Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Dec 08 2022 23:44:09 FOUS30 KWBC 082344 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 643 PM EST Thu Dec 08 2022 Day 1 Valid 01Z Fri Dec 09 2022 - 12Z Fri Dec 09 2022 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Taylor Day 2 Valid 12Z Fri Dec 09 2022 - 12Z Sat Dec 10 2022 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM FAR SOUTHWEST OREGON INTO NORTHWESTERN CALIFORNIA... ....2030z Update... Little change to the inherited Marginal Risk area, with some minor expansion inland to reflect relatively high snow levels (rising to 5000-6000 feet) during the primary period of precipitation (00-12z Saturday). Models remain in good agreement concerning this period of interest, with the 12z HRRR indicating localized hourly rates of 0.50-0.75"/hr. The occasionally repeating nature of these rates may result in localized totals over 6-12 hours as high as 3-4 inches, which may result in isolated instances of flash flooding (with particular vulnerability over burn scars and low-lying areas with poor drainage). Churchill ....Previous Discussion... There is fairly good agreement in the latest model guidance with the amplification of the upper trof off the Pac NW coast late Friday afternoon into the early hours of Saturday. This will strengthen the low level southwesterly flow into coastal sections from northwestern California into Oregon, with 850-700 mb moisture flux anomalies reaching 2 to 2.5+ standard deviations above the mean from far southwest Oregon into northwestern California. This will support heavy precipitation during the 0000-1200 UTC Saturday time period across the marginal risk area, with isolated hourly rates of .50"+ possible. The axis of anomalous onshore flow will, however, be fairly progressive to the southeast. This and only slightly above average precip values over the past week for northwest California will keep the risk level at marginal at best. There were no significant changes from the previous outlook for this period. Oravec Day 3 Valid 12Z Sat Dec 10 2022 - 12Z Sun Dec 11 2022 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE NORTHERN AND CENTRAL CALIFORNIA COASTAL REGIONS... ....2030z Update... The only substantial change for this update was to extend the Marginal Risk northward to encompass nearly the entirety of the northern CA coast (overlapping much of the Marginal on Day 2), as most of the model guidance consensus indicates an addition 1-3 inches of precipitation on Day 3. In fact, the 12z GFS and GEFS indicate more precipitation here than in central CA, whereas the ECMWF and ECENS indicate just as much precipitation in northern CA, but more precipitation (up to 2-4 inches locally) in coastal portions of central CA. Some of the precipitation will be changing to snow in lower elevations (as snow levels begin to rapidly fall on the backside of the system) late in the Day 3 period, but there is enough of a threat during the first half of Day 3 to maintain the overlapping Marginal Risk areas (as the bulk of the heavier precipitation is also during the first half of Day 3). Churchill ....Previous Discussion... The amplifying mid to upper level trof off the Pac NW coast day 2 will continue to press southeastward day 3 across the coastal Pac NW and northern California. The axis of anomalous 850-700 mb moisture flux on the southeast side of this amplified trof will continue to press steadily southeastward, with values 3 to 4+ standard deviations above the mean affecting areas of central to southern California. This will support a period of heavy precipitation, especially through the Sierra and along the central California coastal range, with hourly rates topping near ..50"+/hour. Much of these regions have seen above average precip over the past few weeks. A marginal risk was confined to the coastal sections from the coast range north of San Francisco, south to approximately Santa Barbara where stream flow anomalies from the National Water Model indicate regions of much above normal to high flow from the above mentioned recent heavy precipitation. Additional heavy rains may result in isolated runoff issues, especially across recent burn scars. While heavy precip is also likely in the low elevation upslope of the Sierra, stream flows are mostly at or below average levels. For this reason and after collaboration with WFO STO, no risk area was denoted here. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4bMW_l-OXS0Bj2Yo73euz2_gP1q59OWXdax4vj2RangJ= kKuAsTwVgTL7VPp9vGbDKjl3-kM_JZf2r_throUzrFiRh2c$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4bMW_l-OXS0Bj2Yo73euz2_gP1q59OWXdax4vj2RangJ= kKuAsTwVgTL7VPp9vGbDKjl3-kM_JZf2r_throUzjFEgMIc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4bMW_l-OXS0Bj2Yo73euz2_gP1q59OWXdax4vj2RangJ= kKuAsTwVgTL7VPp9vGbDKjl3-kM_JZf2r_throUzs7hgtJE$=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) .