal forecast from each model, so it's just a matter of fine tuning the more intricate details as we step forward in time. At this point, there was enough of a signal to maintain the MRGL risk inherited over the Great Lakes down into the Ohio and Mid-Mississippi Valley's with only some minor adjustments necessary. Further west, the western fringe of our ridge will maintain a steady southerly low to mid-level flow capable of providing consistent moisture and low-level theta_E advection out of Mexico and tropics, up through portions of the Southwest into the Front Range. Wednesday is shaping up to be active within the terrain from southeast AZ up through north-central NM into the CO Front Range leading to some scattered heavy QPF signatures embedded in the mountains for each zone and across the High Plains of CO. Still not enough of a signal, magnitude-wise to deviate from the inherited MRGL risk, but will monitor closely if there are any targeted upgrade potentials lying within portions of the Southern Rockies or eastern CO. ...Day 5/Thursday... Our aforementioned cold front will make headway through the Mid Atlantic early Thursday with a round of convection likely to ensue with the frontal arrival. A weak surface wave has also been hinted off the Tidewater by Thursday afternoon and evening which could enhance the convective environment locally over the Southern Delmarva into the VA Tidewater. The jury is still out on this particular feature, but it has some merit with the recent ML outputs pinpointing a maxima in that general vicinity. In any case, elevated PWATs along with the frontal approach should be sufficient to provide enough support for locally heavy rainfall for those above areas, extending back into VA through the Ohio and Tennessee Valley's. Cold front will be slowing substantially by Thursday afternoon thanks to resistance from the surface ridge positioned over the Lower Mississippi Valley and Southern Plains. This setup would be conducive for enhanced low-level convergence with the front, and environmental conditioning over the Southern half of the CONUS will be favorable with a rich airmass providing necessary buoyancy to entice thunderstorm activity for yet another day in proximity to the front. A MRGL risk is forecast within areas where the front is expected to be present, or at least slowly move through as the boundary will be the focal point for locally heavier rainfall and potential training/slow-moving cells. The Southwest into the Front Range remains active, once again, as the ridge over the South dominates the pattern and provides general persistence in the flow across the Southwest, along with buoyant air being ushered poleward. A MRGL risk was added to the area in question due to the consistency, but the western extent of the risk is further back into AZ as there is a greater signal for moisture getting pushed further west to enhance the convective scheme back over the Mogollon Rim and surrounding terrain. Kleebauer 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 $$