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March 17, 2023,
18:00 UTC

Heavy rain and snow
09 March - 15 March 2023

Visible satellite image of California
with visible fresh water runoff into the Pacific
Source: NASA Worldview

A strong zonally aligned flow over the Eastern Pacific dominated the weather over the western part of the US since March 09, 2023, leading to the advection of moist and warm air masses into the state of California, bringing heavy rains as well as heavy mountain snow to the state with subsequent river flooding at lower elevations.

As a recurring pattern in this winter season, a strong zonally flow formed over the Eastern Pacific close to the West Coast of the US with a strong trough over the Gulf of Alaska, leading to the advection of very moist and warm air masses from a subtropical origin. This advection of warm and moist air masses is sometimes named pineapple express, due to the moisture transport from the subtropical Central Pacific close to the Hawaiian Islands. Combined with the orography of California, with mountain ranges spanning from north to south, this zonal flow can cause heavy precipitation in the state of California, especially along western facing flanks of said mountain ranges.

On March 09, 2023, a small trough off the coast of the Pacific Northwest increased the geopotential gradient over the Eastern Pacific, leading to a strong zonally aligned flow advecting moist and warm air masses into the state of California, combined with the orographic lift, this flow induced heavy precipitation with large fresh snow accumulations at higher altitudes. Further, the unstable air masses caused rare severe thunderstorm activity over California, with heavy downpours causing flash flooding as well as small-sized hail and funnel clouds being reported in the state.

A second strong low-pressure system affected the state of California between March 13 and March 15, 2023, causing yet again heavy precipitation especially in the central part of California. 72 h-precipitation totals reached yet again more than 125 mm along western facing flanks of the Sierra Nevada, with lower elevations receiving locally more than 50 mm of rain within 3 days and fresh snow accumulations reaching more than 75 cm in the same period. The heavy precipitation accompanied with the higher snow levels led to large runoff, especially in the Central Valley, causing heavy river flooding as well as strong flash flooding in times of heavy precipitation due to the saturated soils in this region. Overall, the heavy mountain snow once again led to many road closures as well as some towns at higher elevations being literally buried in snow with snow depths of more than 5 m. The river flooding damaged homes at lower elevations as well as making many roadways impassable.

Although, the state of California is dependent on these precipitation events to fill the water storages of the state for the dry summer season, especially considering that the state as well as large areas of the western part of the US are suffering from a more than one-decade long drought. Nevertheless, this year's winter season has been nothing but exceptional as many potent low-pressure systems affected this region, bringing much above normal precipitation, with some rivers have running at levels not seen for decades in the past days. In the last 7 days some stations along the Sierra Nevada reported yet again more than 300 mm of precipitation, with radar analysis suggesting local precipitation totals of more than 400 mm/7 days. The heavy precipitation at higher elevations led to significant fresh snow accumulations with the snow-water-equivalent in the Sierra Nevada running at more than 200 % of the seasonal average with values of up to 1500 mm.

7-day accumulated precipitation over California with marker showing sites with precipitation totals > 250 mm (left), source: CNRFC
and snow-water-equivalent of the snowpack in the Sierra Nevada (right), source: CDEC Water

Many sites along the Sierra Nevada have not seen such a snowpack in decades, with the snow totals in the winter season 2022/23 reaching the top 3 spots of the recorded history. The heavy precipitation and the massive snowpack of the Sierra Nevada led to a massive improvement of the drought conditions in California, with nearly half of the state being out of drought conditions.

Further, as above normal precipitation is expected in the medium range forecast for much of the western part of the US, the drought conditions are expected to improve further in the entire region. Though, both the expected precipitation as well as the subsequent spring thaw will increase the concerns for widespread flooding once again.

Text: KG
March 17, 2023

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