Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Dec 30 2022 16:24:18 AWUS01 KWNH 301624 FFGMPD CAZ000-310400- Mesoscale Precipitation Discussion 1180 NWS Weather Prediction Center College Park MD 1122 AM EST Fri Dec 30 2022 Areas affected...Northern California Concerning...Heavy rainfall...Flash flooding possible Valid 301620Z - 310400Z Summary...A strong atmospheric river will bring a prolonged period of localized rainfall rates exceeding 0.5"/hr. Flash flooding is possible, particularly over burn scars and where heavy rainfall coincides with areas of active snowmelt. Discussion...A relatively strong atmospheric river (AR) is fairly broadly oriented across the West Coast (mainly coastal OR/CA) this morning, characterized by a distinct channel of precipitable water (PWAT) values of 1.0-1.5 inches. GOES-West differential water vapor RGB depicts this channel of deep moisture clearly, arcing from near Hawaii northeastward across the northeastern Pacific with a length of over 2000 miles and average width of several hundred miles. While rainfall rates have remained fairly tempered across the western U.S. overnight and early this morning (generally less than 0.25"/hr), this looks to change today as a slug higher low-level (925-850 mb) moisture transport (and integrated vapor transport, or IVT) is expected to make landfall in Northern California. This should result in a rather dramatic increase in localized rainfall rates, occasionally exceeding 0.50"/hr from 30/18z through (at least) 31/04z (10 AM through 8 PM Pacific Standard Time). Corresponding IVT values may reach as high as 700-1000, corresponding to near an AR 3 designation (or 'strong' atmospheric river). These IVT values will be driven by the aforementioned slug of enhanced low-level moisture transport, resulting in an abrupt increase in 850 mb flow to 50+ kts (oriented nearly perpendicular to the coast and Sierra Nevada mountain range). This surge looks to begin around 18z (10 AM PST), peak around 22z (2 PM PST), and wane significantly by 04z (8 PM PST). Given the prolonged potential for localized rainfall rates of 0.5"+/hr, flash flooding will be possible across much of Northern California (primarily immediately along the coast and along upslope portions of the Klamath Mountains, northern Coastal Range, and northern Sierra Nevada). Rainfall totals through 04z are expected to be as high as 3-5 inches locally in the hardest hit areas. Snow levels are also unusually high (around ~8000 feet) given the time of year, which could combine with snowmelt to create more hazardous runoff potential than usual. Concerns for flash flooding (and also potential mudslides) are highest for recently burned areas, as these soils in particular will struggle to keep up with the needed amount of infiltration. Churchill ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!64hlZ4DtQzOp6j2sYDRP9TbFmGBLK4vFdalxk9yK-ArlUna0U0njG4D2WAbhLQyWdBrJ= 7_O4I0Qy9r9NUU27aLOhc1Q$ for graphic product... ATTN...WFO...EKA...MFR...MTR...REV...STO... ATTN...RFC...CNRFC...NWRFC...NWC... LAT...LON 41922335 41882249 41852205 41552168 41222144=20 40662103 40342057 39772039 39452025 38651989=20 38301989 37872023 37782056 37972077 38202086=20 38592100 38982137 39552157 40042188 40242238=20 40122262 39462252 38862245 38482265 38552369=20 39152404 39552431 40122461 40772464 41322450=20 41722427 41892352=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) .