947 FOUS30 KWBC 202015 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 415 PM EDT Thu Aug 20 2026 Day 1 Valid 16Z Thu Aug 20 2026 - 12Z Fri Aug 21 2026 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF FAR EASTERN PA, CENTRAL/NORTHERN NJ, NYC, AND FAR WESTERN LONG ISLAND... 16Z Update... Primary change with the Day 1 ERO was an introduction of a MDT Risk across parts of the Mid-Atlantic. The approaching shortwave over the Upper Ohio Valley and associated showers have been exhibiting warm rain processes and intense rainfall rates even without ample instability due to thick cloud cover. Flash Flood Warnings extend from southwest to central PA as of 16Z. Rainfall rates around 2"/hr without sufficient instability are possible in combination with a favorable right- entrance region of a potent jet streak over the Northeast and New England, as well as a very high melting layer (IAD sounding WBZ height over 13k feet and over the 90th climatological percentile per SPC's sounding climo). Additionally, two frontal boundaries are in play across the Mid- Atlantic to create a focus for additional thunderstorm development as well as an anchor for thunderstorms to remain near and create the duration aspect necessary for higher-end rainfall totals and related flash flooding impacts. The first quasi- stationary boundary was analyzed at 14Z extending across southern PA before lifting northeast into central NJ and following I-95 into southern CT. This boundary and areas to the north will be the focus for 2-4" rainfall amounts, locally higher, due to the repeating nature of thunderstorms ahead of the approaching MCV. Once the MCV exits off the NJ coastline by around 00Z, lingering rainfall is possible to the north and could be moderately heavy given the increasing easterly flow overrunning the lingering boundaries and coastal influence. This may add onto any ongoing flash flooding concerns. 12z HREF/REFS neighborhood probabilities for at least 3" per 6-hours ending at 03Z Fri are 40-80% from eastern PA through central/northern NJ and into the NYC metro region as well as western Long Island. This is a strong signal for an area that is prone to flash flooding due to the urban/impervious ground conditions, which was the primary reason for the upgrade after consulting local forecast offices (OKX and PHI). For NYC specifically, the URRD (Urban Rain Rate Dashboard) from the 06Z run shows over 60% chances for more than 3", with several members over 4" and maximum amounts up to 7". The MDT Risk area highlights not only where numerous instances of flash flooding are likely, but also significant impacts are possible. See MPD 1098 for more information regarding the heavy rainfall potential through 22Z. Be sure to have numerous ways of receiving warnings, follow advice from local officials, and avoid driving through flood roadways. Snell 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 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... 20Z Update... Changes to the previous Day 2 ERO includes expanding the SLGT Risk in the Carolinas and Virginia westward to include parts of the southern/central Appalachians, including northwest NC and Southwest VA. Additionally, a MRGL Risk was added for parts of central NE and north-central KS. For the southern Mid-Atlantic, the meteorology remains consistant to previous discussions. 12Z CAMs and REFS/HREF guidance show solid coverage over 2-4" amounts across central NC into adjacent north- central SC, with localized maximum amounts up to 7". HREF/REFS neighborhood probabilities for >5" are now 30-60%. This includes the Sandhills of the Carolinas where FFGs are higher, but also extends into the Piedmont where terrain is more at risk for flash flooding. Given the expansive QPF westward, the SLGT risk was also expanded. A MDT risk was considered, but given some uncertainty in placement of heaviest QPF and still limited areal coverage of 5"+ probabilities, a SLGT was maintained. For parts of the Central Plains, northwest flow around an upper- level ridge centered over the Southern Rockies will allow for a diving weak shortwave to interact with a frontal boundary to produce locally heavy rainfall. Guidance remains isolated with this risk, but the setup and PWs creeping up to 1.5" support the potential for overrunning showers/storms during the evening and overnight across parts of central NE into north-central KS. HREF/REFS neighborhood probabilities are not too impressive for at least 2", but do reach up to 30% as of the 12Z runs. Snell ...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... 20Z Update... No significant changes made to the previous outlook. Guidance remains mixed along the East Coast with anomalous moisture lurking just offshore and the potential for heavy rain to extend westward to the I-95 corridor. The 12z GFS and NAM were farthest west, with the ECAIFS and AIGFS more offshore. For the western U.S., the inherited MRGL was expanded a bit north as PWs above the 90th climatological percentile also extend wel north. This leads to any thunderstorm developement capable of containing rainfall rates of at least 0.5-1.0"/hr. Thus, the expanded MRGL risk. Snell ...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, PLAINS TO MID-MISSISSIPPI VALLEY, AND NORTHEAST... ...2030Z Update... New Marginal Risk areas were added to Sunday's outlook for portions of the Plains southeast to the mid-Mississippi Valley. "Ridge riders" in the form of shortwaves riding southeast from the Dakotas down through the Ozarks will be the cause of any showers and storms that may produce isolated flash flooding in these areas. Due to fast northwesterly flow aloft, so too will the storms be fast- moving. Thus, the threat for flash flooding is quite low due to that fast cell movement. With that said, the ridge will not be moving much, so multiple rounds of storms may impact that area in prior days, and soils at this time are quite moist/saturated. The terrain of the Ozarks could also locally increase the flash flooding threat, though that looks to be overnight Sunday night. Since the signal for rainfall has been consistent, albeit with low amounts, the Marginal Risk was added with this update. In the Northeast, rainfall ahead of an approaching low will advect into upstate New York and western New England along with 500-1,000 J/kg of instability. This could allow for some modest backbuilding and training. Terrain also could enhance any localized flash flooding where a storm or 2 could linger long enough. For these reasons, a Marginal Risk was added in this area as well. No significant changes were made to the existing Marginal Risk areas across the West. Monsoonal moisture will continue to overspread the west, but not to a degree as to cause widespread impacts. Rather, isolated flash flooding is expected where storms form over flood sensitive areas such as burn scars and slot canyons. Wegman ...Previous Discussion... 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 $$