Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jul 11 2024 20:22:23 FOUS30 KWBC 112022 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 422 PM EDT Thu Jul 11 2024 Day 1 Valid 16Z Thu Jul 11 2024 - 12Z Fri Jul 12 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR SOUTHEAST VIRGINIA AND EASTERN CAROLINAS... ....16Z Update Summary... SLGT risk across the Carolinas into Southern Virginia was expanded to the west to align with the current Flood Watch collaborations with some of the local WFOs. A new MRGL risk was added to the Southeast FL coast to account for a local sea breeze enhancement within the urban corridor extending from just north of Fort Lauderdale down through the Miami Metro. The remainder of the MRGL risk areas across Maine, the Midwest, and Southwestern U.S were maintained given spatial continuity in scattered QPF signatures within their respective synoptic/mesoscale evolutions. Kleebauer ....Coastal Mid Atlantic... ....16Z Update... A sheared axis of heavy rainfall is currently bisecting a portion of the Eastern Carolinas with local rainfall rates exceeding 2"/hr in some of the pulse cells along the stalled frontal boundary. Recent radar/satellite trends indicate a formidable pulse convective environment with totals along and east of NC11 and US13 producing totals of 3-6" in some portions of the above zone. The axis of precip will be reluctant to move as the cells will remain anchored to the boundary over the course of the afternoon which could lead to some totals exceeding 8" within the course of impact. There will be a decay in the current evolution after 21z, but not before a thin band of heavy rain will induce flash flooding concerns for several more hours. The SLGT risk was maintained with a higher end SLGT located within that small corridor where rainfall will continue. Energy from the leading edge of a disturbance approaching from the southeast off the Atlantic will redevelop multiple bands of convective rainfall later this evening through tomorrow morning with a general expanse to the northwest overnight. Some of those cells could produce enhanced rainfall with rates exceeding 2"/hr, especially over Southern and Southeastern VA down into Eastern NC. The current HREF QPF footprint outlined the secondary convective bands very well with a QPF maximum situated over portions of Southern VA with the northern extent reaching Southern MD, extending down into to the NC/VA border. As a result, the SLGT was also expanded a bit to the north with the accompanying MRGL also being spread further northwest to account for the trends. Kleebauer ....Previous Discussion... Stalled frontal boundary will bisect portions of the Eastern Seaboard with elevated PWATs mirroring the alignment of the front to points east. Local environment will be primed for convective potential, but will have some assistance in upper level support as a meandering wave off the Southeast coast lifts northwest on the western fringe of the ridge in the Atlantic, acting as a beneficial forcing mechanism to enhance regional rainfall potential. The best chance will lie from the Southern Delmarva through the Virginia Tidewater down into the Eastern Carolinas. Latest HREF probabilities are fairly aggressive in their signature for local totals between 2-5" as neighborhood probs for 2-3" totals are running between 60-90% with an area of 5" probabilities between 50-70% across Eastern North Carolina. This area has been very dry as of late, and is well reflected within the FFG indices present for 1/3/6 hour markers. Isolated to scattered urbanized flooding and/or flash ponding possible for 2-4 inches/hour rates as per the latest HREF mean hourly QPF output. A Slight Risk was raised for the North Carolina coast and a very small potion of southeast Virginia and northeast South Carolina. A Marginal Risk spans from South Carolina to southern New Jersey. Kleebauer/Campbell ....Northern Maine... ....16Z Update... Remnant moisture and trailing mid-level energy behind Beryl's circulation will keep the threat of convection across Northern and Central ME for at least the next 3-6 hrs before fully escaping into the Canadian Maritimes. Probability fields are not as enthused as yesterday for significant totals, but antecedent conditions along with any convective focus could cause some localized flood concerns in-of the terrain up to the Canadian border near Caribou. The threat was sufficient enough to maintain continuity, especially with the elevated moisture presence still lingering across the northern tier. Kleebauer ....Previous Discussion... Lingering convective threat across Northern Maine could induce some local flash flood concerns, mainly for northern and Downeast Maine. The latest guidance continues to signal the highest QPF across Downeast Maine where the highest PWAT anomalies remain leading to some deterministic output indicating 1-2" of rainfall within the first 3-6 hours. The Marginal Risk was kept for this issuance, but may be considered for removal is convection dissipates sooner than forecast. ....Midwest... ....16Z Update... There are no changes necessary to the previous MRGL risk across the Midwest as scattered thunderstorms within a much lower FFG footprint across the region thanks to impacts from Beryl will create a localized threat for flash flooding as a shortwave trough traverses overhead. Kleebauer ....Previous Discussion... Trailing shortwave trough pivoting southeast around the backside of the mean trough over the Great Lakes and Ohio Valley will strengthen as it moves across Iowa into Illinois by tomorrow afternoon with large scale forcing increasing across the Central Midwest and adjacent Mid-Mississippi Valley. Convection is expected to fire up across Illinois, Missouri and Indiana and within these storms some will be stronger and capable to producing hourly rates of 1 to 2 inches/hour. Much of the region was soaked by Beryl as is passed through and soils have not fully recovered. Antecedent moisture in the top layer of the soils is well-defined with NASA SPoRT moisture percentiles relaying the 0-10cm layer running between 75-90% leading to higher runoff capabilities. Given the sensitive nature of soils and the potential for a few inches, a Marginal Risk for excessive rainfall was maintained for this period. Kleebauer/Campbell ....Southwest... ....16Z Update... No changes to the previous forecast as the Monsoonal pattern and forecasted QPF within zones of higher impact (Burn scars) remains on track. Kleebauer ....Previous Discussion... The persistent monsoonal pattern will continue to support highly isolated convection with the primary areas of interest being the burn scars in New Mexico and small urban threat of towns within complex terrain across Southeast Arizona through much of New Mexico. A Marginal Risk remains in effect for southeast Arizona and a majority of New Mexico. Campbell ....Southeast Florida... Sheared axis of convection is currently positioned off the east coast of FL with accompanying energy slowly migrating southwest towards the Southeast coast of FL. Sea breeze will propagate inland, but will be met with the west coast sea breeze to initiate a strong convergence signal in-of of the urban corridor along the Southeast FL coast. This will include areas like Fort Lauderdale down to the Miami Metro with some guidance fairly bullish in their QPF representation. Totals of 2-4" with locally upwards of 6" will be plausible as the environment remains conducive for heavy rainfall given the forecast sounding signatures of tall, skinny CAPE, SBCAPE around 2500-3000 J/kg, and a steering pattern focused parallel to the coastline. This type of setup has historical precedence in being a sneakier flash flood setup and the addition of a MRGL risk was in agreement with the Miami WFO. Kleebauer Day 2 Valid 12Z Fri Jul 12 2024 - 12Z Sat Jul 13 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL OVER PORTIONS OF THE MID-ATLANTIC AND THE CAROLINAS... ....20Z Update... Previous SLGT risk was expanded further north and west to encompass the I-95 corridor from DC to NYC given the latest trends in proposed QPF. The overall synoptic evolution remains fairly similar from previous discussion, however the expansion of the convective pattern towards the north and west was evident within the latest ensemble QPF output. 12z HREF probability fields were also more pronounced with the neighborhood 3" probability range between 40-60% along and west of I-95 from Philadelphia down through Central NC. 60-90% is located to the east of I-95 with the highest probabilities centered over the Northern Delmarva, south-central VA, and Eastern NC, most of these areas expected to see multi-day impacts from heavy convection from the pattern evolution. Hourly will be highest across the Carolinas up to the Virginia Tidewater as evidenced by the 2"/hr probability fields with a degrading signal north of US50. Environmental factors are more than capable to allow for a higher end evolution, so the current forecast SLGT is bordering the higher-end of the thresholds with a non-zero opportunity for a further upgrade, especially south and east of the I-95 corridor.=20 With flow becoming parallel to the front, there's a greater=20 opportunity for training convection within the confines of the=20 boundary. This is likely the case for the higher neighborhood and=20 EAS probabilities in-of the Mid Atlantic down into the Carolinas.=20 Tropical moisture involvement and those environmental distinctions=20 alone have allowed for a greater emphasis on the potential, and=20 could be exacerbated if the population centers from Richmond to=20 Philadelphia get involved more than currently forecast.=20 Kleebauer ....Previous Discussion... A second round of slow moving convection is expected near the stalled frontal boundary. The greater impacts of scattered instances of flash ponding and flooding be primarily be focused over urbanized areas and areas that see repeat heavy rains from the period prior. The ridge will assist the convection spreading rains further inland. The Eastern Carolinas will remain the=20 primary target for heavier rainfall with some areas in the two day=20 period potentially receiving over 6 inches from the evolving=20 event. Campbell Day 3 Valid 12Z Sat Jul 13 2024 - 12Z Sun Jul 14 2024 ....THERE ARE MARGINAL RISKS IN PORTIONS OF SOUTHERN NEW ENGLAND, COASTAL SOUTHERN MID ATLANTIC, NORTHERN MIDWEST, AND THE SOUTHWESTERN U.S... ....20Z Update Summary... The previous MRGL risk areas across the Upper Midwest and=20 Southwest U.S were generally maintained outside a few modifications to account for the latest QPF trends and First Guess field=20 alignment(s). An additional set of Marginal Risk areas were added=20 to the Hampton Roads area down through Eastern NC as well as across the eastern sections of Southern New England. Kleebauer ....Southwest... ....20Z Update... The threat across the Southwest will be relegated to a typical Monsoon based isolated to scattered local impacts with the main areas of interest occurring within slot canyons, burn scars, and smaller urban corridors within AZ and NM. Precip forecast is relatively benign, but there are pockets of higher QPF littered within the regional domain which is classic for this time of year with the Monsoonal component. The MRGL risk was more than sufficient given the setup. Kleebauer ....Previous Discussion... There will be a resurgence of monsoonal flow back into the Southwest. Diurnal heating will help fire up convection during the afternoon and evening hours with the potential for higher rainfall fall rates. Maintained the Marginal Risk area for much of Arizona, northwest New Mexico and southwest Colorado for this period. Campbell ....North Dakota and Minnesota... ....20Z Update... Scattered convective signals remain across northeastern ND through northern MN with a growing emphasis on potential further southeast into WI. The latter portion of the areas mentioned is still a bit light for a potential expansion of the MRGL risk, but the First Guess Fields have introduced a MRGL risk signal into the WI area as some organized convective prospects could add the area into the mix in future updates. Spread among guidance for exactly where the heaviest convection will develop is still an issue and is one of the reasons for a lack of a higher risk area. That and climatologically the area over northern MN is less prone to flash flooding outside a significant, more organized synoptic scale setup. For this reason, the MRGL was preferred with some time for interrogation of the convective threat the next set of forecasts, this time with the introduction of CAMs. Kleebauer ....Previous Discussion... A shortwave trough will be tracking east across Canada and will send a surface cold front south-southeast through the Canadian Prairie and into the northern tier U.S.. Showers and thunderstorms are expected to form along and ahead of this feature which will predominately focus over North Dakota and northern Minnesota during this period. This part of the country has been above/well- above normal on moisture for the past few months and remain sensitive to additional rainfall. A Marginal Risk is in effect for eastern North Dakota and northern Minnesota. Campbell ....Southeast Virginia and the Carolinas... The back end of the precip threat from D2 will spill over into D3 across the Eastern Carolinas up into the Hampton Roads/VA Beach area on Saturday morning. The front that has been a focal point over the previous periods will finally begin pressing off the Atlantic coast, but very slowly at latitude leaving some remnant moisture and convection lingering across areas that will have seen significant precip prior periods. An additional inch or two is=20 still plausible across the impacted area, more than enough to cause some issues if it occurs over a spot that has seen copious amounts of rain earlier in the week. A MRGL risk was introduced, mainly to cover for the continued threat from D2 with an eventual end=20 sometime late Saturday morning or early afternoon. ....Southern New England... Sheared mid-level vorticity will bisect Southern New England, interacting with the northern extent of the front that has lingered for several days. Elevated PWATs in vicinity of the front, coupled with the focused ascent pattern will generate one last period of thunderstorm activity over Eastern LI up through the eastern half of Southern New England. Precip totals around 1-2" with a max of 3"=20 will be plausible on Saturday morning and afternoon before the=20 threat kicks out of the area and we end the overall threat. An=20 introduction of a MRGL risk was made over the above area with=20 agreement from the CSU First Guess Fields and QPF footprint off general ensemble forecasts. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!918OAUsGktDwL0b5vsJcgOoCWYCfsrZ4k3JGrFgaK3rC= TTF0sCHxsJDw3wYKX-pTmY4MpxDe5qRFbPWATtS5vyz4t4g$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!918OAUsGktDwL0b5vsJcgOoCWYCfsrZ4k3JGrFgaK3rC= TTF0sCHxsJDw3wYKX-pTmY4MpxDe5qRFbPWATtS5IDZwy2k$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!918OAUsGktDwL0b5vsJcgOoCWYCfsrZ4k3JGrFgaK3rC= TTF0sCHxsJDw3wYKX-pTmY4MpxDe5qRFbPWATtS5MOvUpbo$=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .