FOUS30 KWBC 271857 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 257 PM EDT Mon Jul 27 2026 Day 1 Valid 1832Z Mon Jul 27 2026 - 12Z Tue Jul 28 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN EFFECT FOR PARTS OF SOUTHEAST WISCONSIN, NORTHEAST/EAST-CENTRAL ILLINOIS, AND NORTHWEST/WEST-CENTRAL INDIANA... 1830Z Special Update... A SLGT Risk was added to parts of the Midwest from SE WI through much of NE IL and NW IN, including the Chicago, Indianapolis, and Milwaukee metro regions. This is due to ongoing heavy rainfall and the expectation for intense rainfall rates to continue for several hours while the threat slides southward with time. Both 17z runs of the HRRR and RRFS support 2-5" additional rainfall amounts through 06z, which would produce scattered areas of flash flooding. Urban locations could see significant street flooding should thunderstorms train over the region. As the complex pushes southward this evening and overnight across central and southern IL/IN, rainfall rates and the threat of backbuilding storms should diminish as better forcing shifts eastward into the upper Ohio Valley. See MPD 0861 for more information. Snell Previous Discussion... ...Four Corners States onto the Central Plains... The monsoon ridge gets slightly suppressed south today by shortwaves cresting its periphery over the Northern Rockies. This will squelch the higher PW a bit south from previous days, but otherwise the ridge remains in place with weak impulses rotating around it to enhance ascent and resulting convection. Once again, the focus for thunderstorm development will be across the higher terrain late this morning, before expanding across the Four Corners states, with some organization possible through 20-30 kts of bulk shear. While PWs will still be +1 to +2 sigma from AZ through WY, the most impressive CAPE (approaching 750 J/kg) look to be confined to AZ/NM, which can be a better discriminator for excessive rainfall potential within the monsoon. However, any convection has the potential to produce intense rain rates leading to hourly rainfall of 1-1.5", which could produce at least isolated flash flood responses over vulnerable terrain features or burn scars. Storms that develop across UT/WY/CO may feature faster forward propagation (after brief slow downs during initiation) so the MRGL risk was expanded slightly into the Central Plains. However, this faster motion should at least somewhat limit the runoff potential. Farther south into AZ a shortwave moving along the Mexico/Arizona border within the elevated shear may help organized clusters of storms moving more slowly westward, and this is reflected by elevated (but still modest) probabilities for 3" of rain today. Considered a targeted SLGT risk for the TWC CWA but simulated reflectivity from the CAMs and the UA WRF were not sufficient for an upgrade at this time. ...Great Lakes and Ohio Valley into the Northeast US... A potent shortwave diving out of Ontario across the Great Lakes will interact with a complex surface pattern over the region to drive scattered to widespread thunderstorms through the day. Activity this morning is already producing heavy rainfall rates measured via MRMS of over 1"/hr within the pronounced ascent as it impinges upon an impressive overlap of PW/CAPE measured via morning 12Z soundings at APX/GRB of near record values (1.7-2.1 PWs and 4000-5000 J/Kg CAPE). As the aftn progresses, convection will become increasingly widespread as the shortwave dives south and pushes a cold front (and accompanying wave of low pressure) through the Great Lakes and southeast towards the Ohio Valley/Central Appalachians). While the thermodynamic environment will support intense rain rates within any thunderstorms today, and 40-50kts of bulk shear will help organize storms into clusters and/or supercells, mean winds of 30-40 kts suggest convection will remain progressive, and Corfidi vectors do not support much backbuilding/training. So while local impacts due to flash flooding are possible, the overall risk matches best the MRGL risk category so the inherited was changed only cosmetically for new guidance. ...Western Florida Peninsula... Weak shortwave diving along or just west of the Florida Peninsula will work together with broad directional jet- level diffluence to spawn repeated rounds of thunderstorm activity today. This convection will occur within robust PW anomalies reaching +1.5 to +2.0 sigma for the southern half of Florida, which when combined with SBCAPE exceeding 3000 J/kg will support torrential rain rates. While shear will be minimal indicating pulse-type convection, weak flow and resulting cell collisions or outflow mergers, as well as the Gulf breeze, will support scattered to widespread thunderstorms, all of which may contain 2-3"/hr rainfall. Due to the nature of this convection, placing the axis of heaviest amounts remains uncertain, but there is a common signal in both REFS and HREF probabilities for locally more than 5" (20-40% chance) near Tampa Bay, so the inherited MRGL risk was continued for this area, but expanded along the coast for potential beneath the forcing created by the upper diffluence, and added the Florida Keys after coordination with WFO KEY. ...Coastal Mid-Atlantic... A surface trough will sharpen east of I-95 this aftn providing a focus for ascent and convective development. The high-res CAMs suggest development will generally be isolated, but within favorable thermodynamics characterized by PWs above 1.5 inches and SBCAPE of 2000 J/kg or more. This will support intense rain rates within any storms that develop, and both the REFS and HREF neighborhood probabilities reach 50-70% for hourly rainfall of 1 inch or more. While there is some uncertainty into the coverage and placement of activity this aftn, the inherited MRGL risk was continued with just subtle cosmetic modifications since soils are saturated from heavy antecedent rainfall. Weiss Day 2 Valid 12Z Tue Jul 28 2026 - 12Z Wed Jul 29 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE NORTHERN MID-ATLANTIC INTO SOUTHERN NEW ENGLAND... ...Southern Appalachians into New England including northern Mid- Atlantic... The amplification of a mid- and upper-level trough moving out of the Great Lakes/Ohio Valley on Monday leads to the development of surface low pressure over the eastern Great Lakes by the start of the Day 2 period. However...the low stalls over interior NY/PA on Tuesday. There is has been growing certainly on the interior position of this low which then maintains the warm sector over the northern Mid-Atlantic coast into southern New England. Precipitable water values of 2 inches is highlighted through this warm sector which should also have sufficient instability for higher rainfall rates along with divergence aloft to promote growth and repeating activity. A Slight Risk remains over the northern Mid-Atlantic into southern New England with additional focus now across parts of northern New Jersey into southern New York (inland of New York City) into western Connecticut considered to be higher-end Slight. Elsewhere, warm sector moisture and instability is present down the Mid-Atlantic through North Carolina with potential for south moving activity to repeat from reinforcing shortwaves rounding the new low ot the north. ...Four Corners States across the Central Plains... Further suppression of the upper high center south through New Mexico opens the opportunity for additional downstream activity from the Four Corners states across the central Plains. The Marginal Risk was expanded east to the KC metro. The RRFS still highlighted the High Plains of both northeast and southeast Colorado with more typical Monsoonal activity over terrain in the Four Corners states. Bann/Jackson Day 3 Valid 12Z Wed Jul 29 2026 - 12Z Thu Jul 30 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHEAST WEDNESDAY... ...Northeast... A dynamic coastal system takes earlier in the week along/near the New England Coast per model guidance. Strong upper level jet energy upstream diving toward the trough base will amplify the upper trough -- to the tune of 2+ standard deviations below late July means at 500 mb-- which enhances the downstream QG forcing as it encounters deep moisture at low- to mid-levels being drawn in from the Atlantic Gulf stream, with a swath of ~1.75" PWs. As previously noted...given the persistence of favorable cool season dynamics with warm season thermodynamics (high deep- layer TPW/theta-e values), have maintained the Slight Risk area pretty much unchanged across parts of the Northeast, which fits the most recent global guidance rather well. Operational guidance shows 2-4 inches of rainfall across the Slight Risk area from Wednesday into late Wednesday night/early Thursday morning. Frontogenesis of the comma head could on its own lead to efficient heavy rainfall; not much MU/ML CAPE will be necessary to lead to flash flooding concerns. ...Four Corners States into the Central/Northern Plains... Monsoonal flow, likely to be enhanced by Genevieve moving offshore Baja California, brings elevated moisture over the Desert Southwest through the central Rockies and into the central and northern Plains. The more suppressed ridge will allow multiple mesoscale/convectively aided shortwaves to push over the top/across the Rockies into the Plains. The Marginal Risk area here only needed some cosmetic changes to better fit the most recent QPF guidance but the differences did not reflect a change in the forecast reasoning. ...Central Gulf Coast Region into parts of the Southeast... Numerical guidance continued to show heavy...to potentially excessive...rainfall being forced/focused by a cold front associated with the cyclone impacting New England. Still little reason to question whether the moisture will be ample at the same time that the effective bulk shear is sufficient for convective organization. The inherited Marginal Risk areas for Wednesday into Thursday night saw only a few cosmetic changes to better fit the most recent QPF guidance. Bann/Roth Day 4 and Day 5 Valid 12Z Thu Jul 30 2026 - 12Z Sat Aug 01 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE LOWER MISSOURI AND MID TO UPPER MISSISSIPPI VALLEY... ...Lower Missouri Valley into the Mid-Upper Mississippi Valley... On Day 4 (Thursday-Thursday night), convectively-aided shortwave energy progressing east from the central-northern Plains will amplify a bit (dig a bit farther south) upon reaching the Lower MO- Mid MS Valleys) during this time as the ~600DM 500mb high becomes centered farther west toward the Four Corners. There's near to above average discrepancy amongst the 00Z global guidance. The GFS/ GEFS mean/AIGFS are a bit farther east compared to the ECMWF/ECENS mean/EC AIFS, with the CMC camp basically in the middle in terms of eastward progression. In addition, there remains latitudinal differences with the heaviest QPF, with the 00Z UKMet the farthest north. Given the MCS/upscale growth potential with this system, considering the influx of 2-2.25+ PWs per the guidance, concentrated areas of heavy rain are expected, especially across the Corn Belt given the deep-layer moisture profile and high surface dewpoints. Certainly an event that would warrant a Slight Risk, which for now we've confined across eastern SD, far eastern NE and northeast KS, into northwest MO, western 2/3 of Iowa, and southern MN, where the global ensemble probabilities of >3" in 12/24 hours are highest. On Day 5 (Friday-Friday night), have essentially shifted the Slight Risk area farther downstream (east-southeast) per the global guidance consensus. Globals all show spotty heavy 24hr rainfall totals, i.e. 3-5+ inches, though again with some spatial spread in the guidance. ECENS QPF exceedance probabilities have come up from the previous run, and are certainly higher compared to the day 4 period (more expanded area of 3 to 5 inch probabilities). ...Four Corners States into the Central/Northern Plains... Monsoonal flow and anomalous deep-layer moisture will maintain a Marginal (isolated) excessive rainfall risk over eastern AZ and much of MN (except SE portion) into southeast UT and through much of CO. The expanding mid/upper level ridge (600dm heights at 500 mb) will likely limit the coverage compared to recent days, especially by Friday. ...Central Gulf Coast Region into parts of the Southeast... Heavy rainfall is expected along the New England cyclone's trailing front near the FL/GA border. Moisture should be ample and there could be enough effective bulk shear for convective organization. The inherited Marginal Risk areas for Thursday into Friday night saw only a few cosmetic changes to better fit the most recent QPF guidance. Hurley 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 $$