FOUS30 KWBC 311558 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1158 AM EDT Fri Jul 31 2026 Day 1 Valid 16Z Fri Jul 31 2026 - 12Z Sat Aug 01 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF MID TO UPPER MISSISSIPPI VALLEY, AND THE UPPER PENINSULA OF MICHIGAN... ...Central US system... A potent shortwave diving across the Northern Plains this morning will deepen as it ejects southeast into the Upper Midwest by Saturday morning. This deepening will create an extremely anomalous mid-level low for late July, reflected by NAEFS height anomalies falling to below the minimum climatological values at 700mb, and to below the 10th percentile at 500mb. An impressively deep system overlapping late July thermodynamics is a clear indicator of excessive rainfall potential, and this event is no different. As the mid-level low dives southeast, it will drive increasing downstream ascent through divergence and height falls, with the subsequent surface low occluding across Iowa. Within this occlusion (and farther downstream warm sector) PWs will climb to 1.75 to 2.00 inches, above the 90th percentile for the date, and overlap with MUCAPE around 1000 J/kg. The accompanying 850mb inflow will be locally accelerated to 30-40 kts, drawing these pronounced thermodynamics northward to support heavy rain production within thunderstorms. At the same time, the associated theta-e ridge is progged to lift cyclonically, creating an impressive (especially for the end of July) TROWAL which will pivot atop the Upper Midwest. This will not only result in enhanced elevated instability, but also support additional moisture to fuel heavy rainfall. Both the HREF and REFS neighborhood probabilities indicate a 40-70% chance for at least hourly rainfall of 1", and although there continues to be some latitudinal uncertainty to the placement, the consensus of the available high-res CAMs is for locally heavy rain rates within a progressive band this morning, becoming more aligned for training later today. This is most likely beneath the TROWAL where Corfidi vectors and mean 0-6km winds become anti-parallel leading to backbuilding/repeating heavy rain rates. The greatest risk for 3"/24hrs from the high-res ensembles is from near St. Louis, MO northward through southern WI, which is also where there are notable probabilities for 5"/24 hrs (30-60%), especially near the Quad Cities of IA. This is also where the CSU First-Guess ERO field is most aggressive, so despite some dryness in the antecedent soils, an elevated SLGT risk is warranted for this region. Otherwise, cosmetic adjustments were made to both the SLGT and MRGL risks across the region. ...Upper Peninsula of Michigan... The impressive system moving through the Upper Midwest will result in increased deformation between 700-500mb pivoting into the Upper Peninsula this aftn/eve. This will encourage enhanced ascent across the region, despite a slow wane in MUCAPE noted via the various models. The accompanying ascent within this persistent deformation axis will support widespread showers and thunderstorms today, which despite the waning instability will be fueled by PWs as high as 1.5 inches, or above the 90th climatological percentile. This will support rainfall rates that are progged by both HREF and REFS neighborhood probabilities to have a 30-60% chance of exceeding 1"/hr, which will occur within aligned Corfidi vectors/mean 0-6km winds to support training. 3-hr FFG across the area is generally 1.5 to 2.5 inches which has 20-40% chance of exceedance. This suggests at least scattered instances of flash flooding are possible, but will be most likely where any more prolonged training can result in 3 inches (50-60% chance) to as much as 5 inches (10-20% chance) of rain. After coordination with WFO MQT, a SLGT risk was added for the increased excessive rainfall potential today and tonight. ...Northern Florida through the Central Gulf Coast... Robust late-July thermodynamics characterized by PWs of 2-2.25 inches and MUCAPE above 2000 J/kg will spread across the area today and tonight ahead of a quasi-stationary front. This will likely result in two periods of convection capable of producing heavy rainfall. The first will be during peak heating when airmass thunderstorms will develop, aided by a modest vorticity impulse digging into MS/AL around the large mid-level ridge to the west. Weak shear (less than 20 kts) during this time suggests storms that develop will be generally of the pulse variety with limited organization and lifespans, but storm mergers, boundary collisions, and any locally enhanced ascent in the vicinity of this shortwave could increase the overage and intensity of storms during the aftn/eve. These storms should wane with convective overturning during the evening, but a secondary signal has shown up in the guidance for late tonight into early Saturday, especially across the Florida Panhandle and into the Big Bend area. Here, the aforementioned shortwave will slow within an area of thickness diffluence, leading to a longer period of enhanced synoptic lift aided by modest jet- level diffluence. This ascent acting upon any residual boundaries and the land breeze could reignite thunderstorms as the LLJ attempts to back subtly more to the WSW from W as the front approaches, drawing even more impressive PWs along the coast. During this time, Corfidi vectors are progged to collapse to 5 kts or less and become increasingly anti-parallel to the mean wind suggesting some greater backbuilding/training potential with rain rates potentially reaching 3"/hr (10-20% chance). Both the HREF and REFS indicate a 20-40% chance for 5" of rainfall from this nocturnal event, but the deterministic models are quite different in the coverage and placement. At this time, after coordination with WFO TAE, the MRGL risk was left unchanged for this area, but an upgrade to a targeted SLGT risk may be needed with later updates, with the SLGT potentially extending (or being more needed) through D2. ...Four Corners States into the Southern-Central High Plains... The monsoon ridge will get suppressed to the south and west in response to the potent system moving through the Plains/Midwest, with a secondary piece of vorticity energy dropping across the southern High Plains into the Texas Panhandle. This will help push a cold front through the Southern Plains and bank against the terrain of New Mexico later this evening. The overlapping ascent from the cold front and shortwave will act upon favorable thermodynamics as a ribbon of PWs above 1.5 inches and pooled MUCAPE of 1000-1500 J/kg get drawn westward ahead of the front. Coverage of monsoonal thunderstorms may be somewhat less than previous days today due to the re-centereing of the ridge. But at least scattered diurnal thunderstorms are expected from eastern AZ, through NM, and into eastern CO with hourly rainfall up to 1 inch at times. The focus for heaviest rain is likely where the shortwave/front move into NM/TX Panhandle where some organized clusters and better training potential exists. This is reflected by both HREF and REFS exceedance probabilities reaching 15-25% (FFG) and 30-50% (3"/24hrs). This still supports the inherited MRGL risk, and only cosmetic adjustments were made based on the new guidance. Weiss/Miller Day 2 Valid 12Z Sat Aug 01 2026 - 12Z Sun Aug 02 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FRIDAY ACROSS PARTS OF THE UPPER MIDWEST TO MID MISSISSIPPI VALLEY... ...Ohio/Tennessee Valley... A potent low pressure system will supply plentiful instability to the Great Lakes/Ohio and Tennessee Valleys region beneath a deepening positively tilted mid-level low. PW values of 2+ standard deviations above average (2-2.25 inches) will be advecting into the region to support enhanced rainfall. A Slight Risk remains in effect from eastern Illinois to Ohio and southward to Alabama and Georgia. A broad Marginal Risk spans from Wisconsin to western New York south to the eastern Gulf Coast and Southeast. ...Central Gulf Coast Region into parts of the Southeast... Heavy rainfall threat will continue over this region along the southern periphery of the full-latitude trough, south of the quasi-sationary front. Deep-layer moisture and instability should be more than sufficient, especially when coinciding with the weak upper forcing (upstream upper trough w/ embedded convectively-aided shortwaves), for more organized/persistent convective organization. ...New Mexico... During this period general storm motion is expected to be from north to south into an increasingly moist environment (1"+ and 97.5th percentile PWATs) and relatively saturated soils from Friday storms will contribute to the flash flooding threat. A Slight Risk was maintained for portions of central/eastern New Mexico with a Marginal Risk area stretching from southeast Arizona to western Texas and northward to southern Colorado. Campbell/Kebede Day 3 Valid 12Z Sun Aug 02 2026 - 12Z Mon Aug 03 2026 ..THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHEAST... ...Upper Midwest to Northeast and Mid-Atlantic region... The potent mid-level cyclone will continue to track to the east during this period ushering the showers and thunderstorms along with it into the Lower Great Lakes and Northeast region. Streaks of heavy rainfall are also expected to develop across parts of the Mid-Atlantic/North Carolina region. Some of these cells will have the potential to yield 2 inches over a short period during the afternoon and evening hours. A broad Marginal Risk spans from the North/South Carolina border northward to New York and Lower Michigan. A Slight Risk area covers extreme southeast Michigan, northeast Ohio, northern West Virginia, western Pennsylvania and western New York. ...Southeast... Showers and thunderstorms will persist along the stalled frontal boundary thus maintaining an elevated threat for heavy rainfall and potential for local flash flooding. Therefore, a Marginal RIsk area was maintained for eastern Gulf Coast/northern Florida and southern Georgia. ...Southwest... Monsoonal moisture will continue to drive convection across portions of New Mexico and Arizona. Locally heavy rainfall may continue to result in isolated flash flooding across the region. A Marginal Risk area is in effect for southern Arizona and southwest New Mexico. Campbell Day 4 and Day 5 Valid 12Z Mon Aug 03 2026 - 12Z Wed Aug 05 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE NORTHERN MID ATLANTIC INTO SOUTHERN NEW YORK STATE AND SOUTHERN NEW ENGLAND... The well defined upper trof moving through the Ohio Valley day 3, will begin to split into two pieces day 4. While there are differences with the details with this upper trof evolution, there is agreement that an axis of anomalous PW values, 2+ standard deviations above the mean, 2 to 2.25" PW values, will stretch from North Florida, northeastward across the coastal Southeast, Mid- Atlantic and from eastern NY State into New England. Generally favorable right entrance region jet dynamics will accentuate lift in this anomalous PW axis, supporting potential for locally heavy rainfall totals. A slight risk was added over portions of the northern Mid-Atlantic into southern NY State and southern New England where recent heavy rains have lowered FFG values. A marginal risk extends southward through the remainder of the Mid- Atlantic into the Southeast where locally heavy rains are also likely but FFG values are higher. For both day 4 and day 5...a marginal risk was depicted for monsoonal convection across southern Arizona into New Mexico. Not a lot of confidence with qpf details here, but locally heavy rains and isolated runoff issues possible in the above average PW values expected across this area. Day 5...the closed low over the Upper OH Valley will become nearly stationary day 5. An area of anomalous PW values will remain to the east of this closed low, across the Upper OH Valley into the Mid- Atlantic, Southern New England and south into the coastal southern Mid-Atlantic. A broad marginal risk area was depicted in this anomalous PW axis for locally heavy rains and isolated runoff issues. The surface front moving through New England day 4, may become stationary and west to east oriented across southern New England into the northern Mid-Atlantic day 5. A few models are showing more defined heavy rains in the vicinity of this front where there is potential for persistent south southwesterly inflow. Still a lot of spread with qpf details, warranting only a marginal risk at this time, with changes possible with future updates. A surface front expected to push southeastward day 5 from the Upper MS Valley into the Upper Lakes and south through the Central Plains. There is model consensus for an axis of potentially heavy rains from Nebraska, eastward into Iowa and southern MN where the front may become more west to east oriented late Tue into early Wed. Moist southerly low level inflow into the front at this time may support organized convection, 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 $$