FOUS30 KWBC 210824 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 424 AM EDT Fri Aug 21 2026 Day 1 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... For the southern Mid-Atlantic, the 21/00Z CAMS remained consistent with the 20/12Z CAMS in showing solid coverage 2-4" amounts across central NC into adjacent north- central SC, with localized maximum amounts in excess of 6 inches. 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. The picture was similar in the HREF/REFS with respect to placement of the probabilities of 2 inch accumulation per hour later this afternoon in terms of placement but noticeably variable in terms of neighborhood probabilities. 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" and supportive of the previously issued Slight Risk area with only minor modifications needed...especially in more urbanized zones along I-95 and points east. Maintained the high-end SLGT wording as the threat for locally significant rainfall >4" persists. ...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 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 remained in place for much of the Desert Southwest up into the Great Basin...extending east into parts of the Southern Rockies. Bann Day 2 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. 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 NORTHEASTERN US/CENTRAL US AND THE DESERT SOUTHWEST... 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 AND THE CENTRAL PLAINS DAY 5... 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 $$