Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Aug 07 2024 15:54:43 FOUS30 KWBC 071554 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1154 AM EDT Wed Aug 7 2024 Day 1 Valid 16Z Wed Aug 07 2024 - 12Z Thu Aug 08 2024 ....THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH AND NORTH CAROLINA... ....WIDESPREAD AND NUMEROUS INSTANCES OF FLASH FLOODING WITH SIGNIFICANT AND CONSIDERABLE FLOODING LIKELY... ....16Z Update Summary... Minor adjustments to the High Risk across the Carolinas was made to reflect the latest QPF footprint based off CAMs and relative ensemble output. SLGT risk across the Mid Atlantic was expanded west through parts of Central MD and Northern VA. Only modest changes were necessary for the MRGL out west with a small expansion eastward into northern KS for the eventual complex moving out of the Front Range into the area overnight. More details of each located in the respective sub-headings below.... Kleebauer ....Southeast... 16Z Update: Debby continues to meander off the South Carolina=20 coast with a disheveled appearance based on satellite=20 presentation. A lot of drier air has wrapped into the core with the system struggling to shake off the drier environment leading to=20 less vigor within any organized convective patterns around the=20 storm itself. There is still a solid background of tropical=20 moisture associated with the storm as indicated within the latest=20 12z soundings out of both KCHS and KMHX with PWATs hovering between 2.2-2.6" across Eastern SC and Southeastern NC. Diurnal=20 instability will pick up inland with a redevelopment of heavy=20 convection focused on the western periphery of the low center. 12z=20 HREF and associated CAMs were all bullish on an arc of heavy=20 convection from Charleston to points north and northeast, about=20 25-60 miles inland from the coast. A general 3-6" with upwards of=20 8" will be plausible during the afternoon and evening from the band of heavy rainfall with rates hovering between 1-2"/hr at peak=20 intensity. Considering the recent rainfall overhead, the soil=20 moisture percentiles have plummeted with the top-layer soils=20 trending very wet, running through the 80-90th percentile according to NASA SPoRT output. With the a prevalent signal for at least 3"=20 of rainfall based within the latest EAS probability fields (>90%)=20 around the above area, the High Risk was maintained, but some=20 adjustment on the northern and southern fringes where probabilities drop off significantly away from the prospectus area of heavy=20 rainfall.=20 Further away from the main area of interest, scattered heavy thunderstorms will be plausible as they rotate outward from the storms center with embedded heavy rain rates exceeding 1"/hr in the stronger cores. The airmass further away from Debby is drier, but marginal instability and sufficient deep layer moisture could still cause some concern away from the primary focal point. Later this evening, Debby will move back inland with a west-northwest trajectory through the SC Low Country leading to more organized convection spreading inland until it reaches the Carolina Piedmont. This is when probabilities for the heaviest rain focus away from the coast and out towards the I-77 and I-95 corridors with areas like Columbia and Raleigh to Charlotte becoming the areas of focus as we enter the end of the period. This allowed for a small westward nudge in the SLGT and MRGL risk areas with a minor adjustment on the MDT risk based on the timing of Debby's westward progression and some heavier QPF being placed west of the previous forecast. Kleebauer ....Mid Atlantic... 16Z Update: A quasi-stationary front is currently analyzed across the Central Mid Atlantic with a corridor of higher theta-E's and relevant instability located over the Delmarva and adjacent Piedmont across Central MD and Northern VA. This will be the primary focus for scattered convection this afternoon and evening with the initiation occurring during peak diurnal heating along with the passage of modest mid-level perturbation moving east-southeastward out of PA. Majority of the region will see only light totals, but the moist unstable environment in place will allow for multiple cells to produce rainfall rates of 1-2"/hr, easily able to cause issues within the DMV due to higher runoff potential from urbanization, and antecedent wet grounds from last periods rainfall. Neighborhood probabilities are very bullish thanks to the 12z CAMs outputs this morning relaying >3" probs between 50-80% between US50 and the=20 Mason Dixon. This same area is also carrying some elevated=20 probabilities for >5" with a 20-45% footprint running west to east=20 along I-70 to the northern Delmarva. This is the proxy for the=20 frontal location during the period with the CAMs depicting the=20 focal point for convection the next 12-18 hrs. First Guess Fields=20 were also a little more robust in the depiction with a SLGT risk=20 extending out towards the Blue Ridge on the latest update. There=20 was an expansion further west, but only out to US15 with a=20 southern extension encompassing the District, Southern MD, and=20 portions of the Northern Neck of VA through the Southern Delmarva.=20 The best risk will likely be north of US50, but some CAMs were=20 privy to a smattering of convection down across the region which is in the better deep layer moisture extending to the north of Debby. =20 Kleebauer ....Southwest... 16Z Update: Little has changed from the previous forecast across the Western and Central US. HREF probabilities are virtually identical in the depiction with a >3" neighborhood probability still lying within the 20-30% range with the best chance occurring over the Front Range of WY near Cheyenne down into the Northwest corner of KS. There was some CAMs support for the complex of storms to migrate a bit further southeast into Northern KS before dissipating by the morning a cold front associated with the convective pattern pushes eastward and the LLJ ahead of the front gets squashed leading to less of a convergence signal. Still enough to warrant a small expansion across the aforementioned area, but certainly not enough to raise the risk away from the MRGL forecast. The Monsoon convection across AZ will flare up once again later this afternoon and evening with probability fields targeting Southeast AZ near the Huachucas up into the I-10 corridor near Tuscon. More convection will fire along the Mogollon Rim as well, but the QPF footprint was not amped enough to signal anything higher than a standard MRGL. This allowed for a general continuity over the Southwest overall. Kleebauer ....Previous Discussion... The increased moisture flux from the Gulf of California will spread to the north and east across the Four Corners region, helping maintain enhanced rainfall during the afternoon and evening. There is a growing consensus for upstream convective growth around the northern periphery of the ridge with sights downstream into the High Plains off the Colorado/Wyoming Front Range. The nature of this convection is expected to be progressive which should limit the flash flood potential to a low-end MRGL threat, however a narrow corridor of 20-30% probability for >3" exists within the latest HREF output with hourly rates generally between 1-2"/hr at peak intensity. A Marginal Risk area covers much of the Four Corners region and extends into southeast Wyoming, western Nebraska and Kansas. Campbell/Kleebauer Day 2 Valid 12Z Thu Aug 08 2024 - 12Z Fri Aug 09 2024 ....THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH AND NORTH CAROLINA... ....WIDESPREAD AND NUMEROUS INSTANCES OF FLASH FLOODING WITH SIGNIFICANT AND CONSIDERABLE FLOODING LIKELY... ....Southeast and Mid Atlantic... The latest trend with Debby's track is for it to press back inland of the Carolinas with the primary circulation a bit further west compared to the latest forecast. Ensemble trends within the QPF field followed suit with further push towards the Carolina Piedmont leading to a higher risk of heavier rain focused inland from I-77 and points east to around I-95. Current forecast is for a widespread 3-6" with local to 8" of rainfall encompassing the Midland's of South Carolina across into East-Central North Carolina and South Carolina. During this period the highly anomalous airmass will range from +2 to +4 standard deviations above normal for early August for this part of the country and will be drawn northward ahead of an approaching trough/cold front to the east. This will be conducive for heavy precipitation to make it into parts of Southern and Central Virginia with heavy rains extending back near the Southern Appalachians. The setup remains tricky considering the weak steering pattern around Debby as the low gets absorbed into the eventual front. A High Risk remains in effect for central/southern North Carolina and northeast South Carolina with minimal adjustments from the previous forecast. However, the southern bounds of the Moderate and Slight Risk areas were trimmed northward given the threat for excessive rainfall has reduced across southern South Carolina and southeast Georgia. ....Four Corners, Central Rockies and High Plains... The monsoonal pattern should yield a targeted focus for heavy rain across portions of the Central and Southern Rockies. Shortwave energy passing through the Rockies will help provide additional forcing for convection to focus along the higher terrain of Colorado and New Mexico. Model guidance suggests a concentration of cells capable of producing 1 to 2 inches with very isolated maxes greater than 3 inches. A Slight Risk remains in effect for portions of central Colorado and northern New Mexico given the increased risk for isolated to scattered flooding concerns. A Marginal Risk covers much of the Four Corners region, western Kansas and Nebraska, southwest South Dakota, eastern Wyoming and extreme southeast Montana. Campbell/Kleebauer Day 3 Valid 12Z Fri Aug 09 2024 - 12Z Sat Aug 10 2024 ....THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHEAST... ....Mid-Atlantic to the Northeast... During this period Debby is anticipated to transition to extratropical and absorb into the system in tracking through the Northeast. The deep moisture from Debby will feed into the advancing cold front and yield moderate to heavy rain from the Piedmont region of the Maryland northward to Upstate New York, Vermont and New Hampshire. A Moderate Risk area was inherited for this period and was maintained with a small westward shift from the previous area. As such, there was a modest westward adjustment of the Slight Risk area over southern New England. While the new rainfall over the Mid-Atlantic and Southeast region will be greatly reduced there will still be a lingering Marginal/Slight Risk for continued flooding concerns over the extremely sensitive soils. ....Four Corners, Central Rockies and High Plains... Shortwave energy exiting from the Rockies into the Plains will continue to support more organized convection across Colorado and New Mexico and eastward into western Kansas and the panhandles of Oklahoma and Texas. Moderate to heavy rainfall will focus in the vicinity of the higher terrain. A Slight Risk was already in effect for portions of central Colorado and north/northeast New Mexico but with the latest guidance and QPF trends, the Slight was shifted a little to the south. A broad Marginal Risk covers much of the Four Corners region, east into western Kansas and northward into south- central Montana. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_04uR-lckFpHqP2sfoTzDz7m9heOUB4f1dfZrJl9SZk0= JaCz4c60sliuf1UkqRV1IoEmJJsYRuVY6JoR6A8-niNklsQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_04uR-lckFpHqP2sfoTzDz7m9heOUB4f1dfZrJl9SZk0= JaCz4c60sliuf1UkqRV1IoEmJJsYRuVY6JoR6A8-ufhMvXc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_04uR-lckFpHqP2sfoTzDz7m9heOUB4f1dfZrJl9SZk0= JaCz4c60sliuf1UkqRV1IoEmJJsYRuVY6JoR6A8-OaAjZgM$=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) .