Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Dec 02 2022 01:06:35 FOUS30 KWBC 020106 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 800 PM EST Thu Dec 01 2022 Day 1 Valid 01Z Fri Dec 02 2022 - 12Z Fri Dec 02 2022 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS ALONG THE CENTRAL AND SOUTHERN CALIFORNIA COAST... ....01Z Update... Moderate to locally heavy rains continue to edge south down across the central California coastal ranges this evening as a cold front advances gradually southeast in association with an upper-level trough swinging inland across the Northwest. There is a fair degree of Pacific moisture transport noted along the front with a southwest low-level jet of 30 to 40 kts in place, and CIRA-ALPW data plots showing a moderate degree of mid and high-level subtropical moisture advancing northeast out ahead of the larger scale troughing. This is supporting PW anomalies that are 1.5 to 2 standard deviations above normal across the coastal ranges. The orthogonal nature of the onshore flow, with robust upslope flow into the terrain, has helped lead to some rainfall rates over the last few hours approaching and locally exceeding a 0.50"/hour. Some 6-hourly QPEs have been rather heavy across Monterrey County with an emphasis on the windward slopes of the Santa Lucia range where 1.5 to 2.5 inch rainfall totals have been noted. Overnight, the front will continue to settle southward down the coast and should get into the Transverse Range of southern California early Friday morning. Some spotty 0.30" to 0.50" rainfall rates will be possible for a few more hours across the southern parts of the Santa Lucia range moving down into San Luis Obispo County, but as the moisture advances farther south into southern California, the rates are forecast to weaken as the onshore flow slackens a bit. Nevertheless, some additional 1 to 1.5 inch rainfall amounts will be possible overnight. The Marginal Risk area has been adjusted to reflect the latest radar trends which necessitated trimming the northern edge of the outlook area. Orrison Day 2 Valid 12Z Fri Dec 02 2022 - 12Z Sat Dec 03 2022 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF SOUTH-CENTRAL CALIFORNIA... During this period, another fast approaching upper low will drop southward off the Pacific Northwest Coast while a separate plume of moisture remains directed towards the south-central California coast just south of the main upper trough as associated with a trailing cold front. In the Pacific Northwest, the latest guidance continues to trend offshore with the axis of heavier QPF and so the previous marginal risk along the Oregon coast was removed this cycle. To the south, guidance has shifted north with the QPF axis, now directed more towards central California rather than southern, although some uncertainty in placement and amounts continues. Rainfall totals on this day should not be as high as in the day 1 period (as the initial shortwave shifts well inland), although some of the High Res guidance especially continues to suggest some isolated totals near 2 inches in 24 hours, with very localized rainfall rates around 0.5 inch. Despite less favorable heavy rainfall ingredients, a marginal risk was maintained, especially considering heavy rainfall also expected in the day 1 period. Compared to the overnight ERO, the area was pulled northward more consistent with the higher QPF amounts, and as guidance continues to trend much lighter with QPF over southern California. Santorelli Day 3 Valid 12Z Sat Dec 03 2022 - 12Z Sun Dec 04 2022 ....THERE ARE MARGINAL RISKS FOR EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL CALIFORNIA AND THE SOUTHWEST... ....California... The upper low from day 2 will continue to drop south, eventually crossing into Oregon/northern California during the second half of the period. Moisture along a trailing cold front will increase with the upper low approach, with another round of moderate to locally heavy rainfall (areal averaged QPF of 1-2") across portions of the central California coast. Given that this region has measurable rainfall expected during the day 1 and 2 period as well, it should be primed for areas of rapid runoff and increased vulnerability for mudslides and debris flows over sensitive areas (such as burn scars). As such, a marginal risk is in place for much of the central California coastal region. ....Arizona, New Mexico and far western Texas... The frontal boundary is expected to stall over parts of the Southwest before sinking south again as a cold front during the later half of this period. A deep plume of moisture will stream into the Southwest bringing PW values of 0.50-0.75 inches (above 90th percentile) for parts of southeast Arizona and southern New Mexico. As the upper-level low to the north nears the California Coast, southwest flow over the Great Basin and Southwest increases the broad areas of forcing for ascent. Convection is expected to fire up across portions of southern Arizona and New Mexico. Some of the guidance show areal average QPF near or greater than 1 inch, especially in this region. Rainfall in this quantity could reach or exceed local FFG. The Marginal Risk from overnight was maintained with only minor changes needed. Santorelli Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6P3vo654TSH91GdoHdH6jEbFd3njwdnhUg7ZZMShTi-Z= y1C33_fKtm4O0JoUfjEAULi2Rdush80XqIWqBfQIE0QkmH4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6P3vo654TSH91GdoHdH6jEbFd3njwdnhUg7ZZMShTi-Z= y1C33_fKtm4O0JoUfjEAULi2Rdush80XqIWqBfQIVOW44SE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6P3vo654TSH91GdoHdH6jEbFd3njwdnhUg7ZZMShTi-Z= y1C33_fKtm4O0JoUfjEAULi2Rdush80XqIWqBfQIsvaKTyY$=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) .