FOUS30 KWBC 291951 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 351 PM EDT Mon Jun 29 2026 Day 1 Valid 16Z Mon Jun 29 2026 - 12Z Tue Jun 30 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NORTH DAKOTA TODAY... ...Northern Plains and Upper Mississippi Valley... A potent shortwave rounding the deep upper-trough over the western U.S. will be favorably timed to eject over the northern Plains during the afternoon/evening hours. This will lead to a dynamically favorable environment for the development of robust, organized supercell thunderstorms along an arcing frontal boundary through central North Dakota. Plentiful moisture with dewpoints in the upper-60s to low 70s and ML CAPE of 1000-2000 J/KG will promote efficient and very heavy rainfall of 1-2"+ per hour. Southwesterly to south-southwesterly mean flow/storm motions will also lead to the potential for at least a couple repeated rounds of storms over the same areas as the boundary progresses eastward. The latest hi- res guidance suggests areal average amounts of 1-2" and locally 3-4" which will lead to the risk of scattered instances of flash flooding particularly with proximity to the Canadian border. More isolated instances are likely further south. Additional storms will remain possible downstream along a lifting warm front through the Upper Mississippi Valley/Great Lakes along the Canadian border. Locally heavy rainfall with some heftier totals of 2-4" will be possible given the instability in place rounding the northern side of the upper-ridge. However, antecedent conditions and high FFGs should keep the risk of flash flooding isolated. ...Northern Rockies and High Plains... Further west, ongoing generally stratiform rainfall will continue as very moist air wraps around the backside of the surface low into the northern Rockies and adjacent High Plains. Climatologically high PWATs will continue to bring locally heavy rainfall particularly with embedded thunderstorms, and additional rainfall totals of 1-3" are possible. Runoff from the mountains combining with the persistent rain over the past couple of days through the valleys and into the High Plains will contribute to the threat of isolated flooding particularly along local streams, creeks, and rivers. ...Southwest Texas... Another round of thunderstorms is expected with daytime heating ahead of a dryline through southwest Texas. Increasing PWATs (1-1.5") approximately 1-2 standard deviations above the mean will promote heavy downpours with rain rates of 1-2" per hour. However, storms will tend to be shorter-lived and more sporadic natured given minimal shear and high storm-bases favoring quickly developing downdrafts. This should help to keep the threat for flash flooding isolated. Putnam Day 2 Valid 12Z Tue Jun 30 2026 - 12Z Wed Jul 01 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE GREAT LAKES...NEW ENGLAND...OVER SOUTHWEST TEXAS AND THE FLORIDA PENINSULA... ...Southern High Plains to the Great Lakes... There will be a risk of excessive rainfall in many of the same places as on Day 1 as a deep/strong low moves slowly from Saskatchewan to Manitoba while a broad, sprawling upper high expands across the Southern Plains into the Mid-Atlantic and parts of the Northeast United States. This results in broad southwesterly flow that transports Gulf moisture into parts of the Southern High Plains northeastward through the Great Lakes. Embedded shortwave energy within the flow will support increasingly scattered convection in this anomalously moist airmass, with locally heavy rains and isolated runoff issues. The concern is especially heightened over burn scars and dry washes/arroyos of the terrain near the southern Rockies. The latest hi-res guidance shows the potential for 1-3"+ of rainfall with these storms even if they remain rather short-lived and transient due to efficient downpours with rain rates of 1-2" per hour. ...New England... The latest hi-res guidance supports the maintenance/renewed development of thunderstorms in northwesterly flow over New England and portions of Upstate New York, possibly even tied all the way back to ongoing thunderstorms this (Monday) afternoon. The more widespread/organized nature of the convection would bring at least the risk of isolated flash flooding with storm total rainfall of 2-4" possible as suggested by much of the hi-res guidance. Additional refinement and possible upgrade to the risk area may be necessary with more confidence in this scenario depending on the evolution of ongoing convection through the overnight hours. ...Florida... A cold front will be the focus for showers and thunderstorms as it makes its way southward into the northern Florida peninsula. There has been an increasing signal in the models for convection and the storms look to be efficient rainfall producers considering able to tap precipitable water values around 2 inches so added a Marginal risk has been outlined. The chance of showers and thunderstorms pushes westward in conjunction with the front and kept the risk closest to the Gulf coast. Putnam/Bann Day 3 Valid 12Z Wed Jul 01 2026 - 12Z Thu Jul 02 2026 ..THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER THE UPPER MISSISSIPPI VALLEY/UPPER LAKES REGION...SOUTHWEST TEXAS...NEW ENGLAND...AND SOUTH FLORIDA... ...2030 UTC Update... Adjusted the coverage of the Marginal Risk to match regions similar to the day 2 period including portions of the High Plains into southwest Texas. A similar set up over the next couple of days looks to continue into day 3/Wednesday as PWAT values remain climatologically higher (1-2 standard deviations above the mean) and daytime heating supports ML CAPE in the 1000-2000 J/KG range. This environment should remain sufficient to produce a few more robust downpours with the threat for some isolated flash flooding. Another Marginal Risk has also been included for portions of New England. Another round of storms is expected in the northwest flow along the northeastern side of the ridge Wednesday afternoon, possibly intially over southeastern Canada and then progressing southeast into portions of New England. Similar to southwest Texas, PWATs will remain seasonably high and support at least some isolated heavier downpours. Antecedent conditions may also be more sensitive depending on the evolution of more widespread/potentially organized convection on Tuesday. Putnam ...Previous Discussion... Northern US... The large scale flow changes little during the Day 3 period with deep low pressure moving slowly from Manitoba into Ontario during the period while a strong and sprawling upper high is locked in over the eastern US (except for portions of New England). That leaves a broad region of moisture-transport from the central and southern portion of the Rockies front-range and nearby High Plains to the Upper Mississippi Valley and the Upper Great Lakes region. Concern for heaviest rainfall and associated risk of flooding looks to be along the international border states of the Upper Mississippi Valley into the Upper Great Lakes region as shortwave energy rotating around the upper low in Canada results in mid- and upper-level height falls during the latter part of the day and taps an airmass where moisture has been pooling. The GFS showed precipitable water values at or above 1.75 inches...some 2 to 2.5 standardized anomalies above the climatological values for this time of year...being in place. The main concern is for localized flooding where any new rainfall overlaps with the amounts that have recently been observed combined with amounts forecast in the Southeast Florida... The front which is expected to help focus showers and thunderstorms on Tuesday will continue to make its way into the southern Florida peninsula as it weakens on Wednesday. Thinking is that the combined effect of a weakening front and convergence along the sea breeze along with potential for sea breeze collisions will aid the chances of convection capable of producing locally heavy rainfall rates...especially in the highly urbanized areas. Some models produce little to no rainfall...so not confident enough to show more than a Marginal risk. Bann Day 4 and Day 5 Valid 12Z Thu Jul 02 2026 - 12Z Sat Jul 04 2026 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE NORTHERN PLAINS, UPPER MIDWEST INTO THE GREAT LAKES... ...Northern Plains/Upper Midwest/Great Lakes... A broad region of west southwesterly mid to upper level flow is expected to continue day 4 and 5 across the Northern Plains into the Upper MS Valley and Great Lakes between the eastern closed upper high and mean western trough, both of which are in the process of weakening by Saturday morning. Broadly diffluent mid to upper level flow associated with shortwave energy moving northeastward in this west southwest mid to upper level will support active convection, possibly organized, in an axis of slightly above average moisture across these regions. Broad marginal risk areas were depicted both day 4 and 5 to cover the current model spread for locally heavy rains and isolated runoff issues. Confidence remains low with respect to where convection may become organized and produce heavy rains and isolated runoff issues, possibly linked to the changing behavior of the flow across portions of the Central and Eastern US/Canadian border and ridge strength and orientation in the eastern US. Roth/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 $$