08/1257 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 early next week, dry conditions will prevail.
(tdy-Tue), 08/1255 PM.
Upper level ridge of high pressure centered near four-corners area this weekend, with peak heights of around 596 dm today across southern California. While the immediate coast is being moderated by a shallow marine layer, areas away from the immediate coast are seeing warming today. Most areas are expected to see peak temperatures today, with highs in the upper 90s to 109 across the interior (warmest in the Antelope Valley), 90 to 103 degrees in coastal valleys, and 75 to 90 degrees across the coastal plain. These temperatures are about 5 to 10 degrees above normal. Heat Advisories and Extreme Heat Warnings remain in effect through Sunday for many areas away from the immediate coast, and no changes planned at this time. A cooling trend will take shape for early next week as 500 mb heights decline some as the upper- level ridge weakens. An increase in onshore flow will bring a few degrees of cooling by Monday, with more pronounced cooling on Tuesday.
Due to the ridge positioning, the area will sit on the fringe of subtropical moisture being drawn up by the circulation around the high pressure system. Southeast flow aloft will transport marginal amounts of monsoonal moisture into the region. The latest deterministic solutions and some of the ensemble members suggest a slight bump up in cloud cover and possible convection from Sunday afternoon into Monday as weak shortwave impulse moves around the ridge moves over the region. There will be a 5-15 percent chance of mainly afternoon and evening showers and thunderstorms, highest over the LA/Ventura county mountains and Antelope Valley. The latest 12z NAM model does indicate an uptick in elevated instability with MUCAPE values in excess of 1000 J/kg. At this time, there is not much forcing with this enhanced instability. As a result, most likely scenario will be increasing mid level clouds for coastal/valley areas into adjacent coastal waters, but can't totally rule out an isolated shower or thunderstorm in these areas (about 5 percent probability). This will need to be monitored closely in case upper level wave (forcing) strengthens more than advertised. If thunderstorms do occur, there is the possibility that isolated dry lightning may cause fire starts, adding to the fire weather concern. The increase in monsoonal moisture along with the warmer than normal sea temperatures will also increase humidities allowing for warmer than normal overnight lows and add some humidity during the daytime to increase heat impacts.
Increasing onshore flow will likely bring a slightly stronger sea breeze each afternoon and evening and possibly gusty southwest winds 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, 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. 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 (similar to what we saw with the Ridge Fire yesterday).
(Wed-Sat), 08/122 PM.
Heights will continue to fall during the extended period as the ridge of high pressure weakens and shifts to the east. An upper level low pressure system is expected to develop off the coast by Wednesday or Thursday, and potentially linger into Friday. 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 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 next week in southern Santa Barbara County.
08/2339z.
At 1242Z at KLAX, the marine layer was 800 feet deep. The top of the inversion was at 2200 feet with a temperature of 30 C.
High confidence in VFR TAFs for KPRB, KBUR, KVNY, KPMD & KWJF.
Low to moderate confidence in remaining TAFs after 04Z Sun. 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 04Z-15Z Sun. IFR CIGs may return around 10/05Z. No significant east wind component expected.
KBUR, High confidence in VFR TAF. No wind issues expected.
08/120 PM.
Winds and seas are expected to remain largely below Small Craft Advisory (SCA) criteria across the coastal waters through Wednesday night. However, northwest winds locally gusting to 20-25 knots in typically windy areas around and just to the NW and S of Point Conception can be expected each afternoon and evening through Sunday.
Dense fog may affect portions of the coastal waters each night through morning this weekend and possibly into early next week.
08/114 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. 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 Sunday for zones 38-377-381>383. (See LAXNPWLOX). Heat Advisory in effect until 8 PM PDT Sunday 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.
