19/802 PM.
Below normal temperatures will continue through the weekend, with a deep marine layer and a few spots of drizzle, 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-Mon), 20/802 AM.
***UPDATE***
Morning satellite showed limited coastal marine layer clouds south of Point Conception, potentially due to an increase in northwest flow and some sinking motion with dry air intrusion into the marine layer. With the clearer skies, temperatures cooled down to the dew point temperature in many areas, leading to dew formation on vehicles and other surfaces. The clearer skies should also support quicker warming of temperatures through the morning.
The forecast otherwise remains on track for today, with strong onshore flow continuing, breezy onshore winds in the afternoon, and high temperatures this afternoon a few degrees below normal near the coast to 5-10 degrees below normal away from the coast.
***From Previous Discussion***
Overall, 00Z models in good synoptic agreement through the short term period. At upper levels, weak trough will continue over the area through today then a ridge will begin to develop over the area Sunday/Monday as high pressure builds over the desert Southwest. At the surface, moderate to strong onshore pressure gradients will prevail with some weak northerly offshore gradients.
Forecast-wise for the short term, main challenge will be the behavior of the marine layer. Marine inversion remains deep this morning, in the 2000-2500 foot range, but stratus development is very haphazard. So, confidence in the stratus coverage for this morning is low to moderate as well as the potential for any drizzle this morning. For tonight through Monday, the inversion should gradually become more shallow. With continued onshore pressure gradients, the stratus should behave more normally, but exhibit a decrease in areal extent tonight through Monday. Other than the challenging marine layer stratus, skies should remain mostly clear through Monday.
As for temperatures, today's highs should be very similar to Friday although some areas could be a couple degrees warmer, depending on stratus coverage this morning. For Sunday, all areas will see a few degrees of warming, especially away from the coast. On Monday, the area-wide warming trend will continue as the ridge noses in, with coastal temperatures in the 70s and inland areas topping out in the 80s and 90s.
As for winds, the strong onshore pressure gradients will continue to generate gusty southwesterly winds across interior sections in the afternoon and evening hours. However, at this time, expect any advisory-level gusts to remain localized in the Antelope Valley foothills. So, no wind products are expected at this time.
(Tue-Fri), 20/1256 AM.
For the extended, 00Z models continue to exhibit good synoptic agreement. At upper levels, high pressure, over Arizona and New Mexico, peaks in strength on Tuesday then gradually weakens Wednesday through Friday. At the surface, moderate to strong onshore pressure gradients will continue with some continued weak northerly offshore gradients.
Forecast-wise, main concern in the extended will be the heat. With H5 heights peaking, Tuesday will be the warmest day of the extended period with coastal areas 3-6 degrees above seasonal normals and valleys/interior sections 5-10 degrees above normal. For Wednesday, H5 heights lower slightly which will allow for a couple degrees of cooling, but some increase in mid-level moisture will make things feel very similar to Tuesday. On Thursday and Friday, the gradual cooling trend will continue. Moderate heat risk will be likely on Tuesday/Wednesday with a slight improvement on Thursday/Friday. Given the influx of visitors to the area and plenty of outdoor activities, will continue with the EXCESSIVE HEAT WATCH for the Los Angeles county valleys and mountains.
Secondary concern for the extended forecast will be the potential for some precipitation on Wednesday. Deterministic models indicate increasing PWATs (150-200% of normal) moving into the area on Wednesday. This matches up well with ensembles indicating high chances of PWATs greater than 1.00 inch. However, most of this moisture looks to be confined to 700 mb and above. So, at this time, there is a 5-15% chance of showers/high based thunderstorms on Wednesday just about anywhere, but with best chances over the higher terrain. This will need to be monitored closely over the coming days.
20/1820z.
At 1000Z at KLAX, the marine layer was 1200 feet deep. The top of the inversion was near 4700 ft with a temperature of 17 degrees Celsius.
For 18Z TAF package, high confidence in KWJF and KPMD.
For KSMX, KPRB, KSBP, KSBA and KCMA, moderate confidence in 18Z TAFs as timing of flight category changes could be +/- 3 hours of current forecasts.
For KOXR, KSMO, KLGB, and KVNY, low confidence in 18Z TAFs. VFR conditions expected during the day but timing of MVFR/IFR clouds tonight may vary (+/- 3 hours).
KLAX, Moderate confidence in 18Z TAF. Clearing skies through the day. For tonight, moderate confidence in return of CIG restrictions with timing (+/- 3 hours). No significant easterly wind component is expected.
KBUR, Moderate confidence in 18Z TAF. For tonight, there is a 40-50% chance of MVFR/IFR CIGs after 10Z.
20/1256 AM.
For the Outer Waters, high confidence in current forecast. Today through Wednesday, winds and seas are generally expected to remain below Small Craft Advisory (SCA) levels. However, there will be a 10-20% chance of SCA level winds on Sunday and Monday.
For the Inner Waters (both north of Point Sal and south of Point Conception), high confidence in current forecast. Today through Wednesday, winds and seas are generally expected to remain below SCA levels. However, there is a 10-20% chance of SCA level winds across the western half of the Santa Barbara Channel tonight through Sunday night.
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.
