Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Jan 08 2023 00:00:47 FOUS30 KWBC 080000 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 659 PM EST Sat Jan 07 2023 Day 1 Valid 01Z Sun Jan 08 2023 - 12Z Sun Jan 08 2023 ....THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS PARTS OF NORTHERN AND CENTRAL CALIFORNIA, INCLUDING NAPA VALLEY AND NORTHERN PORTIONS OF THE BAY AREA... ....Southwest Oregon into Northern and Central California... The models continue to depict robust QPF totals with this latest AR, especially within the Moderate Risk area, where most CAMs have an additional 3-6". The latest mesoscale guidance maintains the idea of rainfall coverage and rates ramping back up later this afternoon and tonight. Renewed, robust low-mid layer moisture transport should continue to be directed toward the Central CA Coast. The latest current high-res CAM guidance still show hourly rainfall rates peaking between 0.50-0.75+ inch shortly (Sunday late afternoon/early evening), aided by some elevated instability (MUCAPEs 400-600 J/kg), which would pose a flash flood threat over several 2020-2022 burn scars west of the Sierra and south of the Shasta/Siskiyous mountains. Areas were smoothed slightly, but resemble continuity. Recent excessive rains along with several additional inches forecast, there will likely be widespread areas of flooding and if in proximity of recent burn scars, the threat for landslides, debris flows and road damage will elevate. ....Western Gulf Coast into portions of the Lower Mississippi and Tennessee Valleys... Continued with the Marginal Risk of excessive rainfall over this region, which has been recently reinforced by some small scale backbuilding/training just downwind of Houston. Strengthening right-entrance region upper forcing along with a slow moving front has started to draw moisture northward. The area of training thunderstorms across this region appears to increase in scale shortly, which would produce isolated run off problems. Considering the spread in the guidance, will loosely organized convection implied in the splotchy QPF pattern, along with the relatively dry conditions over the past few days, have continued with the Marginal Risk with the expectation that any flash flooding will be isolated to widely scattered at best. ML CAPE is 1000-1500+ J/kg over a broad area, effective bulk shear is 25-30 kts, and precipitable water values are starting to eclipse 1.25" within an area of lower 560s 500-1000 hPa thickness. This should support hourly rain totals up to 2.5" and local amounts to 4" -- small scale Slight Risk-type impacts can't be ruled out. A Mesoscale Precipitation Discussion for a portion of this area will likely be prepared shortly. Roth/Bann Day 2 Valid 12Z Sun Jan 08 2023 - 12Z Mon Jan 09 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF NORTHERN AND CENTRAL CALIFORNIA... ....Southwest Oregon into Northern and Central California... No significant changes were needed in the Day 2 Excessive Rainfall Outlook that begins at 12Z Sunday. The overall environment and synoptic pattern remained similar during this period and forecast reasoning was largely unchanged. There looks to be a relative "lull" for a few hours as one AR wanes and the next approaches.=20=20 Additional rainfall on top of already saturated soils will keep the threat for excessive rainfall and flash flooding elevated despite the relative lull. At the start of the Day 2, one shortwave trough will have moved through while a renewed atmospheric river event initiates as a second...more elongated...approaches Sunday night. Guidance shows PWs ~1.25" approaching the Central CA Coast Sunday night, along with even more anomalous low-mid layer moisture transport compared to the previous AR events. The 12Z SREF and GEFS still depict 850-700 mb moisture flux standardized anomalies in excess of +4 approaching the coast and moving inland by 12Z Monday. While the bulk of this particular event will be during the day Monday (current Day 3), deterministic and probabilistic QPF fields, along with analogs and ERO first guess fields using 24 hr ARI exceedance, would support continuing with the Slight Risk that was inherited from yesterday's Day 3 ERO. This especially taking into account the anomalously wet soils (above the 90th percentile in the top 1 meter layer per the latest NASA SPoRT), owing to the onslaught of recent AR events. Bann Day 3 Valid 12Z Mon Jan 09 2023 - 12Z Tue Jan 10 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS A LARGE PORTION OF CALIFORNIA, INCLUDING THE BAY AREA AND MUCH OF THE CENTRAL VALLEY... ....California... The more significant Atmospheric River or AR event is still expect to impact much of CA ahead of a more amplified upper trough, where a more elongated AR is expected to tap into the subtropical regions of the south Pacific. Guidance continues to show PWs climbing between 1-1.5" (highest along the coast), with southwesterly 850 mb flow peaking between 60-70 kts. Low-mid layer moisture transport will be quite robust, as the 850-700 mb moisture flux peaks in the 4 to 5 standard deviation range above normal per the GEFS. Meanwhile, some elevated instability (at least 200-400 J/kg per the ECMWF and GFS), along with the degree of deep-layer forcing and moisture, would support hourly rainfall rates of 0.50-0.75+ inch, especially within the Moderate Risk area. Overall, guidance consensus remained quite good with respect to the QPF, with 24 hour totals between 2-4+ inches over most areas, with pockets of 5-8+ inches noted along the coast and coastal ranges and across the western slopes of the Sierra. This will be a fairly significant event as far as ARs go, with the latest deterministic QPF indicating the potential for a 5 to 10 year average recurrence interval or ARI within portions of the Moderate Risk area. Considering how wet the soils will be ahead of this event (over the 90th percentile within the top 100 cm across the majority of the outlook areas per the latest NASA SPoRT), impacts from flash flooding or more rapid inundation will become increasingly likely. A High Risk may become necessary in subsequent ERO issuances. Bann/Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7AYSYARu-xuCyxLuGRHohkyjxrDfcGfjy5oDgsc8LKL3= ei8uZsXgwzVCSnNU0WMO-L56_SfSWXYlYUQvGXujNK5xzJM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7AYSYARu-xuCyxLuGRHohkyjxrDfcGfjy5oDgsc8LKL3= ei8uZsXgwzVCSnNU0WMO-L56_SfSWXYlYUQvGXujcKF-cxo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7AYSYARu-xuCyxLuGRHohkyjxrDfcGfjy5oDgsc8LKL3= ei8uZsXgwzVCSnNU0WMO-L56_SfSWXYlYUQvGXujzwOCFEk$=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) .