Dangerous flooding possible in Southern California this weekend

AccuWeather

Several inches of rain in the Los Angeles area in mid-November could cause major problems, with evacuations for burn scar areas already issued days in advance.

Transcript

00:00It's feeding time and in the forecast feed tonight we're very concerned about

00:05serious flooding, dangerous flooding in the second of two waves of moisture

00:11entering California. This weekend southern California we're going to see multiple

00:15inches of rain and that spells major trouble. Let's take a quick look at

00:19what's going on here with our graphics then we're going to dig into the the

00:22models here for a moment. And you can see the satellite loop here as we zoom in

00:26you can see moisture streaming steadily in the southern California but the

00:31initial thrust of moisture is actually aimed more into other sections of the

00:35west with the first batch and again this is going to be rolling in fairly swiftly

00:40as we move on into the day tomorrow. So late tonight through Thursday we're

00:44getting into it but the parent low pressure system is still off to the west so

00:49as we stream this moisture inward you can see this feed of tropical moisture that

00:54will be pretty relentless especially as this dip in the jet stream it gets a

00:59little more amplified and it's going to wander over the southwestern US. So we

01:05want to talk about the topography of southern California here in the major

01:09mountains there is no coastal plain here and the LA basin is at least on a very

01:15localized scale flat but you get just barely north and west northwest or east

01:20even southeast of there you get up into some very impressive mountains here and

01:24the San Gabriel's and many others some of these regional ranges there are areas up

01:28into Ventura County where there is mountain there's mountain coverage right up to

01:34the coastline so the southwest facing slopes are going to get hit very hard with

01:39significant amounts of moisture and it's all about upslope flow and then when we

01:45talk about atmospheric rivers these stripes of abundant moisture like a highway of

01:50moisture in the atmosphere they're going to be pushed up against the west facing

01:53slopes of the mountains you get the coastal range then you have the Sierra and

01:56cold air can contain less water vapor so if you take air that's pretty humid and

02:02you run it upstairs you're going to get colder up there it's colder aloft and

02:06that's going to lead to tremendous amounts of condensation cloud production rain

02:11production and snow production in a major way so quick look at the first wave

02:15we're looking at one to two inches of rain in San Francisco and Los Angeles and

02:19then some of the other areas just north of Santa Barbara two to four inches of

02:23rain that's the first wave there will be some snow that falls into the mountains

02:27initially it's pretty warm out there snow levels will eventually drop down but

02:31again areas like Donner Pass along Interstate 80 and also Echo Summit again

02:36just a little bit of snow there a couple inches two three four five inches

02:40possible at the back end of this storm into Echo and maybe just two to four for

02:43Donner it's mainly a rain event there but the bigger story becomes the weekend and

02:47this is what I really want to focus on with the models here for the next minute

02:51and a half a significant flood risk so on top of the one to two inches of rain in

02:54Los Angeles late this work week we get into another event Friday night Saturday and

03:00into Sunday morning two to four inches of rain in LA some spots four to eight

03:06inches into the northern fringe of the LA basin up into the mountain so let's take a

03:10look at this here we have our big dip in the jet stream there it is through

03:14Thursday and we applauded that low just to kind of make it look a little more

03:18familiar similar to what we did there with our own forecast graphics and as we

03:22take a look at this moving forward in time the core this upper level low is

03:27going to pivot right through by Saturday night the southwest side of California's

03:32coast so this is going to lead to tremendous onshore flow and that's going to be

03:37spelling big trouble so as we take a look about ten thousand feet up into the

03:42atmosphere 700 millibars here you can see I know it's a chaotic map just going to

03:46highlight the state of California here just so you can see with a little more

03:49clarity there's California and LA down this way we're looking at this wobbling

03:55zone of low pressure by Saturday midday it's going to be centered over an area

04:01basically due west of maybe Pismo Beach or maybe even just west of Santa Barbara and

04:06we're going to see tremendous amounts of onshore upslope flow remember I drew the

04:11perspective we highlighted the mountains there where we're going to see a huge lift in the

04:16atmosphere and just a quick look at a couple of model projections here through Friday night

04:21Saturday and now into Sunday and Sunday night Monday you can see the GFS model is producing

04:26basically two to four inches of rain in many of these areas but locally you can see a couple

04:32of hot spots here with 3.9 but if you get up into the mountains away from cities these labels

04:37are plotted over towns there are areas that would see five plus inches and in a second opinion the

04:42European model big numbers here if you look at both storm systems together upwards of five to six

04:49inches from near Los Angeles up into areas around Pismo Beach so big trouble is coming to sections of

04:56Southern California dangerous flooding even evacuations already issued in some places near and just downhill

05:03from some recent burn scar areas that is your forecast feed

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