Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jan 05 2023 15:27:36 FOUS30 KWBC 051527 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1025 AM EST Thu Jan 05 2023 Day 1 Valid 16Z Thu Jan 05 2023 - 12Z Fri Jan 06 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF CALIFORNIA... ....16z update... No changes were made to the current day 1 outlook. Rainfall is expected to continue through the afternoon across California as a secondary plume of moisture advects inland. Rates will be lower, but it won't take much rainfall to cause flash flooding over very saturated soils and ongoing flooding. Downgrades are possible with this evening's update as any impactful rainfall largely comes to an end. ....Previous discussion... A strong cold front and atmospheric river continues to bring periods of heavy rain to a good chunk of California this morning. That rain will continue through much of the day today as various waves of rainfall move inland and sweep northwest to southeast across the state. The most consistent and heaviest rain is with this initial round of rain moving across the Central Valley and approaching southern CA. CAMs guidance shows a renewed round of occasionally heavy rain moving into the northern CA coast later this morning, followed by off and on shower activity through much of the day. A short break from the rain is expected overnight tonight statewide. This second round of rain is expected to impact the Central Valley north and not make it significantly into SoCal. The threat in SoCal is the nearly orthogonal flow of the rainfall into the Transverse Ranges especially, but with some impact into the Peninsular Ranges as well. Kebede/Wegman Day 2 Valid 12Z Fri Jan 06 2023 - 12Z Sat Jan 07 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR COASTAL NORTHERN CALIFORNIA AND SOUTHWEST OREGON... The next round of rain into northern California and southwest Oregon is expected to begin Friday afternoon after a break in the rain from Thursday night. It's a similar setup from Thursday...a second strong low will approach Vancouver Island, and its trailing cold front will drape south into the coast Friday afternoon. While northern areas should see a quick hit of rain before it dissipates, the portion of the front aimed at the Slight Risk area is expected to persist much longer as the long axis of the front aligns orthogonal to the coast and coastal mountains. It is in this area that this front will largely stall...allowing for a prolonged period of heavy rain out of the subtropical Pacific aimed once again at northern California and southwest Oregon. Most notably, the latest guidance has become significantly wetter with the axis of heaviest rain than runs just 24 hours ago, and has also been trending south down the coast. There has been some consistency the past run or two, but further adjustments may be necessary if the guidance hasn't consolidated on this solution. Peak rainfall amounts into northern California's coastal mountains could exceed 4 inches during this period into very late Friday night. Wegman Day 3 Valid 12Z Sat Jan 07 2023 - 12Z Sun Jan 08 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN CALIFORNIA... ....Northern California... Saturday's excessive rainfall outlook features a combination of two different features moving into northern CA. The first is a continuation of the atmospheric river/front slowly moving across northern CA. The front will be enhanced by the approach of an upper level wave that will move into the coast Saturday morning as well. This will both intensify the rainfall for a few hours Saturday morning, but also resume its southward movement towards the Bay area and inland into the Sacramento Valley. Due to this, it appears likely there will be several inches of rain in the valley as well as the surrounding mountains. In addition to obviously very saturated soils, this added rainfall will likely enhance the flash flood potential in the valley as well. Thus, the Slight Risk was expanded to include more valley locations. The initial round of rain will rapidly weaken midday Saturday across much of northern CA. A second reinforcing front and associated upper level wave (which will be much stronger than the first) will quickly follow on the heels of the first Saturday evening. While the bulk of the associated rainfall with this second round will be a little further south, closer to the coastal mountains north of San Francisco, there will be plenty of overlap in heavy rain area not just across far northern CA, but also overlapping areas that have very saturated soils from the rainfall occurring presently. Thus, the Slight Risk includes all of the Bay Area and further south through Monterey Bay. ....Northern Louisiana and Surrounding Areas... Considered introducing a Marginal Risk area for southeast TX through east central MS on Saturday. A stalled out front may be able to draw enough Gulf moisture into it to result in a small area of training thunderstorms across this region. There is significant disagreement in the guidance both on how much rain is expected and where this narrow area will be. Considering the few days that will elapse with dry conditions which will allow previous days rainfall to run through...opted against a Marginal for now until there is better model consensus and more confidence that FFG values will be threatened. Instability is meager as well, which won't help allow any storms to develop upscale to have the possibility of producing heavy rain. Future shifts will analyze this possibility further for potential Marginal Risk introduction at a later time. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!96tjULLT0d5349r-p2NyLFDYxR4QahnfwpfuJ2ZXjkBB= 610EvbOZSvhErKWLpEGcTiryrPdI1SJWoyz-QOvOmM44QtU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!96tjULLT0d5349r-p2NyLFDYxR4QahnfwpfuJ2ZXjkBB= 610EvbOZSvhErKWLpEGcTiryrPdI1SJWoyz-QOvOWJeF_PI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!96tjULLT0d5349r-p2NyLFDYxR4QahnfwpfuJ2ZXjkBB= 610EvbOZSvhErKWLpEGcTiryrPdI1SJWoyz-QOvOZ57yxFc$=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) .