2-pronged storm system to cause dangerous flooding in Southern California

AccuWeather

Months' worth of rain could fall in and around Los Angeles in just two days this weekend, posing major risks for the region.

Transcript

00:01Well, we're going to be dealing with a major flood concern and severe flood dangers, especially

00:08in Southern California, and again, in some areas where we've had significant burn scars

00:14back in January, in places like Pacific Palisades and just downhill from there, there's severe

00:21danger coming up this weekend as, again, the two-pronged southwestern storm system gets

00:27a lot more aggressive Friday night into Saturday in parts of Southern California.

00:31Let's take a look at what's going on here with the forecast feed.

00:34As we take a look at the big satellite loop, you can see that we have a pronounced dip in

00:38the jet stream across the western part of the continent and especially into the East Pacific.

00:45So again, you have an area of low pressure and again, a loft.

00:49You can center it down this way at the surface.

00:52You can see the reflection of that actually with this little swirl here that's just off

00:55to the west of the northern part of California's coast.

00:59But a significant feed of tropical moisture.

01:02It's kind of in two different zones here with a little intermediate zone of drier air.

01:07But we're going to be dealing with a significant, dangerous pipeline of Pacific moisture coming

01:12into California.

01:13And one thing just to point out one more time, we did this yesterday, and again, if you're

01:16just tuning in, we wanted to just show you that these southwest-facing slopes of the mountains

01:22around the LA basin, you go from sea level up to 8,000, 9,000 feet in some spots with

01:27very little distance.

01:29So it's going to be a rapid upslope and cold air can contain less water vapor.

01:34So if you take moisture from sea level and run it all the way up 8,000, 9,000, 10,000,

01:3910,000 feet up into the atmosphere, it chills down and colder air can contain less water

01:44vapor.

01:45So you're going to see huge amounts of rain production as the air saturates just instantly

01:49there.

01:50And this is going to be fueling a dangerous flood event.

01:53Before we get into the models, I wanted to just give you kind of a big picture look at

01:56the second round of precipitation.

01:58This does not include any of the rain that's falling right now.

02:01So we've kind of broken the storm into two maps.

02:04One through Friday, one to two inches of rain in progress into areas around San Diego, or

02:10into places like San Francisco, I should say.

02:11San Francisco is seeing one to two inches now.

02:13But it's this weekend when San Diego and Los Angeles get in on it.

02:17And LA may see a half inch to an inch during the day Friday.

02:20But Friday night, another two to four inches on top of that.

02:23So we're looking at potential for three to five inches over a four-day window in Los Angeles.

02:29That's a big deal.

02:30It's a significant amount of rain and it's going to lead to some severe flood and mudslide

02:35concerns.

02:36And look at this just north of the city where you're going to see another four to eight inches

02:40of rain.

02:41And again, some of these areas might see two to four inches tonight into the day on Friday.

02:46But then Friday night onward, four to eight inches.

02:49So there are areas north of LA that could see half a foot of rain.

02:52Just for a little context, specifically Los Angeles, you could see that if we end up seeing

02:57five inches of rain, that would be basically what we would typically see in September plus

03:03October plus November plus December and into much of January.

03:07Again, we really don't see all that much rain in a typical year.

03:12We might see something like 12, something like 30 to 50%, 30 to 40% of average rainfall just

03:19in this storm setup.

03:21So as we look at the overall pattern, big dip in the jet stream here with this strong, strong,

03:26low pressure system.

03:27It is a, we might say a full latitude trough.

03:30It is really, really deep and amplified.

03:33So it's a huge zone of low pressure that's going to be slinging south and east.

03:37You'll watch as this comes southward over the beginning of the weekend, Friday into Saturday.

03:43Here it comes and it's going to be digging southward and then pivoting right over Southern

03:47California and sending tremendous amounts of moisture.

03:50Here's another model depiction of that.

03:52There's great agreement that this is going to push south and then wobble over Southern

03:55California.

03:56If we look at something called precipitable water, this is the amount of moisture in

04:00the atmosphere in the whole vertical column.

04:03And you can see some significant amounts here with, again, the bright yellows here.

04:10And look at the winds as well.

04:11The winds directing that right into the face, the south face of those mountains around Los

04:16Angeles.

04:17So tremendous amounts of moisture, basically like tropical moisture in Southern California.

04:21So what does that do for us in terms of the numbers for the entire event?

04:26Both waves of moisture.

04:27The GFS model produces between four and six inches of rain in some areas around Santa Barbara

04:33and Ventura County.

04:35And maybe not quite as much, but still four plus inches in some spots, four to six for

04:39some in the European model as well.

04:42So huge dangers coming, you can imagine, in areas near the burn scars and downhill, there

04:47will be significant washouts and debris flows.

04:49So that is your forecast feed, Big Concerns in Southern California.

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