FOUS30 KWBC 280823 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 423 AM EDT Tue Jul 28 2026 Day 1 Valid 12Z Tue Jul 28 2026 - 12Z Wed Jul 29 2026 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE TRI- STATE AREA OF NEW JERSEY, NEW YORK, AND Connecticut... ...Southern Appalachians through New England... An anomalously strong upper low will produce impressive dynamics to create the potential for significant flash flooding across the Tri-State area today and tonight. A closed low will be forming over Upstate NY this evening and then continue to amplify and deepen as it curls southeast into New England. This intense system will help drive increasing divergence downstream, which when combined with the accompanying height falls will result in broad deep layer ascent from the Mid- Atlantic into New England. At the same time, the deepening system will force acceleration of the 850mb inflow, Precipitable water values and the magnitude of the surface-based CAPE will be near climatological maximums (reflected via the SPC sounding climatology), with precipitable water values around 1.75 inches overlapping most-unstable CAPE of 2000-3000 J/kg. Strong ascent in the vicinity of this anomalous closed low and near the accompanying occluded surface low into these strong thermodynamics sets a situation favorable for heavy, training rainfall, and flash flooding. The 28/00Z HREF and REFS both maintain probabilities for 2"/hr rainfall reach 15-25%, and Corfidi vectors aligned anti- parallel to the mean wind which will be aligned to the approaching front, which suggests a strong backbuilding and training potential. This fostered enough confidence for the day-shift to upgrade part of the area to a Moderate Risk. The Moderate risk was maintained with the western boundary of the Slight and Moderate expanded a bit westward--mainly in New York state--due to the model suite slowing down the timing of convective initiation and propagation. ...Four Corners States across the Central Plains... The expansive monsoon ridge will become additionally squeezed as shortwaves rotate anti-cyclonically around it both to the north and south. This will again support at least scattered monsoon convection as precipitable water values at or above 1 inch northward into the Four Corners overlapped with sufficient instability for afternoon development. Once again the risk will likely initiate in the higher terrain during the onset of peak heating, and then there appears to be sufficient shear (20-30 kts) for some organization off the terrain and into the lower elevations, primarily in New Mexico and Colorado, although there is potential for storms to be very slow movers. ...Western Florida Peninsula and the Keys... A shortwave diving along the Florida Peninsula/Western Gulf will interact with broad diffluence aloft to maintain deep layer ascent for convection across the eastern Gulf and into parts of South Florida and the keys. Mean 0-6km winds of just 5-10 kts will be aligned SW to NW, anti- parallel to collapsed Corfidi vectors, and parallel to the Keys. This suggests that convection will repeatedly build across the Gulf where PWs will be pooled at 2-2.25 inches within impressive 2000-3000 J/kg MUCAPE leading to 2-4" of rainfall with locally higher amounts. ...Missouri Valley... Overnight convection leading into the start of the Day 1 period at 29/12Z prompted a Slight Risk area ahead of the storms. The expectation is that the storms will weaken...followed by the next round of storms later today into tonight as a weakening shortwave crests the monsoon ridge before diving southeast. As the shortwave moves along a front tonight...it will help accelerate the already intensifying LLJ to draw robust thermodynamics northward. This will support organizing clusters of convection or an MCS aligning to the front as both 0-6km mean winds and Corfidi vectors become parallel. With rainfall rates likely reaching 1-2"/hr at times, any training has the potential to produce 2-4" of rain (HREF and REFS probabilities for 3+" 20-25%) leading to instances of flash flooding. Bann/Weiss Day 2 Valid 12Z Wed Jul 29 2026 - 12Z Thu Jul 30 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHEAST... ...Northeast... Rainfall rates and additional rainfall amounts will initially be heavy early in the period over the eastern portion of the Northeast before they begin to diminish on Wednesday as an anomalously deep low pressure system begins to occlude. There should be rainfall rates around one inch per hour leading to widespread 1-3" of rain with locally higher amounts. The latest guidance still depicts a sharp gradient of instability which may limit the rates in parts of Vermont, New Hampshire and Maine. This part of the Northeast US should have received less rainfall with Flash Flood Guidance less likely to have been compromised. The previous outlook largely had this covered and changes were fairly minor with this outlook. ...Four Corners States into the Central/Northern Plains... Monsoonal flow persists with isolated to widely scattered convection extending from the Four Corners region north and eastward...perhaps extending as far east as the plains of Colorado. Some of this rain could fall atop soils saturated from heavy rain on Tuesday and Tuesday night. It remains possible that a Slight Risk area may be needed for parts of the High Plains but there was not enough confidence to do so in this outlook. ...Central Gulf Coast Region into parts of the Southeast... A cold front stalling along the Gulf Coast will be a focus for slow moving and training thunderstorms on Wednesday. Guidance continues to indicate a low-probability threat for at least 3" of rain due to training of 1-2"/hr rain rates within favorable thermodynamics. This region can generally tolerate significant rain rates and this amount of rainfall, but any more pronounced training could result in isolated flash flood related impacts. Bann/Weiss Day 3 Valid 12Z Thu Jul 30 2026 - 12Z Fri Jul 31 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... Shortwave energy will be progressing east southeast as it tracks across parts of the central and northern plains on Thursday into Thursday night/early Friday morning. The shortwave energy will be encountering an increasingly moist and unstable airmass with precipitable water values in excess of 2 inches being drawn into the region ahead of the shortwave. Given the MCS/upscale growth potential with this system, concentrated areas of heavy rain are expected, especially across the Corn Belt. The Slight risk area already issued for Thursday still covers the risk....especially where the global ensemble probabilities of >3" in 12/24 hours are highest. ...Four Corners States into the Central/Northern Plains... Monsoonal flow and anomalous deep-layer moisture will maintain an isolated excessive rainfall risk extending from eastern Arizona northeastward across much of western and northern New Mexico into parts of Colorado. The expanding mid/upper level ridge will likely limit the coverage compared to recent days. ...Central Gulf Coast Region into parts of the Southeast... Heavy rainfall is expected along a front associated with a low pressure system which will be pulling away from New England. Heavy rainfall with some potential of flooding will be focused along and near the front...especially over parts of Florida and Georgia line. Bann Day 4 and Day 5 Valid 12Z Fri Jul 31 2026 - 12Z Sun Aug 02 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FRIDAY ACROSS PARTS OF THE MID AND UPPER MISSISSIPPI, AND ON SATURDAY OVER PORTIONS OF THE LOWER OHIO VALLEY, TENNESSEE VALLEY, AND DEEP SOUTH... ...Mississippi Valley-Ohio Valley-Tennessee Valley-Deep South... On Day 4 (Friday-Friday night), minor changes were made to yesterday's Day 5 ERO, including refinements to the Slight Risk area over parts of the Mid-Upper MS Valley, based on the latest (00Z) global deterministic and probabilistic QPFs. The operational models continue to show max 24hr totals in the 3-5" range, particularly over IA-southeast MN-southwest WI, though again with some spatial spread in the guidance. 00Z Operational ECMWF has a bit more areal coverage of the 3+ inch totals, while the ECENS QPF exceedance probabilities have once again come up from the previous run. Based on the latest guidance, along with the most recent ECMWF EFI output (EFI values aoa 0.6), have nudged the southern portion of the Slight Risk area a little farther north across the Mid MS Valley. On Day 5 (Saturday-Saturday night), the globals continue to show a fairly vigorous 500 mb trough/embedded shortwave energy slowly pivoting into the Mid MS Valley. There isn't as strong of a signal in the QPF guidance compared to the previous couple of days; however, the amplifying upper level trough (favorable Qs forcing and increasing effective bulk shear) will combine with a favorable thermodynamic profile (ML CAPEs 1000-2000 J/Kg along a swath of 1.8-2.1" PWs) to generate organized heavy rainfall threat across western portions of the Lower OH Valley into the TN Valley and perhaps as far south as southern AL, depending on how far south the upper shortwave and favorable 0-6KM bulk shear can get. Concern within this area will be heavy rainfall rates (2+ in/hr) given the thermodynamic profile, along with the potential for cell training of convection as the upper shortwave begins to stall and the low-level inflow out ahead of it increases up and aligns more parallel to the mean 850-300 mb flow. Because of these factors, we felt that a Slight Risk was warranted. ...Four Corners States into the Southern-Central High Plains... Monsoonal flow and anomalous deep-layer moisture will maintain a Marginal (isolated) excessive rainfall risk over eastern AZ and much of NM into much of CO. The expanding mid/upper level ridge (600dm heights at 500 mb) will likely limit the coverage compared to recent days, especially on 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, the front that will become more quasi-stationary Thu-Fri before lifting lifting back north on Sat ahead of the approaching upper trough. Deep-layer moisture and instability should be more than sufficient and the guidance shows sufficient effective bulk shear (0-6km magnitudes generally over 20 kts) for more organized/persistent convective organization. As a result, have maintained the inherited Marginal Risk on Day 4 (Fri- Fri night), with a similar Marginal Risk area on Day 5 (Sat-Sat night). 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 $$