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Heavy Rainfall to Bring Flooding Threat to California; Heavy Snow in the Sierra

A strong atmospheric river will bring heavy rainfall to California through the holiday week. The heaviest rainfall is expected on Wednesday in the Los Angeles Basin. Flash and urban flooding is possible. A prolonged heavy snowfall is expected in the Sierra Nevada from the atmospheric river, raising concerns for major snow‐load impacts. Travel may be difficult to impossible over the passes. Read More >

***All information is considered preliminary until investigations are finalized***

Overview

A low pressure system of historic strength led to a variety of high-end weather impacts from the central Plains to the Upper Midwest and Great Lakes December 15-16. An unprecedented December severe weather unfolded over portions of Minnesota and Wisconsin Wednesday evening, with the Storm Prediction Center issuing their farthest-north Moderate Risk for the month of December. A serial derecho moving at 60-80 mph tracked from Kansas to Wisconsin, resulting in over 560 reports of damaging wind and over 60 tornadoes. A total of 57 "significant severe" wind gusts (75+ mph) were reported, breaking the daily record of 53 set on August 10, 2020.

Most of the damage across our area occurred from south-central Minnesota through west-central Wisconsin. Particularly widespread damage occurred in Hartland, Minnesota, and Stanley, Wisconsin where EF2 tornadoes were confirmed. Prior to this event, a tornado has never been reported in Minnesota in December. 

Before the thunderstorms, strong southerly winds led to record high temperatures at many sites across the Upper Midwest. Temperatures warmed into the mid to upper 50s across much of central Minnesota and western Wisconsin and even exceeded 60 degrees across southern Minnesota. The warm temperatures melted what was left of the snowpack from the December 10 winter storm, which led to widespread dense fog through much of the morning and afternoon.

After the thunderstorms, strong gradient winds and widespread wind gusts in excess of 50 mph developed for several hours late Wednesday night and into the overnight hours. The Redwood Falls airport measured a gust of 78 mph at 11:30 PM.

Temperatures quickly fell below freezing across western and central Minnesota Wednesday night, causing rain to change over to a wintry mix of freezing drizzle and snow. Snowfall accumulations of 1-4" inches, along with the strong gusty winds, led to low visibility and travel hazards late Wednesday night into Thursday morning.

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Map of storm reports and the convective outlook from December 15. The "Moderate Risk" is rare any time of year, let alone in December. One has never been issued so far north so late in the year. 
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Warnings Issued December 15 Regional Loop of Radar, Warnings, and Wind Gust Reports (Courtesy: Iowa Environmental Mesonet)  
 
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