Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Jan 06 2023 00:58:15 FOUS30 KWBC 060058 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 757 PM EST Thu Jan 05 2023 Day 1 Valid 01Z Fri Jan 06 2023 - 12Z Fri Jan 06 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF CALIFORNIA... It was still too early to entirely remove the outlook areas from California but the short term radar trends and satellite trends have shown shrinking coverage of rainfall and diminishing rainfall rates. There were still some echoes showing convective nature to the precipitation in parts of Madera and Fresno counties with MRMS depicting some 0.3 to 0.5 inch per hour amounts embedded within a broader area where rates were 0.1 to 0.2 inch per hour ending at 06/00Z. As drier air filters in from the west...the risk for excessive rainfall should continue to diminish rather quickly in areal coverage throughout the evening across southern California leaving most of the area with little or no additional rain by late tonight or early Friday morning. Continued the Marginal Risk area along the coast in the northern portion of California where there should only be a brief intermission from when on-going rain tapers off and the first vestiges of the next system appears in the pre-dawn hours on Friday morning. Bann 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... ....2030 update... No changes were made to the risk areas on day 2. A moderate was contemplated for the northern coastal ranges of California due to increased QPF trends in the NAM GFS and EC, but was ultimately avoided after coordination with local offices. ....Previous discussion... 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. Kebede/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... ....2030 update... Minor changes were made to the day 3 risk areas. There has been a downward trend in 24hr QPF totals in the deterministic GFS, EC and NAM ending with today's 12z suite. That being said we're still forecasting 3"+ for the northern coastal ranges of California down to the northern reaches of the Bay Area, as well as some 2-5" bullseyes south of San Jose and Monterrey. Considered an upgrade to a moderate risk due to antecedent rainfall, including heavy rainfall during the transition period between days 2-3, but ultimately decided, in coordination with local offices, that a slight risk sufficed. A marginal risk was introduced from southeast Texas through the Lower Mississippi Valley and into the Southern Appalachians due to a better signal compared to the overnight issuance and increased QPF trends along an approaching cold front. ....Previous discussion... ....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. Kebede/Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8VhqLECroz7REgs2OI_z6TxVuN6AOkhqEvCRjkOIW4kM= IQ9RJI6fK3_Gh4RdR_czczi5lQbxnLEyiGfLi2WYk_E17rU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8VhqLECroz7REgs2OI_z6TxVuN6AOkhqEvCRjkOIW4kM= IQ9RJI6fK3_Gh4RdR_czczi5lQbxnLEyiGfLi2WY2_aaiMM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8VhqLECroz7REgs2OI_z6TxVuN6AOkhqEvCRjkOIW4kM= IQ9RJI6fK3_Gh4RdR_czczi5lQbxnLEyiGfLi2WYMHmUAwE$=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) .