FOUS30 KWBC 052011 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 411 PM EDT Sat Sep 5 2026 Day 1 Valid 16Z Sat Sep 05 2026 - 12Z Sun Sep 06 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR EASTERN NORTH CAROLINA INTO THE VIRGINIA TIDEWATER... ...Southern Ohio Valley through Carolinas... 16Z Update: Complex from this morning is making a steady progression into VA at this hour with a relatively progressive signature in the storm motion over the past 3 hrs. This steady progression will likely maintain itself over the course of the afternoon and evening as reflected via a majority of the CAMs and even the global interpretation. This has resulted in a slight decrease in the probs for >3" across portions of southern VA into NC with the strongest signal still prevalent over the Tidewater of VA down through eastern NC. This is still the thought for where the best flash flood threat would arise, but sandier soil composition in eastern NC will limit more substantial flash flood prospects and tend to favor isolated to widely scattered instances in more urban settings areas inland, west of Pamlico Sound. Sufficient buoyancy and deep moisture presence over the area outlined in the SLGT maintains a suitable environment overall with the 12z sounding out of KMHX signaling a moist boundary layer and strong SBCAPE output ~3500-4000 J/kg. This would propose any convective threat to be pretty prolific once matured with cells capable of local 2-3"/hr rates which tend to tip the scale for flash flood prospects in that area of the Carolina's. Considering the signal still prevalent, maintained general continuity of the SLGT with a small change in the northern fringe of the risk given the slightly more progressive attribute of this afternoon's convection in the VA. Across the Ohio Valley, signals for scattered heavy rains over IN down through southern OH/WV and KY remain with the frontal approach adding some surface based enhancement as it migrates through the area. Trailing mid-level vorticity around the backside of the trough will aid in upper support favorable for convective prospects through the peak hours of instability this afternoon and evening before sinking southeast into TN and the Carolina's by the back end of the forecast. The MRGL risk was more than sufficient for this threat as modest probabilities for >2" (25-40%) exist over the the aforementioned areas. Kleebauer ..Previous Discussion.. 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. Gallina ...Gulf Coast... 16Z Update: Current forecast MRGL across the Gulf Coast and the southern half of FL remains on track with the morning model consensus maintaining a consistent signature of scattered heavy rain signals with potential for local areas exceeding 3" in the hardest hit locales. Areas of note for the D1 will be over the western side of LA with a rapid development of convection after the 18z time frame that will lead to flash flood concerns in more urbanized footprints, especially those along I-10 in southwestern LA (Lake Charles to Lafayette area). Across FL, expecting rates to be between 2-3"/hr on average leading to localized flash flood concerns for areas south of I-4. HREF neighborhood probs for >3" are greater than 60% for a large portion of South FL with a sharp decline in probs for >5" meaning heavier totals likely in the 3-5" range which historically is sufficient for isolated flash flood potential over that area of the Sunshine state. Kleebauer ..Previous Discussion.. 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. Gallina ...Upper Rio Grande Valley through Central High Plains... 16Z Update: Little change was necessary for the inherited MRGL risk across portions of the Southwest U.S. into the Central High Plains. Models remains consistent in their presentation of heavy rainfall likely confined to the terrain with a few cells drifting off the terrain and impacting the neighboring valley's in AZ/NM before dissipating. Heaviest rainfall will occur over the Central High Plains of northeastern CO into southwest NE and northwestern KS. Modest probabilities for locally >2" exist over southwestern NE and the northeastern corner of CO leading to a solidly MRGL flash flood concern this evening. Kleebauer ..Previous Discussion.. 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 PORTIONS OF THE GREAT BASIN AND SOUTHEASTERN U.S... ...Great Basin.... 20Z Update: Changes to the previous SLGT were to extend the risk down into portions of the Mojave Desert with emphasis on the area encompassing Death Valley and surrounds. Trends are for locally heavy convection to occur near those areas by Sunday afternoon with rainfall rates between 0.5-0.75"/hr possible in the stronger cell cores. These areas are susceptible to flash flooding in these types of environments and projected rates, so wanted to cover for the threat given the susceptibility. Elsewhere into UT, extended the SLGT to include much of Central UT towards the Uinta Range where models have trended higher in their QPF signals as we approach the time frame of interest. Anytime you can advect PWAT anomalies of +3 or greater into the region, the prospects for flash flooding become much greater over this area of the country as past events would concur. QPF between 1-2" is forecast from southwestern UT up through Central UT, including the more favorable slot canyon areas in Zion and Bryce Canyon National Parks. HREF probs for these totals are solidly >60% everywhere in the current SLGT risk issued, a signal that typically correlates to the SLGT risk threat. Kleebauer ..Previous Discussion.. 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. ...Southeast... 20Z Update: The current forecast was maintained for areas encompassed in the SLGT risk over SC into the northern coast of GA. The SLGT was expanded to the south over much of southeastern GA down into northeastern FL as trends remain favorable for a bit more progression of the outlined front, below. HREF neighborhood and EAS probabilities took a notable jump in the latest run with the latter now indicating a 40-60% prob for at least 1", something notably more prevalent with SLGT risks across the Southeastern CONUS. Areal instability will run between 2000-3000 J/kg SBCAPE, on average with PWAT firmly >2" across the area situated along and south of the front. Low-level convergence pattern near the coastal plain from areas south of Charleston, SC down into Jacksonville will ultimately aid in convective initiation Sunday afternoon with a steady progression southward as we move through the period. Heaviest rainfall will begin over SC/GA and migrate into northern FL by late-afternoon, carrying into the evening. In coordination with the local Jacksonville WFO, a the SLGT risk was expanded southward to account for the threat and trends in the latest CAMs. Kleebauer ..Previous Discussion.. 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 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 ...Montana... Broad upper trough with several mid-level shortwaves ejecting northeast through the Northern Rockies and High Plains of MT will create a period of heavy rainfall potential in the form of scattered thunderstorms, some strong to severe in their maturation. PWAT anomalies based on the latest NAEFS show a +2 to +3 standard deviation located over much of central and eastern MT with the maxima located along the northern tier of the state, north of the Great Falls to Glasgow line. Relevant instability on the order of 1500-2500 J/kg MUCAPE along with the elevated PWATs, ample mid- level forcing, and increasing bulk shear will be sufficient for locally heavy rainfall threats within any convective genesis during the Sunday evening and overnight time frame. Rates between 1-2"/hr at peak intensity will offer the threat for localized flash flood concerns, especially in any urban footprint and flashier small streams littered across the region. One caveat to a more substantial risk is the progressive cell motions anticipated as the mean flow will allow for storm motions to average at least 30kts to the north-northeast after their initiation. This should limit more widespread flash flood chances, however this setup is close to something more prominent compared to the, "run of the mill" threat we see this time of year. A MRGL risk was maintained for the area, however this is an area of interest for an upgrade if the signal becomes more aggressive within future CAM forecasts. The risk remains on the upper threshold of MRGL for the time being. ...Northern Plains to the lower Missouri Valley... Multiple shortwaves will round the ridge and enter into the Northern Plains and Missouri River Valley between NE/IA during the course of the D2. Elevated PWATs and a prevailing LLJ in the morning and following evening hours will offer the opportunity for locally enhanced convective patterns with any thunderstorms capable of rainfall rates between 1-2"/hr at their peak intensity. FFG's across western IA into eastern NE remain on the higher side and the antecedent soils are generally dry, even after being impacted by convection earlier this morning. In any case, the prospect for 2-4" of rainfall within the general area above seems more substantial than normal considering the latest HREF and NBM probabilities >60% for at least 2" across the area. Considering the antecedent soils in the area and total rainfall being split between two time frames in the period, opted to maintain the MRGL risk, but a future SLGT is possible with a target mainly for that Missouri River basin where flash flooding concerns in several larger cities/towns along the river are heightened compared to areas away from the river. ...Southeast Texas... Isolated to scattered thunderstorms are forecast across Southeast TX, including the Upper Texas coast into the Piney Woods area that is still recovering from impacts from Edouard. Area FFG's are considerably lower than normal thanks to saturated grounds post storm, so the threat for flash flooding in any cells producing >2"/hr rates is higher leading to a localized flash flood threat. In coordination with the local Houston WFO, a MRGL risk was added over the aforementioned areas. Kleebauer 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... 20Z Update: The previous MRGL risk issuances across the CONUS were generally maintained outside a few minor adjustments overall. The general synoptic pattern will remain active over parts of the northern tier of the CONUS, as well as the Southeast and Southwest, respectively. QPF magnitudes in placement maintained a suitable run to run consistency that alleviated any reason to make noteworthy changes. Considering the consistency in the guidance, maintained continuity to the previous forecast(s). Kleebauer ..Previous Discussions.. ...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 Low Country, southern Georgia, and North Florida. Pereira Day 4 and Day 5 Valid 12Z Tue Sep 08 2026 - 12Z Thu Sep 10 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE SOUTHWEST/CALIFORNIA, PLAINS, GREAT LAKES, AND NORTHEAST... ...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. By Day 5, PW values expected to become increasingly anomalous 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. ...Upper Mississippi Valley into the Upper Lakes... A frontal boundary 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 oriented more southwest-to-northeast 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. The heavy rain threat continues into Day 5, focused from the western central High Plains eastward to the mid-Mississippi Valley where PW values will be +1 to +2 sigma. Some organized convection is possible, with heavy rain and isolated runoff issues. ...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. Fracasso/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 $$