Swell Matrix

Area Forecast Discussion

National Weather Service Los Angeles/Oxnard Ca

1112 am PDT Tue Sep 19 2017

Synopsis

A low will hang over the area this week with overnight low clouds and fog for the coast and some valleys. Cold Canadian air will move in by Wednesday for breezy winds from Santa Barbara and northward. There may be a possibility of preciitation for Los Angeles county on Thursday with major temperature drops inland. Next week, offshore flow should bring a warming trend into midweek.

Short Term

(tdy-Thu)

Marine layer up to around 4000' south of Pt Conception this morning with some patchy drizzle around. Highs will again be several degrees below normal inland and slightly below normal at the coast. Clouds will be slow to clear and some areas may stay cloudy well into the afternoon.

Up north the northwest flow and cool air at lower levels has weakened the inversion and dissipated most of the low clouds. Gusty northwest winds expected today along the Central Coast will keep temps near to slightly below normal in most areas. Further inland the morning started with some low clouds pushing in from the San Joaquin Valley but those should dissipate by late morning with highs 4-8 degrees below normal.

***From previous discussion***

The Axis of the upper low pinwheels to the south and east Wednesday night and Thursday and drags a weak and dry cold front through the area. The lift from the trof along with eddy will keep low clouds across the coasts and vlys of VTA and LA counties. By morning the slight energy imparted by the front should be enough to bring a slight chc of showers across srn LA county, esp the San Gabriel Vly. The clouds should dissipate during the afternoon except for th SAN Gabriel Vly where clouds and a slight chc of showers will persist. Hgts will fall to 565 DM which is about 20 DM below normal. Thursday will be the coolest day of the next 7 with vly temps 10 to 15 degrees below normal. (only 2 to 5 degrees blo normal for the coasts)

The West Coast trough deepens quite a bit Wednesday night into Thursday. With the cooler air aloft the marine layer should deepen quite a bit Thursday morning and it looks like a good drizzle/light rain scenario for areas south of Pt Conception, especially up against the south facing slopes. Probably another reverse clearing day as well and highs a good 10-15 degrees below normal for inland areas.

Long Term

(Fri-Mon)

Not much change Friday. The cold air will have wiped out most of the marine clouds everywhere except for southern LA county. Hgts will rise slightly and the strong onshore flow will relax a little so look for 1 to 2 degree of warming.

On Saturday there will still be some morning low clouds across southern LA county but there will be Sunny skies everywhere else. Hgts will continue to rise and there will be offshore trends. Almost all areas will see 2 to 4 degrees of warming. Max temps will still be below normal but only by a few degrees.

Weak offshore flow will set up Sunday and Monday (and likely a few days past that) it will keep the skies clear and will kick off a decent warming trend. There does not look like there is enough upper or thermal support for a Santa Ana event just enough to warm things up and dry things out. Max temps will climb each day and the csts and vlys will see above normal temps with plenty of upper 80s and lower 90s in the vlys by Monday.

Aviation

19/1800z.

At 1714z at KLAX, the depth of the marine layer was around 3100 ft. The top of the marine layer was around 3700 ft with a temperature of about 19 degrees Celsius.

Low to moderate confidence in the current coastal and adjacent valley TAFs and high confidence in the remaining TAFs. The primary short term concern will be the clearing times for the coastal and San Fernando Valley TAF sites. The latest forecast is more pessimistic as the latest satellite pictures show very slow clearing trends and the clearing times have all been pushed back later in the afternoon. The winds will be strong and gusty in the Antelope Valley this afternoon and forecast winds have been increased some.

KLAX and KBUR, Low to moderate confidence in the 18Z TAF. There is a 30 percent chance that no clearing will occur this afternoon.

Marine

19/900 AM

Northwest winds over the outer waters will be at Small Craft Advisory (SCA) levels early today then increase to Gale Warnings, which are posted from this afternoon through late Wednesday night. The strong winds will likely last into Thursday but may decrease slightly to a strong SCA level.

SCA level winds will develop across the northern inner waters this afternoon through late Wednesday night and may remain strong through Friday. Western portions of the Santa Barbara Channel will have SCA level winds through late tonight and possibly into Wednesday. Winds in the inner waters will be strongest during the late afternoon to evening hours.

Combined seas will increase through Wednesday as building short period waves combine with the first northwest swell of the season. Seas will grow to 6 to 8 feet at 15 seconds today and to 7 to 10 feet at 14 seconds on Wednesday.

Watches, Warnings, Advisories

Ca, NONE. PZ, Small Craft Advisory in effect from 3 PM this afternoon to 3 AM PDT Thursday for zone 645. (See LAXMWWLOX). Small Craft Advisory in effect from 3 PM this afternoon to 3 AM PDT Wednesday for zone 650. (See LAXMWWLOX). Small Craft Advisory in effect until 3 PM PDT this afternoon for zones 670-673-676. (See LAXMWWLOX). Gale Warning in effect from 3 PM this afternoon to 3 AM PDT Thursday for zones 670-673-676. (See LAXMWWLOX).

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