Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jan 10 2023 00:58:30 FOUS30 KWBC 100058 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 757 PM EST Mon Jan 09 2023 Day 1 Valid 01Z Tue Jan 10 2023 - 12Z Tue Jan 10 2023 ....A MODERATE RISK OF EXCESSIVE RAINFALL CONTINUES FROM PORTIONS OF CENTRAL CALIFORNIA SOUTHWARD TO THE TRANSVERSE RANGE... ....NUMEROUS FLASH FLOODING EVENTS LIKELY, SOME POSSIBLY SIGNIFICANT, ESPECIALLY OVER BURN SCARS... Flash flooding tied to a significant Atmospheric River will continue to make its way southward into the southern portion of California this evening and into the overnight hours. 00Z soundings and area 88D showed southwesterly flow in the 40 to 55 kt range over southern California and immediately upstream over the Pacific...resulting in robust moisture transport that the 18Z GEFS shows being at or slightly greater than 4 standard deviations above climatology for January while meso-analysis still shows some elevated instability...comparable with earlier runs of the ECMWF, GFS, and HREF mean). This has supported...and should continue to support...peak hourly rainfall rates of 0.75-1.00+ inch (especially within the Moderate Risk area). Overall, guidance consensus remains quite good with respect to the QPF, with totals through 12z Tuesday 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 Nevada). The latest deterministic QPF continued to have a 5 to 10 year average recurrence interval (or ARI) within portions of the Moderate Risk area. The 09/21Z HRRR focuses another 3 to 5+ inches along the Transverse Range through 10/12Z...with 1 to 3 inches at lower elevations...and the latest HREF 40km neighborhood probabilities of 3 hourly QPF exceeding current 3 hour FFGs are >40% along the Transverse Ranges. Considering how wet the soils were 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. Fortunately for the San Joaquin Valley and central CA region, the axis of heavy rainfall should be shifting out of the area shortly if it has not done so already.=20 Changes to the ERO at 10/01Z is to remove the Moderate Risk aera over most of northern California where most of the rain has already ended...but maintained the Slight Risk with some adjustments due to the next batch of rain coming on-shore and making its way inland overnight. Expected rainfall amounts or rainfall rates in that part of California may not have warranted a Slight Risk area in most cases...but the area remains hydrologically sensitive given the recent rainfall amounts combined with the usual uncertainty about exactly when the next wave arrives and how much rain is delivered through the remainder of the period. No real changes were needed to the on-going Moderate risk area or surrounding Marginal/Slight areas. Bann Day 2 Valid 12Z Tue Jan 10 2023 - 12Z Wed Jan 11 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF SOUTHERN CALIFORNIA... ....Southern California... Maintained a Moderate Risk area for excessive rainfall across portions of Southern California as a continuation of heavy rainfall from Monday night lingers into Tuesday. Models continue to indicate a distinct QPF maxima on Day 2 across upslope portions of the Transverse Ranges (with as much as 3-6" having already fallen on Monday and Monday night throughout this region). Long duration of southwesterly flow will allow for another 2-4 inches of rainfall (locally) along the terrain on Tuesday, with precipitable water values nearing 1.00" and the chance for thunderstorms as low-level lapse rates steepen (and ML CAPE reaching perhaps as high as 500 J/kg). Areas along the coast and in the valleys will receive closer to an inch of additional rainfall on Tuesday, coming with an additional (final) surge of tropospheric moisture (and eventual cold front passage) in the afternoon. This final surge of precipitation will impact the far southern and western facing mountain ranges of southern California, where recent burn scars are most at risk to debris flow and flash flooding. ....Central California... Fast-moving impulses of moderate to locally heavy rain are expected to progress onshore and lead to the potential for additional 1-2"+ amounts across the typical upslope enhanced areas of the Sierra and coastal ranges of central CA. IVT values should remain tame for this next round and rainfall rates are generally expected to remain under 0.75"/hr, but the highly sensitive terrain from an ongoing strong AR events may lower the threshold needed for runoff concerns. Thus, the Slight Risk was expanded northward to include the remainder of the central CA coast and coastal ranges. Snell Day 3 Valid 12Z Wed Jan 11 2023 - 12Z Thu Jan 12 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NORTHERN CALIFORNIA... While only just starting to get a bit of a break by late Tuesday, another significant atmospheric river is at the doorstep of Northern California by Wednesday morning. Another 2-3"+ of rainfall is expected along the coast with locally higher amounts into the elevated terrain, and is generally encompassed by the Slight Risk area. While the forecast IVT values are generally that of a moderate atmospheric river, it's more so the glancing blow of a strong atmospheric river that is expected. This may put somewhat of a limit on rainfall amounts initially, the subsequent stalling of the offshore storm system may also help to keep the frontal zone and warm conveyor belt nearby for a longer period (increasing the risk for subsequent small scale moisture surges beyond Wednesday). Snow levels will start off low Wednesday morning and around 4000 ft across northern California before rising to near 6000 ft by the evening. Churchill/Snell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-Tv4u7K34IRqorVt8bOeGitJa2eYNyvDX2BdFw6Z4xdq= ulw4yJTpb31xbvPQ_LSxcTIyswxRdR1Eb1V0whkYcRRwOrw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-Tv4u7K34IRqorVt8bOeGitJa2eYNyvDX2BdFw6Z4xdq= ulw4yJTpb31xbvPQ_LSxcTIyswxRdR1Eb1V0whkYBjqXOYs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-Tv4u7K34IRqorVt8bOeGitJa2eYNyvDX2BdFw6Z4xdq= ulw4yJTpb31xbvPQ_LSxcTIyswxRdR1Eb1V0whkYb4tLtF4$=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) .