Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Dec 25 2023 08:45:45 FOUS30 KWBC 250844 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 343 AM EST Mon Dec 25 2023 Day 1 Valid 12Z Mon Dec 25 2023 - 12Z Tue Dec 26 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN APPALACHIANS... Upper level difluence will increase during this period as a mid-level closed low slowly moves over the Southeast. 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 MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE BLUE RIDGE MOUNTAINS OF VIRGINIA THROUGH SOUTH CAROLINA... 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 AND NORTHEAST... ....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!48gsKIHLZtHziEZ7ZWF6Nvd5rqk6lRU5AEqXfdvHUhyW= 6a_LBlBrzuMSF9w4dpc90rWrE9t2njfPG3JqLQYPdFetMys$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!48gsKIHLZtHziEZ7ZWF6Nvd5rqk6lRU5AEqXfdvHUhyW= 6a_LBlBrzuMSF9w4dpc90rWrE9t2njfPG3JqLQYPwu-7zco$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!48gsKIHLZtHziEZ7ZWF6Nvd5rqk6lRU5AEqXfdvHUhyW= 6a_LBlBrzuMSF9w4dpc90rWrE9t2njfPG3JqLQYPrPxf0DY$=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) .