20/229 PM.
Below normal temperatures will continue through the weekend, with a deep marine layer, as well as gusty winds over the interior. A significant warming trend will start Monday and peak around Wednesday. A push of moisture midweek will bring at least a low chance for rain.
(tdy-Tue), 21/826 AM.
***UPDATE***
Morning satellite trends are very similar to yesterday morning, with marine layer stratus covering most coasts and valleys north of Point Conception and less, uneven, coverage south of Point Conception. Aircraft reports and the 12Z Vandenberg sounding show the marine layer depth has lowered slightly from yesterday and is sitting around 2000 feet. Pressure gradient readings still show moderate to strong onshore flow and northerly flow over parts of Santa Barbara County. The lowered marine layer inversion will keep air quality poor in the LA basin area. See South Coast AQMD for updated air quality information.
Forecast remains on track for a few degrees of warming (1-4F) today, compared to yesterday, with sunny afternoon skies.
***From Previous Discussion***
Overall, 00Z models in good synoptic agreement through the short term period. At upper levels, ridge will nose in over the area through Tuesday as high pressure develops over southern Arizona and New Mexico. At the surface, moderate to strong onshore flow will prevail with some weak northerly offshore flow.
Forecast-wise, no major changes to thinking of previous shifts. With gradual increase in H5 heights, the marine inversion should become a bit more shallow through the period. So, the expected areal coverage of stratus should gradually diminish through Tuesday. However, given the current chaotic nature of the stratus development, confidence in the exact behavior of the marine layer is moderate at best. Other than any stratus issues, skies should remain mostly clear through Tuesday.
As for temperatures, the combination of the ridge building overhead and less marine influence will allow for a nice warming trend for all areas. For today, there will be about 2-5 degrees of warming for most areas. On Monday and Tuesday, coastal areas will see about 2-4 degrees of warming each day while the valleys and interior sections experience about 4-8 degrees of warming each day. With this warming trend, there will be some areas of moderate Heat Risk on Tuesday, mainly across LA county. Currently, an EXCESSIVE HEAT WATCH remains in effect, beginning Tuesday, for the valleys, mountains and interior coastal plain of LA county. Given the increased tourist activity across LA county, the watch remains reasonable and will likely be upgraded to an advisory by later shifts.
As for winds, no significant issues are expected. The moderate to strong onshore pressure gradients will continue to generate gusty southwesterly winds in the afternoon/evening hours across interior sections. However, any advisory-level winds will remain localized across the desert foothills. So, no wind products are expected at this time.
(Wed-Sat), 21/105 AM.
Through the extended period, models continue to exhibit good synoptic agreement. At upper levels, ridge slowly weakens Wednesday and Thursday with broad cyclonic flow developing on Friday and Saturday. At the surface, moderate to strong onshore flow will continue with some increase in northerly offshore flow later in the period.
Forecast-wise, main issue in the extended period will be temperatures. On Wednesday, there will be some slight warming in most areas. These warm temperatures on Wednesday may be exacerbated a bit due to some increase in mid-level moisture across the area. On Thursday, there will be some slight cooling, but conditions, overall, will remain quite warm. So, the EXCESSIVE HEAT WATCH will remain in effect for parts of LA county through Thursday with a good chance that it will be upgraded to an advisory by later shifts. For Friday and Saturday, there will be several degrees of area-wide cooling each day as the ridge retreats to the east.
Secondary concern for the extended will be increasing mid-level moisture across the area on Wednesday. Current deterministic models indicate increasing PWATs across Ventura and LA counties with values anywhere from 100-150% (GFS) and 125-200% (ECMWF) of normal for late June. The respective ensemble solutions indicate the same PWAT trend. There really does not seem to be any noticeable "trigger" in the atmosphere to kick off any convective activity. However, given the time of year, will indicate 5-15% chance POPs across Ventura and LA counties Wednesday afternoon and evening with the highest POPs over the higher terrain. If anything does develop, it will be high-based, resulting in limited rainfall. If any thunderstorms were to develop, dry lightning and gusty outflow winds would be the main concerns. This will need to be watched closely over the coming days.
21/1125z.
At 1100Z at KLAX, the marine layer was around 1000 feet deep. The top of the inversion was 4300 feet with a temperature of 18 degrees Celsius.
For the 12Z TAF package, high confidence in KSBA, KBUR, KVNY, KPMD and KWJF.
For all other sites, low to moderate confidence in 12Z TAF. Timing of flight category changes could be +/- 3 hours of current forecasts. For KSMO, KLAX and KLGB, there is a 40-50% chance of MVFR CIGs this morning.
KLAX, Low confidence in 12Z TAF. There is a 40% chance that MVFR CIGs develop in the 12Z-18Z time frame. For tonight, moderate confidence in return of CIG restrictions, but low confidence in timing (+/- 3 hours of current forecast). No significant easterly wind component is expected.
KBUR, High confidence in 12Z TAF as CAVU conditions are expected through the period.
21/105 AM.
For the Outer Waters, moderate to high confidence in current forecast. Today through Wednesday, high confidence in winds and seas remaining below Small Craft Advisory (SCA) levels. On Thursday, there is a 30-50% chance of SCA level winds, beginning in the late afternoon hours across PZZ670/673.
For the Inner Waters north of Point Sal and south of Point Conception, high confidence in current forecast. Today through Thursday, winds and seas are expected to remain below SCA levels.
Ca, Extreme Heat Watch in effect from Tuesday morning through Thursday evening for zones 88-368-372-373-379-380-548. (See LAXNPWLOX). PZ, NONE.
