FOUS30 KWBC 060054 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 854 PM EDT Sat Sep 5 2026 Day 1 Valid 01Z Sun Sep 06 2026 - 12Z Sun Sep 06 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE CENTRAL WESTERN HIGH PLAINS AND SOUTHEAST FLORIDA... ...Central Western High Plains... Expanded the Marginal risk area into western NE per rather robust convection that fired early this evening along a surface trough and continues at 01Z. Slower movement of these cells, which have exhibited some merging in roughly the same area, could support a flash flooding risk into the later evening hours. This threat could slip southeastward as well just south of the Sand Hills where instability remains. To the south, isolated convection persists over mostly northeastern New Mexico into southeastern CO and the OK Panhandle, and have adjusted the area a bit eastward. Removed the southern and southwestern portions of the Marginal risk as most activity has become quite isolated. ...Southern Ohio Valley through the Carolinas... Earlier convection over the NC coastal plain has mostly pushed southeastward/offshore, with generally lighter rainfall in its wake. We have removed the Marginal risk with this update. Across the Ohio Valley, isolated convective cells near the frontal boundary remain this evening but have not yielded a heavy rain threat. We have removed the Marginal risk from this area as well. ...Gulf Coast/Florida... Convection over southeastern TX into southern LA has diminished and should continue to weaken through the evening so the Marginal risk areas has been removed. Across FL, convection continues over the central/eastern portion of the southern half of the Peninsula where 2-3" of rain (in an hour or so) has fallen. Coverage should steadily decrease into the later evening hours, but maintained the Marginal risk here for especially the next hour or two before all heavier rainfall likely subsides. Fracasso 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 $$