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A Variety Of Weather Hazards To Start Off The Week

In central Florida, ongoing major to record river flooding across a couple of river basins is expected to continue well into this week and will be slow to recede in the wake of Ian. Locally heavy to excessive rain is possible along the immediate Mid-Atlantic coast. Elevated fire weather conditions are expected in the central Plains. It will be warm in the Northwest and the Plains. Read More >

Overview -  Atmospheric River - October 24-25, 2021 

Early Season Storm Brought Historic Heavy Rains and Strong Winds to the Bay Area

The 2020/2021 Water year ended unceremoniously in the Summer of 2021 with a "status quo" report with little rain during the rainy season leaving dry fuels around the state keeping the region on edge during Fire Season. As the 2021/2022 water season began, the pattern shifted bringing a series of upper level troughs through mid-October toward the Pacific Northwest. As these systems moved toward the California Coastline, it brought light rain in the form of cold fronts along with atypical above average accumulations to the North Bay.

By October 18th, long-range models were indicating yet another low that would approach the Pacific Northwest bringing a chance for rain to the Bay Area. However, these model runs depicted a longer stream of moisture that was tapped into the sub-tropics. By the 20th, models were hinting at the chance for a high to moderate "atmospheric river",  or AR, event with precipitable water values several standard deviations above the norm. There was reason to doubt initial model runs given that this type of an event is unusually early for the region. But as the event grew closer, it became more and more apparent that the Bay Area was to prepare for a substantial rain event.

As the parent low pressure system moved toward Seattle and its central pressure rapidly intensified, undergoing   bomb-cyclogenesis. Light showers already were showing up in the North Bay late on the evening of October 23rd. As the sun rose on the 24th, the water content increased exponentially bringing moderate to heavy rainfall around the Bay Area. Winds out of the southwest increased causing a Wind Advisory to be issued for the entire Bay Area. At higher elevations, winds were observed between 50 to 60 mph with peak gusts between 70 to 80 mph, with gusts at KSFO measured up to 50 mph. Flood Warnings and Flood Advisories were issued throughout the day from the North Bay, down the San Mateo coastline for flood risks in urban locations and small streams. A Flash Flood Warning was issued for the Glass Fire burn scar due to a period of intense rainfall rates. Storm reports that were submitted included flooding, downed trees, power outages, and some minor mud slides. By day's end, October 24th, 2021 will go down as the wettest day for many cities around the Bay Area, setting records along the way. For more information on record rainfall, see the 'Heavy Rain' tab below.

The deep low pressure system driving the atmospheric river also generated historically large and powerful waves along the west coast from Washington to California. Buoys situated along the coast reported significant wave set heights of around 30 feet along and in the wake of the storm, with peak individual wave heights of 50 to 60 feet, making this wave event the 2nd largest in the 23 year history of the Point Reyes (46029) CDIP buoy (Dec 2015 was larger). An early season high surf warning was in effect as these energetic, large waves crashed into our coastline and overran many of our west to northwest facing beaches on Monday the 25th. In addition, excessive runoff from the previous 24 hours of stormy weather led to an overly brown hue to the waves.

Forecasters past and present made similar observations of this wave event: while the breaking waves were indeed very large, they were not the absolute largest waves they had observed around the region. Instead, the amount of big wave energy running up the coast/beaches struck forecasters as most impressive. For example, at Carmel Beach, water overran the majority of the beach Monday morning and rhythmically splashed against the sea wall well at the opposite end of the beach. Similar scenes played out elsewhere as many beaches were still transitioning out of their less resilient summer configurations (ie, not as steep, lacking significant sand bars) when this very large, very early season event arrived.

The atmospheric river gradually moved southward toward Los Angeles on the 25th.

 

CW3E also included a thorough write up on the Atmospheric River here:

https://https://cw3e.ucsd.edu/cw3e-event-summary-19-26-october-2021/

 

Composed by: DKing and MMehle

Satellite loop
Satellite loop showing the evolution of the Atmospheric River impacting the Bay Area and Central Coast.
 

 

Direct Link to Radar Loop

 

 

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