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), 19/159 PM.
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.
19/1717z.
At 1702Z at KLAX, the marine layer was 1000 feet deep. The top of the inversion was at 3500 feet with a temperature of 26 C.
High confidence in VFR TAFs for KPMD, KWJF, KPRB, KBUR, and KVNY.
Good confidence in TAF for KSBP & KSMX, timing of arrival of marine layer clouds could be +/- 2 hours of forecast. Min cig height may be off +/- 200 feet.
Low confidence in remaining TAFs after 06Z. There is a chance for low clouds at KSBA (20%), KOXR (30%), KCMA (30%), KSMO (50%), KLAX (60%), and KLGB (60%). Low clouds would likely develop over LA County between 06Z-12Z, then KOXR/KCMA/KSBA possible after 10Z. Lower confidence in cig heights, but likely between 004-008, except higher at KLGB.
KLAX, Low confidence in TAF after 06Z. There is a 40 percent chance of no low clouds tonight, but if they do occur, likely BKN006-010 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.
19/207 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, 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 afternoon, and again over the weekend, widespread SCA conditions are not likely across the nearshore waters south of Point Conception. Very 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).
