Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Aug 02 2024 15:51:21 FOUS30 KWBC 021551 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1151 AM EDT Fri Aug 2 2024 Day 1 Valid 16Z Fri Aug 02 2024 - 12Z Sat Aug 03 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE MIDWEST, OHIO VALLEY, AND NORTHEAST, ALONG WITH THE EASTERN TENNESSEE VALLEY INTO THE SOUTHERN APPALACHIANS... ....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.=20 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.=20 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.=20 The area across the Ohio Valley will continue to see scattered=20 convective development under the periphery of the upper low, troughWBCQPFERDcenter with a slower storm motion and higher=20 potential for training. The signal is still fairly robust for flash flood concerns on the southern side of Lake Erie out into=20 Northwestern OH and far Southeast MI with the latest HREF=20 probabilities for 3hr FFG exceedance running between 25-45% with=20 the highest located under the expected path of the upper low as it=20 pivots eastward out of IN. This is more than sufficient for a=20 continuation of the previous SLGT risk with only minor=20 modifications from the previous forecast.=20 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 ....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 PORTIONS OF SOUTH FLORIDA ALONG WITH PARTS OF THE MID ATLANTIC AND SOUTHEAST COASTAL PLAIN... ....Eastern U.S... A generally wet and unsettled pattern is likely, with ample moisture (precipitable water values up to 2" for most, 2.25-2.50 across FL along/ahead of the tropical disturbance) and instability in place as a slow moving upper trough and surface/frontal systems all support numerous concentrated areas of convection. Across FL, per collaboration we have hoisted a Slight Risk across parts of South FL given the uptick in QPF ahead of the tropical disturbance. While most of the models and ensemble means keep the max areal-average totals off the SW FL coast (3-5+ inches), TPWs reaching 2.4-2.6 inches along with the possibility of more destabilization east of the tropical distrubance's center will make for the potential of very intense rainfall rates (2.5-3.0+ within an hour) underneath the more isolated stronger cells. Farther north into the Mid Atlantic, inflow at 850 hPa of 25 kts implies some level of convective organization is expected. Hourly rain totals up to 2.5" should be possible here, and local amounts in the 2.5-5" range are advertised by the guidance, which could easily fall within an hour or two. The guidance has come in better alignment in advertising an axis of heavier rainfall across western portions of the coastal plain and eastern Piedmont over the Carolinas into parts of SE VA. This given the infusion of deep-layer moisture surging north of the tropical disturbance toward the mid- upper level trough and surface front. Per collaboration, have included a Slight across this region as well. Elsewhere to the north (north of Richmond VA), there remains uncertainty in placement of the higher amounts, and a decent section of the Washington D.C./Philadelphia megalopolis has descended into drought, raising flash flood guidance values fairly highly. Therefore still believe the Marginal Risk continues best over these areas for the time being. Urban areas are at the highest risk for flash flooding in this region. ....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... ....Eastern U.S.... The Slight Risk across FL and the Southeast was expanded a bit, based on the growing consensus in terms of the forecast track and forward speed of the tropical disturbance. Areal-average QPF per the smart blends/ensembles (including the NBM) is up to 3-5" over parts of the FL peninsula. This given the highly favorable environment (deep layer moisture, tall/skinny CAPE profiles) along and ahead of the tropical disturbance. Per collaboration with the WFOs, have expanded the Slight Risk area, but for now given uncertainty by Day 3 in terms of the eventual track/speed/strength of the potential tropical system, have held off in hoisting a Moderate Risk. ....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!5tbrfl662sUtsXQllKFv8q6CXFJIRAo03bcVolKraMDc= LbgU70HfgqQ-gcnZ5GEld9ry6NaRXSjBZh29eVk3qgFMUmo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5tbrfl662sUtsXQllKFv8q6CXFJIRAo03bcVolKraMDc= LbgU70HfgqQ-gcnZ5GEld9ry6NaRXSjBZh29eVk3lkHaslY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5tbrfl662sUtsXQllKFv8q6CXFJIRAo03bcVolKraMDc= LbgU70HfgqQ-gcnZ5GEld9ry6NaRXSjBZh29eVk3ccHwq0E$=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) .