FOUS30 KWBC 270829 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 429 AM EDT Sat Jun 27 2026 Day 1 Valid 12Z Sat Jun 27 2026 - 12Z Sun Jun 28 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHERN PLAINS AND FROM THE MID MNISSISSIPPI VALLEY TO THE SOUTHERN APPALACHIANS... ...Mid-Mississippi Valley through Southern Appalachians... Another expansion of a high-end Slight Risk area was made along a west to east axis based on convection occurring prior to the start of the outlook period at 27/12Z...with the expectation that the risk persists through much of the Day 1 period in proximity to an east-west oriented stationary front. Low level westerly flow remains parallel to this boundary, raising concerns for repeating/training activity. The 27/00Z HRRR and the 27/00 RRFS QPF placement was similar to each other...but the RRFS appeared to be showings its bias with higher QPF and higher neighborhood probabilities of exceedance. The concern is greatest upon overlap of any additional rainfall with the footprint of recent heavy rainfall events. ...Mid-Atlantic... 00Z guidance offered a split decision of whether or not a frontal zone lingers over the central Mid- Atlantic through tonight rather than shifting shifting south to the Virginia/North Carolina border. Assuming it stays over the Mid-Atlantic...enhanced moisture and instability will remain over the DC area metros and the DelMarVa with a risk of isolated heavy to excessive rainfall otherwise the focus shifts south with time. Despite that uncertainty...any storms that form in such an airmass can easily produce localized downpours and excessive rainfall. This still looks to be a localized flash flood threat, but a focused Slight Risk may be needed eventually. ...Northern Plains... The upper low tracks over the interior Northwest on today with a shortwave troughing ejecting across northern Wyoming and eastern Montana by this evening. Low level forcing east of the approaching wave triggers repeating thunderstorms over eastern Montana through North Dakota from this afternoon into the evening. Increasing Low Level Jet should enhance rainfall within the Slight Risk maintained/expanded eastward in North Dakota. Activity should be fairly progressive, but the route of the upper trough may pivot/allow repeating activity. Bann/Jackson Day 2 Valid 12Z Sun Jun 28 2026 - 12Z Mon Jun 29 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN NORTHWEST MONTANA AND PORTIONS OF THE UPPER MIDWEST... ...Montana... Upper low center pivots over Montana Sunday with the surface low stalling along the international border . This maintains comma head/deformation zone rain shifting into northwest MT where terrain enhances rates at lower elevations and snow generally at or above 6500 feet. This is more of an areal flood concern given a lack of instability, but localized rates may warrant some rapid flooding concerns where precip bands are most stagnant. 00Z consensus rainfall remained in the 1 to 3 inch range around Glacier National Park onto the adjacent high plains in northwest MT. A marginal risk was maintained for this area. ...Upper Midwest... The warm sector of this anomalous low over MT will shift eastward allowing an airmass with robust Gulf moisture advection up the length of the Great Plains through the Upper Midwest. The development of surface low pressure over South Dakota will aid the moisture transport at the low levels and provide a lifting mechanism. Focusing on this...there was an eastward expansion and a bit of a southward expansion of the previously issued Marginal risk area. ...Coastal Carolinas/Mid-Atlantic... Added a small, targeted Marginal along the eastern seaboard where precipitation lingers from the Day 1 period. Blending disparate QPF in such cases tends to yield low amounts...but the area had the best overlap with highest precipitable water values and an area where ensemble members with differing cores tended to cluster. With such high moisture...localized downpours and heavy to excessive rainfall is a concern from any storm that can develop regardless of the meager focusing mechanism. Bann/Jackson Day 3 Valid 12Z Mon Jun 29 2026 - 12Z Tue Jun 30 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE GREAT LAKES REGION AND SOUTHWEST TEXAS... Great Lakes... Shifted the Marginal Risk area from Day 2 eastward into the western portion of the Great Lakes region as the feed of low level moisture from the Plains shifts in association with the parent system. Given recent conditions...only problems are expected in regions of poor drainage or small streams. Southwest Texas... Increasing instability should lead to late day convective development along and near the dry line in southwest Texas. Isolated heavy rainfall may result as moisture from the Gulf gets drawn northwestward in time...with a low end potential for excessive rainfall associated with the most active convection. Bann Day 4 and Day 5 Valid 12Z Tue Jun 30 2026 - 12Z Thu Jul 02 2026 Day 4 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE SOUTHERN HIGH PLAINS, UPPER MISSISSIPPI VALLEY/UPPER LAKES REGION AND ACROSS NORTHERN NY STATE INTO NORTHERN NEW ENGLAND... ...Northern New York State into Northern New England... Shortwave energy riding up over the top of the Ohio Valley upper high across southeast Canada into northern New England will help strengthen the low level westerly flow into the frontal boundary forecast to stretch northwest to southeast from eastern Ontario into northern NY State and northern New England late Tue afternoon into Tuesday night. There is potential for organized convection in the 0000 to 1200 UTC July 1 period along and north of this boundary. The GFS and EC continue to both show PW values becoming very anomalous along and just north of this front, with values 2.5-3+ standard deviation above the mean. While there is still a lot of spread with qpf details at this time range, there is a model signal for organized convection moving northwest to southeast resulting in heavy rains and isolated runoff issues. ...Southern High Plains... PW values expected to rise to 1 to 1.5+ standard deviations above the mean in the broad southwest mid to upper level flow across the Southern High Plains on the west side of the Lower MS closed upper high. Embedded shortwaves in this southwest mid to upper level flow will support increasingly scattered convection in this high PW axis, with locally heavy rains and isolated runoff issues. ...Upper MS Valley/Upper Lakes... Convection expected to become increasingly active late Tue afternoon into Tue night/early hours of Wed along and ahead of the surface front moving slowly across the Upper MS Valley. Favorable right entrance region jet dynamics ahead of this front in an axis of above average PW values will support potential for heavy rains with organized convection along and ahead of this cold front with isolated runoff issues possible. Oravec Day 5 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL THE UPPER MISSISSIPPI VALLEY/UPPER LAKES REGION AND ACROSS NORTHERN NY STATE INTO NORTHERN NEW ENGLAND... ...Northern New York State into Northern New England... Similar to the day 4 time period, additional shortwave energy is expect to ride along the top of the Ohio Valley closed upper high across southeast Canada into northern NY State and New England during day 5. These height falls will be accompanied by another area of anomalous PW values, 2 to 3 standard deviations above the mean, pushing out of southeast Ontario and southern QB into northern NY State and northern New England. While there is the typical amount of spread with qpf details, there is a signal for additional organized convection moving in a northwest to southeast direction across northern NY State into northern New England, supporting heavy rains and localized runoff issues, especially in areas of training. ...Upper MS Valley/Upper Lakes... Another round of shortwave energy pushing northeastward from the Central Plains into the Upper MS Valley/Upper Lakes region will support another round of potentially organized frontal/pre-frontal convection late Wednesday into early Thursday across these regions. .The GFS and EC both show PW values becoming anomalous in the vicinity of this front late Wednesday into early Thursday, supporting heavy rain potential and isolated runoff issues. The marginal risk was drawn to encompass the typical amount of spread in the guidance at this time range. 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 $$