Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Dec 01 2023 08:28:59 FOUS30 KWBC 010828 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 328 AM EST Fri Dec 01 2023 Day 1 Valid 12Z Fri Dec 01 2023 - 12Z Sat Dec 02 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL GULF COAST... Surface troughing over the Gulf will develop ahead of a stationary front over TX. Return flow east of the front will advect an air mass with PWATS approaching 2 inches into the central Gulf coast, pushing north into the Tennessee Valley as a warm front by early Saturday morning. Meanwhile in the upper levels a nearly stationary but strengthening jet streak will form over the stationary front in response to the large gradient in temperature and moisture across the front. Thunderstorms ongoing across the Deep South will diminish through the morning, with redevelopment of showers and storms from late afternoon through the overnight. Instability in the warm and moist air mass will be between 1,000 and 1,500 J/kg...which is capable of supporting training storms near the Gulf. There's considerable uncertainty as to how far north that instability will get as the storms track from the Gulf into the coast. Due to the time of year, disagreement in the guidance, high FFGs, and limited forcing over the Tennessee Valley, the greatest chance of isolated flash flooding should be near the coast, particularly in any urban areas. The pattern is likely to evolve similarly to today, albeit further east, with the potential for training thunderstorms capable of 3 inch per hour rates in the Marginal Risk area. Thus, the Marginal risk was trimmed southward to focus on the urban areas of the central Gulf Coast from near Houston through Pensacola. Given the above limitations further inland, the storms appear unlikely to have the support needed to produce those rainfall rates. The strengthening jet streak should also help to keep the storms moving quickly the further away from the Gulf Coast they get. Wegman Day 2 Valid 12Z Sat Dec 02 2023 - 12Z Sun Dec 03 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL/EASTERN GULF COAST AND THE SOUTHEAST AND FOR THE OREGON AND NORTHWEST CALIFORNIA COAST INCLUDING THE WILLAMETTE VALLEY... ....Gulf Coast and Southeast... Waves of moisture will track northeastward along a strong but slow-moving frontal boundary over the Southeast and southern Appalachians. The upper level jet streak will continue intensifying as a push of cold air begins to move the front more quickly eastward by Sunday morning. Since the front will also be slow-moving, there is a good likelihood that storms that form along and ahead of the front will train over the same areas, increasing the chances of flash flooding. Instability remains the primary limiting factor, though a strong shortwave may make up for that with stronger forcing overnight Saturday night. The Marginal Risk area includes the southern Appalachians, as upslope flow into the Great Smokey Mountains locally increases the rainfall. The greatest rainfall totals and highest chances of flash flooding will be along the Gulf Coast. ....Oregon Coast and Willamette Valley... A series of upper level waves and surface fronts will move into the coast of the Pacific Northwest Saturday. This will result in multiple rounds of rain into coastal Oregon through the Willamette Valley. Colder air in place along and east of the Cascades will keep snow levels low enough that precipitation into the Cascades should largely remain as snow. Soils are drier than normal over this area for this time of year, but this first in a series of rounds of rain could result in isolated flash flooding in sensitive areas supported by topography, especially where the heaviest rain is expected into northwestern OR/along the Columbia River. Rainfall amounts of 2 to 4 inches are possible, particularly in the foothills of the Cascades. Wegman Day 3 Valid 12Z Sun Dec 03 2023 - 12Z Mon Dec 04 2023 ....THERE'S A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF WESTERN WASHINGTON STATE AND OREGON WEST OF THE CASCADES... A strong front will move into the coast of the Pacific Northwest Sunday morning, continuing through the day before the rain tapers Sunday night. An additional 2 to 4 inches of rain with locally higher amounts is likely again on Sunday across much of the Marginal Risk area, though the heaviest rain is expected into northwestern Oregon. Combined with rainfall from previous days, and warmer air over the area raising snow levels, the chances of flooding in the area have been increasing. An upgrade to a Slight Risk in northwestern Oregon is possible with future forecast updates...largely dependent on how much flash flooding and river response occurs as a result of the rainfall in this area on Friday and especially Saturday. With more rainfall than snowfall expected into northwestern Washington State, the Marginal Risk area was extending northward through the Puget Sound, including the Seattle area...though unlike areas further south into Oregon, most of Washington should not have the benefit of antecedent conditions as much of the precipitation Saturday falls as snow. Some snowmelt will of course add to river levels into this area, but the effects of that snow melt may take until Monday/Day 4 to manifest. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-9AXgDPIV1fHO7qEMBpBl9nxE4EqC6YFbnEZIYeyMZTS= OOgZ1hLVQT2ZulIRZK7z-0CE0hmwHDeASnnaLMChvQo0_5I$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-9AXgDPIV1fHO7qEMBpBl9nxE4EqC6YFbnEZIYeyMZTS= OOgZ1hLVQT2ZulIRZK7z-0CE0hmwHDeASnnaLMChMos9muQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-9AXgDPIV1fHO7qEMBpBl9nxE4EqC6YFbnEZIYeyMZTS= OOgZ1hLVQT2ZulIRZK7z-0CE0hmwHDeASnnaLMCh4XXTGXI$=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) .