Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Aug 08 2024 00:52:44 FOUS30 KWBC 080052 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 852 PM EDT Wed Aug 7 2024 Day 1 Valid 01Z Thu Aug 08 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... ....01Z Update Summary... Overall...with Debby moving little and remaining close to the earlier NHC forecast, only minor adjustments were needed to the=20 High Risk area across the Carolinas through the remainder of the night. A minor adjustment was made to the Slight Risk over=20 portions of the Mid- Atlantic based on latest radar imagery. No changes were needed elsewhere with convection expected to weaken and dissipate in the next couple of hours over the Western United States with the loss of solar heating. Bann ....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 coast with a disheveled appearance based on satellite presentation. A lot of drier air has wrapped into the core with the system struggling to shake off the drier environment leading to less vigor within any organized convective patterns around the storm itself. There is still a solid background of tropical moisture associated with the storm as indicated within the latest 12z soundings out of both KCHS and KMHX with PWATs hovering between 2.2-2.6" across Eastern SC and Southeastern NC. Diurnal instability will pick up inland with a redevelopment of heavy convection focused on the western periphery of the low center. 12z HREF and associated CAMs were all bullish on an arc of heavy convection from Charleston to points north and northeast, about 25-60 miles inland from the coast. A general 3-6" with upwards of 8" will be plausible during the afternoon and evening from the band of heavy rainfall with rates hovering between 1-2"/hr at peak intensity. Considering the recent rainfall overhead, the soil moisture percentiles have plummeted with the top-layer soils trending very wet, running through the 80-90th percentile according to NASA SPoRT output. With the a prevalent signal for at least 3" of rainfall based within the latest EAS probability fields (>90%) around the above area, the High Risk was maintained, but some adjustment on the northern and southern fringes where probabilities drop off significantly away from the prospectus area of heavy rainfall. 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 Mason Dixon. This same area is also carrying some elevated probabilities for >5" with a 20-45% footprint running west to east along I-70 to the northern Delmarva. This is the proxy for the frontal location during the period with the CAMs depicting the focal point for convection the next 12-18 hrs. First Guess Fields were also a little more robust in the depiction with a SLGT risk extending out towards the Blue Ridge on the latest update. There was an expansion further west, but only out to US15 with a southern extension encompassing the District, Southern MD, and portions of the Northern Neck of VA through the Southern Delmarva. The best risk will likely be north of US50, but some CAMs were privy to a smattering of convection down across the region which is in the better deep layer moisture extending to the north of Debby. 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 NORTH CAROLINA AND SOUTHWEST VIRGINIA... ....WIDESPREAD AND NUMEROUS INSTANCES OF FLASH FLOODING WITH SIGNIFICANT AND CONSIDERABLE FLOODING LIKELY... ....20Z Update Summary... Significant changes to the previous forecast were made in coordination with the WFOs involved given the recent trends in Debby's forward motion and expected heavy rain footprint. Limited changes were made to the previous SLGT risk across the Rockies, but was generally expanded to the south to match the latest 12z HREF neighborhood probabilities. Updates for each area are located within the sub-headings below.... Kleebauer ....Southeast and Mid Atlantic... 20Z Update: A shift in guidance has led to some wholesale changes in the alignment of the higher risk areas across the Southeast and Mid Atlantic for the D2 period. Debby will continue to move inland with a westward jog over towards the Carolina Piedmont during the morning and afternoon hours of Thursday. To the northwest, an upper trough will scoot eastward through the Great Lakes and Ohio Valley with a surface cold front progged to move quickly to the east with frontal approach into the Mid Atlantic by the end of the period. The remnant circulation of Debby will hold steady through the morning, but the cyclone will be undergoing a transitional state to a more extra-tropical cyclone with more synoptic mid-latitude characteristics customary once inland and tracking northward. The combination of Debby's moisture flux, primary core, and the trough/front approach will allow for a robust swath of heavy rainfall to occur in-of the Carolinas, eventually moving north into the Central Mid Atlantic with sights on the Piedmont from Southwestern PA down through MD/WV and Western VA. This was a significant shift from previous forecast thanks to the forward propagation of Debby to the north and the advection of deep tropical moisture poleward earlier along the frontal boundary. This combination has shifted the primary axis of heavy QPF further north and west with a greater emphasis on the threat further inland. Ensemble bias corrected QPF took a big jump in the areal extent of significant rainfall exceeding 3" with an areal average of 2.5-4" now encompassing the Blue Ridge down through the Shenandoah Valley, arcing back into the eastern side of the Carolina Piedmont between Charlotte and Raleigh. This is due to the extra-tropical transition with a great axis of deformation forming on the northern and western side of the primary circulation left over from Debby. A feed of moisture will wrap around the eastern periphery of the circulation, but in a less organized manner with better feeder bands still present in the morning before becoming diffuse after the extra-tropical transition. Highest totals between 4-8" are forecast across Southwest VA through West-Central and Central NC with the highest precip located between Roanoke, VA down towards Charlotte and over to about 15-20 miles west of Raleigh. This axis is very well agreed upon by both the hi-res and ensemble bias corrected QPF with the HREF blended mean and hi-res bias correction model running at a fairly high correlation leading into the period. HREF EAS probabilities are very high for the >2" and >3" total QPF forecast with the latter running between 50-80% across portions of Central NC into Southwestern VA. This is typically aligned with higher end risk thresholds given the conservative nature of the EAS probability field due to the overlap necessary in guidance. Neighborhood probabilities for >5" and >8" are also very high across Southeast NC extending to the northwest into Southwestern VA with 60-90+% being depicted within both higher-end precip markers. The correlation of higher probabilities with significant totals forecast was enough to garner a shift in the previous High Risk further inland with less emphasis along the coast compared to the previous forecast. A MDT risk extends along the eastern SC Piedmont near I-77 up through much of Southwest VA into the Shenandoah and adjacent Blue Ridge, stopping in Southwest PA just north of the Mason Dixon line. Heavier rains will expand further away from the primary axis of heavier precipitation, but the coverage will be lower due to the confined low to mid-level moisture flux associated with Debby along the leading edge of the trough and advancing cold front. Some heavier tropical downpours will be forecast along the far western side of the setup due to convergent upslope flow against the Appalachians anticipated leading to a general expansion of the risk areas further west from the previous cycle. Eastern flank of the setup will also have a threat of heavier rainfall at times with a sufficient deep moist environment to work with, but the issue for greater coverage is the lack of stronger forcing to work with that is more privy to areas further west. The risk areas along the coast were degraded somewhat to reflect the change, however the threat is still very much non- zero with the prospects of flash flooding well within the range of potential outcomes across Eastern NC up through Central VA into the Central Mid Atlantic, including the DMV. This was emphasized with a continuation of at least a SLGT risk west of the Chesapeake Bay. Kleebauer ....Four Corners, Central Rockies and High Plains... 20Z Update: Only minor modifications made to the previous SLGT risk forecast across the Central and Southern Rockies. The primary threat will be across the Sangre de Cristos thanks to strengthening upslope component as a cold front banks against the terrain with strong upward motion along a prominent theta-E ridge situated across the Southeast CO Front Range into the adjacent terrain. Heavy thunderstorms capable of 1-1.5"/hr will cause issues within the complex terrain and the remnant burn scars that persist within the confines of the Sangre de Cristos. The SLGT was more than worthy given the signal and sensitive grounds lingering over the region. Kleebauer ....Previous Discussion... 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 MID ATLANTIC AND NORTHEAST... ....20Z Update... Previous forecast is still in-line with the current thinking with the core of the heaviest rainfall occurring along the convergent area of the cold front as Debby's remnants get absorbed into the mean trough to the northwest and shunted eastward. The MDT risk was generally maintained, however a small shift south to include portions of the Mason Dixon and Northern MD were included given the latest QPF trends on D3, aligning with some modest probabilities for >2" of rainfall in the area. Debby's remnants will be moving rapidly to the north-northeast with the associated moisture streaming well ahead of any persisting circulation. The storm will have gone through full extra-tropical transition once into Central PA and will advance all the way into Canada by the end of the forecast period. Widespread 2-4" of rainfall are forecast over Central and Western PA through Western NY state and the western side of Northern New England (North Country over into VT) before exiting into the Canadian Maritimes and Eastern Quebec. Rainfall rates of 1-2"/hr are forecast within the zone of highest impact relayed above with the eastern flank of the storm becoming drier as it gains latitude. The current forecast is reflective of some uncertainty in the exact track of the remnant cyclone as it reaches the NY state boarder, but if the trends continue, the eastern side of the risk areas will likely be brought further west to tighten the corridor of heaviest rainfall as is expected with the setup. Look for further updates in future forecasts. Elsewhere, the west remains active with another round of convection centered over the Central and Southern Rockies. There was enough of a signal to expand the SLGT further west into South-Central CO where sufficient destabilization and elevated moisture will prime the environment for another round of scattered thunderstorms with prospects for locally heavy rainfall across the complex terrain. Totals upwards of 1-2" will be possible in the strongest cells, a stronger correlation to flash flooding concerns compared to historical precedence. The rest of the Southwest will still see periods of storms capable of localized flash flood concerns with the best threat located more into the Inter Mountain West and the Mogollon Rim in AZ. Kleebauer ....Previous Discussion... ....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!5eG7FewtRr9t6unFb_YidkabxBD-fTUs0LGwoi00M_k7= G_1Ms-TZZZdylRbplJA1Z6dhUYD4CiLGu0Ok0AjCPfEMyAI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5eG7FewtRr9t6unFb_YidkabxBD-fTUs0LGwoi00M_k7= G_1Ms-TZZZdylRbplJA1Z6dhUYD4CiLGu0Ok0AjCIsO2IgY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5eG7FewtRr9t6unFb_YidkabxBD-fTUs0LGwoi00M_k7= G_1Ms-TZZZdylRbplJA1Z6dhUYD4CiLGu0Ok0AjCbF4daPw$=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) .