FOUS30 KWBC 200052 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 852 PM EDT Wed Aug 19 2026 Day 1 Valid 01Z Thu Aug 20 2026 - 12Z Thu Aug 20 2026 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF INDIANA AND OHIO... A very slow-moving front across Indiana and Ohio is providing plentiful forcing for slow, and in some cases stationary, moving- thunderstorms across central Indiana and Ohio this evening. While the line of storms is generally thin, only about 30-50 miles across from north to south, the very slow movement of the storms is still resulting in multiple inches of rain per hour rates that are occurring across these states. Most of the storms are at least trying to inch southward, but as they approach I-70, several urban areas such as Indianapolis, Dayton, and Columbus, OH are in the path of some of them, especially Indianapolis. Like all urban centers, these areas are especially prone to flash flooding. Further, this portion of Indiana and Ohio have been hit repeatedly with round after round of heavy rain over the past several days to a week, so in addition to ongoing lingering flooding, the soils in this area are fully saturated, so all new rainfall from the ongoing storms will contribute to renewed runoff, and subsequently river, stream, and creek rises. The Moderate Risk was expanded west a county or so into Vermilion County in far eastern Illinois, as this seems to be about where the back edge of the slow-moving line of storms is stopping. The cluster over western Illinois is quite fast-moving, and is likely to start absorbing and ending the persistent rainfall over Illinois over the next few hours. The Slight Risk area was expanded east to include the Pittsburgh metro area. The ongoing storms over Ohio should begin to move more quickly to the east, but given the reservoir of moisture for those storms to work with, and despite a worsening environment, some period of moderate rainfall is likely to impact western Pennsylvania in the predawn hours. Between urban effects and terrain, here too flash flooding could be more widely scattered when the heavier rains move through later tonight. No changes were made out west with the somewhat moving storms towards the southwest have generally kept flash flooding impacts isolated, once impacts due to burn scars are removed. Finally, along the Gulf Coast, the Marginal in northeast Florida was moved into the Panhandle and southern Alabama due to ongoing steady moderate to heavy rain, as well as a few slow-moving storms. Here the FFGs are very high, as is the base level for this area, so despite the heavy rainfall rates with the strongest storms, only isolated flash flooding is possible here. Wegman Day 2 Valid 12Z Thu Aug 20 2026 - 12Z Fri Aug 21 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MID ATLANTIC... ...2030Z Update... The main update for this forecast cycle was to modestly expand and then extend the inherited Slight Risk over the Mid-Atlantic northward to include more of the I-95 corridor from Philadelphia to the New York City metro. As outlined below, a line of thunderstorms is expected to develop along the eastward moving cold front as it reaches the I-95 corridor. The now available CAMs show considerable rainfall potential (local amounts of 3-6") from the Virginia Tidewater north-northeastward through DelMarVa and into far southeastern PA, most of NJ, and the NYC metro. This is backed up by moderate (40-70%) HREF/RRFS probabilities of rain totals greater than 3", and low end (20-30%) probabilities of greater than 5". There remains some uncertainty with the timing/progression of the cold front and when storms form along/ahead of it relative to the I-95 corridor before moving eastward, but again the latest CAM guidance did show notable agreement at least for the northern extent of the risk. The risk remains just to the east further south from Baltimore to DC except for some of the more eastern suburbs/exurbs and towards Annapolis/Chesapeake Bay. In the Southwest, the forecast remains unchanged for the inherited Marginal Risk covering additional Monsoon activity. Putnam ...Previous Discussion... ...Mid Atlantic... Shortwave over the Ohio Valley will continue its progression to east, then northeast as it migrates into the Central and Northern Mid Atlantic by Thursday afternoon. At the surface, a cold front will remain steadfast on its advancement through the Piedmont and the eastern sections of PA by late-Thursday morning leading to a favorable scenario for a line of thunderstorms to develop along and east of the front itself. Increased dynamical forcing and favorable diffluent flow within the RER of a 130kt jet over the Northeast U.S. will lead to a blossoming of afternoon convection within proximity of the front itself. Latest CAMs are quite aggressive on their interpretation of both the magnitude and coverage of the heavy convective signals, especially for areas extending from Northern NJ down through eastern PA, the Delmarva, and the VA Tidewater. Thunderstorm initiation will be very close to the metro areas of DC-Balt-Philadelphia with the majority of CAMs developing cells almost overhead before advancing steadily eastward, away from the I-95 corridor. The threat will be close for the urban centers with the Philadelphia metropolitan area the closest for more substantial impacts from convection when you correlate with the mean QPF output in the region. PWATs between 2-2.3" coupled with a strong theta_E ridge in place ahead of the front promotes a formidable environment capable for producing significant rainfall rates in any convective core that materializes. Analysis of multiple soundings from both the global and hi-res suite at 00z indicated a textbook signal for heavy rainfall with a tall, skinny CAPE signature with moisture layers present from surface to the tropopause. Typically these types of environments favor, at least scattered flash flood prospects in areas where thunderstorms reach their mature phases, which seems to align with areas along and east of I-95. The most notable heavy rain depictions favor NJ/DE/VA Tidewater for the time being as the frontal progression times well with the late-afternoon period, a time frame when diurnal instability has reached its maximum. The signal is pretty close to the NYC metro, but the heaviest may lie just south of the metro area, so it will bear watching the trends over the next 24 hrs to see if a higher risk is necessary for New York to western LI. For now, a MRGL risk encompasses this particular zone with areas just south within the added SLGT risk. The DC/Balt metro area is currently in a MRGL with the new SLGT just to the east over Prince Georges and Anne Arundel counties. Areas east of the Chesapeake are firmly into the SLGT risk, as it stands as the signal is the most robust in that particular corridor. ...Southwest U.S. into the Southern Rockies... Relatively textbook monsoonal pattern across the west will lead to scattered thunderstorm development across much of the Desert Southwest extending north into parts of the Great Basin and through the Southern Rockies. As of now, the signal is mainly seasonal with regards to the PWAT anomalies and general expectation as the most notable areas will likely be in those terrain focused zones across AZ (Mogollon Rim), NM (Sangre de Cristos and Sacramento Mtns.), and UT (Zion National Park). Low probs for >1" on the latest ECENS/GEFS suites and 01z NBM allow for a standard MRGL risk issuance for the area as the setup is genuinely favored for pulse convection and slow cell motions tied to the terrain as a broad 597dm ridge centers over the Four Corners and makes little progression in the D2 window. No changes were necessary to the previous forecast given the consistency within guidance over the last series of model runs. Most favorable areas of impact will include the more flashy streams and arroyos across the Southwest, as well as the slot canyon areas of southwest UT and the burn scars across each of NM and AZ. Kleebauer Day 3 Valid 12Z Fri Aug 21 2026 - 12Z Sat Aug 22 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF VIRGINIA AND THE CAROLINAS... ...2030Z Update... Only one main areal adjustment was made to the inherited Slight Risk based on the latest guidance, with the risk area extended a bit further to the northeast over the Virginia Tidewater region and to the Maryland Eastern Shore. QPF amounts have trended up through this region and this area will also likely have seen an initial round of heavy rain during the day 2 period. Storm coverage overall will shift southward compared to Thursday with development focused along consolidating frontal boundaries through the southern Mid- Atlantic and into the Carolinas. The latest 12Z deterministic guidance continues to depict locally heavy rainfall totals in the 3-5" range. The AI guidance (notably the ECMWF AIFS) is less bullish on the northward extent of the heavy rainfall, leading to a bit more uncertainty into Virginia and the Maryland Eastern Shore. However, as noted, the other deterministic guidance has actually come up in amounts through this region. As during the day 2 period, Monsoonal activity is expected to continue throughout much of the Southwest with at least an isolated risk for flash flooding. The inherited Marginal Risk was mostly left unchanged except for a small expansion over portions of eastern New Mexico to cover more burn scar regions where the latest guidance does depict some storm potential. Putnam ...Previous Discussion... ...Virginia to Carolinas... Cold front will slowly advance south into the Carolinas during the end of the D2 into D3 time frame with an eventual transition to a quasi-stationary front as it reaches the latitude of north GA into the Piedmont of SC. Moisture pooling along the front with continued forcing driven by another shortwave advancing through the Tennessee Valley will lead to another round of heavy convection in- of the Southern Mid Atlantic with the primary axis likely to occur from the southern Delmarva through the VA Tidewater into the Carolina's. Environment across the region outlined will remain very buoyant with PWATs between 2-2.3" for the entire area along and south of the front. Models are consistent with their signature of 1-2" of rainfall on an areal average basis with some of the deterministic outputs approaching upwards of 3-5" where storm materialize and settle as the steering pattern weakens significantly as we step into the D3. Rainfall rates between 2-3"/hr with intra-hour rates upwards of 4"/hr are forecast across the aforementioned area, plenty aggressive enough to cause at least scattered flash flood prospects, especially in more urbanized zones along I-95 and points east. The most notable locations in the guidance are over the Columbia, SC area up through Florence into southeastern NC. With the signal maintaining consistent in the heavy QPF placement and with a favorable environment for locally enhanced rainfall, the SLGT risk was generally maintained from the previous forecast. ...Southwest... Monsoonal activity will continue for another day as the Southwest CONUS lies underneath a broad ridge centered over the Four Corners with multiple mid-level vorticity maxima pivoting under the ridge axis. Seasonal to slightly elevated PWATs will extend throughout the Southwest, even reaching a bit further north into the Great Basin by Friday leading to some locally heavy rainfall even making its presence into portions of NV and central UT while also impacting those same areas that saw activity the prior period. Local totals of 1-2" are forecast, but areal QPF average is pretty much between 0.25-0.5" which is typical for these types of setups. A broad MRGL remains in effect for much of the Desert Southwest up into the Great Basin, extending east into the Southern Rockies located over southwestern CO into NM. Kleebauer Day 4 and Day 5 Valid 12Z Sat Aug 22 2026 - 12Z Mon Aug 24 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE EAST COAST, SOUTHWEST AND INTERMOUNTAIN WEST... ...2030Z Update... Only small changes were needed to the previous forecasts for this weekend. The Marginal on Saturday was expanded north up the New England coast. While there is uncertainty with both the timing of the rain (whether it will be fully off the coast by Saturday) and how heavy it will be (instability will be a limiting factor), urban areas further east, including Providence, RI and Boston, MA both have a higher chance of seeing at least steadier rainfall, as noted by the higher QPF. Given the sensitive nature of those urban areas regarding onset of flash flooding, and potential for at least some instability to support periods of heavier rainfall, the Marginal risk was extended. A Slight Risk may need to be considered for coastal areas of North Carolina as well with future updates if delays that fail to move the front along in that area persist. Monsoon rains continue in the afternoons of both weekend days across much of the West. A bit of trimming was done on the western side, to include western Arizona and southern Nevada due to lack of appreciable rainfall forecast in those areas on Saturday. Transient MCSs may impact portions of the Plains and lower to mid Mississippi Valley this weekend, which may require at least a Marginal Risk to be issued with future updates, but large uncertainty about where they will be precluded that issuance at this time. Wegman ...Previous Discussion... ...East Coast... The mid to upper trough deepens further over the eastern U.S. on Saturday, but the northern stream energy currently appears separated from the deeper moisture plume further south and east. While adequate instability and upper level forcing will support scattered thunderstorm development across portions of MI, OH and NY, PWs appear insufficient to support flash flooding, particularly given expected quick cell motions. Thus the Marginal risk was removed across this region. The better threat for heavy rainfall will remain focused from the coastal Carolinas into the Mid-Atlantic, where a lingering stationary front interacts with the best overlap of instability and high PW values. Forecast confidence in a continued flash flood threat is highest along the coastal Carolinas, where stronger instability is more likely, and antecedent rainfall in prior days may lower FFG. Confidence is lower across the coastal Mid- Atlantic, as the axis of instability and moisture could get shunted offshore. However, the Marginal risk was maintained here as a few models still indicate locally heavy rain potential and the CSU machine learning ERO guidance highlights a small Slight risk area. No Marginal risk is delineated on day 5 (Sunday) given spatial uncertainty and the higher probability that the digging trough does not tap into the better moisture pool. However, if the upper trough digs more and takes on a negative tilt, deeper moisture could get drawn back into the system, resulting in some flash flood risk over New England. Thus we will continue to monitor trends. ...Southwest and Intermountain West... Broad Marginal risks remain in place across portions of the West this weekend. PLentiful monsoonal moisture and daytime instability will promote localized flash flooding both days. Because convective development will remain largely tied to terrain driven forcing, overall coverage is expected to stay below Slight risk thresholds. On Saturday the highest PW anomalies are forecast to be centered over UT, WY, ID and southern MT, where values are expected to exceed the climatological 90th percentile. Sufficient instability should be in place, along with some large scale forcing on the periphery of the upper ridge...to result in scattered convection and localized flash flood potential. The Marginal risk was expanded northward to include these areas. Chenard 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 $$