21/244 PM.
A shallower marine layer is expected over the next few days and a significant warming trend will start Monday and peak around Wednesday with well above normal afternoon temperatures. A push of moisture midweek will bring at least a low chance for rain and then drier conditions return and temperatures cool heading into the end of the week and this weekend.
(tdy-Wed), 21/232 PM.
Overall weather conditions today are similar to yesterday, under continued zonal flow. High temperatures this afternoon will be around 1-3 degrees warmer than yesterday for most areas, with the exception of the valleys of San Luis Obispo County. Midday temperature readings across the county were up to 5-10 degrees warmer and than yesterday and should see high temperatures pushing the low to mid 80s. Elsewhere, coastal areas will top out mostly in the 70s and beaches will mostly be in the upper 60s to around 70F.
Heading into Monday, H5 heights will start rising, as high pressure strengthens over the Desert Southwest, and low level (925mb) temperatures will also steadily climb through Wednesday. This will lead to a notable warming trend at the surface that will continue through at least Wednesday and model guidance continues to show Wednesday being the warmest day of the short term and the entire week. Afternoon high temperatures away from the coast are forecast to warm up to 4-10 degrees above normal beginning Tuesday. Coastal areas will see high temperatures in the middle 70s to lower 80s, while many inland valleys will reach the low to middle 90s. These temperatures will lead to widespread Minor HeatRisk, with areas of Moderate HeatRisk in the warmer valleys. While this may be considered borderline advisory-level heat, the risk for heat impacts and the vulnerability to heat is greater, especially across the LA Area, with an increase in people not from the area and many larger outdoor events. The Extreme Heat Watch for Tuesday-Thursday was converted to a Heat Advisory for portions of LA County.
In addition to the heat, there remains good confidence in a mid-level moisture push from the south late-Tuesday into Wednesday, along with potentially a weak shortwave moving through. Guidance continues to show a 5-15% chance for rain with this push. Ensembles continue to refine the best area for the rain chances, with the focus mainly from Ventura and LA Counties eastward. Model soundings indicate potential for some weak mid-level instability. So, some elevated convection cannot be ruled out. The overall odds of measurable rain is low under this regime and much of the precipitation that falls may evaporate before reaching the ground due to much drier low levels.
At the surface, moderate to strong onshore flow will prevail with some weak northerly offshore flow continuing. 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 the short term. 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 and then there will be more mid-level clouds heading into Wednesday. The lowered marine layer inversion will keep air quality poor in the LA basin area for as long as the warehouse fire continues. See South Coast AQMD for updated air quality information.
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.
(Thu-Sun), 21/241 PM.
Through the extended period, models continue to exhibit good synoptic agreement. At upper levels, ridge slowly weakens into the second half of the week 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.
Heat-wise, late week into the weekend, there will be several degrees of area-wide cooling each day, especially away from the coast, as the Desert Southwest high pressure flattens and dips southward in favor of increased troffing across the Western CONUS. The increased troffing will also bring about breezier conditions, potentially advisory-level winds in some areas, across the region and increase coastal water impacts as well as fire weather concerns.
21/1817z.
At 1540Z at KLAX, the marine layer was around 800 feet deep. The top of the inversion was 4300 feet with a temperature of 19 degrees Celsius.
For the 18Z TAF package, moderate to high confidence in KSBA, KBUR, KVNY, KPMD and KWJF.
For all other sites, moderate confidence in the 18Z TAF. Timing of flight category changes could be +/- 3 hours of current forecasts. For KSMO, KLAX and KLGB, there is a 40-60% chance of MVFR CIGs Monday morning.
KLAX, Moderate confidence in 18Z TAF. 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 CAVU conditions through the day and into the evening. Moderate confidence in overnight conditions with a 40-50% chance of MVFR/IFR CIGs after 10Z.
21/132 PM.
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. Chances for SCA level winds increase late Thursday afternoon through Friday morning as a trough tightens pressure gradients over the area. Probabilities increase to 30-60% from north to south across PZZ270/273/276.
For the Inner Waters north of Point Sal and south of Point Conception, high confidence in current forecast. Localized gusty winds up to 25 kts possible during the afternoon hours through tomorrow in the western Santa Barbara channel, south of Point Conception to around the Channel Islands.
Ca, Heat Advisory in effect from 8 AM Tuesday to 9 PM PDT Thursday for zones 88-368-372-373-379-380-548. (See LAXNPWLOX). PZ, NONE.
