Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Jan 13 2023 20:18:37 FOUS30 KWBC 132018 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 317 PM EST Fri Jan 13 2023 Day 1 Valid 2014Z Fri Jan 13 2023 - 12Z Sat Jan 14 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHERN CALIFORNIA COAST AND ADJACENT MOUNTAINS... The upper trough just off the California coast will make landfall later today while a cold front associated with as a potent Pacific cyclone also makes its way ashore. NAEFS by 18Z today showed a 400 kg/m/s IVT oriented SSW-NNE from just off the central Coast of California on north into the Sacramento Valley. PWs are set to range between 0.75-1.0" between 18Z Fri and 06Z Saturday, which are close to the 90th climatological percentile. The heaviest precipitation rates (>0.5"/hr) are expected along the North Coast of California through late morning, then reaching the Shasta just before midday and finally the northern Sierra Nevada this afternoon. These same rainfall rates will extend as far south as Monterey Bay with the 06Z HREF showing 0.5"/hr probabilities as high as 50-70%. Northern and central California (frankly, the vast majority of California) remains exceptionally saturated with most of the Coastal Range, the Great Valley, and along the Sierra Nevada and Shasta sporting 0-40cm soil moisture percentiles >90% according to NASA SPoRT-LIS. With some of these ranges picking up over 20" of precipitation spanning the last 2 weeks, soils remain sensitive enough that rapid runoff and quick rising streams and creeks can occur from even hourly rainfall amounts of ~0.5"/hr. In addition, localized flooding on roads is possible along with the ever present threat for mudslides. Maintained the Slight Risk this forecast period along the North Coast of California, but with rainfall rates gradually diminishing by this evening in wake of the cold frontal passage, it is possible the Slight Risk could be dropped for the 01Z update this evening. Northern and central California should get a brief reprieve in the ongoing barrage of rainfall overnight before the next round of heavy rain arrives early Saturday morning. Mullinax Day 2 Valid 12Z Sat Jan 14 2023 - 12Z Sun Jan 15 2023 ....THERE ARE SLIGHT RISK OF EXCESSIVE RAINFALL FOR AREAS SPANNING FROM NORTHERN CALIFORNIA TO THE TRANSVERSE RANGE AND THE SAN BERNADINO 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). According to NAEFS, geopotential height levels in the 1000-500mb levels (and all mandatory heights in between) feature climatological percentiles <1%, implying the storm system will be exceptionally deep and anomalous for the time of year. At 250mb, a robust 190kt jet streak will place it diffluent left exit region over the southern half of the Golden State on Saturday and into the Lower Colorado River Valley by late Sunday night, allowing for strong vertical ascent in the upper levels of the atmosphere. In terms of moisture, PWs are set to be above the 90th climatological percentile across central and southern California with actual PW values topping out close to 1.0" along the southern California coast. 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 WSW 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'. Did expand the Marginal Risk to cover more of the San Joaquin Valley where, while totals are not as heavy as the more elevated areas, still could result in localized flooding. Mullinax Day 3 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 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 generaly out of the west. The heaviest periods of rainfall arrive Sunday evening with the most anomalous PW values set to move over southern California. 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. This will be a fast moving shield of precipitation, so the duration is short enough that it helps to limith the extent of the excessive rainfall potential. But the westerly 30-40 knot 850mb winds Sunday could be oriented favorably into the Peninsular Ranges of southern California to cause rainfall rates up to 0.5"/hr. Have extended the Marginal Risk to include the Peninsular Ranges and including the San Diego metro area. The Marginal Risk remains in place along the remainder of the California coast given hourly rainfall rates of 0.25-0.5"/hr atop overly saturated soils could still result in minor, localized flooding in poor drainage areas and near hydrologically sensitive locations. Mullinax Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5ueE2jO7uamYWOhK-O9tOnPdVLc8rOr_QhZglCV8mrYX= LQWC12rZ3w_ZkLElwgZFhNx3uEy7J3HUs-jV-xAuBVCxVX4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5ueE2jO7uamYWOhK-O9tOnPdVLc8rOr_QhZglCV8mrYX= LQWC12rZ3w_ZkLElwgZFhNx3uEy7J3HUs-jV-xAuTg2FBzU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5ueE2jO7uamYWOhK-O9tOnPdVLc8rOr_QhZglCV8mrYX= LQWC12rZ3w_ZkLElwgZFhNx3uEy7J3HUs-jV-xAuTbUEniQ$=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) .