Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Dec 26 2023 15:54:42 FOUS30 KWBC 261554 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1053 AM EST Tue Dec 26 2023 Day 1 Valid 16Z 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 VIRGINIA, NORTH CAROLINA AND SOUTH CAROLINA... ....16Z Update... The overall synoptic evolution and expected zone of heaviest rainfall remains steadfast from the previous forecast issuance. Currently have a slog of heavy rain across the SC Piedmont up into western NC where low-level convergence signatures from prevailing eastern flow allow for upslope flow and focused ascent within the eastern slopes of the Blue Ridge/Appalachians over the interior Southern Mid-Atlantic and Southeast. Totals of 2-3" are already a commonality across extreme northeast GA into the SC Piedmont where Flood Advisories are in effect for the rise of water along creeks and rivers located upstream of Greenville. With the anticipated heavy rainfall the next 2-4 hrs over the same areas, have brought the SLGT risk down to encompassed the area that's been hit the hardest and still has multiple hours left of heavy rain before the overall footprint shifts northeast with the surface low evolution. Removed the MRGL risk from GA as the precip has lightened or moved out of the area and conditions improve for the remainder of the period. The other addition to the forecast was the expansion of the MRGL risk to the east to include the Midlands of SC up through central NC and south-central VA. The reasoning is two fold as the convective probability over those areas will be highest later this afternoon and evening thanks to a tongue of low-level theta-E advection wrapping north on the eastern flank of the surface low over the southeast leading to better instability axis centered from I-20 in SC up through Raleigh and points west. The prospects for heavy rainfall are highest within the SC Midland's into southern NC where ensemble neighborhood probabilities have jumped for both 1" and 2" totals for the period compared to past runs. This correlates well with the latest deterministic indicating some convective swaths in the QPF field that even when blended into the mean can be seen on the latest 12Z HREF. The HRRR is the most aggressive with this stance, but other CAMs are hinting at local maximums within the Columbia to Charlotte axis, both areas of which are more prone to flash flooding due to urbanization factors. The threat is still not as prolific compared to areas northwest where heavy rain is occurring, but the ingredients are there for isolated FF concerns with convective parameters meeting the criteria, along with plentiful moisture for the time of year. Latest CIPS analogs also peg 12/25/2009 as best 500mb and PWAT comparison for the area highlighted which produced some flash flooding across portions of SC, so there's some historical precedence, but that setup was better overall, so we are not anticipating a repeat, but it shows some premise to the threat. Kleebauer ....Previous Discussion... Scattered to widespread rain will continue across the Appalachians and surrounding area as another surface low will develop along the tightening low-level baroclinic boundary near the Gulf Stream off the coast. The steady rainfall will increase in intensity thus 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. The latest guidance shows an uptick in coverage and amounts expected for parts of the Southeast up through the northern Mid-Atlantic region. The Slight Risk was broadened to include more of southern Virginia, western North Carolina and just over the border of South Carolina where amounts of 1 to 2+ inches with isolated higher amounts are expected to focus. 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. Campbell Day 2 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... As the systems progresses up the coast it will somewhat weaken and slow while spreading rains over the Mid-Atlantic through the day and reaching southern portions of New England by Wednesday evening/overnight. 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. While some uncertainty remains, there's growing consensus for the highest amounts to focus over eastern Pennsylvania and parts of New Jersey. Most of the guidance suggests 1 to 2 inches for this part of the country. Very minimal adjustments were made to the Marginal Risk that was in effect. ....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. The latest guidance keeps areal averages of 1 to 3 inches for this period. The Marginal Risk was maintained, albeit with a few adjustments to reflect the latest trends. Campbell/Wegman Day 3 Valid 12Z Thu Dec 28 2023 - 12Z Fri Dec 29 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!41ri2G0zY4rp6KVVNBuo8Ph_hT7vQhtBCur_xWtdnWYe= Bc-sP0LBanohB7VCuC3M__yk57iSY0sYTRC2ExVGT1sbU9w$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!41ri2G0zY4rp6KVVNBuo8Ph_hT7vQhtBCur_xWtdnWYe= Bc-sP0LBanohB7VCuC3M__yk57iSY0sYTRC2ExVG7vsplvU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!41ri2G0zY4rp6KVVNBuo8Ph_hT7vQhtBCur_xWtdnWYe= Bc-sP0LBanohB7VCuC3M__yk57iSY0sYTRC2ExVGF7beWAo$=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) .