FOUS30 KWBC 270820 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 420 AM EDT Mon Jul 27 2026 Day 1 Valid 12Z Mon Jul 27 2026 - 12Z Tue Jul 28 2026 ...THERE ARE MARGINAL RISK OF EXCESSIVE RAINFALL AREAS ACROSS THE FOUR CORNERS STATES ONTO THE CENTRAL PLAINS AND GREAT LAKES/OHIO VALLEY INTO THE NORTHEAST... ...Four Corners States onto the Central Plains... The center of an upper high will shift southward into New Mexico...which will direct more ridge topping diurnally- driven activity from the central Rockies onto the central Plains later today and through much of the upcoming night. Favored focus areas for QPF are again along the Mogollon Rim in Arizona north/east into Colorado. Only a minor expansion to the Marginal Risk is noted with the HREF and RRFS which would favor the central Plains activity into southwestern South Dakota. ...Great Lakes and Ohio Valley into the Northeast US... A shortwave that helped trigger convection over parts of the Minnesota Arrowhead and nearby portions of northwestern Wisconsin overnight leading into the the start of the Day 1 period at 27/12Z will continue to shift towards the southeast across portions of the Upper Great Lakes this afternoon and evening. This is in northwest flow downstream of the upper level ridge axis stalled over the western Dakotas. Increasingly difluent flow ahead of the shortwave will lead to the consolidation and the strengthening of a low pressure system later tonight...which in turn increases the forcing for heavier rainfall over the Great Lakes/Ohio Valley. No major changes were made to the Marginal Risk area, ...Tampa Bay... Troughing occurs over the western side of the Florida peninsula which shifts the focus for repeating thunderstorm with locally heavy rainfall towards the greater Tampa Bay from later this morning into the afternoon. Monday morning into the afternoon. Consensus average QPF is 2-4" with potential for 6" from both the HREF and RRFS. A Marginal Risk for flash flooding was maintained especially due to urbanized drainage concerns. Bann/Jackson 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 $$