19/245 AM.
Night through morning marine layer clouds will be confined to the coastal areas, mostly north of Pt Conception, through much of this week. Warmer and drier conditions are expected away from the immediate coast, with the heat peaking today and Thursday, when moderate heat risk is expected in warmest locations.
(tdy-Sat), 19/739 PM.
***UPDATE***
It is the end of another hot day across Southern California. Most locations were similar if not a few degrees warmer than yesterday. 100+ degree readings were observed across Antelope, Salinas, San Fernando, Santa Clarita, and San Gabriel Valleys. Woodland hills reach 106 degrees. Expecting max temperatures to remain similar tomorrow.
Hot and dry conditions are expected to continue into next week. ECWMF guidance keeps higher H5 (3 dm difference) with offshore flow compared to the GFS which keeps it onshore. Both solutions result in above normal temperatures, but better potential for heat products with ECWMF solution.
Both solutions keep Socal in the western periphery of an upper level ridge situated over the four corners region. This would allow the influx of mid-level monsoonal moisture to continue into next week bringing a 5-15% chance of a mountain shower or thunderstorm (focused over LA/VTA counties).
For Thursday, mtn showers and thunderstorms chances are about 5% due to lack of mid-level moisture and no well defined perturbation. Friday. CAMs are in good agreement for perturbation with sufficient mid-level moisture (475mb-700mb) to arrive in LA county around midnight. This activity should move mainly over LA/VTA counties through Friday morning. The timing should dampen instability over the area and keep chances for lightning lower (5% chance max). Have increase POPs across the interior to 15% for this timeframe. If this activity materializes this may dampen afternoon convective chances especially across Lockwood valley - but with uncertainty decided to not make any changes.
Sundowners winds are beginning to pick up but are expected to remain below advisory levels. However, Gaviota or Refugio may gust up to 45 mph later this evening.
***From Previous Discussion***
Overall, 12Z models in good synoptic agreement through the short term period. At upper levels, high pressure develops over the 4-Corners area through Friday then shifts eastward on Saturday. At the surface, moderate onshore flow will persist to the east with weak northerly offshore flow.
Main issue in the short term will be the heat. With high H5 heights and thicknesses, as well as a limited marine influence, afternoon high temperatures will remain quite warm away from the coast through Thursday (mid 90s to low 100s). With the addition of warm overnight lows, widespread moderate heat risk can be expected across inland areas. So, current lineup of HEAT ADVISORIES for the mountains, LA Valleys and interior valleys of San Luis Obispo and Santa Barbara counties still look on track and will remain valid. On Friday, temperatures will cool slightly, but an uptick in relative humidity will still keep things uncomfortable. On Saturday, the humidity will decrease a bit, but temperatures will warm slightly in some areas. So, at this time, there is a chance that some of the heat advisories may need to be extended into Friday or even Saturday. People should exercise caution with any outdoor activities the next few days as above normal heat will even continue along the coast.
The secondary concern in the short term will be the potential for some monsoonal moisture. With the upper level pattern, southeast flow aloft will allow for some mid-level moisture to filter into the area on Friday. At this time, no huge influx of moisture is expected, but should be enough to warrant some slight chance POPs for showers/thunderstorms over the eastern San Gabriel Mountains and the northern Ventura county mountains Friday afternoon. If any thunderstorms do develop, they would likely be a mix of wet and dry storms. On Saturday, the moisture is forecast to decrease which means about a 5-10% chance of thunderstorms Saturday afternoon.
Other than the heat and thunderstorm chances, sensible weather concerns are minimal. The marine layer will continue to impact the Central Coast each night and morning. South of Point Conception, stratus/fog will be very limited tonight, but may increase in coverage across the immediate coastal plain Thursday night and Friday night. Winds do not appear to be much of an issue, other than locally gusty northerly winds across the western Santa Ynez range the next couple of evenings.
(Sun-Wed), 19/159 PM.
For the extended, 12Z models continue to exhibit decent synoptic agreement. Main feature of note will be the upper level high over the desert Southwest. Through the extended period, it is forecast to just meander around the 4-Corners area.
Forecast-wise for the extended period, main focus will continue to be the heat. With this continued synoptic pattern, temperatures will remain noticeably above seasonal normals through the period. The one thing to watch will be the surface pressure gradients. The GFS indicates weak to moderate onshore gradients through the period. Conversely, the ECMWF actually indicates some weak offshore gradients by the Tuesday/Wednesday time frame. If the ECMWF is correct then conditions will be warmer than currently forecast which will be watched closely.
As for winds, there will be a gradual increase in northerly winds, in the usual locations (Santa Ynez Range and I-5 Corridor), through the period as northerly offshore gradients increase. As for any easterly offshore winds, the ECWMF forecast of weak offshore gradients would result in some northeasterly winds, but nothing approaching advisory concerns.
This pattern will limit the areal coverage of marine layer stratus and fog through the period, with the best coverage likely across the Central Coast. As the upper high meanders about, there will be the continued potential for some incursions of monsoonal moisture. So, there will be a 5-10% chance of mountain showers/thunderstorms Sunday and Monday with about a 10-15% chance on Tuesday and Wednesday.
19/2325z.
At 2250Z at KLAX, the marine layer was 800 feet deep. The top of the inversion was at 3000 feet with a temperature of 29 C.
High confidence in VFR TAFs for KPMD, KWJF, KPRB, KBUR, and KVNY.
Good confidence in TAF for KSBP & KSMX, timing of flight cat changes may be off +/- 2 hours of forecast. Min cig height may be off +/- 200 feet.
Low confidence in remaining TAFs, especially 06Z-18Z. There is a chance for low clouds at KSBA (15%), KOXR (30%), KCMA (20%), KSMO (50%), KLAX (60%), and KLGB (60%). Low clouds would likely develop over LA County between 08Z-12Z, then KOXR/KCMA/KSBA possible 10Z-16Z. Lower confidence in cig heights, but likely between 003-008, except higher at KLGB. Lowest heights possible at KOXR/KCMA/KSBA.
KLAX, Low confidence in TAF 06Z-18Z, good confidence otherwise. There is a 40% chance of no low clouds tonight, but if they do occur, likely BKN005-010 arriving between 06Z-12Z. Any east wind component is expected to remain below 8 kt.
KBUR, High confidence in VFR TAF. No wind issues expected.
19/722 PM.
Moderate confidence in periods of Small Craft Advisory (SCA) level winds (20-30 knots) across the Outer Waters each afternoon and evening into next week, with lowest chances Thursday (40-60% chance of SCAs to be issued). Local gusts of 20-25 kt will be possible across the nearshore waters along the Central Coast through tomorrow, then a moderate chance for more widespread wind gusts reaching 25 knots Friday-Sunday.
Aside from gusts of 20-25 knots reaching the western half of the Santa Barbara Channel this evening, and again each afternoon/evening over the weekend, widespread SCA conditions are not likely across the nearshore waters south of Point Conception. Localized gusts near 20 knots are possible near Point Dume and the San Pedro Channel at times.
Ca, Heat Advisory in effect until 8 PM PDT Thursday for zones 38-88-343>345-351-353-371>373-375-376-379-380-548. (See LAXNPWLOX). PZ, Small Craft Advisory in effect until 3 AM PDT Thursday for zones 670-673-676. (See LAXMWWLOX).
