Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jan 15 2023 15:51:11 FOUS30 KWBC 151551 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1050 AM EST Sun Jan 15 2023 Day 1 Valid 16Z Sun Jan 15 2023 - 12Z Mon Jan 16 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN CALIFORNIA COASTAL RANGES... 1600Z Update... Changes with this update include the introduction of a Slight Risk for mainly the Peninsular Range of southern California given the timing and strength of the next offshore shortwave and attendant punch of moisture transport/higher IVT values which will be arriving late tonight and early Monday. Sufficiently strong enough onshore flow in conjunction with orographic ascent should yield a burst of heavy shower activity for areas of southern California in the 06Z to 12Z time frame, and the latest 12Z HREF has trended a tad wetter compared to overnight and suggests some 2 to 3+ inch rainfall totals locally going through 12Z/Monday. Given the wet antecedent conditions and additional rainfall, there may be some runoff concerns with at least a localized flooding threat. Elsewhere, very minor tweaks were made to continuity given little overall change in the synoptic scale setups, but we did opt to advance the Marginal Risk a farther east in central California to include areas of the Central Valley where the 12Z HREF suggests some localized west to east swaths of heavy showers today in association with additional weak shortwave energy offshore that will be moving inland. Additional rainfall amounts of locally up to 1 inch will be possible today through this evening across parts of the Central Valley, and this may encourage some additional runoff problems given the wet/saturated soil conditions across the region. Orrison Previous Discussion... ....Washington and Oregon coasts... Low pressure off the Oregon coast will continue to move north and northeastward...allowing for a fetch of moisture to be transported onshore from Oregon into Washington through this evening.=20 Precipitable water values on the order of 0.5 to 0.75 inches will be drawn inland toward the coast and the coastal ranges by low level winds of 20 to 35 kts. The global models show an increasing amount of air with lower precipitable water values getting wrapped around the southern flank of the upper low which should help to lead to decreasing rates. QPF spaghetti plots tend to show 1 to 2 inch amounts clustered along the coastal ranges while the HREF was more aggressive in showing 24-hour probabilities over 70 percent of 2 inches in 24 hours ending at 16/12Z close to the Columbia River Delta. The previous outlook had this covered and few changes were needed here. ....California... There should be a relative break in the rainfall across California during the day as one system pulls away and before the next system makes its appearance. Moisture transport should be ramping up during the late afternoon/early evening prior to the arrival of the surface low and associated upper level trough later in the evening. IVT values increase to around 500 kg/m/s in southern California near the Peninsular Range as the winds become normal to the range axis. Thinking is that rainfall amounts of 1 to 2 inches could occur in the mountains with a few spots in excess of 0.5 inches in the upslope region. The biggest change here was to provide a break in the Marginal Risk area farther north early in the period from the second area of rain and potential excessive amounts farther south. Given uncertainty with the timing and placement...maintained the Marginal Risk from the Bay area southward. ....Arizona... A Marginal Risk area was maintained for portions central and southeastern AZ with this update due to the potential for convection early where 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. Bann Day 2 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... .....Southern California Transverse and Peninsular Ranges... Continued moisture transport into the Transverse and Peninsular Ranges will keep the risk of excessive rainfall lingering into the Day 2 period...as well as over an increasingly small area of the upslope region of the Sierra before winds switch and drier air begins to move into the region.=20 ....Central Arizona... Upslope flow south and west of the Rim will be a primary component driving the rainfall ahead of an approaching wave and precipitable water values between 0.5 to 0.8 inches get drawn northward at about the same time as the upper level divergence maximizes during the afternoon. With temperatures aloft falling during the late afternoon and evening...instability from steepening low/mid level lapse rates may locally enhance rainfall rates...with 0.5 inches per hour possible. That has the potential to result in minor, localized flooding in poor drainage areas, near hydrologically sensitive locations, washes and arroyos. Bann Day 3 Valid 12Z Tue Jan 17 2023 - 12Z Wed Jan 18 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG PORTIONS OF THE PACIFIC NORTHWEST COAST... Low level winds alof the Washington coast are expected to strengthen and back to a south-southwesterly direction as a surface front approaches. This will draw air with precipitable water values approaching 0.75 inches toward the coast and be in place at about the same time that height falls aloft arrive.=20 Amounts of an inch or two of rain on increasingly wet soils could lead to increased run off that leads to isolated flooding mainly in areas of poor drainage. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!78OKbzVwf5PKU204ZTnz6hlLiFgkyX-llI7MVVsxLjUV= UiurxGDpLsidLWNQQD7PxpAMpuKRf-SelHG9pxvdr-KTMZg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!78OKbzVwf5PKU204ZTnz6hlLiFgkyX-llI7MVVsxLjUV= UiurxGDpLsidLWNQQD7PxpAMpuKRf-SelHG9pxvd0eZix3k$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!78OKbzVwf5PKU204ZTnz6hlLiFgkyX-llI7MVVsxLjUV= UiurxGDpLsidLWNQQD7PxpAMpuKRf-SelHG9pxvdOAE7zlY$=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) .