Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jan 11 2023 00:09:44 FOUS30 KWBC 110009 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 708 PM EST Tue Jan 10 2023 Day 1 Valid 16Z Tue Jan 10 2023 - 12Z Wed Jan 11 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF CENTRAL AND NORTHERN CALIFORNIA... ....01Z Update... The bulk of the atmospheric river has progressed southward across southern California, bringing an end to the heaviest rainfall across parts of the region through tonight. Closer to the closed mid/upper level low near the CA/OR border, steadier and more widespread showers continue across parts of central and northern California. Here, probabilities of seeing 0.5"/hr rain totals are elevated enough to warrant keeping the Slight Risk in place, which could lead to isolated to scattered instances of flash flooding. Toward the end of the period, the next round of heavier rainfall will begin to move onshore, though the bulk of that AR will fall after 12Z Wednesday. Further south, the Moderate and Slight Risks were removed given the ending threat of heavy rainfall. Taylor 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... ....16Z Update... Now available Hi-Res guidance concurs with the prior forecast, indicating 1-3" totals, locally higher, within the Slight Risk area. The plume of higher moisture associated with the next Atmospheric River event will arrive at the northern California coast near the start of the period, with the axis of moisture narrowing and orienting more north/south and parallel to the coast later in the day/overnight as ridging builds inland to the east. The Slight Risk covers the region with flow favoring upslope along areas of higher terrain oriented more east/west and thus more perpendicular to the flow compared to those oriented north/south and parallel to the flow. Putnam ....Previous Discussion... 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 SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NORTHERN CALIFORNIA... A Slight Risk has been introduced for portions of the northern California coast south of Eureka and north of the Bay Area. Uncertainty remains with where the axis of higher moisture associated with an Atmospheric River will set up along the West Coast as the next incoming Pacific storm system stalls offshore. The orientation will become more parallel to the coast with time on day 2 as ridging builds to the east inland. Even small changes in the flow could have a significant effect on the eventual rainfall totals along the coast. However, the 12Z global guidance trended more onshore, with higher totals between 2-5" possible.=20 Also, given the very wet pattern and overlap of the heaviest forecast rainfall in favorable upslope regions between day 2 and day 3, some flood risk can still be expected even if more modest totals are eventually realized. In addition, a Marginal Risk has been added for the Olympic Peninsula in Washington. Warm, moist air will approach from the south associated with the latest Atmospheric River event ahead of a Pacific storm system to the west. Snow levels are expected to rapidly rise over 6000 feet, and most precipitation during the period even at higher elevations in the Olympic Mountains should fall as rain. Forecast rainfall amounts range between 2-4", with locally higher amounts possible. Putnam Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5USKdXQ8ZAOOccrh6DQlFeCDWVVVsjYlL-GjtxzTf06s= ZH0WNhPBZMT63RWfEpn3fQD6RXaK0s-2rjxGZVf6AcnGdgI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5USKdXQ8ZAOOccrh6DQlFeCDWVVVsjYlL-GjtxzTf06s= ZH0WNhPBZMT63RWfEpn3fQD6RXaK0s-2rjxGZVf6QedvLXk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5USKdXQ8ZAOOccrh6DQlFeCDWVVVsjYlL-GjtxzTf06s= ZH0WNhPBZMT63RWfEpn3fQD6RXaK0s-2rjxGZVf6AXJuUfw$=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. 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