Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jan 11 2023 08:11:21 FOUS30 KWBC 110811 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 310 AM EST Wed Jan 11 2023 Day 1 Valid 12Z Wed Jan 11 2023 - 12Z Thu Jan 12 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NORTHERN CALIFORNIA... Yet another significant atmospheric river is at the doorstep of Northern California this morning, as indicated by a stream of tropospheric water vapor directed nearly orthogonal to the coastline (per GOES-West infrared imagery). Another 1-3" of rainfall is expected across much of Northern CA and far southwestern OR, with locally higher amounts possible into the idealized upslope portions of the terrain. It is in these localized areas where the uplift enhanced by the terrain may result in rainfall exceeding 3" (generally encompassed by the Slight Risk area, which was expanded inland towards Mount Shasta with this update). While the forecast intergated vapor transport (IVT) values directly along the coast are still generally that of a 'moderate' atmospheric river (i.e. AR2), in reality it's more so a 'strong' atmospheric river (i.e. AR3) providing more of a glancing blow to the already hard hit region. This is due to the warm front of the parent system (and best low-level moisture transport) stalling just offshore later today (as ridging builds inland), resulting in a narrowing band of moisture transport becoming nearly parallel (oriented north-south) to the coast. It is this stalling of the front (and lessening low-level moisture transport) that should keep the risk capped at a Slight for today (with the bulk of the QPF concentrated prior to 00z Thursday when the flow is more normal to the coastline). Churchill Day 2 Valid 12Z Thu Jan 12 2023 - 12Z Fri Jan 13 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NORTHERN CALIFORNIA... By Thursday morning, the aforementioned stalling warm front is beginning to make slow progress (again) eastward towards the Northern California coastline. This may result in an additional 1-3" along a relatively narrow portion of the coastline, as the AR will be increasingly oriented due north-south with the slow eastward translation of the efficient moisture transport. This introduces a fair amount of uncertainty (compared to Day 1), with model consensus depicting the heaviest QPF just offshore (as high as 3-5", more akin to an AR3). The Slight Risk was maintained for a narrow portion of the Northern California coastline where locally totals may exceed 3", as the expectation is that coast will continue to be grazed by the strong AR (with unidealized upslope flow, given the parallel orientation to the coastline). Given uncertainty is higher than usual, subsequent updates my necessitate an upgrade (to Moderate Risk) OR a downgrade (to Marginal Risk), depending on the trends of the CAMs (as more HREF members come into full focus with the afternoon update). In the meantime, a more narrow Slight and Marginal Risk were maintained for this update for Northern CA and far southwestern OR (with a Marginal Risk for the Olympic Peninsula of WA as well with 2-5" expected totals, falling mostly as rainfall, over the course of 24 hours). Churchill Day 3 Valid 12Z Fri Jan 13 2023 - 12Z Sat Jan 14 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NORTHERN AND CENTRAL CALIFORNIA, WESTERN WASHINGTON, AND FAR SOUTHWESTERN OREGON... Increased uncertainty continues into Friday morning, as much of the day may end up being a break between atmospheric river events. The aforementioned strong AR should be weakening and shifting inland during the morning hours, followed by a midday lull before the next (likely weaker) AR begins to approach from farther offshore. The Marginal Risk areas mainly reflect the lingering effects of the former (strong, but weakening) AR, and subsequent targeted upgrades to Slight may well be necessary down the line. The current expectation is for an additional 1-3" of rainfall, mostly during the morning hours on Friday (including much of the CA coast, upslope portions of the Sierra Nevada, the Olympic Peninsula of WA, and far southwestern OR). Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!42RcG0R7I1_eQyFmEE4g7szkBk2J1DJzpp-0OIAT86Ls= KrrtaHu7xL24Ec-5U6vRXGtOnYzBDcndDe_zUG721QVvv3Y$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!42RcG0R7I1_eQyFmEE4g7szkBk2J1DJzpp-0OIAT86Ls= KrrtaHu7xL24Ec-5U6vRXGtOnYzBDcndDe_zUG72rbpWUTA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!42RcG0R7I1_eQyFmEE4g7szkBk2J1DJzpp-0OIAT86Ls= KrrtaHu7xL24Ec-5U6vRXGtOnYzBDcndDe_zUG720Q7gqgs$=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) .