Swell Matrix

Area Forecast Discussion

National Weather Service Los Angeles/Oxnard Ca

919 pm PDT Wed aug 12 2020

Synopsis

12/900 PM.

There is a slight chance of light showers and afternoon thunderstorms Thursday as moisture streams into the region from the south. Temperatures will rise Thursday, becoming hot Friday especially for valley and interior areas. The heat is expected to last into early next week. Night and morning low clouds and fog will be minimal, and confined near the coast.

Short Term

(Wed-Sat), 12/918 PM.

An upper-level ridge of high pressure is centered over the Borderland region of the Desert Southwest this evening, while a surge of middle and high level moisture pushes north from post- tropical storm Elida. High pressure aloft brought warm temperatures away from the coast today and a warm air mass is slated to remain in place through the weekend. Although Thursday could be cooler as far as temperatures due to increased cloudiness, it will be more humid and could push heat index higher in some areas. The heat risk will increase on Friday as the moisture move away from the area and the high pressure builds west into Arizona.

The cloud mass, seen well on infrared imagery this evening, will continue to push north this evening as Elida continues to weaken. The model solutions show the bulk of this moisture moving along the Central Coast and scraping Los Angeles and Ventura Counties Thursday, but linear satellite trends bear watching overnight tonight. PoPs has been nudged higher for late tonight and into Thursday across the region. With the instability from Elida moving across the area tomorrow, convection cannot be ruled out on Thursday afternoon and evening, especially near the Lake Fire. Plume-dominated behavior could develop again on the fire as instability increasing mixing heights, similar to today.

An update will issued shortly.

***From Previous Discussion***

Friday will see quite a dramatic warmup with widespread 100 plus readings quite certain over inland areas. Still some uncertainty in the models, both deterministic and ensemble versions although even the low end of the spectrum is enough for potential heat risk concerns. Have upgraded the watch to a warning starting on Friday for all but the coastal areas, which should have enough of a sea breeze with just enough onshore gradient to keep it cooler. The one exception will be the LA coast whose temperatures will be influenced by a weak northerly pressure gradient. Went ahead and put a heat advisory for this zone, but some conditions on the eastern side of the zone up against the valleys will likely have a higher heat risk. Have left the coastal zones of VTU, SBA, and SLO counties out of the heat products at this time, although temps away from the beaches will still be 3 to 6 degrees above normal. The models that were indicating potential convection on Friday have backed off a little. Not much of any instability forecast in the NAM, which yesterday had quite a bit for Friday. Have trimmed chances back quite a bit and just left some slight chances in a small part of the VTU and SBA county mountains. Wouldn't be surprise to see some cumulus building up most mountains, however, with just enough mid level moisture around.

Saturday has been consistently shown as the hottest day of the period, even though Friday will feel quite hot as well. There appears to be better agreement among all the models on high temps, which are forceast to reach the 105 to 109 range in the hottest locations. Overnight lows will also be very warm including not dropping below 80 in some foothill locations of LA county.

Long Term

(Sun-Wed), 12/237 PM.

Models have had a tough time with Sunday temperatures, but the overall trend will be for high temperatures to be a few degrees cooler for coastal and valley areas, but jumping up a degree or two further inland. Temperatures are still very warm away from the coast and in the valleys where the warmest temperatures will still be in the low 100s, especially the LA county valleys. Additional cooling for Monday, but kept the excessive heat warning going through Monday due to temperatures continuing to be unusually warm. However, there should be enough cooling over the LA county coast to end the heat advisory Sunday evening. That may need to be extended if high temperatures are adjusted with later forecasts. The one area that will continue to be very hot will be the Antelope Valley, which will also continue to be very hot on Tuesday. Thus their heat warning goes into Tuesday.

High pressure remains well entrenched in the southwest into Tuesday and beyond into at least the rest of the week to keep temperatures above normal across all areas. Will need to keep an eye on the high pushing any monsoon moisture this direction from the east. For now nothing is obvious but the position of the high is such that it typically favors that scenario.

Aviation

13/0359z.

At 03Z, there was a surface-based inversion up to around 2800 feet with a temperature around 29 degrees Celsius.

There is a chance of LIFR to IFR conditions at coastal terminals and a chance of IFR to MVFR conditions in smoke for desert terminals. Otherwise, VFR conditions will likely persist.

KLAX, There is a 30 percent chance for IFR conditions between 10Z and 16Z.

KBUR, VFR conditions are expected through the period.

Marine

12/901 PM.

Winds and seas will likely remain below Small Craft Advisory (SCA) levels through Friday across the waters. There is a 20-30 percent chance of SCA level gusts during the afternoon and evening hours each day from Point Conception to San Nicolas Island. There is a better chance of winds increasing from Point Conception to San Nicolas Island Friday afternoon and Saturday with a 50 percent chance confidence level for SCA level winds.

Patchy fog, locally dense with visibility under one mile, will likely continue through mid-week across all the waters. Moisture from the remnants of Tropical Storm Elida will move across the coastal waters, bringing a slight chance of rain Thursday morning.

Watches, Warnings, Advisories

Ca, Heat Advisory in effect from 11 AM Friday to 9 PM PDT Sunday for zones 36-41. (See LAXNPWLOX). Excessive Heat Warning in effect from 11 AM Friday to 9 PM PDT Monday for zones 37-38-44>46-51>54-88-547. (See LAXNPWLOX). Excessive Heat Warning in effect from 11 AM Friday to 9 PM PDT Tuesday for zone 59. (See LAXNPWLOX). PZ, NONE.

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