FOUS30 KWBC 050751 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 351 AM EDT Sat Sep 5 2026 Day 1 Valid 01Z Sat Sep 05 2026 - 12Z Sat Sep 05 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR EASTERN NORTH CAROLINA INTO THE VIRGINIA TIDEWATER... ...Southern Ohio Valley through Carolinas... Persistent west-northwesterly flow regime aloft will see a change starting today as large scale closed low/trough over northeastern North America retrogrades today with stronger north to south push of drier Canadian air crosses the Great Lakes into the Ohio Valley by the end of the day. The lingering deep layer moisture plume and conditionally unstable air mass will start to shift through the Upper Ohio Valley and through the Central Appalachians with some ongoing WAA convective line SE OH, E KY and WV at the start of the forecast period. This 'back-door' cold front will also continue a southward push through the Mid-Altantic and interact with the Ohio Valley wave toward peak diurnal period in the afternoon across VA into N NC interacting with the higher theta-E air and higher moisture over the Carolina coastal Plains. Total PWats to 1.75-2" will support hourly rates of similar range though forward propagation and interaction with the sea-breeze boundary will likely result in the greatest scattered totals ranging from 2-5". HREF probs of 3"/24hrs (mainly coastal) are around 50% but also across sandier, higher FFG values. So scattered incidents of flash/rapid inundation flooding are possible. Overall little change was made for the low-end Slight Risk for this cycle. ...Eastern South Dakota/Southwest Minnesota/Northwest Iowa... A complex of elevated thunderstorms will be ongoing across eastern South Dakota at the start of the forecast period. Strengthening large scale ridging eastward across the central MN through the Mississippi River Valley accompanied by distant passing of a shortwave through the northern High Plains of Montana has resulted in increased diffluent flow along the northwest side of the ridge over the Dakotas. Low level jet has responded with strengthened S to SSWly warm advective flow in proximity to a surface front across the Upper Midwest toward the deepening surface reflection associated with the wave moving through MT. Conditionally unstable air with MUCAPE to 3500 J/kg resides across the Sand Hills of N NEB and Missouri Valley; with deep layer moisture/TPW around 1.5" pooled across the boundary. The strengthening and slight veering of the WAA flow should help to maintain a NNW to SSE convective line that will be slow moving eastward due to downstream ridging building in. Scattered cells capable of 1.5"/hr and increased residency suggests isolated additional totals after 12z may be in the vicinity of the higher FFG values (2-3"/3hrs) in the area. The area is very hard due to prolonged drought with 0-40cm relative soil moisture values below 10%. So, overall totals and lesser rates that may slowly absorb into the dry soils may limit overall coverage for flash flooding potential. However, with ARW/ARW2/RRFS solutions suggesting 2-3", a small Marginal Risk will be introduced for the isolated to widely scattered nature of FFG exceedance expected. ...Gulf Coast... Return flow along the southern edge of the synoptic ridge with weak embedded waves will continue to support enhanced regions of convective activity at diurnal peak. Broad zone of 2-2.25" total PWats and narrow, skinny profiles supporting 1500-2000 J/kg of MLCAPE will be sufficient to support stronger cells with intense rates over 2"/hr. Guidance is in fairly good agreement for a few areas of enhanced confidence for increased coverage and organization. The larger/broader shortwave will be progressing westward from S MS to SE TX with enhanced 2-3"/24hr probabilities across the saturated areas of SE TX into SW/S LA, as well as, a secondary maxima expected across S AL and western Panhandle of Florida. Model trends show some isolated to scattered activity between across northern Florida, but minor enough to remove from the Marginal Risk. Overall coverage and increased easterly flow across the Bahamas into Southeast Florida should also see an increase in convective activity with increased probabilities through much of the southern half of the peninsula, favoring the eastern side due to the slightly increased convergence. ...Upper Rio Grande Valley through Central High Plains... Day 1 period will see a lull in available moisture and convective convergence relative to prior and future days. However, the return flow around the western edge of the synoptic ridge. Low level flow/upslope through the Rio Grande/Pecos River Valleys will overlap with weak southwesterly flow across NW Mexico. Mountain circulation will likely drive initial convection and pose a risk mainly for known/prone burn scars across much of NM. However, deeper layer flow will continue to transport moisture northward through the Central High Plains. Weak convective clusters are expected to grow upscale after 00z across NE CO/SW NEB/W KS, where increasing pooled moisture could support slower clusters with internal cell mergers. HREF probs of 40-50% for 2" and 20% spots of 3" in an area of similar FFG values. As such, have expanded the Marginal Risk of Excessive Rainfall north-northeast to encompass the area up to the southern SandHills of NEB. Gallina Day 2 Valid 12Z Sun Sep 06 2026 - 12Z Mon Sep 07 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHERN NEVADA AND NORTHWESTERN ARIZONA TO CENTRAL UTAH AND OVER PORTIONS OF SOUTH CAROLINA AND GEORGIA... ...Western U.S.... An upper low centered along the Pacific Northwest coast early in the period is expected move inland, reaching the northern Intermountain West by the close of the period. Meanwhile, a series of shortwaves moving along the base of the associated trough will track across southern California before lifting northeast through the Great Basin. In addition to this energy, southwesterly flow will tap tropical moisture associated with Maria, supporting an impressive PW anomaly axis (+3 std dev) extending from southern California to central Utah. This moisture along with large-scale ascent afforded by the aforementioned shortwaves and upper jet forcing is expected to set the stage for widespread shower and thunderstorm activity. However, limited instability remains a potentially factor with regards to rainfall rates. A Slight Risk remains where models continue to signal where moisture, lift, along with orographic effects will support an enhanced threat for heavier amounts. Only minor adjustments were made to the previous Slight Risk area extending from southern Nevada and northwestern Arizona to central Utah, including a small shift to the north that reflects the latest trend in the guidance. Remaining in this area are some of the more flash flood prone areas in southwestern Utah. Farther east, more typical monsoon moisture will support diurnal activity, with the potential for isolated flash flooding across southeastern Arizona, New Mexico, and southern Colorado. Elsewhere, a broad Marginal Risk was maintained to the north, covering portions of the Northwest, northern Rockies, and High Plains. The previously noted upper low/trough is expected to support widespread activity. Instability west of the Rockies will remain quite limited, however rates may become sufficient to raise isolated concerns, especially across recent burn scars. ...Northern Plains to the lower Missouri Valley... While the models are showing plenty of differences in the details, there is a growing signal for heavy amounts impacting at least a portion of the area as energy spills off of the top of a persistent ridge centered over the Plains. This energy is forecast to interact with a strengthening low level jet and axis of deepening moisture (PWs ~1.75 inches). There is indication in some of the models, including the ARW, NAM, and RGEM, that heavy rains may be ongoing or developing near the start of the period across portions of the eastern Dakotas and western Minnesota. This activity will eventually drop south, followed by additional storms developing late in the period and possibly impacting some of the same areas. Model spread did not offer enough confidence to go higher than a Marginal Risk. However, this is an area where a potential upgrade may be necessary, especially if the hi-res guidance begins to show a more unified signal. ...Southeast... Energy dropping into the base of a broader-scale trough centered along the East Coast is expected to support a weak wave developing along a slow-moving, moisture-laden front extending from the coast near the South Carolina-Georgia border back across the Southeast. Models continue to signal heavy amounts near the coast, but also some potential for heavy rains and potential flash flooding extending as far north as Upstate South Carolina and western North Carolina as deep moisture is drawn northwest by the developing wave. Reflecting the overnight model trends, the Slight Risk was shifted a little farther south, but also extended northwest along the South Carolina-Georgia border. Pereira Day 3 Valid 12Z Mon Sep 07 2026 - 12Z Tue Sep 08 2026 ...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHWEST, NORTHERN ROCKIES, NORTHERN PLAINS, UPPER MIDWEST, AND THE SOUTHEAST... ...Northern Rockies to the High Plains... An upper low/trough will continue to translate east, reaching the northern High Plains/southern Canadian Prairie by Tuesday morning. Mid-level energy and upper jet forcing interacting with notable PW anomalies (+2 std dev) will contribute to widespread precipitation, with some orographically-focused heavier amounts. Rainfall rates will likely be checked by limited instability, but may be sufficient to cause isolated runoff concerns, especially in vulnerable areas. ...Northern Plains into the Upper Mississippi Valley... Moisture will continue to deepen across the region as southerly flow intensifies ahead of the previously noted upper trough moving out of the West. This is expected to fuel showers and storms developing ahead of a strong cold front moving across the northern Plains, with heavy amounts possible, especially across portions of northern Minnesota. ...Southwest... Additional showers and storms along an axis similar to the previous day are expected across portions of eastern Arizona, western to central New Mexico, and southern Colorado - where PW anomalies will remain around 2 std dev above normal. ...Southeast... Deep moisture will remain centered along a front that will continue to sink slowly south across the Southeast. PWs around 2 inches, along with weak mid-to-upper level forcing, will help accentuate diurnal activity -- raising the potential for heavy amounts and possible flash flooding across parts of the South Carolina Lowcountry, southern Georgia, and North Florida. Pereira Day 4 and Day 5 Valid 12Z Tue Sep 08 2026 - 12Z Thu Sep 10 2026 Day 4 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE SOUTHWEST AND OVER PORTIONS OF THE UPPER MISSISSIPPI VALLEY AND UPPER LAKES... ...Southwest... An axis of above average PW values will stretch from the Southwest, northeastward through the Central Plains and into the Upper Mississippi Valley and Upper Lakes region. Scattered monsoonal convection likely to continue across the Southwest day 4, supporting locally heavy rains and isolated runoff issues. ...Upper Mississippi Valley into the Upper Lakes... Farther to the northeast, while the frontal boundary overall is expected to be progressive day 4 moving from the Upper Mississippi Valley into the Upper Lakes, the southwestern portion of the front will become become more southwest to northeast oriented along the southern edge of the westerlies. At this time the low level inflow into this front is also expected to become more parallel to the boundary, supporting potential for a period of training along the front Tuesday night into early Wednesday. Day 5... ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS EASTERN CALIFORNIA, THE SOUTHWEST, THE CENTRAL PLAINS INTO THE MID MISSISSIPPI VALLEY AND OVER NORTHERN NEW YORK STATE INTO NORTHERN NEW ENGLAND... ...Sierra, southeast California, the Southwest and Southern Rockies... PW values expected to become increasingly anomalous day 5 across the Southwest, southern to central California and into the Great Basin. This is the result of the re-amplification of the mid to upper level trof off the northern to central California coast that will be capturing tropical moisture from in the vicinity of Marie and advecting it northeastward. Scattered convection in this anomalous PW axis, 2-3+ standard deviations above the mean, will support locally heavy rains and isolated runoff issues from the Sierra, into southeast California, the Desert Southwest and portions of the Southern Rockies. ...Central Plains into the Lower Missouri and Mid Mississippi Valley... A surface frontal boundary is expected to become west to east oriented along the southern edge of the westerlies from the Central Plains into the Lower MO and Mid MS Valley region. Shortwave energy pushing eastward along the southern edge of the westerlies will enhance uvvs along the west to east oriented front, supporting potential for organized convection in what is expected to be an axis of PW values 1 to 2+ standard deviations above the mean. Heavy rain and isolated runoff issues possible with this organized convection. ...Northern New York State into Northern New England... The northern and eastern extension of the front over the Central Plains to Mid MS Valley region is expected to remain progressive day 5 as it pushes from the Great Lakes into the Northeast. There is the potential for organized frontal convection in an axis of PW values 1.75-2" moving eastward across NY State into New England. A marginal risk was depicted where FFG values are relatively low across northern NY State into Northern New England. The very progressive nature of the front will be a detriment to any widespread runoff issues. Oravec 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 $$