Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jan 10 2023 15:57:36 FOUS30 KWBC 101557 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1056 AM EST Tue Jan 10 2023 Day 1 Valid 16Z Tue Jan 10 2023 - 12Z Wed Jan 11 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF SOUTHERN CALIFORNIA... ....16Z Update... No updates to the outlook areas were made as the current forecast remains on track. One last round of heavier rainfall will move southward from central into southern California ahead of a cold front pushing through the state. MRMS has estimated some hourly rain rates peaking between 0.25-0.5" with this rainfall which is expected to continue through this afternoon. There remains a locally higher threat in the outlined Moderate Risk for terrain sensitive areas along upslope portions of the Transverse Ranges where MRMS estimates 2-3"+ of rain has fallen in the last 12 hours. Rainfall continues over these areas this morning and another 2-4 inches (locally higher) are forecast as the cold front pushes through. Otherwise, areas of heavy rainfall are anticipated both along the cold front and in the post-frontal regime over central/northern California. Very wet antecedent conditions and high streamflow anomalies are present along the coast and upslope region of the Sierra due to the recent heavy rainfall. Some similar rainfall totals may be realized north of the current Slight Risk area in coastal northern California/southern Oregon. However, this region does not look to be as sensitive relative to areas further south which were much harder hit over the last day. It is also likely that rainfall will end sooner than later behind the cold front where a dry slot pushes in, but given the antecedent conditions, expect that any additional lingering rainfall will still pose a threat for flooding. Putnam ....Previous Discussion... ....Southern California... Maintained a Moderate Risk area for excessive rainfall across portions of Southern California as a continuation of heavy rainfall from the overnight periods lingers into today. CAMs continue to indicate distinct QPF maxima along upslope portions of the Transverse Ranges (with as much as 1-3" having already fallen on Monday night over the Moderate Risk area). The relatively long duration of southwesterly flow will allow for another 2-4 inches of rainfall (locally) today, with precipitable water values already near 1" (well above the 90th percentile per VBG and NKX sounding climatology) with a chance for thunderstorms as low-level lapse rates steepen (and ML CAPE reaching perhaps as high as 500 J/kg with peaks of daytime heating). 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 (again) northward to include more of the northern CA coast and coastal ranges. Churchill/Snell Day 2 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 1-3" 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 3 Valid 12Z Thu Jan 12 2023 - 12Z Fri Jan 13 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NORTHERN CALIFORNIA... As the aforementioned atmospheric river becomes more oriented due north-south just offshore the Northern California coast on Thursday, there is more uncertainty with regard to additional rainfall amounts along coastal Northern California. Given the strong nature of the AR just offshore, additional accumulations of 2-4" will be uncomfortably close to the coastline.. with 24-hour amounts on the order of 1-2" much more likely directly onshore (on par with more of a 'moderate' AR). Given the inherent uncertainty involved this far out, have opted for a Marginal Risk area that may need to be upgraded locally to a Slight Risk for portions of the coastline (should the AR stall with a more significant upslope component directly into the coastline). Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6FdrmORODv0ed42EpCWPFcZyEgKD2pDnjgP24gSKdPSZ= 06AOCMTUb8QP5K_HOT04I5IZhBe1Js7aHaDuKqhNCMEdWTc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6FdrmORODv0ed42EpCWPFcZyEgKD2pDnjgP24gSKdPSZ= 06AOCMTUb8QP5K_HOT04I5IZhBe1Js7aHaDuKqhNSekbHgg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6FdrmORODv0ed42EpCWPFcZyEgKD2pDnjgP24gSKdPSZ= 06AOCMTUb8QP5K_HOT04I5IZhBe1Js7aHaDuKqhNUDFmpvw$=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) .