FOUS30 KWBC 070826 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 426 AM EDT Mon Sep 7 2026 Day 1 Valid 12Z Mon Sep 07 2026 - 12Z Tue Sep 08 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR A PORTION OF NORTHWESTERN MONTANA... ...Northwest Montana... A well defined shortwave expected to rotate northeastward from the Northern Rockies into the Northern High Plains Monday. Model consensus is for the development of a comma head/deformation precip band to the northwest of this feature across northwestern portions of Montana. There will likely be a period when this comma head/deformation band pivots over northwestern Montana, supporting heavy precip totals. The slight and marginal risk areas across this area were changed slightly from previous issuances to align better with the latest HREF and RRFS neighborhood probabilities for 1 and 2"+ amounts. Marginal risk areas ...Upper Mississippi Valley... The height falls ejecting northeastward from the Northern Rockies into the Northern Plains will strengthen the low level southerly flow along and ahead of the associated frontal boundary, raising PW values to 1.75-2.00"+. Overall favorable large scale conditions will support potential for organized pre-frontal/frontal convection over the Upper MS Valley. At the moment, this convection should be very progressive, limiting the excessive rainfall potential to marginal. No significant change made to the previous outlook area across this region. ...Eastern Arizona into western to central New Mexico... PW values expected to remain above average day 1 from eastern Arizona into western to central New Mexico. Widespread scattered convection possible in this high PW axis,supporting locally heavy rains and isolated runoff issues. ...Northeast Florida into coastal Georgia and coastal southern South Carolina... An axis of above average PW values will persist day 1 from the coastal Southeast into northeast FL. Both the HREF and RRFS neighborhood probabilities are fairly high in this above average PW axis for 2 and 3"+ precip totals. Not a lot of confidence in qpf details, but potential for locally very heavy rains and isolated runoff issues, especially in more urbanized areas. The previous marginal risk was reduced along the northeastern Gulf coast to where the HREF and RRFS probabilities are greatest. Oravec Day 2 Valid 12Z Tue Sep 08 2026 - 12Z Wed Sep 09 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER IOWA... ...Upper Mississippi Valley into the Upper Lakes... The surface front pushing east into the Northern Plains day 1 will remain very progressive day 2 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. This is shown in the latest guidance that has consensus for a heavier axis of qpf here. A small slight risk area was added across portions of central to southern Iowa for this training potential. RRFS neighborhood probabilities are fairly high for 1 and 2"+ amounts across the slight risk area, with a smaller 50%+ probability for 3"+ amounts over east central Iowa. Marginal risk areas ..Eastern Arizona into western to central New Mexico... No significant changes day 2 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 2 in this anomalous axis. The previous marginal risk area over northeast Montana was removed given latest model qpf consensus. The broad marginal was also removed along the Gulf coast into FL given less organized precip than day 1 Oravec Day 3 Valid 12Z Wed Sep 09 2026 - 12Z Thu Sep 10 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE SIERRA INTO SOUTHEAST CALIFORNIA, THE SOUTHWEST, THE MID MISSISSIPPI VALLEY INTO THE OHIO VALLEY AND NORTHERN NEW YORK STATE INTO NORTHERN VERMONT... ...Sierra, Southeast California, Southwest... A very broad area of anomalous PW values, 2 to 3+ standard deviations above the mean, will stretch day 3 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 3. The marginal risk was expanded northward from day 2 into the Sierra to capture any locally heavy precip amounts in the much above average PW axis. ...Lower Missouri, Mid Mississippi, Ohio Valley region... A surface cold front expected to sink southward through the Mid MS, Lower MO and into the OH Valley region day 3. PW values expected to remain anomalous along this front with values 2"+, supporting potential for widespread moderate to locally heavy totals. The front remains very progressive to the south, which along with relatively high FFG values, will limit widespread runoff potential. ...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 4 and Day 5 Valid 12Z Thu Sep 10 2026 - 12Z Sat Sep 12 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL APPALACHIANS AND UPPER OHIO VALLEY ON THURSDAY... ...Central Appalachians and Upper Ohio Valley... A fast-moving cold front tracking east across the Midwest will provide the forcing, coincident with a strong upper level shortwave trough, for numerous showers and thunderstorms to impact the central Appalachians and upper Ohio Valley Thursday afternoon and evening. Ahead of this front, deep tropical moisture from off the Gulf will advect northeastward into the region. PWATs are expected to exceed 1.75 inches, perhaps exceeding 2 inches in spots as the thunderstorms rumble through. The front itself will be aligned parallel to the mountains, moving southeastward/perpendicular to the mountains. Thus, ideal uplift will be in place as the front hits the mountains all at roughly the same time. The northwesterly flow behind the front running straight into the mountains will locally enhance the uplift, and thus strengthen the numerous storms expected Thursday afternoon. Soil moisture levels are about normal for this time of year. Fortunately, no significant rainfall is expected in this area until Thursday, which should help mitigate some of the resultant flash flooding as the storms move through. However, with ideal uplift, there will likely be some training and backbuilding as continued uplift through the afternoon and evening allows repeat thunderstorms to develop on the windward sides of the mountains, then continue advecting southeastward. Given the abundant moisture and forcing present, as well as the flash-flood-prone nature of many of the mountain and river valleys of the central Appalachians, as well as any urban concerns from Pittsburgh south through Charleston, a Slight Risk upgrade was introduced with this update. Model guidance is in good agreement on the position of the front during this period, with the storms moving through at peak heating...maximizing their rainfall output. ...Marginal Risks... Across the Southeast and Florida Peninsula, rounds of afternoon showers and thunderstorms are expected both Thursday and Friday. Typical heavy rains are expected along the Gulf Coast. While any one storm is unlikely to cause flash flooding, the widespread coverage of storms and the decently high likelihood that storms will impact more flood-sensitive urban areas led to the continuation of the Marginal Risk for these areas. The monsoon in the Southwest will be ongoing both Thursday and Friday, but its coverage and intensity should be subdued compared to recent days. Thus, the Marginal Risks are generally confined to Arizona and New Mexico. A strong shortwave will track along the Montana/Canada border Thursday night into Friday. The heaviest rains associated therewith are likely to remain in Canada, but enough guidance suggests enough convection impacts the Montana side to have a Marginal Risk for the northern half of the state on Friday. 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 $$