Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Dec 26 2023 01:56:13 FOUS30 KWBC 260155 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 854 PM EST Mon Dec 25 2023 Day 1 Valid 0154Z Tue Dec 26 2023 - 12Z Tue Dec 26 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN APPALACHIANS... ....01Z Special Update... A band of convection located near South Carolina coastal areas was expected to remain nearly stationary through the next several hours while spreading moderate to occasionally heavy rainfall over areas that received 5+ inches of rain on December 17. Ground conditions are still saturated, and a combination of coastal flooding and excessive runoff is possible in a few locales. Per coordination with WFO CHS, a Marginal Risk area has been added for coastal South Carolina areas to address flood/flash flood potential. Cook ....01Z Update... No changes were made to the inherited forecast for risk areas in the southern Appalachians and the Pacific Northwest. Moderate to heavy rainfall continues across portions of northeastern Georgia northeastward into western North Carolina, and additional convection has developed across northern Georgia that should move into tha same area through the night. An additional 1.5-3.5 inches of rainfall is possible through 12Z Tuesday morning, with the bulk of that rainfall occurring through approximately 07Z.=20 For additional details, refer to Mesoscale Precipitation Discussion #1220. Periodic episodes of rainfall will continue to occur through the overnight hours across Washington State. The previous discussions below address this scenario well and no changes are needed at this time. Cook ....16Z Update... No changes were made to the inherited forecast for either risk area. The rain is beginning to move into the Appalachians of NC/SC/GA, and rates are expected to intensify when the main band, denoted as the ongoing squall line across central AL, moves northeast to the mountains. Expect 2 to 4 inches of rain with isolated totals as high as 5 inches into the southern Appalachians through tonight. Into Washington State, light rain has moved into the Olympics, though the primary event will occur overnight, as a front and associated strong shortwave move in tandem into the coast. Expect totals of 3 to 5 inches into the Olympics through Tuesday morning. Wegman ....Previous Discussion... Upper level difluence will increase during this period as a mid-level closed low slowly moves east. Moisture transporting in from the Gulf/Atlantic will be around +2 sigma (PW values of 1-1.25 inches) will override a warm front/stationary boundary over Georgia and Carolinas. This as the strengthening SE-SSE low-level flow provides some upslope enhancement, as most of the models show a heavier rainfall footprint across this region with areal-average totals between 1.5 and 3.0+ inches. The favored location for the highest amounts and the greatest threat for flooding concerns will by along/east of the extreme Northeast corner of Georgia/South Carolina/North Carolina border in the higher terrain. However, any elevated instability will be relatively weak/shallow, with MUCAPEs generally under 500 J/Kg. This will limit the 1 and 3 hourly rainfall rates somewhat, which along with the current low 0-100 cm soil moisture percentiles (less than 20% per NASA SPoRT), will likely lead to a more marginal excessive rainfall threat with the expectation of at most localized short-term runoff issues. Also limiting the potential for a Slight or better risk over much of the Southeast region will be the faster progression of the leading cold front, thus increasingly transient quasi-linear convective segments over most areas. ....Washington... Model guidance continue to depict 3 to 5 inches of rain, with snow levels remaining high enough to only impact the highest peaks of the eastern Olympics. While the event is only expected to be 24 hours long, the forecast rainfall amounts are high enough (and have been increasing) such that flash flooding and riverine flooding of the rivers that drain the Olympics are possible. Some isolated locations could see maximum values approach the upper single digits in accumulations. The Marginal Risk that was already in a effect was adjusted a bit further east while reducing the north some to remove the mostly snow areas. Campbell/Hurley Day 2 Valid 12Z Tue Dec 26 2023 - 12Z Wed Dec 27 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE BLUE RIDGE MOUNTAINS OF NORTH CAROLINA AND VIRGINIA... ....2030Z Update... In coordination with GSP/Greer, SC and RNK/Blacksburg, VA forecast offices, an upgrade to a Slight was introduced with this update for a portion of the Blue Ridge Mountains for much of NC and a small portion of far southern VA. Off-and-on rainfall has already begun in this region, and is likely to continue through the overnight. As the primary forcing in the form of a front and a slowly moving upper level low approach the area Tuesday, the steady rainfall will increase in intensity, with most of the heaviest rain happening after 12Z (Tue) and in the Day 2 period. Thus, think the long duration of the steadiest rain should allow for widely scattered instances of flash flooding on the south/east facing slopes and immediately adjacent foothills. Further south towards SC, the Marginal remains in place for the tail-end of the rainfall that is largely forecast to occur overnight tonight in the Day 1 period. Finally, the forcing and instability will be weaker further north into VA, so the Marginal remains unchanged up to the mountains adjacent to the Shenandoah Valley. Wegman ....Previous Discussion... The deep, cutoff low over the central U.S. will slowly pivot across the Lower Missouri Valley. Meanwhile, along the eastern seaboard, another surface low will develop along the tightening low-level baroclinic boundary near the Gulf Stream off the Carolina coast. Broadening upper level difluence across the Mid Atlantic, along with increased left-exit region forcing within the right exit region of the strong subtropical jet streak, will favor a fair quick uptick in deep-layer moist isentropic ascent. A majority of the guidance show areal averages of 1 to 2+ inches of rain spanning from the far southern portions of the Appalachians to the Virginia Blue Ridge Mountains, the the higher amounts near the areas of the highest elevation. The latest trends supported the expanding the southern end of the Marginal Risk area further east along the North Carolina/South Carolina border. Any ongoing flash flooding from Day 2 in the Blue Ridge of the Carolinas may continue into Tuesday morning. Campbell/Hurley Day 3 Valid 12Z Wed Dec 27 2023 - 12Z Thu Dec 28 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR NORTHERN CALIFORNIA AND SOUTHWEST OREGON AND FOR THE NORTHERN MID-ATLANTIC... ....2030Z Update... For the West Coast, no significant changes were made. QPF continues to hover in the 1 to 3 inch range with soil moisture levels around average, which should allow for maximum absorption of the rainfall into the soils before there's significant runoff. Thus, the Marginal remains in place. For the East Coast, a somewhat slower and weaker progression of the rainfall up the Eastern Seaboard should keep the steadiest rainfall out of most of New England until overnight Wednesday night. Despite a developing low moving up the New England coast, the moisture and instability should keep most areas from seeing rainfall amounts before 12Z Thursday at or below an inch, which should preclude flash flooding in this area despite the nearly saturated soils in northern New England. There's good consensus that the heavier rainfall will occur over the Mid-Atlantic from the Catskills/Lower Hudson Valley/NYC south and west. Upslope may enhance rainfall totals a bit in the mountains of eastern PA and northern NJ as mentioned in the previous discussion below, so the Marginal remains in place for these areas down to the DC area, where rainfall will be ongoing from the Day 2/Tuesday period. Wegman ....Previous Discussion... ....East Coast... The deep cutoff low will still be slowly traversing the Ohio Valley during this period will the surface cold front advances through the eastern Ohio Valley into the central/northern Appalachians while a coastal boundary lifts up the coast. Scattered to widespread precipitation will span from the Carolinas to Maine. There is some uncertainty with the exact location of the highest amounts however there is a growing consensus for eastern Pennsylvania and parts of New Jersey. Most of the guidance suggests 1 to 2 inches for this part of the country. A Marginal Risk that was in effect for the previous day 4 period was maintained with very minimal adjustments. ....West Coast... Onshore flow paired with an approaching Pacific cold front will usher in scattered to widespread precipitation to the Pacific Northwest and northern California. Recent rains has increased the soil sensitivity thus reducing the amounts required to lead to local flooding concerns. Most of the guidance has 1 to 2 inches of rain moving inland, with the higher amounts focusing over favored higher elevations. The previous day 4 Marginal Risk was maintained for this period with minor reshaping to reflect the latest WPC forecast. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6IeOGFPm8r5_KSQ5otUhtvrjOZm10kw24AN_uFWXjzbf= _XpKhJW1O5-wvz-9x9XiXgi0yysUmLgPhwX0rU8oIciRQ-4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6IeOGFPm8r5_KSQ5otUhtvrjOZm10kw24AN_uFWXjzbf= _XpKhJW1O5-wvz-9x9XiXgi0yysUmLgPhwX0rU8oCMzwjCU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6IeOGFPm8r5_KSQ5otUhtvrjOZm10kw24AN_uFWXjzbf= _XpKhJW1O5-wvz-9x9XiXgi0yysUmLgPhwX0rU8og9yaq_I$=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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .