11/103 PM.
Several degrees of cooling are expected Tuesday and Wednesday as an upper low moves through northern California. Low clouds and fog will move into the coastal valleys then clearing to near the coast by afternoon. Gusty winds are possible at times across the mountains and deserts. Slight warming is expected Thursday and Friday.
(Mon-Thu), 11/842 PM.
High pressure aloft and a slight reduction of the marine layer brought some warming today across interior areas as expected, with many areas climbing well into the 90s. The onshore flow kept a marine influence and cooler temperatures near the coast. For tonight, a shallow and strong marine inversion remains in place and starting to see some visibilities dropping across portions of the coast. In evening update, have added patchy dense fog for tonight and some areas may require a dense fog advisory. By Tuesday morning, cross sections show some deepening of the marine layer, with low clouds and fog likely extending into some of the coastal valleys. Looking for a slow to no clearing day on Tuesday afternoon for portions of the coast.
High pressure will start to weaken and shift east Tuesday, with the upper level flow turning more southeasterly. The lowering heights and much stronger onshore pressure gradients will bring 5 to 10 degrees of cooling across much of the forecast area on Tuesday, except only a few degrees of cooling across the Antelope Valley where highs will still climb into the 90s. In fact, latest forecast pressure gradients showing the potential for LAX-Daggett and LAX-Bakersfield gradients to exceed +9 mb which is rather significant for this time of year. Looking at historical pressure gradient percentile information, a +9 LAX- Bakersfield gradient would be near all-time highs for the month of May, and LAX-Daggett would exceed the 97th percentile. With this in mind, looking for a significant increase in coverage and strength of onshore winds across the interior on Tuesday afternoon/evening, with gusts mostly ranging in the 30 to 45 mph range. In evening update, have included a Wind Advisory for Tuesday afternoon/evening for the Antelope Valley and adjacent western foothills where gusts up to 45 mph are expected, and could locally reach 50 mph in a few of the favored foothill locations. This will likely generate some blowing dust and sand which could locally restrict visibilities. Another area of concern for stronger onshore winds will the interior valley of San Luis Obispo county where gusts of 35 to 45 mph will be likely, and may require a Wind Advisory. The strengthening onshore winds combined with lingering warm/dry conditions will bring elevated to locally brief critical fire weather conditions across the interior on Tuesday afternoon, thereby increasing the risk of fast-moving and large grass fires.
Another item to keep an eye on for Tuesday will be some influx of mid level moisture and instability as the upper flow turns southeasterly. Models have been consistent with embedded vorticity areas sweeping through the interior on Tuesday afternoon, with some elevated instability (NAM lifted index values of -4 to -6 and MUCAPE values of 500 to 700 J/kg). The main limiting factor will be available mid level moisture, which looks rather minimal at this point. For now, will keep with a 5-10 percent chance of a brief shower or thunderstorm across the interior, with the best chances over the San Gabriel mountains and Ventura county mountains. If any convection were to develop, forecast model soundings showing that isolated dry lightning and gusty downdrafts would be the main concerns, with DCAPE values exceed 1000 J/kg.
An upper low will move through Northern California on Wednesday. Following the upper low passage northerly flow aloft will increase across the Santa Ynez Range and western Transverse Range creating increasing northerly winds in those areas, mainly from late afternoon through the early morning hours. Forecast ensemble gradients are still in the -3 to -5 range, which would likely be strong enough for wind advisories in the Santa Ynez Range. Further cooling is expected on Wednesday, with little change in temperatures on Thursday.
(Fri-Mon), 11/144 PM.
Building high pressure and continued offshore flow from the north will greatly reduce or eliminate the low clouds on Friday. There will be a decent bump up in temps of 3 to 6 locally 8 degrees across the area.
Further warming Saturday with limited or no marine layer clouds will bring max temps into the 70s across the csts and 80s in the vlys or 3 to 6 degrees over normal.
Weak troffing will move into the area next Sunday. In addition there will be an increase in the onshore flow to the east. Max temps will likely fall a degree or 2 over most areas.
In addition the northerly gradients will likely bring advisory level wind gusts to the western portion of the SBA south coast late in the afternoons and evenings.
11/2340z.
High confidence in KWJF and KPMD TAFs.
Low to moderate confidence in remainder of TAFs. Timing of flight category changes may be off by 3 hours and one flight category.
KLAX, Latest satellite trends showing more rapid onset of low clouds into KLAX, likely in the 01-02z time window. Otherwise moderate confidence in TAFs, with cigs likely to persist through aftrnoon hours on Tuesday.
KBUR, Moderate confidence in TAF. There is a 30 percent chc of VFR conditions remaining through period, but latest trends indicating increasing potential for IFR/LIFR cigs around sunrise.
11/814 PM.
Winds and seas will remain below Small Craft Advisory (SCA) levels through Wednesday morning.
Wednesday afternoon through the weekend, winds and seas will start to strengthen/build, with SCA winds across the outer waters starting late Wednesday night and Seas reaching steep and choppy SCA levels around Friday night. There is a 20-40 percent chance of SCA winds across the inner waters, with the highest chances each afternoon and evenings. GALE Force winds will be possible across the outer waters as early as Thursday afternoon, with highest (30-50 percent) chances on Saturday into Sunday.
Large SCA level seas of 10-12+ feet will be possible for the outer and northern waters by this weekend.
Dense fog will be possible this evening through mid morning Tuesday.
Ca, Wind Advisory in effect from 2 PM to 10 PM PDT Tuesday for zones 381-383. (See LAXNPWLOX). PZ, NONE.
