After the flooding rains on Sunday many were asking, “Why so much rain?” “What happened?’ The short answer is it was all thanks to a Convergence Line but I think most people are going to need more than that so here is the long answer.
A Convergence Line (specifically the Shasta County Convergence Zone) is a localized weather phenomenon where winds from different directions collide, forcing moist air upward to form a stationary band of heavy rain, clouds, and sometimes thunderstorms.
Strong south or southeast winds blowing up the Sacramento Valley meet opposing winds or are forced upward by the surrounding mountains known as orographic lifting.
These lines often stall over a specific area—typically from Redding northeast toward Burney along Highway 299—leading to extreme rainfall totals in a very short period.
They are notorious for producing torrential downpours, gusty winds, accumulating hail, and sometimes frequent lightning.
Because the band of rain can remain stationary for hours, it frequently causes localized flash flooding that overwhelms urban storm drains.
Specifically on the Sunday, it all started with the strong south/southeast winds with a peak gust of 51 mph recorded at the Redding Airport. The winds were forced up the mountains creating upward lift which squeezed out the warm moist air in the atmosphere and a Convergence Line had developed.
Typically these lines only last for a few hours, wobble north and south and rarely cause major flooding.
However, the line on Sunday lasted for over 10 hours and although it did wobble a little north and south it stayed over the same general area. As a result, some areas saw upwards of 5 to even 8 inches of rain.
The winds were a key factor to amount of rain that fell because the rest of the valley on average saw less than an inch of rain on Sunday. If the winds were much calmer the Convergence Line would not have developed and it would have just been another rainy day.
Moving forward, we have more rain and wind in the forecast. The winds are once again expected to gust in the 30 to 50 mph range Tuesday night and then again Wednesday night into Christmas Day. Although the winds could create another Convergence Line the chances of repeating what we saw on Sunday are slim. That said, Convergence Line or not, flooding will still be a concern along with the potential for more mud/rockslides and debris flows. With a soft, saturated ground the strong winds will most likely blow down more trees bringing with it the potential for power outages.
Oh, let’s not forget about snow dropping to as low as 3,500 feet leading to travel delays and chain requirements just in time for holiday travel on Wednesday night, Christmas Day and Friday.
This all should come to an end this weekend with drier but still continued cool conditions.
You can always check the latest forecast at https://weathernorcal.com/, the free Weather NorCal app and the 24 hour streaming channel available on Roku, Firestick and anywhere YouTube is available.
