Swell Matrix

Area Forecast Discussion

National Weather Service Los Angeles/Oxnard Ca

526 pm PDT Sat jul 20 2019

Synopsis

20/235 PM.

Night through morning low clouds are expected over the coast tonight into Tuesday. A strong area of high pressure will move into the region Sunday yielding much warmer temperatures over inland areas through the end of the week. There will also be a slight chance of afternoon thunderstorms over the mountains and deserts Monday and Tuesday.

Short Term

(tdy-Tue), 20/228 PM.

The marine inversion early this afternoon was around 1400 ft deep at LAX. Low clouds had cleared from most coastal areas early this afternoon, with sunny skies prevailing across most of the region. Little change can be expected for the rest of the day. Good onshore gradients (NAM fcst +8.9 mb LAX-DAG at 00Z) will contribute to some gusty S to W winds mainly for the foothills, mtns and deserts into early this evening. Temps will be several degrees below normal for most areas this afternoon. Highs in the warmest vlys and lower mtns will reach the 80s overall, except lower to mid 90s in the Antelope Vly.

A large upper level high over the central plains today will expand into the western U.S. and form a strong upper level high over the four corners region by late Sun. The center of this upper high will move slightly north to the UT/CO border for Mon and Tue. The western portion of the upper high will build into the forecast area thru Mon then persist over the region thru Tue. 500 mb heights will increase from 587-590 dm today to 591-593 dm by Mon then be around 593 dm on Tue. The upper level flow will be from the SW through this evening then turn more S to SE Sun thru Tue. This flow pattern should draw some monsoonal moisture into srn CA by early next week.

The upper level ridging will help to lower the marine inversion down to about 1500 ft tonight, then further to below 1000 ft Sun night thru Tue. Night and morning low clouds and fog will affect mainly the coastal plain and locally into the vlys tonight into Sun morning, then just the coastal plain Sun night into Mon morning. Even less coverage is expected Mon night into Tue morning, with just the immediate Central Coast and southern coast of L.A. county expected. Otherwise, mostly clear skies can be expected thru Sun night, with some mid and hi level clouds moving into the area Mon thru Tue for partly cloudy skies at times.

The 12Z NAM was indicating increased instability and moisture moving into to the area on Mon. LI's are forecast to drop to about -1 to -2 over the mtns and deserts Mon afternoon, and to as low as -3.5 in the eastern San Gabriel Mtns. MUCAPE values drop to as low as 300- 600 J/kg Mon afternoon in these areas as well, and PWAT values increase to near 1.35 inches. 850 mb dewpoints are also expected to increase, and are forecast to be in the 6-7 deg C range Mon afternoon. With the moisture in place combined with instability and strong heating, there will be a slight chance of afternoon and early evening showers and thunderstorms in the L.A./VTU/SBA County mtns and Antelope Vly Mon afternoon into early evening. There will be some residual moisture and instability over the area on Tue as well, with a slight chance of an afternoon thunderstorm expected for portions of the L.A./VTU County mtns and Antelope Vly. Any thunderstorms that develop could produce heavy rain, gusty winds and small hail.

Although onshore gradients should diminish slightly for Mon and Tue, there will still be enough of an onshore push in the afternoon and early evening for locally gusty S to W winds in the foothills, mtns and deserts.

Temps will to warm to near normal Sun, near normal to several degrees above normal Mon, and several degrees above normal for most areas on Tue. The warmest vlys and lower mtns are expected to reach into the upper 80s to mid 90s Sun, 90s to near 100 on Mon, and mid 90s to about 102 on Tue. The combination of heat and some higher humidities may eventually bring the need for a Heat Advisory for the vlys and mtns for Tue.

Long Term

(Wed-Sat), 20/230 PM.

The EC and GFS are in generally good agreement with the strong upper level high drifting S to the four corners region by Wed, then building slowly W and into srn CA by Sat. There may be some monsoonal moisture over eastern parts of the forecast area on Wed, then the moisture should remain just E of L.A. County Thu and well E of the region Fri and Sat. There will be a less than 15 percent chance of a shower of thunderstorm Wed afternoon over the eastern San Gabriels, but POPs are too low to mention in the fcst.

The marine inversion is expected to be very shallow and probably down to 400 to 600 feet deep during the period. Varying amounts of low clouds and fog are still expected for the immediate Central Coast and southern L.A. County coast each night and morning. Otherwise, some mid and hi clouds should bring partly cloudy skies at times to the area thru about Thu, then mostly clear skies are expected Fri thru Sat.

Temps will peak on Wed with highs well above normal away from the coast, then cool slightly each day thru Sat. Highs in the warmest vlys and lower mtns should be in the upper 90s to 105 Wed, 90s to around 100 Thu, 90s Fri, and upper 80s to mid 90s Sat. The Antelope Vly should be in the low 100s each day for the most part. A Heat Advisory may be needed for Wed, with some vly and mtn areas possibly approaching excessive heat criteria.

Aviation

21/0025z.

At 2230Z, the marine layer depth was around 1550 feet deep at KLAX. The top of the inversion was around 3000 feet with a temperature near 22 degrees Celsius.

Low-to-moderate confidence in the current forecast. Higher confidence in timing, less confidence in flight categories.

North of Point Conception, Predominantly, LIFR to IFR conditions will likely spread in through 13Z. Less confidence in exists in sub-VFR conditions for KPRB. There is a 20 percent chance of VLIFR conditions between 10Z and 14Z. VFR conditions will develop as soon as 16Z or as late as 18Z.

South of Point Conception, IFR to MVFR conditions will likely spread into coastal and lower valley terminals through 13Z. There is a chance of IFR conditions for valley terminals between 10Z and 15Z. VFR conditions could develop up to one hour later on Sunday.

KLAX, MVFR conditions will likely spread into KLAX as soon as 06Z or as late as 10Z. VFR conditions should develop as soon as 17Z or as late as 19Z.

KBUR, There is a 30 percent chance of IFR conditions and a 50 percent chance of MVFR conditions between 10Z and 15Z. Otherwise, VFR conditions will persist.

Marine

20/155 PM.

For the Outer Waters, Conditions will remain below Small Craft Advisory (SCA) level through at least Monday. There is a sixty percent chance of SCA level winds across the northern and central outer waters on Tuesday through Thursday and a thirty five percent chance for the southern outer waters.

For the Inner and Nearshore Waters, Conditions will remain below SCA level through at least Thursday.

Watches, Warnings, Advisories

Ca, NONE. PZ, NONE.

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