09/152 PM.
A gradual cooling trend begins today, with cooler than normal temperatures expected across most of the areas by Tuesday. More robust marine layer cloud coverage is expected, expanding into most coastal valleys by midweek. There remains a low chance of thunderstorms each afternoon through much of the week, highest chances in the mountains through Tuesday.
(tdy-Wed), 09/145 PM.
Today will be the warmest day through the forecast period as a cooling trend kicks off on the heels of this prolonged period of above normal temperatures. The Four Corners High Pressure will finally begin to move east as a weak trough slowly sags down the West Coast over the next couple of days, helping push the higher pressure into the Southeastern United States. This feature will remain fairly stagnant through much of the week, with the trough hanging out just off the coast of California.
The pattern allow for a gradual cooling trend, with most areas cooling a few degrees each day. The marine layer will likely become more consistent and expanisve in coverage starting tonight, more reminencist of a "June Gloom" like pattern rathern than that of mid-August. Low clouds are anticipated to expand to some lower valleys as early as late tonight, reaching most coastal valleys by Tuesday or Wednesday. Onshore flow will be enhanced each day, especially Tuesday or Wednesday as the upper level flow pattern becomes more southwesterly. Some interior areas such as the Antelope Valley and foothills will see winds approaching advisory levels each afternoon, with Tuesday and Wednesday having a low to moderate chance of requiring wind advisories. The increased onshore flow will make clearing of the marine layer clouds difficult at some beaches by midweek.
The most uncertain aspect of this forecast is the potetnial for convection. Once again our area is on the fringes of the monssonal thunderstrom risk. PW's will increase to around 1.50 inches this afternoon, remaining similar each day through the short term period. MUCAPE values peak over the mountains of LA/Ventura/Santa Barbara Counties and Antelope Valley each afternoon, bringing a 10-15% chance for thunderstorms/showers each day, except for today, where chances of convection remain over Eastern LA County. At 500 mb, the flow pattern is solidly southeasterly, but at 700 mb the flow is much more southerly each day, which could be one of the main reasons CAMs keep most activity to the east of LA County. Another interesting aspect of this is an influx of mid- level instability tonight south of Point Conception, bringing a 5% chance of thunderstorm/shower development to these areas overnight.
As the upper level trough eeks closer to the California coast, there is a low chance it can tap into the leftover monsoonal moisture and result in thunderstorm development (10% chance) Wednesday.
(Thu-Sun), 09/145 PM.
The weak trough off the coast will remain in place into the weekend with heights slowly rising Friday into the weekend. However, there is some disagreement amongst models as to how progressive the increase in heights will be, and to what extent. What does appear likely is that high temperatures are likely to remain below seasonal normals through the extended period, although the slight warming trend Friday into the weekend will bring maximum temps closer to normal. As soon as next Sunday is when we start to see significant differences between the GFS and Euro. The Euro favors another, potentially slightly stronger, trough swinging down through California, bringing another round of cooler temperatures into next week. Meanwhile, the GFS favors the pattern transitioning into a more zonal flow with slightly increasing 500 mb heights.
Southwest flow aloft will help aid the increase of onshore flow region-wide, with enhanced southwest to west winds. Increasing onshore flow over the interior will bring a moderate chance for wind advisories Thursday, lower chances Friday. Marine layer coverage is expected to be expansive, with most folks across the coastal valleys waking up to cloudy skies. The coverage should shrink somewhat this weekend, but still expecting some coastal valley communities to see marine layer clouds into the weekend. Similar to midweek, some immediate coastal areas may not see much in the way of marine layer clearing Thursday due to that enhanced onshore flow.
As mentioned in the short term, the threat of thunderstorms due to the upper level trough along the coast potentially drawing in leftover monsoonal moisture will continue through Friday (5-15% chance), highest chances along the Central Coast. Confidence is quite low in this convective potential, but there is a signal for elevated MUCAPE values, especially Wednesday and Thursday. We will continue to monitor as we get closer.
09/1745z.
At 1722Z at KLAX, the marine layer was 900 feet deep. The top of the inversion was at 2800 feet with a temperature of 27 C.
High confidence in VFR TAFs for KPRB, KBUR, KVNY, KPMD & KWJF.
Low to moderate confidence in remaining TAFs after 03Z Mon, with lowest confidence for sites south of Point Conception. Mid-level moisture and warm SSTs will likely continue to disrupt the formation of marine layer clouds S of Point Conception. There is a 40%-50% chance that conds remain VFR at KLAX, KSMO, and KLGB through Mon morning. Timing of flight cat changes may be off by 3 hours, and any CIG heights should be accurate within +/- 200 ft of current forecast.
KLAX, Low to moderate confidence in TAF especially tonight into Mon morning. There is a 50% chance that conds remain VFR through the fcst period. Small chance (20%-30% chance) of intermittent CIGs (007-010) after 04Z Mon. Better chance (50%) of these cigs 14Z-17Z Mon. No significant east wind component expected.
KBUR, High confidence in VFR TAF. No wind issues expected.
09/1238 PM.
Conditions will remain below Small Craft Advisory (SCA) across all the southwestern CA coastal waters through Friday night. The only exception is a 20% chance of SCA level NW winds across the extreme northwest portion of the outer waters north of Point Sal (PZZ676) through this evening.
09/1239 PM.
High astronomical tides are expected to affect low-lying beaches south of Point Conception with the possibility of minor coastal flooding starting with the evening high tide on Monday and continuing through at least Wednesday. As a result, a Coastal Flood Advisory has been issued.
Predicted tide heights of up to 7.5 feet are expected between about 8 and 10 PM each evening. These tides, combined with a small increase in a long-period southerly swell, could result in minor flooding of sea water around the time of high tide over low-lying coastal areas such as parking lots, beaches, and walkways. Beaches most susceptible to potential minor coastal flooding issues include those around Point Dume and near Long Beach.
Ca, Extreme Heat Warning in effect until 8 PM PDT this evening for zones 38-377-381>383. (See LAXNPWLOX). Heat Advisory in effect until 8 PM PDT this evening for zones 88-342>345-348-349-351>353-356>358-367>376-378>380-548. (See LAXNPWLOX). Coastal Flood Advisory in effect from 6 PM Monday to midnight PDT Wednesday night for zones 349-350-354-362-366. (See LAXCFWLOX). PZ, NONE.
