Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Dec 01 2022 01:01:44 FOUS30 KWBC 010101 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 800 PM EST Wed Nov 30 2022 Day 1 Valid 01Z Thu Dec 01 2022 - 12Z Thu Dec 01 2022 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS THE COASTAL RANGES OF NORTHERN CALIFORNIA... ....01Z Update... ....Coastal Ranges of Northern California... A deep upper trough continues to advance in across the Pacific Northwest which is allowing a cold front and a pre-frontal advance of Pacific moisture to cross the coastal ranges. The front has made it south down into northern California, and the latest radar imagery shows the heaviest rainfall and corresponding transport of Pacific moisture focused across Del Norte, Siskiyou, Humboldt, and Trinity counties. Hourly rainfall rates have been rather modest over the last several hours, with rates only occasionally getting up to a 0.25"/hour, and with 6-hour QPE values generally near or under 1 inch for areas farther north across the Oregon coastal ranges as the front was settling south down the coast. The latest HRRR guidance suggests sufficient orographic forcing and moisture transport for some additional 1 to 1.25" amounts going through 12Z for the northern California coastal ranges south of Eureka and north of the Bay Area. The Marginal Risk was adjusted to reflect this. Any excessive rainfall concerns are expected to be highly isolated. Orrison Day 2 Valid 12Z Thu Dec 01 2022 - 12Z Fri Dec 02 2022 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS ALONG MUCH OF COASTAL CALIFORNIA... ....2030z Update... No changes to the inherited outlook, as model agreement on the overall QPF footprint remains solid through the day 2 period. With the inclusion of the full HREF suite, there is general agreement that localized rainfall rates will approach (and perhaps occasionally exceed) 0.5"/hr along much of the California coast. The HRRR is notably the most intense of the CAMs, depicting localized rainfall rates occasionally exceeding 0.75"/hr. This is quite an outlier amongst the CAMs, particularly along upslope portions of the Santa Lucia Range and San Rafael Mountains where output indicates localized totals as high as 2-4" (compared to the HREF mean of 1-2"). This will need to be monitored for a potential targeted Slight Risk upgrade(s), but given the outlier status of the HRRR (and so far underperformance of the ongoing event across coastal OR/CA) the Marginal Risk will be maintained for the time being. Churchill ....Previous Discussion... During this period the trough/surface low will continue to sink southward and move inland while spreading rain and mountain snow to western Oregon and California. The plume of moisture directed onshore with this system will be increasing through the day reaching +1.5 to +2 sigma. The nearly perpendicular flow to the terrain will result in a fair amount of orographically enhanced rainfall and hourly rainfall rates may exceed 0.50 inch/hour. Recent rains will have already raised soil saturation and increase sensitivity prior to this period. With colder air filtering into the Pacific Northwest and northern California the snow levels will be falling thus reducing the threat for excessive rainfall. The northern portion of the inherited Marginal Risk area was trimmed down to only cover portions of central and southern California. Campbell Day 3 Valid 12Z Fri Dec 02 2022 - 12Z Sat Dec 03 2022 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN CALIFORNIA, NORTHWEST CALIFORNIA AND WESTERN OREGON... ....2030z Update... No changes to the inherited outlook for this update, as reasoning remains the same for both Marginal Risk areas. That said, there is still appreciable model differences with regard to the timing and intensity of both features, so confidence in the outlook is lower than usual for Day 3. This means the spatial placement has the potential to change significantly in subsequent outlooks, with outright removal of the Marginal Risk area over Southern California possible (with most support for it now coming solely from the ECMWF/ECENS). Churchill ....Previous Discussion... During this period the system mentioned during the day 1/2 will be moving through the Southwest/Southern Rockies with its trailing front passing through southern California. Showers are expected to continue across the Los Angeles and San Diego metro areas and including the San Rafael, San Gabriel, San Bernardino and San Jacinto ranges. With additional rainfall expected over some of the same locations that maybe soaked on day 2, the threat for excessive rainfall will remain elevated. Further north, another fast approaching Pacific system will usher in another plume of deep moisture that will produce areal averages of 1 to 3+ inches over recently saturated soils. Once again, the southwest Oregon and northwest California region looks to be where the greatest QPF will focus. This stretch of the coastline and the adjacent terrain will be sensitive to runoff, debris flow and localized flash flooding. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6sYEAOENjBCtP2r5ZTannE-rKzEtlWEywzWfkexdnddh= poaUK1asc1oGUOPxlTOA5OHj5UHGvG5TEjMjnpJyOacqowE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6sYEAOENjBCtP2r5ZTannE-rKzEtlWEywzWfkexdnddh= poaUK1asc1oGUOPxlTOA5OHj5UHGvG5TEjMjnpJylJSvn9I$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6sYEAOENjBCtP2r5ZTannE-rKzEtlWEywzWfkexdnddh= poaUK1asc1oGUOPxlTOA5OHj5UHGvG5TEjMjnpJyrT6sIv0$=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) .