256 FOUS30 KWBC 061955 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 355 PM EDT Mon Jul 6 2026 Day 1 Valid 16Z Mon Jul 06 2026 - 12Z Tue Jul 07 2026 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF COASTAL NEW JERSEY TO SOUTHEAST MASSACHUSETTS... 16z Update: Tricky day 1 ERO update this morning. Don't really think any of the 12z high res or even recent HRRR runs have a good handle on the convective situation over the Mid-Atlantic into southern New England. Unusual to have a MDT risk in an area with such low current and forecast instability. However, we should see a rather well defined surface low develop today somewhere near or south of Long Island. If this low becomes defined enough we could see strong enough low level convergence and lift to generate efficient warm rain into southern New England even without much of any instability (which seems to be what the 12z HRRR is showing). Confidence in this is low...and could very well see the better convection and higher rates stay from Long Island and points offshore. But since the HRRR like scenario of low topped convection/warm rain tonight is a possibility, we will maintain the MDT risk area. Overall, higher confidence of flash flood impacts from coastal NJ into portions of Long Island where enough instability should be present to drive higher rainfall rates. Locally significant impacts are a possibility. Lower confidence over CT/RI/MA, and certainly some chance the higher rainfall rates and flash flood risk stay south of these areas. However, there is still a chance that strong low level lift focused within the warm cloud layer could drive higher rates and a flash flood risk tonight even without deep instability. Thus we will maintain continuity with the MDT risk here and continue to reassess through the day. Elsewhere, we expanded the Slight risk into portions of northern OH and southwest NY where slow moving convection near an MCV and along a front will drive an isolated to scattered flash flood risk through this afternoon. Also added a Marginal risk across portions of the eastern Dakotas into MN where organized convection is expected by this evening. While this activity should be progressive, high rainfall rates and a few cell mergers could drive a localized flash flood risk. Expanded the Marginal risk to include more of TX, with a well defined MCV dropping south expected to trigger at least some slow moving convection later today. Chenard ...Previous Discussion... ...Mid-Atlantic and Northeast... Waves of low pressure ripple along a latitudinally wavering stationary front causing multiple rounds of training thunderstorms. During this period the front will likely be sprawled from central Ohio to central New Jersey. Two distinct shortwaves moving along this front will help track dual waves of low pressure eastward, each of which will locally enhance ascent (one across western Pennsylvania, the second from New Jersey into southern New England). Broad areas of ascent, instability with PW values exceeding 2" will be aligned near this front to yield a significant risk for heavy rainfall capable of producing flash flooding. The latest guidance mainly focuses the heaviest QPF over southern New England and portions of northern/coastal New jersey. Although the placement widely varies among the hires guidance there continues to be consensus for areal averages of 3 to 6 inches with local maximums of 8+ as reflected by the REFS and HREF PMM. The neighborhood probabilities reach above 40% for coastal New England and Long Island for 3"/24 hr and 15% for 5"/24 hr. Rainfall rates at times reaching 2-3"/hr combined with aligned mean wind/Corfidi vectors which are parallel to the boundary will drive training/backbuilding to support these heavy rainfall amounts. A Moderate Risk cover far northeast New Jersey, Southeast New York and Long Island, southern Connecticut, southern Rhode Island and portions of southeast Massachusetts. A bit farther west, a secondary area of low pressure interacting with the same thermodynamic environment will produce streaks of heavy rain producing convection from western Pennsylvania through the vicinity of Washington, D.C. While uncertainty is a little higher in this area as to the coverage, speed, and intensity of the thunderstorms, both the HREF and REFS depict a 20-40% chance of 2"/hr rain rates with at least short-term training potential. FFG across western PA is quite low (1-1.5"/3hrs) and HREF/REFS exceedance potential is elevated as a result. Farther south into the D.C. area, FFG is higher, but these intense rates atop more urban regions could still pose a flash flood concern so a Slight Risk was maintained. ...ArkLaTex east through the Carolinas... A slow moving cold front will drop slowly southward, reaching a line from southern Arkansas through eastern Tennessee by Tuesday morning before stalling. This front will sit within a plume of overlapping elevated PWs (1.75 to 2.00 inches) and MUCAPE (1000-2000 J/kg) to support hourly rainfall of 1-2" with higher short-term rates. 0-6km bulk shear is progged to be minimal (less than 20 kts) so storms that form will likely be of the pulse variety. Repeating rounds and possible training of heavy rainfall will keep an elevated threat for flash flooding. A Marginal Risk was maintained. ...Florida... Diurnal heating and onshore flow will help trigger convection along the eastern coastline of Florida with 1 to 3 inches and isolated local maximums up to 5-6 inches. A Marginal Risk area was raised for this period. Weiss/Campbell Day 2 Valid 12Z Tue Jul 07 2026 - 12Z Wed Jul 08 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE NORTHERN PLAINS AND MID-ATLANTIC... 20z Update: A Slight risk was added across portions of the Mid- Atlantic where cell mergers near a backdoor front should lead to isolated to scattered flash flooding. Pretty good agreement between the 12z HREF and REFS on this axis from portions of southwest PA into central VA and northeast NC, with rainfall expected to locally exceed 3". The Marginal risk was expanded across the TN Valley and into portions of the lower MS Valley. Convection along this corridor should be disorganized and pulse in nature...however lingering high moisture and instability will support heavy rainfall rates and a localized flash flood risk where cell mergers occur. The Slight risk across portions of the Dakotas into MN still looks in good shape, and so only minor adjustments were made. Chenard ...Previous Discussion... ...Northern Plains... During this period a shortwave rounding the ridge anchored over the Four Corners will make its way through central Montana to western Minnesota. This shortwave will drive downstream PVA and height falls while concurrent but modest RRQ diffluence in the tail of a distant jet streak helps enhance synoptic lift. At the surface, a wavering boundary with a wave of low pressure moving along it will try to shift north in response to subtly increasing southerly low- level flow, with the resulting isentropic ascent aiding lift across the region. This robust ascent will act upon PWs that will surge to 1.25 to 1.75 inches, or around +2 sigma, collocated with a plume of MUCAPE exceeding 2000 J/kg. Areal coverage of convection will increase through the day, tracking from west to east. The environment will be favorable for backbuilding or training. Hourly rainfall as much as 0.75"-1.0"+ is likely, which could create locally 2-3" of rainfall as reflected by modest (5-15% chance) of 3"/24 hrs. Soils across this part of the country have been recently wet thus increasing sensitivity. A Slight Risk remains in effect from the eastern Dakotas to western Wisconsin. A broad Marginal Risk extends from eastern Montana to north-central Wisconsin. ...Mid-Atlantic States... Much of this region will continue to have scattered to widespread thunderstorms as weak impulses traverse through the trough overhead. In general, these storms should be of the pulse variety and move E/NE on 15 kts of 0-6km mean wind. However, the impressive thermodynamics in place (PWs over 1.75 inches overlapping MUCAPE of 1000-2000 J/kg) will support rainfall rates of 1-2"/hr, through which any short-term training or repeating rounds could lead to instances of flash flooding. ...Eastern Massachusetts & Rhode Island... A wave of low pressure will be tracking south of Rhode Island during this period keeping some showers and thunderstorms in place across southern New England. Although new accumulations are expected to be modest hourly rates of 1+ inches/hr will be possible. A Marginal Risk area was maintained for this period. Weiss/Campbell Day 3 Valid 12Z Wed Jul 08 2026 - 12Z Thu Jul 09 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE UPPER MISSISSIPPI VALLEY/UPPER GREAT LAKES... The Slight risk over the Upper Midwest still look pretty good, although we did nudge it a bit south with this update. Convection should be ongoing at 12z Wednesday moving from west to east across MN and WI. While this activity could be moving along and weakening during the morning, we should see renewed development along the front by afternoon from southeast MN into WI. Approaching shortwave energy and an upper level jet will help drive this uptick in activity, with PWs above the climatological 90th percentile and CAPE approaching 2000 J/kg supporting heavy rainfall rates. While a progressive squall line should eventually evolve, there will be an opportunity for some training and backbuilding during the development and initial upscale growth phase of convection. Rainfall totals of 1-2" should be most common, but localized swaths over 3" are probable and supported by the 12z RRFS. A Marginal risk was added across portions of the OH/TN Valley into the Mid-Atlantic. A shortwave moving across KY/TN and a lingering backdoor front over WV/VA should both act as a focus for convective development Wednesday. Not expecting anything too organized or widespread, but enough moisture and instability will be around to support locally heavy rainfall rates. Chenard Day 4 and Day 5 Valid 12Z Thu Jul 09 2026 - 12Z Sat Jul 11 2026 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE OHIO VALLEY AND CENTRAL APPALACHIANS ON DAY 5... 20Z Update: Additional information provided on an upgrade for the D5 period with a maintenance of a broad MRGL for D4, below. Stronger flow will initially remain along the northern tier of the CONUS early to mid-week with mean upper-ridging across the western to central U.S. Embedded shortwaves rounding the upper-ridge will eventually lead to deepening troughing across the the Midwest and adjacent areas on Thursday. The southward shift of stronger flow aloft into the central Plains east through the Middle Mississippi/Ohio/Tennessee Valleys and Lower Great Lakes will bring more scattered thunderstorm chances across the region. Seasonably high instability and moisture pooling along a southward progressing, wavy cold front will support locally heavy rainfall bringing the threat for at least isolated instances of flash flooding, and a broad Marginal Risk of Excessive Rainfall exists for D4 (Thursday) with enough confidence to upgrade a large portion of the Ohio Valley and Central Appalachians to a SLGT risk for D5 (Friday) as increasing low-level convergence ample mid-level ascent will likely trigger widespread convection with training potential over the region, compounding with any impacts from convection the previous forecast period. Consensus grew over the past series of NWP updates with the ensemble mean QPF structure signaling a large area of 1-2" areal average totals with embedded higher amounts positioned over the southern-third of IL/IN/OH through much of KY into WV. This is coincident with the progression of both the front and a strengthening shortwave trough migrating out of the Central Plains into the region as we move into Friday. ML output is also favoring the Ohio River Basin into the Central Appalachians extended from southwestern PA down through WV as terrain influences along with the synoptic evolution will couple into a targeted threat for heavy rainfall and local maxima sufficient for scattered flash flood prospects. PQPF forecast for at least 1" signals a broad 20-40% probability with a bullseye of 50-80% located over the Central Appalachian front in northern WV. This is a relatively bullish signature at this lead, enough to warrant an upgrade to a SLGT risk over the aforementioned areas from IL to WV. Putnam/Kleebauer 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 $$