Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 03 2024 00:34:47 FOUS30 KWBC 030034 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 834 PM EDT Fri Aug 2 2024 Day 1 Valid 01Z Sat Aug 03 2024 - 12Z Sat Aug 03 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE EASTERN TENNESSEE VALLEY INTO THE SOUTHERN APPALACHIANS... ....01Z Update... Removed the Slight Risk area in the Northeast as afternoon=20 convection has peaked and moved out of the area. Upper low over Ohio will still support a localized threat of heavier rain but the coverage should be less than during the afternoon hours, so we removed the Slight Risk there as well. The Marginal Risk should cover the region through the overnight hours. Maintained the=20 Slight Risk area over the southern Appalachians given low FFG=20 values and the continuation of scattered convection for at least a=20 couple more hours. For the Southwest, late afternoon convection=20 continues for a few more hours, so we mostly maintained the broad=20 Marginal Risk outline from earlier.=20 Fracasso ....16Z Update... Upper energy across Western PA will pivot eastward through the afternoon with large scale ascent focusing more into Central and Eastern PA into portions of Southern NY state near the Lower Hudson Valley by this evening. The departure of the focused ascent across Western PA will lead to a cull, of sorts with regards to the convective coverage that has been plaguing the area all morning. In coordination with the local Pittsburgh WFO, have removed the previous SLGT risk located across much of Western PA, but a non- zero threat of a few stronger cells could cause some localized flash flood concerns within the aforementioned zone. Elsewhere, the forecast remains on track with the broad forcing shifting eastward into the Northern Mid Atlantic allowing for upscale growth of convection from the Susquehanna Valley to points east as the southwesterly regime ahead of the mean trough in the Ohio Valley pushes the main vorticity maxima downstream. Latest 12z HREF blended mean QPF footprint continues to depict the heaviest totals (>1.5") across Eastern PA into Southern NY where the greatest large scale forcing will coincide with the deeper moisture returns that have advected northeast through the morning, and will continue into the afternoon and evening. 12z soundings out of KPIT this morning indicated PWATs greater than 2" which is the environment that is forecast to advect downstream in the area of interest, placing much of the Mid Atlantic in 1-2 deviations above normal PWAT anomalies based off the latest ECMWF ENS and NAEFS output. This is a sufficient environment for locally heavy rainfall and will allow for a heightened threat of flash flooding within any slower moving cores and/or areas with a larger urban footprint. The SLGT was maintained through Central and Eastern PA into part of the I-95 corridor southwest of NYC down into Northeastern MD. Across the Southern Appalachians, a mid-level speed max will rotate around the base of the trough to the north with the nose of the jet core entering into the eastern half of TN and Western NC by the afternoon. Decent surface buoyancy on the order of 1000-2000 J/kg SBCAPE will be located across the I-75 corridor with a max of 2000-2500 J/kg located over the terrain in Western NC. The combination of buoyancy and increased large scale ascent will aid in scattered to widespread convection later this afternoon with neighborhood probabilities for at least 3" still hovering around 20-30% for a broad area encompassing the above region. Considering the lower FFGs across the area thanks to a multi-day convective episode that has already led to flooding in areas highlighted, the threat will remain on the higher side for flash flooding concerns despite a relatively lower magnitude of rainfall compared to recent days. Thus, the SLGT risk was maintained for continuity. The area across the Ohio Valley will continue to see scattered convective development under the periphery of the upper low, trough center with a slower storm motion and higher potential for training. The signal is still fairly robust for flash flood concerns on the southern side of Lake Erie out into Northwestern OH and far Southeast MI with the latest HREF probabilities for 3hr FFG exceedance running between 25-45% with the highest located under the expected path of the upper low as it pivots eastward out of IN. This is more than sufficient for a continuation of the previous SLGT risk with only minor modifications from the previous forecast. Lastly, the MRGL risk was extended to the north across Southwestern UT for increasing signals of isolated heavy rainfall within the Zion National Park region which includes several flood prone slot canyons in the vicinity. In coordination with the Salt Lake City WFO, the MRGL was added to encompass the more sensitive areas in that region. Kleebauer ....Previous Discussion(s)... ....Eastern U.S.... Slowly advancing upper trough and surface front will focus broad- scale ascent for scattered-numerous convective elements across much of the eastern CONUS. Two areas of particular concern include a W-E corridor from SE MI and the OH Valley eastward to NJ and southern NY, where the deep-layer forcing (DPVA) ahead of the opening mid-level low will be a bit more robust. The combination of favorable deep-layer instability, especially during peak heating hours (MLCAPEs 1000-2000+ J/Kg) along with TPW values of 1.75-2.00+ inches will make hourly rainfall rates of 1-2+ inches likely underneath the strongest cells, with potentially higher hourly rates (2-3") farther east toward eastern PA, NJ, and southern NY given the closer proximity to the max TPW and low-level moisture transport axes. Farther south, the guidance is also favoring eastern portions of the TN Valley into the southern Appalachians of northern GA-western NC for more widespread convection and an enhanced risk of flash flooding (i.e. Slight). This given the upslope low-level component along with a more vigorous vort lobe traversing the area. Elsewhere, any flash flooding is expected to be more isolated/localized, thus the Marginal Risk areas. ....West... With the upper level ridge continuing to build across the Intermountain region and monsoonal moisture lingering near the International Border (precipitable water values up to 1.75"), another round of late day/evening showers and thunderstorms is expected. Continuity was generally maintained, though we did expand the Marginal Risk across the Sierra Nevada, Transverse Ranges, and adjacent foothills. Hourly rain totals to 2" should be possible. Locally heavy rainfall would be most problematic in any area burn scars and dry washes/arroyos. Hurley Day 2 Valid 12Z Sat Aug 03 2024 - 12Z Sun Aug 04 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE SOUTHERN FLORIDA PENINSULA, FLORIDA KEYS, AND PORTIONS OF THE SOUTHEAST AND SOUTHERN MID ATLANTIC... ....20Z Update... Minor adjustments were made to the MRGL risk areas across the Southwest, West, and Upper Midwest. The general consensus for those areas remain a more isolated threat for convective flash flooding with each area exhibiting modest instability and focused large scale ascent thanks to shortwaves rippling through the outer edge of a broad ridge centered over the Interior West. The Southwestern U.S will remain the better risk of flash flooding due to the remnant burn scars and more sensitive locales encompassing parts of NM/AZ and UT. The MRGL risk was expanded into portions of the Central and Southern High Plains as a stronger mid-level perturbation continues to pivot south out of the Front Range with a redevelopment of convection expected Saturday afternoon in-of Western KS into the OK and TX Panhandles. Enough forcing and instability have allowed for more emphasis on the potential compared to previous forecasts, so wanted to include the area of interest given some modest probabilistic depictions via the HREF and upper quartile NBM output. The MRGL risk was maintained across the Central Mid Atlantic and Northeast, however there is an opportunity for a targeted upgrade in future updates pending the outcome of today's convective impacts. Latest UFVS First Guess Fields do place a SLGT risk across the Lower Susquehanna Valley through much of Eastern PA. This is likely due to the total QPF output and signal for heavy rainfall Saturday afternoon and evening along the frontal convergence zone in place from NoVA through the Lower Hudson Valley. Area FFGs are running higher in those areas compared to normal, but the priming from today's precip could bring those down to more reachable levels, thus implying a better chance for an upgrade. This will be reassessed in later updates so long as guidance remains at least continuity or increases their heavy rainfall footprint. The SLGT risks are updated below within their respective sub- headings... ....South Florida... Tropical wave over Eastern Cuba will continue to become better organized over the Florida Straits the next 24 hours with a stronger core allowing a more defined circulation with heavy rain encompassing the circulation and general surrounds. Latest National Hurricane Center (NHC) forecast has labeled the convective cluster as AL4 alluding to an expected tropical disturbance sometime in the next 24-36 hours. This has prompted a greater emphasis on heavy rain prospects within the Southern FL Peninsula down through the Florida Keys who will see the initial brunt of the wave beginning Saturday morning. A persistent onshore flow across the Southeast FL coast will lead to increasing frictional convergence with outer banding feeding into the storms eastern flank of the circulation. This is classic with evolving tropical disturbances and a sneaky way for areas well away from the storm center to see heavy rainfall in the form of repeated bands and training that would allow for flash flood threats to arise. Ensemble guidance, both hi-res and global resolutions signal a stronger case for more widespread heavy rain between 2-5" expected along the population centers based from Miami up to just south of West Palm Beach. This area is notorious for flash flooding due to the focused lower FFGs within a sprawling urban footprint that runs up I-95. The threat for heavy rain is also present within the Florida Keys with the combination of the center of the wave moving overhead, as well as trailing bands remaining persistent in-of the entire island chain. Totals of 3-6" with locally higher are forecast across the Keys with the highest totals located near and south of Marathon down into Key West. In coordination with the local Miami and Key West WFOs, the SLGT risk from the previous period was expanded southeast to include all of the Southern FL Peninsula, as well as the entire island chain of the Florida Keys. Further northwest into the Southwestern side of FL, the threat for significant rainfall has increased substantially with NHC's forecast depicting a close proximity of what is trending to be the next tropical storm as the circulation wanders just off the Southwest FL coast. A steady south to southwest flow around the eastern flank of the low will create an onslaught of very heavy, efficient tropical rainfall poised to migrate onshore around the Southwestern Everglades up through the populated areas of Naples and Fort Meyers. HREF neighborhood probabilities are about as high as you could expect for a nearby tropical disturbance with >5" signals running between 60-80% along the coast with >8" a formidable 25-45% positioned right over the coastal areas of Lee and Collier counties. The previous SLGT risk has plenty of merit with even some inference of a potential upgrade given the signals present. There is some modest uncertainty on the exact location of the circulation as it makes progress to Southwest FL latitude as a further westward track would mitigate some of the heavier rainfall to just off the FL coast. The the threat is currently sufficient to label as a high-end SLGT with an upgrade likely over the Southwest coast if there begins to be increasing confidence in the exact track hugging the western FL coastline. This is an evolving situation, so stay tuned for future updates as we move closer to the event. ....Southeast and Southern Mid Atlantic... Increasing convergence pattern along an approaching cold front will allow for greater convective coverage and magnitude across portions of Southern VA down through the Central Carolinas into far Eastern GA. A surface trough will bisect the above locations with a cold front migrating eastward out of the Carolina Piedmont. Destabilization from diurnal heating will present a strong instability pattern located over the interior portions of the Carolinas with several deterministic aligning a 3000-4000+ J/kg SBCAPE signature from Columbia up through Southeastern VA. In combination with PWATs running ~1 deviation above normal, this will be sufficient for locally heavy rainfall with higher threat of urban flooding, especially considering the mean flow being lighter as the trough continues to lift northward. The best signature for heavy rain lies within Central NC where 12z HREF EAS probabilities are running between 50-80% for at least 1" of rainfall, a strong signal for the EAS with historical precedence for at least a SLGT risk when depicted across most areas east of the Appalachians. This was enough consensus, along with relatively high neighborhood probabilities for at least 2" and 3" respectively in the same locations with low-end >5" probabilities within the Carolina Piedmont. The SLGT risk from previous forecast was maintained with little change in the orientation. Kleebauer ....Previous Discussion(s)... ....Southwest... Monsoonal convection over the Southwest (precipitable water values up to 1.75" near the International Border) continues to support a Marginal Risk area. Locally heavy rainfall would be most problematic in any area burn scars and dry washes/arroyos. ....Sierra Nevada into Oregon Cascades, Blue Mountains, and parts of the Northern Rockies to western portions of the Great Basin... Anomalous moisture (TPW anomalies topping 3-3.5 standard deviations above normal per the GEFS) will advect northward through the West on the backside of the upper ridge and ahead of the shortwave trough off the PAC NW coast. The guidance gets TPW values up to 1-1.25", which would be most anomalous in terrain. Given these moisture profiles, along with the diurnal heating (MLCAPEs peaking at least ~500 J/Km), spotty hourly or sub-hourly rainfall rates aoa 1" will make for localized runoff issues, especially over burn scares. ....Upper Midwest... The guidance is generally showing a modest signal for heavy rainfall in and near Minnesota this period. Precipitable water values rise about 1.25" with a warm advection pattern. This led to an introduction of a Marginal Risk here, as flash flood guidance values are modest after recent heavy rainfall. Hurley Day 3 Valid 12Z Sun Aug 04 2024 - 12Z Mon Aug 05 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF THE FLORIDA PENINSULA AND PARTS OF THE SOUTHEAST COASTAL PLAIN... ....20Z Update... Only minor changes were necessary to any of the risk areas across the CONUS with the largest change occurring over the Northern Plains and Upper Midwest with the previous MRGL risk expanded to the east to reflect a faster forward propagation of a shortwave traversing the area. The main concern for the period continues to be across Florida into the Southeastern Coastal Plain thanks to an eventual tropical disturbance/storm migrating north and northeast along the western FL coast before cutting across the northern half of the Peninsula. There is still too much uncertainty on the exact location of the general circulation of the disturbance, so wanted to maintain continuity based on the ensemble mean QPF and some of the better probabilities for heavy rainfall within the 75th percentile of the NBM 24hr QPF. The area across the Mid Atlantic and Northeast is subject to the location of a stationary front where the primary focus for convection will be located as this will ultimately determine the western extent of the flash flood threat. The setup would allow for better urban flash flood prospects with the frontal positioning bisecting the urban corridor from DC up through Southeast New England. The MRGL risk was maintained with some minor adjustments based on the 12z ensemble bias corrected QPF footprint. ....Florida through the Southeast Coastal Plain.... AL4 will eventually evolve into a defined tropical cyclone with the latest NHC forecast developing the system into our next tropical storm sometime during D2, carrying through the D3 time frame. The uncertainty of exact location of the low given the timing and interaction with the approaching trough/front to the north has limited the capabilities for discerning specifics for the Sunday into Monday period. There is some general consensus on heavy rainfall potential within the Western FL coastal plain, including areas like Sarasota up through Tampa and Clearwater before the ensemble forecast takes the low up through the north-central FL Peninsula. If this track verifies, this would send a dearth of heavy rainfall across interior areas with prevailing onshore flow to the north ransacking the Southeastern U.S coastal areas in GA into SC. Some models try to develop a Predecessor Rain Event (PRE) across the Southeast coast thanks to the timing of the system's approach with interaction of the coastline. This something to monitor in future updates as these phenomena are tough to gauge at range, but can be focused significant rain events where they occur. Regardless if a PRE forms or not, the highly anomalous tropical moisture flux and persistent heavy rain banding around the storm circulation will cause locally enhanced flash flood concerns across much of FL into the GA/SC coast. The SLGT risk was maintained given the ensemble QPF continuity and expected NHC track with the opportunity for future upgrades pending increased confidence on the storms forward motion. Kleebauer ....Previous Discussion... ....Montana into the Northern Plains and Upper Mississippi Valley... Aforementioned axis of anomalous TPW out West during Day 2 will ultimately get absorbed in the upper level jet streak and mid- level vort lobe that will cross MT and the Dakotas on Day 3. Dynamical forcing along with the thermodynamical profile will favor more widespread showers and thunderstorms, though the shear profiles will certainly favor more progressive/forward-propagating cell motions and thus make for more of an isolated flash flood threat. Areas across northwest MT, including Glacier NP, would be the most susceptible given the lower FFG (1" or less within 3 hours for most areas). An upgrade to Slight may be needed over this region in later outlooks, especially if we start seeing a consensus of heavier QPF from the models. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4Cf97LiJpEbg8Ew8FwoGj5DjrfY5b2g8PSbtzQLb8F0o= UE62H3CCOCeQr1n479rp6hwZQ6YWA34wadSkWXZg5qfZz0U$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4Cf97LiJpEbg8Ew8FwoGj5DjrfY5b2g8PSbtzQLb8F0o= UE62H3CCOCeQr1n479rp6hwZQ6YWA34wadSkWXZgELRELGM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4Cf97LiJpEbg8Ew8FwoGj5DjrfY5b2g8PSbtzQLb8F0o= UE62H3CCOCeQr1n479rp6hwZQ6YWA34wadSkWXZgP8ATjSQ$=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) .