07/1247 PM.
Night through morning low clouds and patchy dense fog will be common over coastal areas and the coastal valleys through the weekend. Over the interior areas, hazardous heat will continue through this weekend and expand into the coastal valleys, with near to above normal temperatures around the beaches. Other than a small chance for mountain showers during the afternoon and evening hours through Sunday, dry conditions will prevail.
(Fri-Mon), 07/752 PM.
***UPDATE***
Generally fair weather covered the forecast area early this evening. However, the Ridge Fire, which broke out near the Grapevine area of Interstate 5 around mid afternoon has caused a large amount of smoke in that area and and Air Quality Alert was issued by the South Coast AQMD.
Low clouds were starting to move back into the immediate Central Coast and edging toward the L.A. County coast early this evening. Overnight, low clouds and fog will spread to many coastal areas and move inland some as well. The marine inversion was near surface-based at VBG and only around 700 ft deep at LAX, with a warm layer around 80 deg at 4000 to 5000 ft. The rather strong inversion is expected to help contribute to the formation of at least patchy dense fog especially along the coast overnight. Otherwise and elsewhere, mostly clear skies will prevail thru the night. Warm overnight lows in the 70s will contribute to heat stress across many interior areas as well.
Current forecast is on-track and no evening updates are anticipated.
***From Previous Discussion***
As has been the case for the last several weeks, a large and wide ridge of high pressure continues to remain centered over the Four Corners region through the short term period. 500 mb heights will increase from 593 dam to 596 dam through Sunday, bringing gradual warming, with the heat likely peaking on Saturday. High temperatures will be in 95 to 108 range for the interior areas (including the lower mountains/foothills), 90 to 100 across the coastal valleys, and 75 to 90 across the coasts/inland coasts. Somewhat less confidence in the coastal area temperatures due to slight variations in the onshore flow and marine layer clouds. If the night through morning clouds become more expansive, then temperatures across the coastal area may be on the cooler end of the range (and vice versa). Heat Advisories and Extreme Heat Warnings remain in effect for the interior mountains (including the foothills) and interior valleys (such as the Antelope Valley and Cuyama Valley). Additionally for this weekend, Heat Advisories have been expanded toward (but generally not reaching) the coasts, to include coastal valleys -- as pressure gradients trend offshore (onshore gradients weakening). Even through most immediate coastal locations are not included in these advisories, residents will need to be mindful of the afternoon heat and take appropriate precautions.
Monsoonal moisture will continue to move over the area through this weekend, due to the position of the upper high over the Desert Southwest region. This flow pattern aloft will draw subtropical moisture across the area, mainly at levels above around 600 mb, and precipitable water values (PWs) of 1.0 inch to near 1.25 inches. There will be a 10-15% chance of afternoon/evening convective showers or thunderstorms through the weekend, mainly across the mountains of Los Angeles and Ventura Counties, and expanding into the Santa Barbara County Mountains this weekend. If thunderstorms do occur, there is the possibility that dry lightning may result, and may cause fire starts, adding to the fire weather concern.
The increase in monsoonal moisture will also increase humidities closer to the surface, at least for the coast and near-coastal areas. Along with the warmer sea surface temperatures, this will tend to increase heat impacts during the daytimes and keep overnight lows warmer than normal.
Onshore winds are expected to be gusty and persistent through the short term, increasing during the afternoon and evening hours across interior sections including the Antelope Valley. The strongest onshore winds are expected to be on Sunday, based on projected surface pressure gradients. The gusty winds across the interior in combination with the hot and dry conditions will produce elevated to brief critical fire weather conditions.
Low clouds and fog will continue to affect coastal areas and some of the coastal valleys during the night to morning hours through the short term. This pattern could be disrupted depending on the amount of monsoonal moisture aloft, which could slow or prevent marine layer formation. Also, some of the fog could be locally dense as the marine layer becomes increasingly shallow through the weekend.
The aforementioned ridge of high pressure will start to finally weaken over the area on Monday as a series of upper level troughs move into the Pacific Northwest down into NorCal. This will correspond to a cooling trend for the beginning of next week.
(Tue-Fri), 07/1117 AM.
500 mb heights will continue to drop to at least 592 dam through Thursday, and maybe even lower. At the same time, onshore flow will increase both to the north and the east becoming strongly onshore in both directions in the afternoons. The marine layer will respond to this and low clouds will likely begin to extend into the valleys. Look for slower than normal clearing 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 2 to 3 degrees of cooling each day through Thursday. Maximum temps will approach normals on Tuesday and then most areas will see below normal maximum temps on Wednesday and Thursday, with valley temps in the mid 80s to lower 90s.
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. By the end of next week, medium-range model guidance indicates 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 next Thursday and Friday.
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 next week in southern Santa Barbara County.
08/0001z.
At 2209Z at KLAX, the marine layer was 900 feet deep. The top of the inversion was at 1800 feet with a temperature of 27 C.
High confidence in VFR TAFs for KPRB, KBUR, KVNY, KPMD & KWJF.
Low to moderate confidence in remaining TAFs after 05Z Sat. Mid-level moisture and warm SSTs will disrupt the formation of Marine layer. This will likely result in intermittent CIGs especially for KLAX, KSMO, & KLGB. CIG heights accurate within +/- 200 ft of current forecast.
KLAX, Fair confidence in TAF. Intermittent CIGs (007-010) possible 07Z-15Z Fri. No significant east wind component expected.
KBUR, High confidence in VFR TAF. No wind issues expected.
07/748 PM.
Winds and seas are expected to remain largely below Small Craft Advisory (SCA) criteria across the coastal waters thru Wednesday night. However, northwest winds could locally gust 20-25 kt around and to the south of Point Conception each afternoon/evening thru Saturday. Local gusts may reach or exceed 21 kt nearshore Point Dume (Saturday evening) and across NW portions of PZZ670 (Sunday evening).
Dense fog may affect portions of the coastal waters each night through morning this weekend and possibly into early next week.
07/110 PM.
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 Monday and continuing through at least Wednesday.
Predicted tide heights of up to 7.5 feet are expected between about 8 and 10 PM each evening. These tides, combined with an increase in a long-period southerly swell, could result in 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 Sunday for zones 38-377-381>383. (See LAXNPWLOX). Heat Advisory in effect from 10 AM Saturday to 8 PM PDT Sunday for zones 88-342-343-348-349-351-352-356>358-367>374-548. (See LAXNPWLOX). Heat Advisory in effect until 8 PM PDT Sunday for zones 344-345-353-375-376-378>380. (See LAXNPWLOX). PZ, NONE.
