Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
524 AM PDT Sun Sep 4 2022

SYNOPSIS...03/221 PM.

A prolonged period of very hot conditions with minimal coastal clouds is expected as high pressure aloft remains anchored over the West.  Triple digit heat will be common for many valley and mountain locations through the middle of next week.  Record breaking heat will produce a very high risk of heat illness.  There is also a slight chance of afternoon and evening mountain and desert thunderstorms through Monday.  Cooler conditions are likely at the coast Tuesday and everywhere by Friday.

SHORT TERM (TDY-TUE)...04/352 AM.

It has been quite an active morning over the coastal waters adjacent to Orange and San Diego Counties with several clusters of weak storms moving out to sea.  This activity is associated with Post-Tropical Cyclone Javier hanging around 450 NM south of Los Angeles.  There were a few lightning flashes detected from satellite lightning networks near Catalina Island but storms quickly fell apart.  There will be a slight chance of storms lingering through the morning over the waters south of the Channel Islands and there is a strong likelihood these storms will avoid land areas.

To no one`s surprise, heat continues to be the main story through much of this week.  Coastal areas will see the warmest temperatures of the week today with weak offshore flow in the morning helping to increase the heating.  Highly populated cities along the coast, such as Los Angeles and Long Beach, may reach 100 degrees and even 90s are likely within a few miles of the coast.  It will be dangerously hot in the valleys with temperatures above 105 highly likely. There will be a lot of record highs in the territory of being broken, mainly over the valleys.  There is also the threat of additional monsoon convection this afternoon and evening.  Precipitable water values are expected to climb to between 1.5 and 1.8 inches across the forecast area, and 850 mb dewpoints to around 15 degrees Celsius. 06z NAM model continues to show impressive instability values with lifted index values in the -6 to -8 range across the local mountains and surface based CAPE values of 1500 to 2000 J/kg.  With model soundings showing a much wetter profile, heavy rain and associated flooding will be a greater concern for thunderstorms today.  At this point, will hold off on a Flash Flood Watch as steering flow appears to be too strong (in the 25 to 35 knot range), so storms should be moving fairly rapidly.  But there is some concern for training storms which could therefore increase the flash flood threat if that materializes.  With such significant CAPE values being projected for this afternoon, will definitely have a concern for isolated stronger thunderstorms with frequent lightning, larger hail, and strong wind gusts.  Also of note is that thunderstorm threat is expected to be much greater in areal coverage, with a slight chance of thunderstorms for the mountains and valleys of LA, Ventura, and Santa Barbara counties, as well as the Antelope Valley.