09/733 PM.
A gradual cooling trend is expected this week, with cooler than normal temperatures expected across much of the area 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 the week, with the highest chances over the mountains through Tuesday.
(Sun-Wed), 09/858 PM.
***UPDATE***
Another warm day on the books, with highs ranging from the mid 100s in the deserts to the 90s to 105 for the valleys, with much cooler values near the coasts, especially north of Point Conception. Coastal clouds are continuing to limit warming in this area, with a less consistent marine layer south of Point Conception. A few monsoonal showers managed to move into the San Gabriel Mountains this afternoon, but otherwise it was a fairly benign day. This slight chance for convection continues, with rather low confidence. High resolution guidance suggests Monday afternoon could see a stray shower/thunderstorm for the Ventura/LA County mountains, and perhaps better chances Tuesday afternoon/evening.
***From Previous Discussion***
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 expansive in coverage starting tonight, more reminiscent of a "June Gloom" like pattern rather 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 potential for convection. Once again our area is on the fringes of the monsoonal thunderstorm 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.
10/0329z.
At 2356Z at KLAX, the marine layer was 300 feet deep. The top of the inversion was at 3500 feet with a temperature of 28 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 coastal sites south of KSMX. Cigs may be intermittent through the night, and timing of flight cat changes may be off by 3 hours.
KLAX, Low to moderate confidence in TAF especially tonight into Mon morning. Expecting cigs at some point, but may clear out a few times through Monday morning. No significant east wind component expected.
KBUR, High confidence in VFR TAF. No wind issues expected.
09/735 PM.
Conditions will remain below Small Craft Advisory (SCA) across all of the southwestern CA coastal waters through Friday night.
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, 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.
