A collection of weather phenomena off the west coast of North America, including a bomb cyclone, will give northern California record rainfall over the next day and a half:
Amid an exceptional drought that has wrought havoc on California for years, a Level 5 out of 5 atmospheric river is soaking the region, dumping double-digit rainfall totals and up to six feet of mountain snow. This heavy precipitation will help ease the drought but produce dangerous mudslides and debris flows in areas recently devastated by fires.
Atmospheric rivers are long, narrow swaths of exceptionally moist air, sometimes sourced from the tropics, that can produce excessive amounts of precipitation.
“It will be a wild 24 to 36 hours across northern California as we will see an extreme and possible historic atmospheric river push through the region,” wrote the National Weather Service in Sacramento, calling it a “dangerous, high-impact weather system.”
Flash flood watches are up for most of Central and Northern California, blanketing some of the same areas that went upward of six months without a stitch of measurable rain. Sacramento recorded its first 0.01 inches of rain last week since March 19, capping off a record-setting 222 days without precipitation. Now it is bracing for more than half a foot of rain and flooding.
The storm offshore of the Pacific Northwest driving this atmospheric river into the coastline rapidly intensified at a staggering rate. Meteorologists refer to a storm as a “bomb” if its minimum central air pressure drops by 24 millibars or more in 24 hours, signifying intense and speedy “deepening.” That sucks in more air and allows the storm to intensify.
This particular storm strengthened at twice that rate as a “double bomb” with its pressure tanking to 943 millibars, which, according to the Weather Service, would make it “the strongest known system for the area.”
Over the mainland United States, such low atmospheric pressure is practically unheard of. Even on the East Coast, where cyclones regularly strengthen into massive nor’easters, pressures seldom fall this low. The powerful March 1993 “superstorm,” among the most intense and damaging storms ever to strike the United States, bottomed out at “only” 960 millibars.
Welcome to climate change: seven months without rain, then nothing but rain for two days. Fun.
Here in Chicago we just have a normal gray rainy October afternoon. I'm glad we don't have to worry about 150 mm of rain in the next four hours.