779 FOUS30 KWBC 192007 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 407 PM EDT Wed Aug 19 2026 Day 1 Valid 16Z Wed Aug 19 2026 - 12Z Thu Aug 20 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL PLAINS, MID-MISSISSIPPI AND OHIO VALLEYS, AS WELL AS SOUTHERN ARIZONA... ...16Z Update... Only some minimal areal adjustments were needed for the inherited Slight Risk covering portions of the Missouri to Middle Mississippi and Ohio Valleys after review of the updated 12Z guidance, with the CAMs well clustered within the risk area. Storm coverage and rainfall amounts indicated by the guidance have notably come up along the southward progressing cold front stretching generally west to east from the Middle Missouri to Upper Ohio Valleys. This includes portions of central Illinois, Indiana, and Ohio which remain more sensitive to additional rainfall given very wet antecedent conditions remaining after the past week's rainfall and ongoing river flooding. Locally heavy rainfall totals in the 2-4" range will easily lead to the risk of additional scattered instances of flash flooding. Further south, the guidance also continues to depict a separate focus area for heavy rainfall over the Middle Mississippi to Lower Ohio Valleys as storms develop and then progress to the southeast along a similarly oriented frontal boundary. The updated CAMS continue to show the potential for totals of 2-4", possibly as high as 6". The HREF/RRFS also both show moderate probabilities (40-60%) for rainfall greater than 3" with low end probabilities (20-30%) for rainfall greater than 5". Despite the potential for these heavier totals and some backbuilding to help contribute to longer duration rainfall, the storms are expected to remain relatively progressive overall which should lead to mainly scattered instances of flash flooding. Antecedent conditions also remain rather dry and FFGs generally high despite some initial rainfall from storms this morning. The prior forecast also remains on track for the Southwest to Central Plains for locally heavy rainfall from increased Monsoonal storms with at least an isolated risk for flash flooding. Two more focused areas continue to be noted over south-central Arizona, where a Slight Risk was introduced, and along the High Plains of southeastern Colorado/northeastern New Mexico where shortwave timing/terrain influences will support locally greater storm coverage. The latest 12Z CAMs continue to show greater potential for locally heavy totals of 1-2"+ for south-central Arizona compared to southeastern Colorado/northeastern New Mexico, and FFGs are generally lower, so have maintained the inherited Slight for south-central Arizona with a Marginal Risk into the central High Plains. Putnam ...Previous Discussion... ...Central Plains to Ohio Valley.. Current satellite and radar composite depicts the initial stages of our shortwave ejection from the High Plains making headway into the Missouri Valley as we move through the evening. Expectation is for this maturing area of convection to propagate southeastward into the Mid-Mississipi regions of MO, leading to the first round of convection being the remnants of the overnight MCS with an expected decay of the complex later this morning as we lose the nocturnal LLJ and we approach the diurnal instability minimum across the western Ohio Valley. A cold front currently analyzed over the Northern Plains extending east into the Great Lakes will also continue its progression south and southeastward through the morning hours with sights on the Ohio and Missouri Valley during the period with a bit more latitudinal loss further west compared to downstream. By Wednesday afternoon, we'll see the introduction of another potent shortwave into the center of the CONUS leading to broader amplification of the longwave trough positioned east of the Mississippi allowing for greater large scale ascent focused over the western Ohio Valley back into the confluent area of the Ohio and Mississippi River between the MO/TN/IL area. This has been consistently depicted over the past series of forecast runs with the current 00z CAMs suite maintaining this particular zone as one of the more favorable areas of organized convection as step into the second half of the forecast period (Wednesday evening into early Thursday AM). After 00z, nocturnal LLJ will kick in across the Mississippi Valley with the nose of the jet acting as a low-level convergent feature and aiding in an increasing regional bulk shear to provide a necessary element to organize and sustain convective development. The added bulk shear (30-40kts) and increasing upstream forcing with the addition of the next shortwave is typically a signature that correlates to a focal point for heavy convection with back- building potential until the eventual frontal passage is complete leading to some heavier rainfall lingering for several hours before the threat subsides. Considering PWATs between 2-2.25" centered across the tri-state region outlined above, and the added ascent factors in both the boundary layer and mid-levels (sfc-500mb), there will likely be an area that sees a period of 1-2"/hr rainfall for at least 3-6 hrs. leading to flash flood prospects carrying overnight into early Thursday morning. Latest 00z HREF 1" EAS probs depict the area of far southeastern MO through southern IL and western KY as the primary region of focus coming through with 30-60% probabilities, a threshold typically reserved for more appreciable organized convective setups leading to higher confidence in this area seeing at least a scattered flash flood threat when you couple with the current FFG indices. Overwhelming consistency in the signal has allowed for a continuation of the previous SLGT risk with the main area of interest lying between southeastern MO into southern IL and western KY/TN. Some areas are liable to see 2-4" of rainfall with a max of up to 6" plausible considering the environment at large. The last few iterations of the CAMs have added another wrinkle to the new D1 as the frontal progression and the preceding environment have increased the overall threat of heavy rain across areas of the Central Plains over into the Ohio Valley as thunderstorm genesis in these areas are liable to cause scattered flash flood instances, especially in those more prone areas across the Ohio Valley from eastern IL to OH. Latest HREF neighborhood probabilities for >2" of rain has increased significantly over the past 24 hrs. with the newest set of probs firmly settled between 50-80% for the threshold with even a "bullseye" of closer to 90% over portions of eastern NE near Omaha. The upstream areas across NE and MO are a combination of both the frontal ascent posed by the addition of the cold front, as well as the added mid-level diffluence provided by the trailing shortwave trough descending out of the Northern High Plains on the backside of the mean trough. There has been a consistent signature of locally heavy rainfall materializing between 15-18z today upstream over northeastern NE, moving southeast towards the Missouri River Basin by mid-afternoon to impact the Omaha area and points southeast into northwestern MO. Rainfall rates in some of these cells could eclipse 2"/hr at times, especially when you couple the intra-hour rate variables that ultimately cause a majority of the flash flooding incidents. Local totals of 2-4" are certainly viable in this setup over the above locations leading to an addition of a SLGT risk now across those area of eastern NE down through northern MO and far southern IA. Further east into the Ohio Valley areas stretching from eastern IL through IN/OH, antecedent soil moisture anomalies remain into the 90-95th percentile over a large area leading to a lower threshold necessary for any convective elements to cause trouble, especially ones that linger for multiple hours at a time. We still have hydrologic concerns aplenty for this region of the CONUS as several Flood Warnings remain due to the swollen rivers and streams in recovery from last weeks onslaught of heavy rainfall. The latest HREF is keen on developing a long line of thunderstorms along the advancing cold front with some of the activity likely to push rates between 1-2"/hr by late-afternoon as we see a slow increase in regional shear (25-35kts) as the upstream pattern evolves and provides a better environment for sustaining mesocyclone development. Mean flow will also run fairly parallel to the boundary, a signature that typically acts as basis for repeated convection for a time until the cold front finally trudges past the area of interest. This signature is anticipated to extend from IL through IN/OH, eventually with the eastern fringes possibly reaching as far as western PA where the front will be located by the evening hours. Considering the sensitivity of so many areas bounded within this region, and in coordination with the local Ohio Valley WFO's, a SLGT risk was added to encompass a broad area of the Ohio Valley with emphasis on those sensitive areas spanning eastern IL, central IN, and much of central/eastern OH. Kleebauer ...Southwest into the Central High Plains... The previous forecast remains on track with the latest 00z CAMs maintaining agreement on scattered thunderstorm activity likely to transpire later this afternoon, carrying into the evening hours. Areas of the Front Range and southern AZ are the most notable for seeing more organized convection in the setup thanks to added shear and regional ascent attributed to a shortwave proxy off the Central Rockies to the north, and a deeper moisture layer and instability maxima near the International border. The general synoptic pattern has remain persistent in its presentation the last several forecast cycles with the consensus unchanged on where convection will occur. The expected evolution of the setup across the Southwest and Front Range begins with an upper ridge centered over the Four Corners, re- centering and amplifying while an inverted trough pushes west on Wednesday. This will promote a more favorable setup for monsoonal convection as a surge of thermodynamics (PWs above 1.25" overlapping MUCAPE of 1000-2000 J/kg) lifting northward behind the inverted trough. At the same time, weak shortwaves embedded within the flow and rotating around the ridge will produce areas of locally enhanced ascent, which into the improving thermodynamics suggests more widespread convective activity is likely on Wednesday. While a more substantial risk for greater coverage and intensity exists just beyond D1, a large MRGL risk for excessive rainfall appears supported from the Deserts of Arizona northeast into the Central High Plains of CO/KS. With rainfall rates of 0.5-1"/hr likely at times, and mean storm motions of just 5-10 kts, (higher in CO but with stronger bulk shear for organization) this will support at least isolated instances of flash flooding, especially over vulnerable terrain, urban areas, or recently saturated soils. In coordination with the Tuscon WFO in southern AZ, a SLGT risk was added over the areas along and west of the Huachuca Mtns through the I-19 corridor into the eastern sections of Pima/Pinal counties due to a more robust axis of convection expected across far southern AZ by later today. Forecast is for a cluster of thunderstorms maturing and migrating off the Huachucas, advancing westward through Santa Cruz and southeastern Pima counties as we head into the evening. HREF probs for >1" have ballooned upwards of 60-80% over the I-19 corridor in southern AZ, including over the city of Tuscon lending a favorable potential for flash flooding, even into the metro area. Rainfall rates between 0.5-1"/hr are anticipated in the strongest cells, plenty aggressive enough to exceed the already low FFG thresholds in place across the southern AZ deserts. Expecting scattered flash flood potential given the current signal, more than sufficient for a targeted SLGT risk upgrade. ...Florida into Far Southeast Georgia... A small mid-level vorticity maxima will slowly organize over central and northwestern FL with a proposed surface trough likely situated very close to the northeastern coast of the area. The combination of the two features and generally moist and unstable airmass presence will allow for quite a convective burst to occur by the afternoon hours, especially as you add the typically diurnal components of the sea breeze as a focal point for convection. HREF probabilities are pretty aggressive for >3" locally (60-90%) extending from Orlando up through just south of Jacksonville. We saw some isolated flood instances crop up from storms earlier in the previous period from an even less favorable setup compared to what will transpire today. Hourly rates of the heaviest cells will likely be between 3-4"/hr after assessing individual deterministic, which makes sense considering PWATs easily pushing 2.1-2.4" over the entire stretch of the east coast of FL and all of North FL into the Panhandle. Given the signal, a MRGL risk was added across that northeastern extent of FL into far southeastern GA, south of Brunswick. Kleebauer 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 $$