09/205 AM.
Night through morning low clouds and patchy dense fog will affect some coastal areas and coastal valleys through the weekend. Away from the coast, hazardous heat will continue into this evening, with near to above normal temperatures around the beaches. Starting Monday, a gradual cooling trend will last through the week, with near to below normal temperatures by late week. Other than a small chance of mountain showers during the afternoon and evening hours through early next week, dry conditions will prevail.
(tdy-Tue), 09/319 AM.
A strong ridge of high pressure aloft remains centered near the four corners area, with high pressure extending westward across southern California. Despite the high pressure system remaining in place, it will start to weaken, and 500 mb heights over the region will start to fall from here on out through the forecast period. While 500 mb heights will fall to around 593-594 dam today, compared to the peak of 596 dam yesterday, and a little bit of cooling will be possible. However, moderate to high heat risk will still impact the area. High temperatures today for areas away from the coast will be in the low 90s to over 100 degrees, and in the warmest locations, readings were up to 108 degrees will be possible (warmest over the Antelope Valley, northern Ventura County Mountains, and Cuyama Valley, where Extreme Heat Warnings remain in place through this evening). Along the immediate coast, the marine layer will keep the high temperatures a little cooler, in the 70s and 80s, although more humid conditions persist there due to warm ocean temperatures. Highs in most areas have been running about 5 to 10 degrees above normal. Heat Advisories and Extreme Heat Warnings remain in effect through this evening for all areas away from the immediate coast as well as the interior coast of San Luis County.
More significant cooling is anticipated for early this week as the upper high weakens and heights diminish over the region, combined with increasing onshore flow. There will be a few degrees of cooling into Monday, and more significant cooling Tuesday with all locations likely to drop into the 90s or lower.
Meanwhile, southeast flow aloft will result in clockwise flow around the four-corners ridge aloft, effectively drawing monsoonal moisture into southwest California. While the moisture is somewhat marginal, a weak shortwave disturbance moving around the ridge will bring a slightly increased chance of convection to the area this afternoon into Monday. It may result in some increased mid to upper level cloudiness, along with a 5-15 percent chance of afternoon and evening showers and thunderstorms, highest over the LA/Ventura county mountains and Antelope Valley. There is also a lower (5%) chance of an isolated shower or thunderstorm over the coast and valley areas, extending out over the coastal waters.
If thunderstorms do occur, there is a chance of isolated dry lightning with a risk of fire starts, adding to the ongoing fire weather concerns. The increase in monsoonal moisture along with the warmer than normal sea temperatures will also increase humidities, especially for the coast and coastal valley areas. This will result in warmer overnight lows and more humid daytime conditions, adding to the heat impacts.
Increasing onshore flow will cause stronger sea breezes each day during the afternoon and evening hours across interior sections, including the Antelope Valley. The strongest onshore winds are expected to be on Sunday and Monday. The gusty winds, along with the hot and dry conditions will produce elevated to brief critical fire weather conditions. With the enhanced instability over the region, there is also an increased risk of large vertical plume growth with any new fires across the interior this weekend.
(Wed-Sat), 09/229 AM.
The ridge of high pressure will finally move eastward and weaken as an incoming trough starts to move into the West Coast on Tuesday into Wednesday. At the same time, an upper level low/disturbance will sit off the Central Coast mid-week, getting absorbed by the trough and potentially lingeringering into late Friday. At the same time, onshore flow will increase both to the north and the east becoming strongly onshore in both directions during the afternoons. The increased onshore flow will allow the marine layer to push into the valleys each night through morning. Slower than normal clearing skies will be likely as the onshore flow strengthens, and some west facing beaches may see only limited clearing each afternoon and evening. The combination of a deeper marine layer, stronger onshore flow, and falling heights will bring cooling Tuesday through Thursday. Maximum temps will largely be below normal by Wednesday and Thursday.
PWs will remain elevated through the period, however. In combination with warmer than normal ocean temperatures off the immediate coast, conditions will continue to feel a little more humid than normal. It will also keep overnight lows a little warmer than normal. The region will remain on the fringe of the monsoonal moisture through at least Wednesday, with the elevated moisture and increased instability keeping the low chance (10 to 15 percent) of afternoon mountain convection going. In the Thursday-Friday time frame, medium-range models continue to indicate the potential for an upper low to develop off the Central California Coast, east of which large-scale ascent and convective chances could modestly increase sometime between Thursday and Friday. At this time, the convective chances are still in the 5-15 percent range across the area, with the highest chances focused across the Central Coast. Latest ECMWF ensemble date showing a weak convective signal in that Thursday-Friday time frame with a slightly uptick in MUCAPE values. If convection were to develop, the main concern would be the potential for isolated dry lightning strikes with minimal rainfall, adding to the fire weather threat. This will need to be monitored closely especially considering the dry fuels in place after this prolonged heatwave.
Additionally, the strong onshore flow to the east will produce near advisory level wind gusts across the western Antelope Valley and the western foothills each afternoon and evening. Gusty sundowner winds will also be possible early this week in southern Santa Barbara County.
09/1300z.
At 1211Z at KLAX, the marine layer was 700 feet deep. The top of the inversion was at 3300 feet with a temperature of 28 C.
High confidence in VFR TAFs for KPRB, KBUR, KVNY, KPMD & KWJF.
Low to moderate confidence in remaining TAFs through 16Z Sun and after 04Z Mon, with lowest confidence for sites south of Point Conception. Mid- level moisture and warm SSTs will disrupt the formation of Marine layer clouds. There is a 40% chance that conds remain VFR at KLAX, KSMO, and KLGB through at least 03Z. Timing of flight cat changes may be off by 3 hours, and CIG heights accurate within +/- 200 ft of current forecast.
KLAX, Low confidence in TAF through 16Z Sun, then high confidence through 03Z Mon. There is a 40% chance that conds remain VFR through 03Z Mon. Intermittent CIGs (007-010) possible after 04Z Mon. No significant east wind component expected.
KBUR, High confidence in VFR TAF. No wind issues expected.
09/201 AM.
Conditions will remain below Small Craft Advisory (SCA) across the coastal waters through at least Wednesday night. The only exception is a 20% chance of SCA level NW winds across the extreme northwest portion of the outer waters north of Point Sal (PZZ676) this afternoon and evening.
Dense fog may affect portions of the coastal waters through this morning and possibly the next night through morning periods through Tuesday.
09/201 AM.
High astronomical tides are expected to affect low-lying beaches south of Point Conception with the possibility of minor coastal flooding starting with the evening high tide on Monday and continuing through at least Wednesday. As a result, a Coastal Flood Advisory has been issued.
Predicted tide heights of up to 7.5 feet are expected between about 8 and 10 PM each evening. These tides, combined with a small increase in a long-period southerly swell, could result in minor flooding of sea water around the time of high tide over low-lying coastal areas such as parking lots, beaches, and walkways. Beaches most susceptible to potential minor coastal flooding issues include those around Point Dume and near Long Beach.
Ca, Extreme Heat Warning in effect until 8 PM PDT this evening for zones 38-377-381>383. (See LAXNPWLOX). Heat Advisory in effect until 8 PM PDT this evening for zones 88-342>345-348-349-351>353-356>358-367>376-378>380-548. (See LAXNPWLOX). Coastal Flood Advisory in effect from 6 PM Monday to midnight PDT Wednesday night for zones 349-350-354-362-366. (See LAXCFWLOX). PZ, NONE.
