FOUS30 KWBC 280049 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 849 PM EDT Mon Jul 27 2026 Day 1 Valid 01Z Tue Jul 28 2026 - 12Z Tue Jul 28 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN EFFECT FOR PARTS OF CENTRAL INDIANA AND ILLINOIS... ...Great Lakes and Ohio Valley... Shortwave crossing the Upper Great Lakes tonight within a northwesterly flow regime extending into the Ohio Valley and East Coast will drive additional thunderstorm and heavy rain chances. Of most concern is an area of west-northwest flanking storms that continue to exhibit training/back-building potential in parts of central and southern IL/IN this evening. High levels of MLCAPE (4000-5000 J/kg) continue to be estimated in the inflow sector across southern IA, IL and into southern IN, which will help storms maintain themselves after sunset. 23Z experimental WoFS depicts 40-70% chnaces for additional rainfall amounts above 2 inches within the SLGT risk area and along the IL-IN border. However, the mid- level flow does begin to become less favorable for back- building after about 03Z and better forcing quickly shifts eastward due to the deepening trough over the Northeast. Thus, the scattered flash flood threat over IL/IN remains, but is short- lived. For the remainder of the Great Lakes and Ohio Valley region, the left-exit region of a potent jet streak rounding the northwest periphery of the anchored upper ridge over the southern High Plains will spawn overnight thunderstorm development. Most storms should be progressive within the fast northwest flow, but brief periods of training may lead to a localized flash flood threat. The greatest potential for this appears to be over the eastern L.P. of MI and possibly the Upper Ohio Valley into far western NY State depending on upstream development. CAMs have been fairly unreliable today, but the latest 23Z HRRR seems to capture current radar representation well and has a realistic outlook for tonight. ...Northeast... Slow-moving isolated thunderstorms with impressive rainfall rates continue across the Northeast along I-95 from Baltimore to the Lower Hudson Valley, but should decrease in coverage this evening following sunset. Otherwise, increasing mid-level southwesterly flow and PWs over 1.5" will lead to addition thunderstorm chances across parts of central PA. A few inches of rainfall could lead to localized flooding concerns. ...Four Corners States into the Central Plains... The vast south-central U.S. ridge remains in place with weak impulses rotating around it to enhance ascent and resulting convection. Thunderstorms are ongoing on the western and northern periphery of this ridge from southeast AZ and NM to NE/KS. Most storms are scattered in nature and not organized enough for a risk level above MRGL. However, any convection has the potential to produce intense rain rates leading to hourly rainfall of 1-1.5", which can still produce at least isolated flash flood responses over vulnerable terrain features or burn scars. Over the Central Plain, ongoing convection will eventually try to congeal along an area of low-to-mid level convergence along the NE/KS border, but should remain progressive enough within westerly flow to limit flash flooding. Maintained the MRGL risk here and preferred the 23Z HRRR vs the 20Z RRFS solution. Snell Day 2 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 on Tuesday. A shortwave moving across the Great Lakes today will deepen into a closed low over Upstate NY Tuesday evening, and then amplify as it curls southeast into New England. NAEFS 500-850mb heights fall to below the 1st percentile (or even below the minimum climatology) according to NAEFS by 00Z Wednesday. 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, reflected by NAEFS percentiles in the 850mb wind to above the 99th percentile, forcing robust moisture convergence into the Mid- Atlantic and Northeast. As is typical for late July, PWs and SBCAPE will be near climatological maximums (reflected via the SPC sounding climatology), and this event will be no different as PWs are progged to be around 1.75 inches overlapping MUCAPE 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. Both HREF and REFS 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, suggests a strong backbuilding and training potential. While there is still quite a bit of timing spread among the various CAMs as to when the strongest convection will pivot through, the exceedance probabilities for both 5"/24hrs and 8"/24hrs are impressive (40-70% and 10-30%, respectively) despite some uncertainty in position. This combined with the 100-yr RI exceedances exceeding 20% indicate increasing confidence for widespread flash flooding with locally significant events possible. After coordination with the affected WFOs, a MDT risk has been introduced, focused in the Tri-State region where FFG is already more vulnerable due to recent rain and the general urban corridor. Despite the MDT risk in place for the Tri-State area, it should be noted that the REFS probabilities are a bit displaced south from the HREF, and focus the heaviest rainfall closer to Philadelphia. The surrounding SLGT risk has been expanded to account for this potential, and there is potential for additional adjustments to the higher-end risk areas in place for Tuesday/Tuesday night. ...Four Corners States across the Central Plains... The expansive monsoon ridge will become additionally squeezed Tuesday as shortwaves rotate anti-cyclonically around it both to the north and south. This will again support at least scattered monsoon convection as PWs above 1 inch rise 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 NM and CO. Storm motions will again be weak as reflected by 0-6km mean winds of just 5-10 kts, and locally chaotic motion is likely where storm clusters interact with other boundaries or mergers/collisions. The CAMs, as well as HREF/REFS PMM suggest a local enhancement to QPF from far southeast Arizona through western New Mexico and into eastern Colorado, which is where multiple rounds of storms are more likely. After coordination with the affected WFOs, a SLGT risk was introduced for this axis where a greater risk for flash flooding exists, especially across vulnerable terrain features. ...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. This will support widespread activity which is reflected by high-res CAM simulated reflectivity. Within these impressive thermodynamics, rainfall rates of 2-3"/hr are likely at times, leading to 2-4" of rainfall with locally higher amounts. Some of these areas will be sensitive from heavy rain on D1, so a MRGL risk was added for the potential for isolated impacts on Tuesday. ...Missouri Valley... A weakening but notable shortwave and accompanying vorticity impulse will crest the monsoon ridge to the rest and then race steadily southeast over the Central Plains and into the Missouri Valley by Tuesday night. This feature will help push a cold front southward towards the Ozarks, but become aligned parallel to the mid-level flow causing it to stall in a NW to SE oriented fashion. As the shortwave moves along this front Tuesday night 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. Weiss 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... ...Northeast... An occluding surface low beneath an anomalous mid-level closed low will bring heavy rainfall to parts of the Northeast and New England on Wednesday. Strong mid-level divergence combined with upper diffluence will produce deep layer ascent into robust thermodynamics characterized by a plume of 1.5 to 1.7 inch PWs and MUCAPE of 500-750 J/kg. At the same time, the deepening system will accelerate the 850mb inflow to 30-40 kts, tapping Atlantic/Gulf Stream moisture to further enhance the potential for heavy rain. This will support an expansive area of heavy rainfall for the Northeast, with rainfall rates of 1"/hr or more possible at times, leading to widespread 1-3" of rain with locally higher amounts (SREF, GEFS, and ECENS all have around a 10% chance for 3+" of rain). There will be a sharp gradient of instability which may limit the rates into VT/NH/ME, and FFG is elevated farther north due to recent dryness. For this reason, and after coordination with the WFOs, the SLGT risk was modified to encompass primarily southern New England, with a broader MRGL risk covering most of the Northeast. ...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. It is possible that some of these could result in locally heavier rain across the High Plains of CO, and this could fall atop soils saturated from heavy rain on D2. At this time confidence is not high enough for an upgrade risk due to low probabilities and uncertainty about storm coverage/speed, but it is possible a SLGT risk may be needed for parts of the High Plains with later updates. ...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. Weiss 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... 20Z Update... Forecast remains largely unchanged. Only adjustments to the Day 4/5 EROs was to extend the SLGT risks a tad farther south given latest model QPF and the tendency for thundertsorms in north-northwesterly flow to extend and potentially backbuild farther south than initially modeled. Snell Previous Discussion... ...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 12Z 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 $$