21/825 PM.
A shallower marine layer is expected over the next few days and a significant warming trend will start Monday, peaking 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. Temperatures are expected to cool heading into the end of the week and the weekend.
(Sun-Wed), 21/901 PM.
***UPDATE***
Marine layer clouds were fairly widespread this morning for northern portions of the region, with clouds reaching deep into the valleys. South of Point Conception, stratus was much more inconsistent. Clouds cleared fairly quickly over most of the area though, and high temperatures warmed a touch from yesterday, especially over interior San Luis Obispo/Santa Barbara Counties and the Antelope Valley.
Tonight into Monday morning, expecting eddy circulations in the Santa Barbara Channel and south of Santa Cruz Island to support marine layer clouds developing and pushing into LA, Ventura, and southern Santa Barbara Counties. However, clouds have been slow to form south of Point Conception so clouds may not arrive until the early morning hours and could struggle to the valleys. Current forecast looked on track, with developing heat as high pressure builds over the area. The temperature forecast will be tricky for the coasts and coastal plains, with the ongoing potential for low clouds and patchy fog.
***From Previous Discussion***
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.
22/0413z.
At 0400Z at KLAX, the marine layer was around 1200 feet deep. The top of the inversion was 5100 feet with a temperature of 18 degrees Celsius.
For the 06Z TAF package, high confidence in KPMD and KWJF.
For all other sites, low to moderate confidence in 06Z TAFs. Timing of flight category changes could be +/- 3 hours of current forecasts. CIG categories could be either MVFR or IFR.
KLAX, Moderate confidence in 06Z TAF. Timing of flight category changes could be +/- 3 hours of current forecasts. There is a 50% chance that CIGs will not drop to IFR levels overnight. No significant easterly wind component is expected.
KBUR, Moderate confidence in 06Z TAF. There is a 50% chance that MVFR VSBYs do not develop overnight. There is a 30% chance of IFR CIGs in the 11Z-17Z time frame.
21/821 PM.
For the Outer Waters, moderate to high confidence in current forecast. Through Wednesday there is 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 PZZ670/673/676.
For the Inner Waters north of Point Sal high confidence in conds remaining below SCA levels through Thursday, with a chance of winds increasing to SCA levels Friday night.
For the inner waters south of Point Conception, high confidence in current forecast. Gusts up to 20 kts are affecting portions of these waters, in the western SCA Channel and nearshore LA County. Locally gusty winds 20 to 25 kts will be possible during the afternoon/evening hours again tomorrow in the western Santa Barbara channel, south of Point Conception to around the Channel Islands. Otherwise, conds are expected to remain below SCA levels through the period, except for a chance of SCA level gusts Friday evening in the western channel.
Ca, Heat Advisory remains in effect from 8 AM Tuesday to 9 PM PDT Thursday for zones 88-368-372-373-379-380-548. (See LAXNPWLOX). PZ, NONE.
