Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jan 09 2023 00:23:36 FOUS30 KWBC 090023 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 722 PM EST Sun Jan 08 2023 Day 1 Valid 01Z Mon Jan 09 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... A more robust and elongated Warm Conveyor Belt with deep-layer subtropical fetch from the central Pacific approaches overnight, mainly after 07z, which results in renewed heavy rainfall...especially across portions of central and northern California as precipitable water values within recent RAP guidance show PWs ~1.25" approaching the Central CA by the end of the period -- 12z Monday -- along with even more anomalous low-mid layer moisture transport compared to the previous AR events.=20 Inflow at 850 hPa is forecast to be 70+ kts, which even with extremely little instability, is sufficient for hourly totals of 0.5"+ -- 0.75" is the projected hourly maximum prior to 12z. The 850-700 mb moisture flux is around five sigmas above the mean for early to mid January. While the bulk of the heavy rainfall with the overall event should occur beyond Monday 12z, the brief period of heavy rain over saturated soils with the bonus of some snowmelt contribution under ~7000 ft elevation due to the warm advection pattern expected would support continuing with the Slight Risk.=20 The Slight Risk area was adjusted northward per the 18z HREF probabilities of 0.5"+ in an hour and 3"+ in 12 hours (00z-12z).=20 Roth Day 2 Valid 12Z Mon Jan 09 2023 - 12Z Tue Jan 10 2023 ....A MODERATE RISK OF EXCESSIVE RAINFALL CONTINUES ACROSS A LARGE PORTION OF CALIFORNIA, INCLUDING THE BAY AREA AND MUCH OF THE CENTRAL VALLEY... ....NUMEROUS FLASH FLOODING EVENTS LIKELY, SOME POSSIBLY SIGNIFICANT, ESPECIALLY OVER BURN SCARS... ....2030Z Update... Models continue to advertise a significant event ramping up early in the period that propagates southeastward during the period. There has been a subtle southward shift in placement over the past several runs...which necessitate extending the Moderate Risk area a bit farther eastward along the Transverse Range in the southern portion of California while trimming a northern portion of the Slight Risk area just a bit. Overall...the forecast reasoning was similar and the models/ensembles continued to show impressive forcing and moisture transport as well as modest instability.=20 Bann ....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 1.25-1.5" (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 AT 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 peak hourly rainfall rates of 0.75-1.00+ 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, along with pockets of 5-8+ inches along the coast-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 a 5 to 10 year average recurrence interval or ARI within portions of the Moderate Risk area. In addition, the latest HREF 40km neighborhood probabilities of 3 hourly QPF exceeding current 3 hour FFGs climb to >50% within the Moderate Risk area after 15Z Monday. 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 -- especially across the several burn scars since 2020. A High Risk may become necessary in subsequent ERO issuances, especially if the guidance shows another uptick with the QPF, along with consensus in terms of where the peak amounts or bullseyes will be. Hurley Day 3 Valid 12Z Tue Jan 10 2023 - 12Z Wed Jan 11 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF SOUTHERN CALIFORNIA... ....California... Introduced a Moderate Risk area for excessive rainfall in areas of southern California. The models have been suggesting a subtle southward trend in the QPF that continues into Day 3...with a corresponding uptick in rainfall amounts resulting from a longer duration of flow directed into the Transverse range, precipitable water values that are now forecast to increase into the 1.00 to 1.25 inch range, and the chance for thunderstorms as low-level lapse rates steepen. Additional rainfall amounts of 2 to 4 inches of rain are possible on Day 3 near higher terrain from Orange county northward to the Transverse Range (with several inches at lower elevations) on top of any rainfall that falls on Monday into early Tuesday morning. The expectation is that increasingly saturated soils or urbanized areas will be at an increased risk of excessive rainfall even if rainfall rates are not as robust compared with Day 2. Localized totals of 3+ inches are possible along the western slopes of the Sierra. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-trvFJ4qDNHzNBmc_-Pa-AQEcIkZYnBQGNNQX47HUPgm= 29ksb1ymKij9yeHx7SlrZMlHQ-l5TmkU8jkkiJVK13aC9KA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-trvFJ4qDNHzNBmc_-Pa-AQEcIkZYnBQGNNQX47HUPgm= 29ksb1ymKij9yeHx7SlrZMlHQ-l5TmkU8jkkiJVKxJ_rz1M$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-trvFJ4qDNHzNBmc_-Pa-AQEcIkZYnBQGNNQX47HUPgm= 29ksb1ymKij9yeHx7SlrZMlHQ-l5TmkU8jkkiJVKGjdyex4$=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) .