Swell Matrix

Area Forecast Discussion

National Weather Service Los Angeles/Oxnard Ca

810 pm PDT Mon may 25 2020

Synopsis

25/732 PM.

High pressure will bring mostly clear skies and hot temperatures over the next few days. Interior areas will be hottest with Antelope Valley reaching the low 100s. The coast will stay cooler. Areas of overnight to morning clouds are expected mainly for coastal regions. The heat will diminish slightly on Thursday, then becoming near normal by Sunday.

Short Term

(Mon-Thu), 25/808 PM.

***UPDATE***

Overall, a quiet evening across the district. Latest satellite and surface observations indicate clear skies except for some stratus over the coastal waters south of Point Conception. Latest sounding data indicates strong, but shallow, marine inversion with a depth around 600 feet.

For the immediate short term, main forecast issue will be the extent of the coastal stratus/fog. Overnight, weak onshore gradients will persist with a weak eddy circulation likely over the coastal waters. So, stratus/fog should return to the coastal plain, especially south of Point Conception. However, not too confident in how widespread the stratus/fog will be. Current forecast indicates patchy/areas wording for coastal plain which sounds good at this time. With the shallow inversion, dense fog will be a concern overnight. Depending on how widespread stratus formation is, Dense Fog Advisories may be needed. However will let the night shift monitor that potential.

For the evening, no major updates expected for the forecast. Did issue an update earlier to remove the Heat Advisory headline from the Central Coast as it expired at 700 PM. Otherwise, current forecast package has good handle on the immediate short term.

***From Previous Discussion***

No major changes to the forecast this afternoon. It's a tale of two regimes. The hot temps inland will get the headlines but at the coast it will be much cooler with some dense fog at times that could impact air travel. The marine inversion remains around 1500' based on the latest Simi profiler but that slopes down to around 300' or less near Pt Conception. An eddy circulation will continue off the LA/Orange County coast the next few days but high pressure aloft will force warmer air towards the surface and squash the marine layer under 1000' in LA County, likely by Tuesday morning. Uncertain how widespread clouds will be but based on coverage this morning clouds will likely be fairly solid south of Pt Conception. Low clouds did manage to sneak north of Pt Conception just barely today and will probably see this again tomorrow and possibly as far north as srn SLO County as the northeast flow there drops off. Temps will likely cool slightly there too for the same reason.

Temperature adjustments were minimal and mainly to account for the cooler temps right at the beaches due to the expanded low cloud coverage but inland temps were left alone or raised slightly. Still looks like most lower elevation locations, including the valleys, will see decent overnight relief as temps drop into the upper 50s to mid 60s in many areas so that's a significant reason why heat advisories so far have not been issued for the inland areas aside from the Antelope Valley.

Very little change Wednesday from Tuesday, though onshore gradients will be increasing, possibly leading to some gusty southwest winds in and around the AV. Continued low humidities and decreasing fuel moistures there will present some increased risk for enhanced fire behavior but not for long enough duration to consider any sort of red flag issuances.

Temps should start trending down for the coast and coastal valleys Thu with the increasing onshore flow but still 10-15 degrees above normal.

Long Term

(Fri-Mon), 25/142 PM.

Models continue to show differences with respect to when or even if more significant cooling will begin. While both models do shift the ridge axis to the east by the weekend the EC is much weaker with the trough west of the ridge and thus is slower to cool things off. Have added some additional EC (warmer) influence into the temperature forecasts through the weekend to factor in the possibility of less cooling and to match up better with neighbor forecasts. Will see how this plays out over the next few days but blocking patterns like this tend to last longer than initially expected. With increasing onshore flow a pretty fair bet expect at least coastal areas will be cooler with night and morning low clouds and fog.

The GFS is deeper with the approaching trough and actually has some moisture with it, enough that a few of its ensemble members actually have some light precip across nrn SLO County Saturday as it clips the area. At the very least we'd see some increasing mid/high clouds as it passes to the north but again still too much disagreement in the models to have much faith in either solution yet.

Aviation

25/2229z.

At 2215Z, the marine inversion at KLAX was based at 800 feet. The top of the inversion was 2600 feet with a temperature of 26 degrees Celsius.

Overall, low to moderate confidence in 00Z coastal TAFs and high confidence in valley/desert TAFs. Shallow, but strong, inversion will persist overnight. With onshore flow continuing, stratus/fog is possible at coastal sites overnight with LIFR/VLIFR conditions likely, but only low confidence in timing of arrival and dissipation.

KLAX, Low confidence in 00Z TAF. There is a 30% chance that VFR will persist overnight. If stratus does develop, high confidence in LIFR/VLIFR conditions, but low confidence in timing (could be +/- 4 hours of current 11Z forecast). No significant easterly wind component is expected.

KBUR, High confidence in 00Z TAF as CAVU conditions are anticipate through TAF period.

Marine

25/808 PM.

For all the coastal waters, high confidence in current forecast. Tonight through Saturday, winds and seas are expected to remain below Small Craft Advisory (SCA) levels.

Patchy dense fog with visibility one nautical mile or less at times is likely late tonight through Tuesday morning, mainly for the waters south of Point Conception. A Marine Weather Statement will be in effect from tonight through noon Tuesday for these waters. Mariners should be prepared to reduce speeds and use GPS navigation if available.

Watches, Warnings, Advisories

Ca, Heat Advisory in effect until 7 PM PDT Thursday for zone 37. (See LAXNPWLOX). Excessive Heat Warning in effect from 11 AM Tuesday to 7 PM PDT Thursday for zone 59. (See LAXNPWLOX). PZ, NONE.

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