12/335 AM.
Several degrees of cooling are expected today and Wednesday as an upper low moves through northern California. Low clouds and fog will move into the coastal valleys with clouds lingering at some coastal areas through today. Gusty winds are expected times across the mountains and deserts. Slight warming is expected Thursday and Friday.
(tdy-Thu), 12/825 AM.
***UPDATE***
Wind Advisory looks good for this afternoon to evening for the Antelope Valley to foothills where gusts up to 50 mph are likely. Other interior areas will be breezy, but winds should be 5-10 mph too weak to warrant Wind Advisories elsewhere.
On track for a 5-10 percent chance of a high based shower or thunderstorm this afternoon to early evening for interior areas, especially northern Ventura to northeastern Santa Barbara County. Dry lightning fire starts and gusty winds would likely be the main hazards with little if any rain.
***From Previous Discussion***
This morning cross sections show deepening of the marine layer, thus low clouds and fog will likely extend into some of the coastal valleys. Looking for a slow-to-no clearing day through this afternoon for portions of the coast.
High pressure will start to weaken and shift east today, 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, 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 this afternoon/evening, with gusts mostly ranging in the 30 to 45 mph range. A Wind Advisory is in effect for this 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 30 to 45 mph will be likely, and may also 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 this afternoon, thereby increasing the risk of fast-moving and large grass fires.
Another item to keep an eye on today 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 this 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), 12/429 AM.
In the long term, most temperatures will be within a few degrees of normal, with a very gradual warming trend each day. 500 mb heights will likely increase into Saturday, followed by broad weak troughing over the area. However, weakening onshore to briefly onshore surface pressure gradients will allow the warming trend to generally continue, most noticeably at the coasts, where temperatures will reach a few degrees above normal by Monday.
12/1751z.
At 1725Z at KLAX, the marine layer was 2100 ft deep. The top of the inversion was at 4600 ft with a temperature of 22 C.
High confidence in VFR TAFs for desert airfields (KPMD & KWJF).
Low to moderate confidence in remainder of TAFs. Timing of flight category changes may be off by 3 hours and by one flight category.
KLAX, Moderate confidence in TAF. MVFR CIGs 015-025 expected thru 13/21Z. Periodic IFR CIGs 008 may occur through 13/15Z. 50% chance CIGs persist throughout entire forecast period. No significant east wind component.
KBUR, Moderate confidence in TAF. IFR to MVFR CIGs likely to persist through entire forecast period. 15% chance of LIFR CIGs 004 13/08Z-15Z. No wind issues expected.
12/754 AM.
Winds and seas will remain below Small Craft Advisory (SCA) levels through Wednesday morning.
SCA conditions (winds and seas) are expected to develop across the northern Outer Waters Wednesday afternoon, expanding in coverage to include all Outer Waters by Wednesday night.
These hazardous conditions will persist into the weekend reaching portions of inner waters (along the Central Coast and SB Channel) at times.
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.
Seas will continue to build into the weekend peaking 10-15 ft across the outer waters.
Dense fog will be possible through mid morning Today.
Ca, Wind Advisory in effect from 2 PM this afternoon to 10 PM PDT this evening for zones 381-383. (See LAXNPWLOX). PZ, NONE.
