Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jan 14 2023 20:22:29 FOUS30 KWBC 142022 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 321 PM EST Sat Jan 14 2023 Day 1 Valid 16Z 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... ....16Z Update... A few changes were made with this morning's update. Starting up north, the Slight Risk area along the western slopes of the Sierras was expanded westward further into the Central Valley. Please see MPD #41 at https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/metwatch/metwatch= _mpd_multi.php?md=3D00__;!!DZ3fjg!933P3zAIezP0lJnYgMqIzawvt6ckdjhZSHa4SmxWQ= o932DvlM9eIUqV3wI-x6jz646sOgYoohpRVso9apoVNq2i5T0Q$=20 41&yr=3D2023 recently issued for a summation of the finer details. Elsewhere, the Slight Risk for the Peninsular Ranges east of San Diego was expanded south all the way to the Mexican border with this update. Strong forcing associated with a rapidly moving shortwave helping dig the longwave trough further south will enhance the flow orthogonal to the ranges starting in a few hours and continuing into the overnight. The heaviest rainfall in this area will be this evening, where 0.5 to 0.8 in/hr rainfall rates are possible. Total rainfall into the Peninsular Ranges is likely to exceed 2 inches through tonight, but locally heavier amounts are likely. With recent burn scars and rainfall, the soils are certainly wetter than normal, if not fully saturated, so it won't take much of that rain to saturate the soils, with additional rainfall beyond that resulting in flash flooding. Finally, a new Marginal Risk area was introduced for much of northwest Arizona west of the Mogollon Rim, the southern tip of Nevada, and portions of Southeast California with this update. Shower activity ongoing now in this area will help prime the soils for much heavier rainfall overnight tonight as the same forcing causing heavy rain into coastal SoCal moves inland. Most of the guidance shows enhancement, with even some embedded convection as the AR (atmospheric river) moves over this area. 24 hour totals in this area are around 1.5 inches, but all of that is likely to fall in a 3-6 hour period tonight.=20=20 Wegman ....Previous Discussion... 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 FOR THE ENTIRE CALIFORNIA AND OREGON COASTLINE AND FOR PORTIONS OF CENTRAL AND SOUTHEAST ARIZONA... ....2030Z Update... ....Oregon... A couple changes were made to the Marginal Risk areas across the West with this update. The Marginal Risk along the entire CA coast was expanded all the way up the Oregon coast as well. A slow moving low centered off the Oregon coast will move northeastward towards the Columbia River Delta by Sunday night. This will allow a front with ample Pacific moisture to wrap around the low center to its south and west, resulting in a forecast local maximum of rainfall into the Oregon Sunday. Snow levels will be low, so the rain is only expected to impact the coastal mountains. Most of the precipitation that falls into the Oregon Cascades is likely to be snow once again due to the low snow levels. ....Arizona... A Marginal Risk area was introduced for portions central and southeastern AZ with this update. This is primarily focused on the convection that will be moving into the area at the start of the forecast period early Sunday morning. A few of the CAMs suggest there may be some convective elements embedded within the atmospheric river as it moves eastward across the state. Areas south of the Mogollon Rim will have the advantage of additional upslope with impressive IVT resulting in PWAT approaching 3/4 inch, which is 3 sigma above normal. Thus, the main factor determining the extent of flooding will be the hourly rainfall rates, which may briefly exceed 0.5 inches/hour, depending on how much convection is embedded within the main line of rain as it moves across the state. Soils in this area are wetter than normal, though not totally saturated either. With that said, burn scars, slot canyons and perhaps the more urbanized northern/eastern suburbs of Phoenix are at the greatest risk for isolated flash flooding. Wegman ....Previous Discussion... 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 additional rainfall on already saturated soils could still result in minor, localized flooding in poor drainage areas and near hydrologically sensitive locations. Bann Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!933P3zAIezP0lJnYgMqIzawvt6ckdjhZSHa4SmxWQo93= 2DvlM9eIUqV3wI-x6jz646sOgYoohpRVso9apoVNP9_gw7Q$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!933P3zAIezP0lJnYgMqIzawvt6ckdjhZSHa4SmxWQo93= 2DvlM9eIUqV3wI-x6jz646sOgYoohpRVso9apoVNedvMxmU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!933P3zAIezP0lJnYgMqIzawvt6ckdjhZSHa4SmxWQo93= 2DvlM9eIUqV3wI-x6jz646sOgYoohpRVso9apoVND8EiMN8$=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) .