Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jan 14 2023 23:56:00 FOUS30 KWBC 142355 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 655 PM EST Sat Jan 14 2023 Day 1 Valid 01Z Sun Jan 15 2023 - 12Z Sun Jan 15 2023 ....THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL IN AND NEAR PORTIONS OF CALIFORNIA... A deep layer cyclone offshore Oregon has directed and atmospheric river (AR) at California, now moving through the southern portion of the state. At 250 hPa, a robust 190kt jet streak will place its diffluent left exit region over the southern half of the Golden State later tonight aiding deep-layer strong vertical ascent in the upper levels of the atmosphere. Precipitable water values of ~0.75 inch is seen in GPS data along the OR/CA coastal border and in the Sierra Nevada, with 0.7-1.3" values in southern California. These values are in the 95th percentile for January and over 3 standard deviations above climatology. Winds at 850 hPa will be around 35-45 knots on average, which with WSW flow aloft would favor orographic enhancement for rainfall rates, particularly along the windward slopes of the Sierra Nevada where ML CAPE is 250+ J/kg as a bonus. Farther north and closer to the deep layer low, 850 hPa winds of 40-50 knots will be coming out of the west southwest with enhanced moisture transport aimed at the north coast of California and the southern coast of Oregon. There is also instability with SPC mesoanalyses showing 250-500 J/kg of MU CAPE, which should aid rainfall efficiency in/near the northern CA coast. California remains quite susceptible to heavy rainfall due to saturated soils. Portions of the state have picked up 15-30" of rain and >600% of normal rainfall in the last few weeks. NASA SPoRT-LIS still shows much of the area 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. The main change to the previous issuance was to remove areas where the 18z HREF indicates that 0.5"+ in an hour are fairly low from here on out (coastal central CA). Have kept the Marginal Risk in the Desert Southwest as a precaution/bow to continuity; it should be noted that 18z HREF indicates that the probability of hourly rain totals of 0.5"+ are quite low in this area at any point overnight. However, the guidance can underdo the QPF in that area so figured it was smarter to leave it be than remove it. Roth 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 Valid 12Z Mon Jan 16 2023 - 12Z Tue Jan 17 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL AND SOUTHERN CALIFORNIA AND CENTRAL ARIZONA... ....2030Z Update... ....Central Valley... Just a small trimming of the previous Marginal Risk on the northern end of the inherited area. Most guidance suggests there will be relatively little precipitation north of the latitude of the Bay Area as the entire pattern sinks southward with time. Otherwise no changes. ....Southern California Transverse and Peninsular Ranges... No changes were made to the inherited Marginal Risk. Of the three areas on the map, this is the one most likely to need an upgrade to a Slight Risk with future updates as model agreement improves. ....Central Arizona... The primary change was to extend the inherited Marginal a bit further north and west along the Mogollon Rim. Just as in Day 2/Sunday, upsloping south and west of the Rim will be a primary component driving the rainfall, with PWATs of 2/3 to 3/4 inches. Expect there to be more precipitation here Monday Night as compared with the round of rain tonight into Sunday morning, as the forcing will be significantly stronger. The upper level shortwave driving the rainfall will be intensifying and the longwave trough digging southwestward with time. Thus, expect additional instability that will further enhance rainfall rates over those on Sunday. Given the rounds of rain occurring in rapid succession, areas further west should have their soils primed as well for this second round of rain. Wegman ....Previous Discussion... 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!46jlu5tbrJgCEMolHvKA9JBXqnYJZdWocc20GsR9Yv2b= oqlZlBzSfnRvj_nPbZc28_RlXLwg2OdOcWmHI08aeZZYEE8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!46jlu5tbrJgCEMolHvKA9JBXqnYJZdWocc20GsR9Yv2b= oqlZlBzSfnRvj_nPbZc28_RlXLwg2OdOcWmHI08aT4l051Q$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!46jlu5tbrJgCEMolHvKA9JBXqnYJZdWocc20GsR9Yv2b= oqlZlBzSfnRvj_nPbZc28_RlXLwg2OdOcWmHI08aH0_DdoE$=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) .