Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Aug 08 2024 19:38:44 FOUS30 KWBC 081938 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 338 PM EDT Thu Aug 8 2024 Day 1 Valid 16Z 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... ....16z Update... The current High risk and lower risk areas associated with Debby are still on track through the period. Some minor nudging of the High Risk further east across North-Central NC and South-Central VA was implemented given the threat of outer banding streaming northwest out of Eastern NC bisecting the above area with consensus from all CAMs and signaled well within the latest HREF blended mean QPF. Debby will begin undergoing the transition to extra tropical through the period with a more "lopsided" look within the precip field, exhibiting a bit of a deformation axis currently across Western NC into portions of Upstate SC along I-77. The MDT risk was adjusted to include more of the I-77 region with Charlotte the focal point for significant flooding potential due to urbanization factors. There is not much of a shift in the overall QPF magnitude across VA with substantial rainfall expected across almost all of Southwest VA, excluding the area near Bristol, extending north up the I-81 corridor within the Shenandoah Valley. Persistent rainfall rates this evening between 1-2"/hr for several hours with intra-hour bursts of over 2"/hr are forecast overnight leading to widespread flooding/flash flooding with some potentially major impacts due to high runoff capabilities along the Blue Ridge into the neighboring valleys. 4-8" with local to 10" is forecast within much of Western VA leading to very high 100-year exceedance probabilities between 50-80% encompassing much of the region from Roanoke up to about Front Royal and surrounding locales. This is an upper quartile type depiction lending to confidence in the High Risk and high-end MDT risk extending as far north as the bordering WV Panhandle. HREF EAS probabilities for >3" continues to remain very high with a large coverage of 50+% probs located across much of Southwestern VA up through the Shenandoah. Some signals to the northeast of Raleigh are also spurring up compared to previous HREF runs which correlates well with the line forming along the northeast flank of the low. Further south across South Carolina into far Eastern GA, a strengthening in-flow area on the underside of the low will likely amount to a line of convection that will stream counter-clockwise through the Low Country of SC into Southeast NC before it pivots back north into the low. This is pretty consistent within the CAMs with some fairly aggressive in the signature on a band of heavy rain over a very water logged region currently. This was the reason for some minor adjustment on the risk areas along the southern periphery of the low to cover for the threat. Out west, the MRGL was expanded into the Northwest Panhandle thanks to increasing confidence in scattered convection forming along the NM/TX line and shifting eastward with heavy rain potential of 1-2"/hr at peak intensity. Flash flood threat will be pretty isolated, but was enough to warrant the MRGL risk. The SLGT risk was maintained with little to no changes given model consistency. Kleebauer ....Previous Discussion... ....Southeast and Mid Atlantic... There was a trend with the latest guidance for an eastward shift given the Debby's forward motion and expected heavy rain footprint. Debby will continue to move inland with a westward jog over towards the Carolina Piedmont during the morning and afternoon hours. 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 Pennsylvania down through Maryland/West Virginia and western Virginia. The High Risk area covers portions of the southeast North Carolina, north-central North Carolina and much of central/northern Virginia where areal average of 6 to 10 inches are forecast. A Moderate Risk extends along the eastern South Carolina Piedmont near I-77 up through much of Southwest Virginia into the Shenandoah and adjacent Blue Ridge, stopping in Southwest Pennsylvania 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. With reduced forcing closer to the coastline the threat for heavier rain will be less than areas to the west, but non-zero. ....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. 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. 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 2 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 Summary... Some minor adjustments were made to the northern periphery of the MDT risk with the removal of VT and the Capital District of NY as a pronounced QPF minimum is forecast across guidance within the Valley's along and east of I-87. A SLGT risk expansion was created along the SC coast up through Eastern NC as a trailing axis of heavier precip is forecast to impact the Eastern Carolinas where much of the region will be sensitive to additional rainfall. Elsewhere, a SLGT risk was brought eastward through the TX/OK Panhandles with a growing consensus on the prospects of a strong complex developing over the Southeast CO and migrating southeast along the northern periphery of the ridge out west. More on these individual setups in the appropriate sub-headings below.... Kleebauer ....Mid-Atlantic to the Northeast... 20Z Update: Guidance remains in good agreement on the axis of=20 heaviest rainfall bisecting much of West-Central and Northern PA up through Upstate New York with the heaviest precip forecast across=20 North-Central PA between Centre county and the bordering counties=20 near NY like McKean over to Western Tioga county. Secondary max=20 will be focused within the Adirondacks where some easterly upslope=20 flow coupled with primed surface convergence will lead to a=20 bullseye of enhanced precip outlined solidly within the=20 neighborhood probability fields and blended mean QPF output. A=20 widespread 2-4" with locally as high as 6" will be forecast across=20 the above area with a standard 1-3" on the outer periphery of the=20 MDT forecast. The area across VT and the Capital District was=20 removed from the MDT risk considering a stark shift in the axis of=20 heaviest QPF removing much of the area along I-87. This was=20 coordinated with the local Albany and Burlington WFOs with=20 agreement from the probability fields in the entire stretch=20 becoming a regional minimum with regards to QPF and potential=20 flash flood concerns.=20 Another area of interest compared to the previous forecast is=20 across the Eastern Carolinas where trailing moisture on the inflow=20 region of the extra-tropical cyclone will interact with the=20 approaching surface cold front with strengthening low- level=20 convergence in-of the moist unstable environment lingering over the region. The threat for locally heavy rainfall of 1-2"/hr will be=20 plausible from Charleston up the SC/NC coastal plain with a strong=20 probability signature of >3" (70-90%) located within the above=20 region(s). Considering the sensitivity of the area from the recent=20 onslaught of rainfall, a SLGT risk upgrade was created over SC=20 coast up through much of Eastern NC.=20 The rest of the forecast area due to impact from the remnants of Debby was relatively unchanged with perhaps a slight bump in the flash flood threat across the DMV and Northern Delmarva due to increasing low-level convergence pattern within moisture anomalies running between 2-3 deviations above normal as the cold front quickly ushers eastward through the area. An organized line of convection will blossom to the east of the Blue Ridge with embedded heavy rain cores situated within the forecasted line. A marked increasing in the potential >2" (70-90+%) was noted within the latest neighborhood probability fields, a signal very sufficient for flash flooding concerns within a mostly urbanized region. The prospects for a MDT risk extending into the DMV was entertained, but the progressive nature of any convective threat was a detriment to the confidence in a more widespread flash flood prospect. The SLGT risk was maintained, but would likely lean on the higher-end of SLGT, especially over Central MD for areas east of US15.=20 Kleebauer ....Previous Discussion... The core of the heaviest rainfall is anticipated to occur along the convergent area of the cold front as Debby's remnants get absorbed into the mean trough to the northwest and shunted eastward; from the Piedmont region of the Maryland northward to Upstate New York, Vermont and New Hampshire. 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 Pennsylvania through western New York 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 Moderate Risk was maintained with very minor adjustments to account for the latest WPC forecast. There continues to be some uncertainty in the exact track of the remnant cyclone as it reaches the New York state border, 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. With rain diminishing across the Appalachians and western portions of the Carolinas and Virginia the Slight and Marginal Risks were significantly trimmed eastward. Some heavier bands are expected to setup over coastal areas of North Carolina that will keep an elevated threat for flooding concerns during this period. ....Four Corners, Central Rockies and High Plains... 20Z Update: The previous SLGT risk was generally maintained, but the areal extent was adjusted to include more areas downstream across the TX/OK Panhandles, as well as inclusion of the Sacramento Mountains due to the very sensitive burn scar area located near Ruidoso. The area across Northeast NM through the Panhandle(s) has come up considerably in the probability fields for enhanced heavy rain potential thanks to an expected complex initiating over Southeast CO and ejecting southeast along the northeast periphery of the mid-level ridge. Signals of >70% chance for up to 2" of rainfall with a correlating probability of 20-30% for up to 2"/hr are generally good enough to warrant a SLGT risk. The upgrade also has support from the latest UFVS First Guess Fields which have broadened the areal coverage of the higher risk compared to previous forecasts. The remainder of the Southwest maintained general continuity with isolated pockets of heavier rain focused over much of the west with a continuation of the Monsoon pattern.=20 Kleebauer ....Previous Discussion... Showers and thunderstorms are expected to fire up across the Southwest and Rockies as shortwave energy passes through and encounters the ongoing monsoonal moisture in place. There will be increased focus for more organization and higher amounts concentrating near the higher terrain of south/central Colorado and northern New Mexico, where a Slight Risk area remains in effect. Accumulations 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 Arizona. Convection will also spread eastward into western Kansas and the panhandles of Oklahoma and Texas. A broad Marginal Risk covers much of the Four Corners region, east into western Kansas and northward into south-central Montana. Campbell/Kleebauer Day 3 Valid 12Z Sat Aug 10 2024 - 12Z Sun Aug 11 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHWEST, CENTRAL ROCKIES INTO THE PLAINS... ....20Z Update... The biggest change for the period was the removal of the MRGL risk across ME as the trends in the forward propagation of Debby's remnants has shifted to a faster exit into the Canadian Maritimes with a very small lingering QPF footprint located over Northern ME with the heaviest precip generally out of the picture prior to 15z Saturday. The consensus was enough to remove the previous risk area with the NBM 90th percentile even too low to warrant the risk coverage. The remainder of the forecast was basic continuity, but will be assessing the setup across the Southern and Central Plains, mainly over Southern KS into OK for a potential strong MCS with a large heavy precip footprint located over much of the state. A lot of the setup is contingent on the placement of the mid-level ridge axis and the handling of a shortwave exiting out of the Central Rockies. Ensemble bias corrected QPF was aggressive in the latest update with widespread 1.75-2.5" totals located over Northwest and Central OK by the end of the D3. This may necessitate a SLGT risk so long as these trends continue. The rest of the Southwest will remain parked under a broad ridge with Monsoonal convective development anticipated once again in the afternoon and evening leading to isolated flash flood concerns.=20 Kleebauer ....Previous Discussion... ....Southwest, Central Rockies and Plains... Diurnal heating and monsoonal moisture will keep showers and thunderstorms developing during the afternoon and evening hours for much of the Four Corners region. The area of more organized convection triggered by shortwave energy from the D2 period will be making its way eastward into the foothills and spreading out into the high Central/Southern Plains. There is a fair amount of uncertainty with how far east into the plains the convection reaches, location of heaviest amounts and the overall accumulations. Most of the consensus is suggesting 1 to 3 inches however a couple models have maxes near/over 4 inches. A Marginal Risk covers much of the Four Corners, eastern Wyoming, western Nebraska and Kansas, Oklahoma, northern Texas, southwest Missouri and northwest Arkansas. If amounts trend up then a Slight Risk area may be need with further updates. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_P54j6LLDP6y1aR3Efomb-PYXbzr3lv7ZQ4gTc2TvyHN= LIWVBj2cDtKAZjTrhUWo8yaZOeAmhnWGPvyLRcuy64wWPZQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_P54j6LLDP6y1aR3Efomb-PYXbzr3lv7ZQ4gTc2TvyHN= LIWVBj2cDtKAZjTrhUWo8yaZOeAmhnWGPvyLRcuyMzZj6tA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_P54j6LLDP6y1aR3Efomb-PYXbzr3lv7ZQ4gTc2TvyHN= LIWVBj2cDtKAZjTrhUWo8yaZOeAmhnWGPvyLRcuy2H6lN4A$=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) .