Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Aug 09 2024 00:57:08 FOUS30 KWBC 090056 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 856 PM EDT Thu Aug 8 2024 Day 1 Valid 01Z Fri Aug 09 2024 - 12Z Fri Aug 09 2024 ....THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF WESTERN VIRGINIA AND EASTERN WEST VIRGINIA ... ....WIDESPREAD AND NUMEROUS INSTANCES OF FLASH FLOODING WITH SIGNIFICANT AND CONSIDERABLE FLOODING LIKELY... ....01Z Update... Changes to the previous outlook was based largely on recent observational trends and the latest hi-res guidance runs.=20 A High Risk was maintained along the track of Tropical Depression=20 Debby as the system tracks north through Virginia and the eastern West Virginia Panhandle near the Blue Ridge, Shenandoah Valley,=20 and the Potomac Highlands overnight. Deep tropical moisture (PWs=20 2.25-2.5 inches) along with upslope flow will contribute to=20 efficient, heavy rainfall rates. Neighborhood probabilities from=20 the 18Z HREF indicate that additional accumulations of 5 inches or=20 more are likely within the High Risk area. Expect a very tight=20 gradient to the west, with amounts dropping off quickly west of the Eastern Continental Divide.=20 Further to the south and east, slow-moving, inflow bands, will continue to produce intense rainfall rates and locally heavy rainfall accumulations. While areal average amounts will be generally less across this region, a Moderate Risk was maintained back through eastern North Carolina and Slight Risk through coastal South Carolina. The HREF indicates that some of these bands may produce an additional 2 inches or more, especially across eastern=20 North Carolina and south-central Virginia, which given the=20 antecedent soil conditions, is likely to create additional runoff=20 concerns. In the central to southern Rockies and adjacent High Plains, Slight Risk areas were left in place from portions of the southern Front Range to the Sangre de Cristo and Sandia Manzano mountains, where moist upslope flow near a front pressing south through the region will continue to fuel slow-moving storms, with additional locally=20 heavy rainfall through the evening.=20 Pereira 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 heaviest rainfall bisecting much of West-Central and Northern PA up through Upstate New York with the heaviest precip forecast across North-Central PA between Centre county and the bordering counties near NY like McKean over to Western Tioga county. Secondary max will be focused within the Adirondacks where some easterly upslope flow coupled with primed surface convergence will lead to a bullseye of enhanced precip outlined solidly within the neighborhood probability fields and blended mean QPF output. A widespread 2-4" with locally as high as 6" will be forecast across the above area with a standard 1-3" on the outer periphery of the MDT forecast. The area across VT and the Capital District was removed from the MDT risk considering a stark shift in the axis of heaviest QPF removing much of the area along I-87. This was coordinated with the local Albany and Burlington WFOs with agreement from the probability fields in the entire stretch becoming a regional minimum with regards to QPF and potential flash flood concerns. Another area of interest compared to the previous forecast is across the Eastern Carolinas where trailing moisture on the inflow region of the extra-tropical cyclone will interact with the approaching surface cold front with strengthening low- level convergence in-of the moist unstable environment lingering over the region. The threat for locally heavy rainfall of 1-2"/hr will be plausible from Charleston up the SC/NC coastal plain with a strong probability signature of >3" (70-90%) located within the above region(s). Considering the sensitivity of the area from the recent onslaught of rainfall, a SLGT risk upgrade was created over SC coast up through much of Eastern NC. 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. 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. 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. 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!_uOjkRcKKWkXMRvi45IMb1ag7GVH9NTUDkk-HrI0719d= B3P1InX_1gVYPkZ_XROQ1mj5lU63H2q9GEG_XEprIfbDPHI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_uOjkRcKKWkXMRvi45IMb1ag7GVH9NTUDkk-HrI0719d= B3P1InX_1gVYPkZ_XROQ1mj5lU63H2q9GEG_XEprN6dHAQY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_uOjkRcKKWkXMRvi45IMb1ag7GVH9NTUDkk-HrI0719d= B3P1InX_1gVYPkZ_XROQ1mj5lU63H2q9GEG_XEpreSotZnc$=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) .