240 FOUS30 KWBC 200755 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 355 AM EDT Thu Aug 20 2026 Day 1 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... ...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 later this afternoon. At the surface, a cold front will remain steadfast on its advancement through the Piedmont and the eastern sections of PA by late 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 southern CT down through the NYC metro, Northern NJ, eastern PA, into 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 latest CAMs have really provided a more significant heavy rainfall footprint across the NYC metro and surrounds with several of the CAMs members within the latest HREF depicting localized maxima between 3-5" across NYC proper, neighboring areas of Northern NJ, western LI, and the southwest reaches of CT. 00z HREF neighborhood and EAS probabilities have increased further since the last few forecast cycles with the signal now firmly greater than 50% (60-80%) for both the neighborhood >3" prob and >1" EAS probability over the corridor extending from Trenton up through the NYC metro area. Further south towards the Baltimore to Philadelphia area, the signals are not as prolific as the above areas further up I-95, but they are still pretty robust compared to where they were last evening and even a bit higher than the 12z probabilities. The question becomes, "Where does the line initiate?", and, "Will the storms have enough vigor behind their development near the Balt/Phi metro corridor to entice more flash flooding potential than recently thought?" The answer to both questions haven't been thoroughly resolved for this particular zone, however the environmental conditions in place would absolutely favor the threat if storms were to materialize a bit upstream to provide a more substantial impact to that particular area of the Mid Atlantic. 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 (1-3"/hr likely according to the latest HREF hourly rate probabilities). 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 NYC area has trended to a more formidable signature in part to the region potentially situated in an area that sees a more prolonged heavy rain potential as flow becomes backed locally along the northern periphery of what would be a bookend vortex that materializes from an organized MCS structure that is anticipated for areas of eastern PA into NJ. This has been a signature within multiple CAMs, mainly the RRFS/HRRR outputs that place Northern NJ into NYC within a position that favors back-building potential for a period of time early this evening that would ultimately lead to significant rainfall across the metro corridor. The caveat behind this is these signals can be hard to parse out perfectly at range. These types of scenarios are best defined as we move closer to the initiation phase and as the organization begins for the anticipated complex. As of now, the indications are the metro area between Philadelphia to NYC into all of Northern NJ is the best target for this evolution. This signal provided enough confidence to warrant an increase in the magnitude of rainfall potential and more significant language to the incoming threat. That said, we still have some uncertainty on the exact positioning of the organized complex and eventual bookend vorticity. Unless we get overwhelming consensus, felt the maintenance of the SLGT risk with a high-end SLGT positioned on a line from Balt-Phi-NYC along I-95 and all the areas in-between. This is a delicate situation considering the urban footprint providing ample run-off opportunities that tend to loom large when dealing with expected rainfall rates of 1-3"/hr with intra-hour rates very likely to exceed this locally. We will continue to monitor the trends closely over the next several hours leading into the event. ...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 D1 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 2 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... ...Virginia to Carolinas... Cold front will slowly advance south into the Carolinas during the end of the D1 into D2 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 as we step into the D2. 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 east-central NC. 00z HREF neighborhood probabilities for >3" within the 12z Fri to 00z Sat period are running between 50-90% with the above zone with the maxima focused between I-74/95 just southwest of Raleigh. Instability across the area will be fairly robust with 1000-2000 J/kg of MLCAPE on average across NC with upwards of 2500-3500 J/kg located down into SC. Deep, moist atmospheric profiles are prevalent with the latest CAMs suite, as well relaying a textbook heavy convective signature across much of the Carolina's, especially along and east of the Piedmont. With the signal maintaining consistent in the heavy QPF placement and with a favorable environment for locally enhanced rainfall, the SLGT risk was maintained from the previous forecast with an addition of high-end SLGT wording as the threat for locally significant rainfall >4" has increased steadily since the last few updates. ...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 3 Valid 12Z Sat Aug 22 2026 - 12Z Sun Aug 23 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG MUCH OF THE EAST COAST, AND OVER THE WESTERN U.S... ...East Coast... Large scale pattern will continue its evolution into a much more amplified setup with a deepening mid-latitude trough extending from Canada all the way into the Eastern CONUS, especially for areas east of the Mississippi. Embedded shortwave disturbances will be migrating through the flow with a few robust disturbances anticipated to motion north-northeast around the front side of the trough leading to heavy rainfall prospects up and down the eastern seaboard. Current model consensus is a moderate to heavy QPF footprint extending from coastal SC all the way up into ME with the heaviest precip core located over Southern New England as of the latest ensemble depictions. One of the positives in this setup is the lack of a truly buoyant airmass with significant instability maxima located within proximity of any of the disturbances. Despite the lack of prime instability factors, enough mid and upper level forcing coupled with some fairly rich tropical moisture being pulled poleward will likely entice periods of heavy rainfall, both stratiform and embedded convective pulses located within the main precip shield. Ensemble mean QPF is between 1-2" for the D3 period across coastal NJ up through southeastern MA into coastal ME with 0.5-1" located a bit further west with incrementally less the further west you move away from the coastal plain and into the Piedmont. Considering the above factors, the previous MRGL risk was maintained with only some fine tuning across coastal ME, and closer to the I-95 corridor from DC to NYC. Unless we see significant changes in the timing of the disturbances and/or amplification prospects that indicate more backed flow for coastal enhancements, this could very well maintain a MRGL risk through the entire series of forecasts. ...West... Our persistent ridging across the Western U.S. will finally begin to shift back to the east over the Plains as a robust trough over the Pacific makes headway into the Pacific Northwest and begins pushing the pattern downstream and flattening the ridge up top. The progression of the trough will lead to an influx of both regional moisture and ascent as the trough makes progress into OR/WA as we move into the middle part of the weekend. PWAT anomalies are anticipated to surge upwards of 2-3 standard deviations above normal as far north as ID and western MT with the rest of the West firmly entrenched within a +1 standard deviation PWAT anomaly, good enough to maintain a persistent monsoonal type pattern over the Desert Southwest and Great Basin, extending into the Central Rockies. The previous periods have been slowly creeping further north and northwest with the MRGL risk and this period continues the pattern. A broad MRGL risk will encompass much of the Western U.S, east of the Pacific coast states. Kleebauer Day 4 and Day 5 Valid 12Z Sun Aug 23 2026 - 12Z Tue Aug 25 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHWEST, INTERMOUNTAIN WEST, AND ROCKIES... The overall pattern is expected to remain relatively unchanged through the weekend into early next week, with an upper ridge remaining anchored over the Southwest and southern Plains. Moisture and energy moving along the western and northern periphery of the ridge will contribute to daily shower and thunderstorm activity and the potential for isolated flash flooding extending from the Southwest to the northern Intermountain West and Rockies. However, do expect a diminishing threat along the northwestern extent of the Marginal Risk as drier air behind a cold front spreads into parts of the northern Intermountain West/Great Basin Sunday into the early part of next week. Pereira 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 $$