Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jan 14 2023 08:29:17 FOUS30 KWBC 140829 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 328 AM EST Sat Jan 14 2023 Day 1 Valid 12Z Sat Jan 14 2023 - 12Z Sun Jan 15 2023 ....THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR AREAS SPANNING FROM NORTHERN CALIFORNIA TO THE TRANSVERSE RANGE AND THE SAN BERNARDINO MOUNTAINS... A progressive upper low off the Oregon coast will direct yet another atmospheric river (AR) at California. Compared the previous ARs in recent weeks the good news is this particular event will be faster moving, meaning California will be subject to a shorter duration of excessive rainfall rates (maxing out between 0.5-0.8"/hr) and somewhat only marginally lower precipitation amounts. At 250mb, a robust 190kt jet streak will place it diffluent left exit region over the southern half of the Golden State by later tonight aiding deep-layer strong vertical ascent in the upper levels of the atmosphere. In terms of moisture, Precipitable water values of 0.75 inches or greater will cover much of California south of the Transverse Range by late this afternoon with values at or slightly greater than an inch along the coast....placing it in the 95th percentile for January and over 3 standard deviations above climatology. Lastly, 850mb winds will be around 35-45 knots on average, which with WSW flow aloft would favor orographic enhancement for rainfall rates, particularly along the central coast of California and the windward slopes of the Sierra Nevada. Meanwhile, farther north and closer to the upper low, 850mb winds of 40-50 knots will be coming out of the west southwest with enhanced 850mb moisture transport aimed at the north coast of California and the southern coast of Oregon. Similar upslope enhancement into the coastal range is expected and has led to global guidance trending wetter in recent runs. Have expanded the Slight Risk there farther north to account for more of southwest Oregon where soils are also sensitive, just not to the extent of their California neighbors. Impacts to California are unlikely to be as significant as some of the predecessor atmospheric rivers over the last couple weeks, but the state remains quite susceptible to heavy rainfall. Portions of the state have picked up 15-20+" of rain and >600% of normal rainfall in the last two weeks. NASA SPoRT-LIS still shows much of the California coast, the Great Valley, the Sierra Nevada, and into southern California with soil moisture percentiles >90%. Most observed rainfall rates are likely to be in the 0.25-0.5"/hr range, but some localized areas could approach 0.8"/hr. Additional flooding is possible along complex terrain, along with the potential for mudslides and ponding on roads. Slight Risks for Excessive Rainfall remain in place from Big Sur on south to the Transverse Ranges, as well as the windward slopes of the Sierra Nevada where snow levels are high enough to warrant periods of heavy rainfall below 6,000'...and saw little reason to make any change to the Marginal Risk placed over the San Joaquin Valley where, while totals are not as heavy as the more elevated areas, still could result in localized flooding. Bann Day 2 Valid 12Z Sun Jan 15 2023 - 12Z Mon Jan 16 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG THE ALONG AND NEAR MUCH OF THE CALIFORNIA COASTLINE... California will enjoy a brief break in the action Sunday morning, but as the upper low off the Oregon coast from Day 1 weakens and tracks into western Washington, another upper level disturbance will approach the California coast Sunday night. This will will result in another surge of 850-700mb moisture flux directed toward the Golden State with mean winds in the 850-300mb layer generally out of the west. NAEFS shows PWs of 0.75-1.00" that are above the 90th climatological percentile tracking into southern California by 06Z Monday. The IVT will be quite strong, reaching as high as 750 kg/m/s west of San Diego Sunday night. Given the westerly 30-40 knot 850mb winds being favorably oriented into the Peninsular Ranges of southern California...rainfall rates up to 0.5 inch per hour and rainfall totals that may locally exceed an inch over already waterlogged areas...saw little reason to change the on-going Marginal Risk areas given the potential for additioal rainfall on already saturated soils could still result in minor, localized flooding in poor drainage areas and near hydrologically sensitive locations. Bann Day 3 Valid 12Z Mon Jan 16 2023 - 12Z Tue Jan 17 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER A PORTION OF SOUTHERN CALIFORNIA AND A PORTION OF SOUTHEAST AZ... Maintained a Marginal Risk at lower elevations along the Sierra Nevada given a continuation during the first 6 to 12 hours of the period of the 850-700mb moisture flux surge at lower elevation being directed upslope by mean winds in the 850-300mb layer generally being out of the west. Gradually the focus for any heavier rainfall amounts/rates shifts southward where upper level difluence/divergence increases ahead of the mid-level wave.=20 Precipitable water values should briefly peak around 0.75 inches near the international border at about the same time that the better upper level difluence arrives which may locally result in rainfall rates briefly approaching 0.5 inches per hour and isolated maximum rainfall amounts in the Peninsular Range and in southern Arizona of 1 to 1.25 inches which has the potential for minor, localized flooding in poor drainage areas, near hydrologically sensitive locations, washes and arroyos. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!99ZxjhJQAGmFOKyd1bXQI_SLfeBHkLdZzxT8yIi4x9db= Zwe58zq7wSGUgzB3c4oHvgJvS50QzTgbusYEXL9rpnysn-U$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!99ZxjhJQAGmFOKyd1bXQI_SLfeBHkLdZzxT8yIi4x9db= Zwe58zq7wSGUgzB3c4oHvgJvS50QzTgbusYEXL9rdOKHa-Q$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!99ZxjhJQAGmFOKyd1bXQI_SLfeBHkLdZzxT8yIi4x9db= Zwe58zq7wSGUgzB3c4oHvgJvS50QzTgbusYEXL9rIuLyGiA$=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) .