UCLA Weather

from the Department of Atmospheric and Oceanic Sciences.

Tuesday - 7:53am

f

74°

High - 74°

Low - 66°

Today:

Five Day Forecast

Latest Average Hourly Temperatures

NOTE: OCCASIONAL SPURIOUS TEMPERATURE/HUMIDITY SENSOR READINGS RETURNED ON 24 AUGUST. SINCE THE CAUSE IS STILL A MYSTERY (CLEANING APPEARS TO ONLY TO PROVIDE A TEMPORARY FIX), MORE OF THESE BOUTS ARE LIKELY TO CONTINUE FOR THE FORESEEABLE FUTURE.

A large, upper level low pressure is located a few hundred miles southwest of the Bay Area (been like that since the past weekend). It has brought mostly modest, but relevant precipitation to parts of northern California. Within a narrow frontal band, significant rainfall has occurred (from the Sacramento Valley down through western Santa Barbara County (front now dissipated). Maximum rainfall exceeded four inches at Rancho San Julian in Santa Barbara County (one to two inches more common in that part of the County). Though some minor rain did fall in western Ventura County (mostly under a tenth inch), nothing made it into L.A. County (to my knowledge). The aforementioned, low pressure helped disrupt the low cloud field in L.A. County.

All the computer model predict that the off-shore low pressure will weaken and move inland over northern California (continuing on through the northern Rockies states later in the week). Additional, scattered showers are forecast through tomorrow in that part of the state. The southern extent of the showers may reach northwest San Luis Obispo County, but additional rainfall should be under a tenth inch there. The weather effect on most of southern California, however, should be little change tomorrow from today's weather.

High pressure aloft is forecast to slowly build across most of the West once the aforementioned low pressure moves east of the Rockies. It's expected to reside in the Southwest well into next week (probably the the entire week). Most numerical, model forecasts don't show high pressure getting as strong as with the record heat wave earlier this month. However, a predicted, marginal, off-shore flow will likely promote widespread 90 degree weather by Sunday or Monday (potential for triple digit heat in some valley locales). At this point, beach temperatures aren't expected to exceed the mid-80s (might stay several degrees lower there if on-shore flow toward low desert can be maintained as some model forecasts show). In addition, the upcoming heat should be less so in the evening and overnight hours. Less predicted, surface, water vapor and lengthening night hours should contribute to less warmth in the evening hours (relative to what occurred in the first days of the month).

The predicted heat spell isn't unusual for early autumn (astronomically speaking, fall begins at 6:03 PM this Thursday). However, the predicted high pressure is expected to pull some moisture aloft from Arizona into southeast California (mostly staying around the Colorado River Valley). That's "monsoon" moisture that's currently being advected northward from Mexico into Arizona by the California, off-shore low pressure. So, we may see variable mid/high clouds over parts of the Southland by the weekend. At this point, none of the models show sufficient moisture/instability for a risk of mountain/desert showers or thunderstorms. Such a scenario is very rare by this time of year (even the current wet weather in Arizona is a little unusual for late September).

The upcoming hot weather is expected to be more humid than the typical, autumn, off-shore flow event. It, however, will be far less muggy than with the almost Florida like weather of earlier this month. The air mass just won't possess the bone dry quality of genuine Santa Ana wind events that are bound to occur later in the autumn. Also, if a diluted marine layer persists by the coast (even a very shallow one), a little muggy feeling is likely to occur (assumes air temperature rises into the 80s).

Most of the longer range models show only slow weakening of high pressure in the Southwest. A cooling trend should occur sometime in the second half of next week. Based on today's model consensus, it may take until after the subsequent weekend (beginning of October) before temperatures return to seasonable or slightly below normal levels.

Next issued forecast/synopsis should be on Tuesday, 27 September (back to once a week, issued synopses...for the time being).

UCLA Forecasts and analyses are written by James Murakami

– posted: 2022-09-23

Data and technology is provided by UCLA's Department of Atmospheric and Oceanic Sciences

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