676 FOUS30 KWBC 212026 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 426 PM EDT Fri Aug 21 2026 Day 1 Valid 16Z 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... For the southern Mid-Atlantic, the latest CAMs remain consistent in showing solid coverage 2-4" amounts across central NC into adjacent north-central SC and far southern VA, with localized maximum amounts in excess of 6 inches. HREF neighborhood probabilities for >5" are 35-45%, and REFS neighborhood probabilities for >5" are 40-60%. 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. HREF/REFS show similar placement of the probabilities of 2 inch accumulation per hour later this afternoon, and show probabilities up to 50-70%. Environmental conditions will remain very buoyant with PWATs between 2-2.3" for the entire area along and south of a quasi- stationary front. Global-scale models remained 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" in the Carolinas. Overall, models are supportive of the previously issued Slight Risk area, including the urbanized zones along I-95 and points east. The Slight Risk area was expanded southwards with the 16Z update to include the Augusta, Georgia metro area, and high-end Slight impacts continue to look possible, with locally significant impacts if conditions align this afternoon. A broader Marginal Risk surrounds the Slight, stretching from the Mid- Atlantic to the central Gulf Coast where locally heavy rainfall along the frontal boundary will be possible. Portions of the central Appalachians and central Gulf Coast down to New Orleans were added with the 16Z update to account for increasing QPF trends in a favorable environment and increased sensitivities in urbanized areas. ...Southwest... Monsoonal activity will continue for yet 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 farther north into the Great Basin and into portions of NV and central UT. This moisture will lead to some locally heavy rainfall that could impact some areas that saw convective activity yesterday. 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 Marginal Risk area remains in place for much of the Desert Southwest up into the Southern Rockies and was expanded with the 16Z update to include more of the Great Basin. ...Southern Wisconsin to Northern Illinois... A Marginal Risk area was introduced with the 16Z update that covers portions of southern Wisconsin and northern Illinois to cover heavy rain threats with pre-frontal convection. A linear convective complex was moving south across southern Wisconsin late this morning, and produced rainfall rates in excess of 1.5 in/hr, which exceeds the FFGs for this area. CAMs suggest convection will continue to be supported through the afternoon and gradually push south into northern Illinois. The environment will continue to be supportive of locally heavy rainfall with PWAT values up to 1.6" and decent lift and instability in the vicinity of the advancing cold front. ...Southern Nebraska to Northern Kansas... A small Marginal Risk area remains in place for portions of Nebraska southeast of the Sandhills and far northern Kansas. An upper level shortwave is forecast to move south across this region as a cold front sinks south across the Plains, which is expected to trigger a burst of convective activity overnight tonight. This wave should be fairly progressive, which will limit the flood potential, but there is some chance for repeating/training storms. CAMs suggest some locally heavy rainfall with rates of 1-2 in/hr will be possible. The current 1 hr FFGs for this area are 1.5 in/hr, which have a chance of being exceeded. Dolan/Bann Day 2 Valid 12Z Sat Aug 22 2026 - 12Z Sun Aug 23 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE MID- ATLANTIC... ...20Z Update... The Slight Risk area over the Mid-Atlantic was trimmed up a bit to account for QPF trends/placement in the most recent CAM runs. This area also aligns well with where the HREF and REFS have the highest probabilities (50-60%) of 2" in 6 hrs. Localized areas within the Slight that received heavy rainfall yesterday and today may see high-end Slight impacts. Elsewhere, the surrounding Marginal Risk area was maintained and expanded to include portions of the Ohio Valley and eastern Great Lakes where QPF has trended up in the vicinity of a strengthening low pressure system. This pattern may allow for some lake-enhanced precipitation across portions of Ohio, Pennsylvania, and New York. Given recent heavy rains in the Ohio Valley, the Marginal was expanded to include portions of Ohio and West Virginia that may be sensitive to flooding with additional precipitation on Saturday. The Marginal Risk area across the Intermountain West was also maintained and expanded to include more of the northern Rockies as anomalous moisture pushes north across the region. Dolan ...Previous Discussion... ...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. This places the entrance region of an upper level jet in favorable position over an airmass with precipitable water values of 2 to 2.25 inches as a few robust disturbances rotate around the leading edge of the trough which will support heavy rainfall along the eastern seaboard. Current model consensus is a moderate to heavy QPF footprint extending from coastal SC all the way up into ME. Primary concern for excessive rainfall is along and east of I-95 in the Mid-Atlantic due to the heavy/excessive rain that occurred on Thursday as much as the with expected rainfall amounts in the Day 2 period. ...West... Persistent ridging across the Western U.S. will finally begin to shift back over the Plains as a robust trough over the Pacific Ocean 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 for this time of year as far north as Idaho and western Montana 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 into the Central Rockies. A broad MRGL risk over much of the Western U.S, east of the Pacific coast states will suffice. Bann Day 3 Valid 12Z Sun Aug 23 2026 - 12Z Mon Aug 24 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE CENTRAL US AND THE SOUTHWEST... ...20Z Update... The previous forecast remains on track. Small adjustments were made to the existing Marginal Risk areas over the West and Great Plains. Anomalous moisture will continue to support locally heavy rainfall in the West, with some heavy rainfall potential spilling over into the central High Plains as well. The Marginal over the Great Plains and Midwest remains in place, but was adjusted to match the QPF footprints from the latest model runs and WPC forecast. The Marginal Risk in the Northeast was removed with this issuance given a more progressive trend with the cold front, resulting in lower QPF values across the region. Dolan ...Previous Discussion... ...Northeast... Cyclogenesis north of the Great Lakes will help push a cold front eastward across New England on Sunday. Until it clears the area...southerly flow ahead of the front will continue to draw an airmass with precipitable water values of 1.8 to 2.2 inches across the coastal region. This should support a low-end risk of excessive rainfall mainly where low level flow interacts with the terrain. Note that the areal coverage of the Marginal Risk area has decreased in response to the quicker forward speed compared with previous runs. ...Midwest... Energy approaching from the west begins to flatten the northern portion of the ridge...allowing for showers and thunderstorms to develop across the northern Plains which can build southeast into the Ozarks. Given the potential for repeat convection...maintained the Marginal risk area even though individual cells should be progressive. ...Western US... The overall pattern is expected to remain relatively unchanged with the upper ridge remained 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. Bann Day 4 and Day 5 Valid 12Z Mon Aug 24 2026 - 12Z Wed Aug 26 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHWEST, INTERMOUNTAIN WEST, AND ROCKIES DAYS 4/5, SOUTHEAST DAYS 4/5, AND THE CENTRAL PLAINS DAY 5... ...2030Z Update... Marginal Risks were added for portions of the Southeast on both days for potential slow-moving storms associated with southeastward moving storms down the lower Mississippi Valley. Plentiful moisture will be in place for these storms with PWATs over 2 inches. Urban areas that could be impacted stretch from Jackson, MS through Birmingham and Atlanta. An isolated instance of flash flooding or 2 can't be ruled out in these areas. Wegman ...Previous Discussion... An upper ridge will remain anchored over the Southwest-southern Plains into midweek, with moisture and energy on the backside of the high contributing to diurnal showers and storms that will raise the potential for isolated flash flooding both days from parts of the Southwest to the northern Rockies. Areas of heavy rainfall leading to potential flash flooding are expected to develop across portions of the central Plains Tuesday into early Wednesday as shortwaves emanating from the West spill off of the top of the ridge. Uncertainty in the timing and placement of these features limited confidence in the location of these heavier amounts. However, a corridor of deepening moisture and increasing instability along a cold front dropping south into the region, combined with the potential for training, prompted the introduction of the Marginal Risk area extending east of the Rockies across portions of the central Plains on Day 5. 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 $$