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483 FOUS30 KWBC 082009 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 409 PM EDT Tue Jul 8 2025 Day 1 Valid 202Z Tue Jul 08 2025 - 12Z Wed Jul 09 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE EASTERN MID-ATLANTIC STATES AND SOUTHERN NEW ENGLAND, AS WELL AS FOR PARTS OF THE ARKANSAS AND EASTERN OKLAHOMA... 2008Z Update: SLGT risk was expanded to include more of the Mid- Mississippi Valley into Illinois to account for an evolving threat in the region. More on this threat can be found in the latest MPD #0622. Kleebauer ...Mid-Atlantic and Northeast... 16Z Update: The overall synoptic and mesoscale scheme are still relevant from previous forecast, although one note to point out is the increasing potential for more locally enhanced flash flood concerns focused across portions of Southeast VA. 12z CAMs are much more bullish on the area between I-95 to the mouth of the Potomac in VA for the highest precip output in the period. A lot of the setup is driven by the training prospects in the vicinity of Richmond through Williamsburg up to the areas running adjacent to the Rappahannock River. 12z HREF neighborhood probs are running between 40-60% for >3", but between 60-80% for >2" leading to a strengthening signal compared to previous forecast output as noted in the previous discussion below. This aligns with the increased low-level convergence pattern as the mean layer wind becomes generally unidirectional after 00z across Central and Southeast VA with the approach of a shortwave expected to eject out of Southwest VA. With coordination from the local Wakefield, VA WFO, have expanded the SLGT risk into all of Southeast VA to account for the threat. Further north into the Megalopolis of the Mid Atlantic and Northeast, the best threat for heavy rainfall will likely be situated over Southern New England down into NYC where a secondary focus will occur as a shortwave analyzed over Western PA lifts to the east-northeast and centers over the above zone by later this evening. Environment remains favorable for locally stronger convective cores to produce rainfall rates in excess of 1"/hr, especially with the assessment of 12z KOKX PWATs coming in ~2", a reading encroaching the 30-day moving max, and eclipsing the daily max for the 8th. The high urbanization factor always constitutes close monitoring for locally impactful flash flooding, so the threat remains well within the threshold for a SLGT risk leading to general continuity for the forecast. Kleebauer ..Previous Discussion.. The remnants of post-tropical cyclone Chantal are in the process of transitioning into an open trough while exiting east of Cape Cod to start the period, as it continues to get caught up in increasing southwesterlies downstream of a trough approaching from the Great Lakes. This trough will continue to translate eastward through the day, but will be slow to advect due to downstream Bermuda-type ridging. Between these two features, southwest flow will remain prevalent over the area, with 850-500mb winds being nearly unidirectional from the W/SW at 15-20 kts. This will result in a continued extremely moist environment with PWs likely eclipsing 2" from eastern VA through southern New England Tuesday afternoon, coincident with a plume of SBCAPE exceeding 2000 J/kg (per 00z HREF mean fields). Into these impressive thermodynamics, a subtle shortwave embedded within the mean flow will traverse northeast ahead of a cold front and along a surface trough, providing additional ascent atop the already impressive convergence on these boundaries. The 00z hi-res CAM consensus suggests showers and thunderstorms will become numerous to widespread along and near the front during the mid to late afternoon, and storm motions are expected to initially be quite slow (0-1 km mean flow near 10 kts) with the potential for localized backbuilding and training along the surface trough and front (as Corfidi vectors collapse to around 5 kts with the development of deep convection and resulting outflow boundaries). With warm cloud depths potentially eclipsing 14,000 ft, this will support efficient warm-rain processes and rain rates above 2"/hr at times (and resulting localized hourly totals of 1-2"). Where training occurs, this could result in total rainfall in excess of 3" (per 00z HREF 40-km neighborhood 3" exceedance probabilities ranging from 10-40% across the SLGT risk area). Scattered instances of flash flooding are possible across much of the I-95 corridor from Richmond, VA to Boston, MA. Churchill ...Southern Plains through the Ohio Valley... 16Z Update: A targeted area across Eastern OK into Western AR now exists as a higher-end SLGT with some locally significant impacts plausible over the aforementioned area. Current WV satellite and UA analysis depicts a prevalent mid-level shortwave located within the confines of Northeast OK and Southeast KS dipping southeast over the last several hrs. Enhanced sfc-500mb convergence pattern over Eastern OK to the AR/OK line is already materializing with a solidified low-level inflow pattern advecting unstable air into the vicinity of the mid-level circulation. Latest RAP surface analysis pins a target of 1000-2000 J/kg of SBCAPE located over the axis of greatest convergence with PWATs noted to be ~2" as 12z KLZK sounding just to the east depicted a PWAT observation of 1.99", above the 90th percentile daily output. The combination of a pronounced area of buoyancy and focused surface convergence on the southern flank of the shortwave will create an opportunity for locally enhanced hourly rates between 2-3+"/hr through the next 3-6 hrs before the threat wanes with the progression of the shortwave migrating eastward. For more information on this threat, please see MPD #0619 for the latest. More convection will fire this evening across Southern MO with the strongest cells likely to induce some 2-3"/hr rates for a short time, enough to warrant some attention for flash flood prospects across the Ozarks due to the local topography. The SLGT risk was expanded to include those areas where 12z CAMs have highlighted the potential. No additional changes were made across the Midwest and Mid- Mississippi Valley, but will monitor the progression and expected output from the MCV as it migrates into IL this afternoon and evening. Some spots may see 2-4" of rainfall which could very well induce flash flooding, especially in any urbanized areas where run off prospects are highest, generally in-of and between St. Louis to Springfield, IL. Kleebauer ..Previous Discussion.. Farther north near the base of the trough (into MO/IL), an MCV has the potential to locally organize convection with better overall dynamics (DPVA and shear from right-entrance region of jet streak) compared to farther south (despite overall tropospheric moisture likely to remain a bit lower with PWs less than 2.0" (but still at or above 90th percentile). CAMs are not as aggressive with this area, but will need to be monitored closely today given the MCV and added daytime instability (HREF indicating SBCAPE rising to 1500-2500 J/kg). Churchill ...Upper Midwest... 16Z Update: No changes to the previous forecast as the general synoptic and mesoscale pattern remains on track with little run to run deviation. Kleebauer ..Previous Discussion.. A wave of low pressure and accompanying surface trough will move progressively eastward from Minnesota into Michigan the first half of today. While this will be generally fast moving, it will impinge into a favorable environment to support heavy rain rates which will likely eclipse 1"/hr, especially where it overlaps a ribbon of MUCAPE reaching 1000-2000 J/kg. The fast motion will limit total rainfall, but in some places that receive multiple rounds of heavy rain, event totals could reach 2-3". This falling atop sensitive soils (FFG as low as 1-1.5"/3hrs) could produce localized instances of flash flooding. Churchill ......Southeastern New Mexico... 16Z Update: The pattern remains favorable for another diurnally driven convective pattern in-of the terrain of NM, especially in the confines of the Sacramento Mtns. up to the Sandia`s to the east of ABQ. No changes were necessary from the previous forecast MRGL risk issuance. Kleebauer ..Previous Discussion.. Another daily round of showers and thunderstorms is expected to develop in the high terrain and then drop southeast into the High Plains and southeastern portions of New Mexico. Although storms are expected to be generally progressive as they come off the terrain (0-6km mean winds from the north around 15 kts), they will move into an axis of more favorable thermodynamics characterized by PWs of above 1" and MUCAPE approaching 1000 J/kg. This could support briefly heavy rain rates above 1"/hr, supporting an isolated risk for flash flood impacts across urban areas or sensitive terrain and burn scars. Churchill/Weiss Day 2 Valid 12Z Wed Jul 09 2025 - 12Z Thu Jul 10 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID ATLANTIC... ...Mid-Atlantic... 20Z Update: The previous SLGT risk issuance was generally maintained with some expansion further west into the interior Mid Atlantic and far Eastern KY, including portions of Southwest VA and much of WV. The signal within the CAMs output is greatest across Western NC up through Southwest VA and the southern reaches of WV. Despite that depiction, the ML is very consistent with a marker of heavy rainfall a bit further north into WV with ample instability and elevated PWATs located over the southern half of the state. When assessing the synoptic setup, the amplified shortwave progression from the remnant MCV exiting into the Ohio Valley will allow for a better axis of diffluence downstream of the advancing disturbance which should increase convective prospects. A tongue of elevated theta_E`s forecast within the deterministic bisect all of Southern WV, especially along and east of I-79 within the terrain leading to MUCAPE signatures between 1500-2500 J/kg during time frame of interest, Wednesday afternoon. ECMWF AIFS indicates a convectively driven QPF bullseye to materialize over areas near and just northeast of Charleston, WV, a bit further north compared to what is inferred from hi-res at this time. Given the consistency and general amplification with the approach of the shortwave/MCV, would not be surprised to see a touch more meridional conjecture within the convective regime over the area. The threat still looks pretty solid within the Southwest VA area as there`s pretty good agreement overall in the prospects for local totals >3" with the latest HREF running between 60-80% within the neighborhood probs and even some low-end 15-25% probs for >5" in the same area. Considering the current outline of higher probs and the consistency within ML depiction of a bit further north threat of heavier precip, have expanded the SLGT risk to cover the more sensitive areas across Western NC up through Southwest VA into WV with the greatest threat likely over Southwest VA and Southern WV. Further east to the I-95 corridor, the signal has ramped up for heavier precip to focus over Northern MD along the M/D from Parrs Ridge over into Cecil County expanding northeast along I-95 through Philadelphia and Southern NJ up towards Toms River. This area is coincident with the frontal positioning and pre-frontal trough alignment across the Eastern Mid Atlantic leading to focused convective elements and potential training. HREF neighborhood probs for >3" are running hot (50-70%) from Richmond up through DC/Balt and Philadelphia which is very favorable for scattered flash flooding prospects due to a large footprint of impervious surfaces within the southern half of the Megalopolis. PWATs between 2-2.2" will lead to near or exceeding daily PWAT maxima by the time we reach Wednesday afternoon and evening, an environment suitable for heavy rain potential with enhanced rates collocated with the strengthening low-level convergence regime. Hourly rates between 1-2"/hr are likely with intra-hour rates reaching between 2-4"/hr at times within the heavier cores that initiate over the area. Areal average rainfall forecast lies between 1-2" with a maxima of 2.5" centered near Richmond with a secondary maxima positioned over the Delaware River basin near Philadelphia and adjacent South Jersey urban areas. This threat is suitable for the previous SLGT risk with an elevated threshold within the risk that could lead to a targeted upgrade if the signal is amplified further overnight. Kleebauer ..Previous Discussion.. Quasi-stationary front from Tuesday will linger in a general east- to-west fashion on Wednesday across the Mid-Atlantic States. Along this boundary, a much more potent shortwave (likely enhanced in the latest guidance from the aforementioned MCV currently in the Plains) will press east within the approaching longwave trough, leading to weak low pressure development moving east along this front. The accompanying ascent will act favorably into continued robust thermodynamics with PWs around 2 inches (above the 90th percentile according to NAEFS ensemble tables) and 1000-3000 J/kg SBCAPE during the afternoon. Increasing 0-6km bulk shear and aligned 0-6km mean winds to the front suggests storms will repeatedly develop to the SW and then train ENE across the Mid- Atlantic states. Not only will this result in heavy rainfall which may exceed 3 inches in some areas, but it will occur across primed soils from heavy rainfall on prior days, enhancing the flash flood risk. An inherited SLGT risk was maintained and expanded based on the new guidance, and high-end (25%+) SLGT probabilities are indicated from central NC through much of VA and into southern MD. Upon the full evaluation of the new 12z CAM suite later today, a targeted upgrade to MOD risk may be necessary for portions of the SLGT risk (particularly given the more vulnerable antecedent conditions). Churchill ...Central Texas through the Southern Ohio Valley... 20Z Update: Spread within the CAMs still maintains a posture of medium confidence in convective development across Central TX, but lower confidence in where the heaviest precip could occur, as well as coverage ranging from generally isolated to numerous pending hi-res output. Analysis currently indicates an expansive TUTT migrating into the TX Gulf Coast and Northeast MX south of Brownsville. This feature will remain a driver in what will occur upstream as the system moves inland and rotates around the western flank of the ridge downstream. How much this feature interacts with the stationary front will decide how much of a potential impact the area will receive from area convection. Environment will be conducive for locally heavy rainfall with sufficient deep layer moisture and relevant instability as referenced below... HREF EAS probs for at least 1" are actually very low (<15%) but neighborhood probs for >1" and >2" are generally high (60+%) meaning the models agree on the potential, but vary with the spatial recognition of where exactly this will occur. It`s also an inference for more isolated prospects in those higher totals, so it will be something to note as we move through the next few forecast cycles. A better signal and greater overlap from guidance will likely lead to a targeted upgrade, especially for those highly compromised areas over Hill Country to the west of the I-35 corridor as they are still recovering from the significant impact over the weekend. For now, maintain a MRGL risk, but certainly an area to watch closely. Downstream over the Southern Ohio Valley, the signal is relatively similar to the threat across TX with isolated to widely scattered flash flood instances plausible from the Lower Mississippi Valley up through TN/KY down into northern AL/MS as they align with the stationary front. Look for the highest risks to occur within the fronts confines with emphasis on more urbanized zones. The MRGL remains for the region mentioned above with local totals between 2-4" possible in the highest impact areas. Kleebauer ..Previous Discussion.. Stalled frontal boundary will gradually begin to lift northeast on Wednesday over the OH Valley, but persistent ascent along this front as weak shortwaves cross it will maintain a convective risk through the day. Some DPVA and subsequent forcing for ascent from storms in the Plains may be deflected southward into the warm, moist air mass of the Southern Plains (possibly as far south as more vulnerable areas of central TX). Storms will fire in response to thermodynamics characterized by PWs around 2 inches collocated with SBCAPE during peak heating that will approach 2000 J/kg. While convection is likely to be scattered, any storm which trains along the boundary with intense 1-2"/hr rain rates may result in rapid runoff capable of producing instances of flash flooding (with training/repeating of efficient rates less likely into the Plains and Lower MS Valley, but will come down to mesoscale details that are difficult to discern in the 24-48 hour timeframe). Churchill/Weiss ...Central Plains... A shortwave rotating along the northern periphery of a ridge centered over the Southwest U.S. will eject out of the northern extent of the Front Range leading to focused ascent in-of NE into the northern Missouri Valley and adjacent Midwest. HREF neighborhood probabilities for >3" are fairly modest (20-35%) across the aforementioned area as convection fires with the introduction of a low-level jet Wednesday evening. Sufficient ascent and increasing regional 0-6km bulk shear should enhance convective initiation and maintenance through the evening Wednesday into Thursday morning with the heaviest precip located along the surface trough bisecting NE down into Northeast KS. Jury is still out on whether this thunderstorm development can grow upscale and migrate southeast as more of a developed MCS which has been implied by some of the latest CAMs. If that case, a more organized complex would certainly lend favor to at least an isolated flash flood threat. A MRGL risk was added across the 4 state area of NE/MO/IA/KS for the risk of nocturnal flash flooding potential. Kleebauer Day 3 Valid 12Z Thu Jul 10 2025 - 12Z Fri Jul 11 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL MID-ATLANTIC STATES, AS WELL AS FOR PORTIONS OF THE CENTRAL PLAINS AND UPPER MIDWEST... ...Tennessee Valley into the Mid-Atlantic and New England... 20Z Update: SLGT risk for the Mid Atlantic was maintained with an expansion up into portions of Southeast VA as the area will likely be impacted for a third day in a row with heavy rainfall. Soil moisture anomalies leading in will be on the high side with perhaps some regionally compromised soils located over the region between Richmond and the mouth of the Chesapeake. It will not take much rainfall in these areas to cause problems and with the environment remaining favorable, adjusted to reflect the prospects. The rest of the forecast remains on track with good run to run continuity from ensembles and associated deterministic. Kleebauer ..Previous Discussion.. A warm and anomalously moist air mass looks to remain in place across much of the Southeast and Eastern U.S. into Thursday continuing a broad flash flood threat into yet another day. Although upper-level forcing looks less impressive overall relative to Wednesday, still expect another round of primarily diurnally driven convection focused along the terrain and associated leeward trough of the Appalachians, perhaps becoming best organized a bit farther south than prior days as the combination of best forcing and moisture/instability looks concentrated across the southern Mid-Atlantic into portions of the Southeast. Maintained an inherited SLGT risk area for more vulnerable portions of central NC and surrounding far south-central VA, but this area will likely be modified over subsequent forecast cycles with dependencies on both trends in the guidance and how the forecast ultimately pans out for days 1 and 2 (as antecedent conditions will be a big factor). The latest trends in the guidance suggest the threat will be more muted from much of VA northward (with subtle height rises aloft from sub- tropical ridging tries to offset the more favorable parameter space for heavy rainfall). Churchill ...Northern and Central Plains into the Upper Midwest... 20Z Update: Consensus continues to grow with the expected approach of a potent shortwave trough ejecting out of WY and the Central Rockies, flattening the ridge and making headway into the Central and Northern Plains. A well-defined axis of diffluence ahead of the approaching trough should entail a pretty stout convective pattern within a very favorable environment in the grand scheme. Despite a relatively consistent forward propagation of the disturbance, low- level inflow within the southern flank of the shortwave and attendant low will likely enhance from potential for back-building along the southwest side of the disturbances progression. As of now, this has been noted over Southern and Eastern NE, a potential overlap with some areas that will have seen rainfall the period prior. The correlation would likely enhance flash flood prospects, in a local sense, but the coverage this go around should be more favorable for scattered flash flood occurrences. The SLGT was maintained and expanded to account for the recent trends within guidance. Kleebauer ..Previous Discussion.. By Thursday the persistent ridge aloft over the Southwest U.S. begins to break down, as an upper-low off the northern CA coast opens up into a shortwave trough on Wednesday and ejects eastward ahead of a digging longwave trough over western Canada. These two features look to interact over the Northern and Central Plains, but there are still substantial differences between models in how these features evolve and interact. Maintained an inherited SLGT risk area in the vicinity of where the best QPF signal overlaps (west- central IA into adjacent portions of NE/SD/MN). PWs of 1.5"+ are expected (near or above 90th percentile for the region) with ample instability and dynamics for organized convection and subsequent high rainfall rates. Churchill 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