Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Jan 09 2023 15:58:50 FOUS30 KWBC 091558 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1057 AM EST Mon Jan 09 2023 Day 1 Valid 16Z 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... A more significant, elongated Atmospheric River (or AR) event is ongoing across much of CA ahead of a more amplified upper trough, effectively tapping into the subtropical regions of the north Pacific. PWs as of 14z this morning register over 1" throughout the central CA coast and inland over much of the Sacramento Valley, with widespread rainfall rates up to 0.5"/hr. The 12z sounding from VGB reported a PW of 1.29", which breaks the record for the date and well above the 90th percentile moving average. Southwesterly 850 mb flow peaking between 60-70 kts will continue to produce efficient and robust moisture transport, with the 850-700 mb moisture flux peaking at 5 standard deviations above normal (per the GEFS), centered around the central CA coast and weakening after 00z tonight. Meanwhile, some elevated instability (at least 200-400 J/kg per the ECMWF, GFS, and HREF mean), 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 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). This will be a fairly significant event as far as ARs go, with the latest deterministic QPF indicating a widespread 5 to 10 year average recurrence interval (or ARI) within portions of the Moderate Risk area. Within this large Moderate Risk, areas of particular concern for intense rates over 1.00", which could linger for a few hours between 16z and 00z, extends from the Santa Lucia Range to parts of the eastern San Joaquin Valley. Rainfall amounts for the Santa Lucia Range are most likely to exceed 6" within this timefame, as potential 2-4"+ amounts occur near the Fresno area and nearby foothills up against the Sierra Nevada. 12z hires guidance continues to depict the potential for 2"+ amounts within 6 hours leading up to 00z near Fresno, CA which alone could threaten a 24-hour rainfall record for the city. In addition, the latest HREF 40km neighborhood probabilities of 3 hourly QPF exceeding current 3 hour FFGs are >40% within the Moderate Risk area from the Santa Lucia to 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 quickly shift southward after 00z tonight. The 16z ERO update included some very small trimming along the northwestern edge of the outlook area where the heavier precipitation is already coming to an end, as well as a small extension of the MRGL on the southern end to capture the potential for modest rainfall to shift into the southern CA coastal areas right before 12z Tuesday. For more detailed information regarding today's atmospheric river event, view MPD 0032 and MPD 0033. Snell 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... Maintained a Moderate Risk area for excessive rainfall across portions of Southern California. Models continue to indicate a distinct southward shift in the QPF maxima on Day 2, with significant rainfall already expected to be ongoing Tuesday morning across upslope portions of the Transverse Ranges (with as much as 3-6" having already fallen on Monday and Monday night). Long duration of southwesterly flow will allow for another 2-4 inches of rainfall (locally) along the terrain on Tuesday, with precipitable water values of 1.00-1.25" 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. Churchill 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 locally is expected along the coast, 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). Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-ALC94LOIxNlwGsZKKUhZK0KvYE6fjZMZk7vO6V3hSOr= Iabwypksa4h7TmBG_MkR_B9-NIvIj6cNgIjR-6LKErG0aZA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-ALC94LOIxNlwGsZKKUhZK0KvYE6fjZMZk7vO6V3hSOr= Iabwypksa4h7TmBG_MkR_B9-NIvIj6cNgIjR-6LK148Dkhs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-ALC94LOIxNlwGsZKKUhZK0KvYE6fjZMZk7vO6V3hSOr= Iabwypksa4h7TmBG_MkR_B9-NIvIj6cNgIjR-6LK_72JsKI$=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) .