FOUS30 KWBC 082008 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 408 PM EDT Tue Sep 8 2026 Day 1 Valid 16Z Tue Sep 08 2026 - 12Z Wed Sep 09 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS IOWA... 16Z Update.. Only a minor expansion to the inherited SLGT Risk area was needed into northeast Iowa and through far southwest Wisconsin. The previously mentioned meteorological setup remains valid, and adjustments were made based on the latest suite of global and CAMs guidance. The surface front forecast to move through Iowa will support a line of a progressive and efficient line of thunderstorms. PWAT values are forecast to exceed 2", and in some scenarios approach 2.25", and will support heavy rain rates, with HREF and RRFS probs suggesting high chances (65% and 85% respectively) of 1"/hr rates. SREF Spaghetti Plots also suggested greater consensus of 2" amounts over the Day 1 period in northeast Iowa, which further supported the expansion. Otherwise, only cosmetic adjustments were needed for the inherited MRGL Risk area to match the latest run of global and CAMs guidance. Blanco-Alcala ...Previous Discussion... ...Iowa... The surface front currently pushing east into the Northern Plains will remain very progressive day 1 as it moves into the Upper Mississippi Valley/Upper Lakes. The progressive nature of the front will continue to be a detriment to widespread runoff issues. However, the southwestern portion of this front will slow somewhat compared to the farther northern portion in the vicinity of eastern NE /northeast Kansas, east into Iowa. Across these regions there is a better chance of a period of training of cells late Tuesday/early hours of Wednesday. A slight risk area was maintained where current HREF and RRFS neighborhood probabilities are high for 1, 2 and 3"+ amounts this period. Marginal Risk areas ..Eastern Arizona into western to central New Mexico... No significant changes day 1 to the axis of above average PW values extending from eastern portions of Arizona into western to central New Mexico. Continued low confidence with qpf details, but potential for another round of widespread scattered convection across this area. Locally heavy rain and isolated runoff issues again possible. Anomalous PW values also expected to push farther north from Southern CA into Central CA. At the moment, models are not showing precip developing day 1 in this anomalous axis. ...Central to South Florida... No changes made to the previous marginal risk across central to south Florida. PW values are expected to remain anomalous day 1, 2.00-2.25"+ to the south of the frontal boundary across northern to central FL. HREF and RRFS neighborhood probabilities are fairly high for 1,2 and 3"+ totals. Areas of heavy rain likely with isolated runoff issues possible, especially in urbanized regions. Oravec Day 2 Valid 12Z Wed Sep 09 2026 - 12Z Thu Sep 10 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER FAR SOUTHEAST KANSAS INTO CENTRAL MISSOURI AND THE SOUTHWEST... 21Z Update.. A Slight Risk upgrade was introduced in southeast Arizona and far western New Mexico in coordination with the local forecast offices. The early availability of CAMs guidance indicated notable FFG Max Exceedance probabilities, especially in the RRFS. Anomalous mean PWATs are expected to exceed the 90th percentile throughout the period. Scattered thunderstorms will form and be fueled by this anomalous moisture, and will be locally supported in the upper- levels, with precip occurring along the eastern periphery of a ridge, promoting some areas of divergence aloft. A building upper- ridge should confine precipitation to be in the form of scattered thunderstorms mostly along southern Arizona, but will have the potential to bring heavy rainfall. Elsewhere, the inherited SLGT risk was expanded to include more of eastern Kansas and central Missouri. Updated HREF & RRFS precipitation probabilities suggest medium to high chances (~55% & 75% respectively) of rainfall exceeding 3" during the day 2 period. In terms of hourly rates, both HREF and RRFS neighborhood probabilities suggest medium chances ~50-60% of rainfall exceeding 1"/hr over a 3-5hr period. The general storm motion should be fairly progressive however, which resulted in maintaining the Slight. The inherited MRGL Risk areas were also expanded and introduced through the Ohio Valley and Lower Great Lakes, as well portions of the Southern Plains, to capture the latest run of global models and ensembles. An introduction to a SLGT was considered over the Ohio Valley given higher QPF totals and higher rain rates per the HREF, but the expected quick progression of storms was the main deciding factor to not upgrade to a SLGT. Later runs may necessitate the need for a SLGT, particularly if storm motions slow. Blanco-Alcala ...Previous Discussion... ...Lower Missouri Valley... A surface cold front expected to continue to sink southward through the Mid MS, Lower MO and into the OH Valley region day 2. PW values expected to remain anomalous along this front with values 2"+, supporting potential for widespread moderate to locally heavy totals. A small slight risk area was added over far southeastern portions of Kansas into central Missouri for potential of training of cells in this anomalous PW axis. Much of this area has, however, been very dry over the past month, which may mitigate potential for runoff issues. RRFS neighborhood probables are high for 1 and 2"+ totals day 2 in the slight risk area and RRFS probabilities of 3 hour precip exceed FFG values are in the 20-30%+ range at 0000 UTC Sept 10. These probabilities are not available for the 0000-1200 UTC period, but given the overall favorable pattern along this front for heavy rains, similar probabilities likely will continue along the front into early Thursday morning. Marginal risk areas. ...Sierra, Southeast California, Southwest... A very broad area of anomalous PW values, 2 to 3+ standard deviations above the mean, will stretch day 2 from central to southern California, across the southern Great Basin and into the Southwest. Latest models do show scattered precip in this high PW axis from the Sierra into Southeast California and over the Southwest. The heaviest totals continue across similar areas day 1 and 2 from eastern Arizona into western to central NM where a marginal risk continues day 2. The marginal risk was expanded northward from day 1 into the Sierra to capture any locally heavy precip amounts in the much above average PW axis. ...Northern NY State into Northern Vermont... The northeastern portion of the front moving south into the Mid MS Valley/OH Valley region will also remain very progressive to the east across the eastern Lakes, northern NY State and northern New England. PW values expected to be anomalous along this portion of the front also, with values 1.75-2.00"+. No changes to the marginal risk area for far northern NY state into northern VT where model consensus is for moderate to heavy precip totals. The expected very progressive nature of the front will be a detriment to widespread runoff issues, but the anomalous PW values and potential for a brief period of training may produce isolated issues. Oravec Day 3 Valid 12Z Thu Sep 10 2026 - 12Z Fri Sep 11 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER THE UPPER OHIO VALLEY INTO THE CENTRAL APPALACHIANS... 21Z Update.. Virtually no change was made to the inherited Day 3 outlook other than cosmetic adjustments and minor tinkering to the SLGT Risk area to match the latest run of global and ensemble guidance. An uptick in the multi-model ensemble bias-corrected QPF was noted as a potential concern, particularly in Kentucky, although there was a lack of agreement in the global models, and particularly high QPF in the NAM when compared to the other global models. As such, no expansion was needed here, although it could be considered in future updates if CAMs guidance also decide to go the NAM route. Blanco-Alcala ...Previous Discussion... ...Upper Ohio Valley into the Central Appalachians.. PW values 2+ standard deviations above the mean are forecast along the west-east oriented frontal boundary sinking south through the Ohio Valley, Central Appalachians and Mid-Atlantic region day 3. A slight risk was maintained where there is an overlap of relatively low FFG values and expected moderate to heavy precip totals. H85 flow parallel to this front will support training potential and also enhance uvvs in the upslope regions of the Central Appalachians. Marginal Risk areas. ...Southwest... A marginal risk area was continued over portions of the Southwest for potential for local heavy rains and isolated runoff issues. Model consensus is for a smaller area of potential heavy rains compared to the previous two days, with the marginal risk area drawn for a smaller geographic area over southeast Arizona and southwest New Mexico. Confidence remains low with qpf details day 3. Oravec Day 4 and Day 5 Valid 12Z Fri Sep 11 2026 - 12Z Sun Sep 13 2026 ...THERE IS A MARGINAL RISK FOR FLASH FLOODING FOR MUCH OF THE SOUTHEAST AND PORTIONS OF THE MIDWEST, PLAINS, AND WEST... ...2030Z Update... Few changes needed. The previous ERO still covered the few places where the 12Z suite of guidance still showed the highest QPF totals (along and ahead of the upper Midwest cold front...Western US...Southeast US...). Bann Previous Discussion... Large sections of the country are in a Marginal Risk for flash flooding on both Friday and Saturday. Across the Southeast, abundant tropical moisture will remain in place under weak ridging/a rather zonal jet over the Northeast. From the lower to mid Mississippi Valley east across the Ohio and Tennessee Valleys, and into VA/NC, the proximity to fronts or the Texas dryline will generally decide where more organized and concentrated showers and thunderstorms will develop Friday afternoon into the evening. Meanwhile, a strong low well up into Canada will push along a cold front over the upper Midwest/upper Mississippi Valley. While the front will be more progressive into the upper Lakes, the front may stall further south near KS/MO. This area near the end of a front is always a dangerous spot to be as far as flash flooding concerns go, with abundant moisture, forcing, and slow-moving storms all concentrated in one area. Thus, this area will be closely watched in the coming days for a potential Slight Risk upgrade. Across the West, the monsoon will continue across the Four Corners region both days. By Saturday, a strong "bowling ball" upper low will swing across the Pacific Northwest, providing the forcing for widespread shower and thunderstorm activity over generally flood- prone areas due to the usual aridity over the area and the plentiful terrain. Isolated instances of flash flooding are possible. Wegman 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 $$