910 FOUS30 KWBC 291551 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1151 AM 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...NORTHERN 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 into the portions of 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 risk of locally heavy rainfall also exists in portions of New England as a moisture ridges around the periphery of the upper high and approaches from the west/northwest in a region of weak height falls east of the upper ridge axis. 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 area. The chance of showers and thunderstorms pushes westward in conjunction with the front and kept the risk closest to the Gulf coast. Bann Day 3 Valid 12Z Wed Jul 01 2026 - 12Z Thu Jul 02 2026 ..THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL THE UPPER MISSISSIPPI VALLEY/UPPER LAKES REGION AND SOUTH FLORIDA... 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 Day 1 and 2 period. 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 MISSISSIPPI VALLEY INTO THE UPPER LAKES REGION ...Northern Plains/Upper MS Valley/Upper 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 trof. 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 in an axis of slightly above average PW values 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. 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 $$