10/534 PM.
A cooling trend will continue through Thursday, with cooler than normal temperatures expected across the area into the weekend. There is a chance of mountain and occasional lower elevation convection through at least Wednesday night. More robust marine layer cloud coverage is likely this week, however, higher clouds may disrupt the formation of the marine layer at times.
(tdy-Thu), 10/206 PM.
After a long and persistent period of widespread above normal temperatures, Southwest California is finally on the downward trend temperature-wise. While today will remain seasonably warm across much of the area, we're expecting the gradual cooling trend to result in below normal temperatures virtually everywhere by Wednesday. The upper level ridge of high pressure responsible for the above August-normal temperatures will finally be pushed east while a weak trough moves down the West Coast by Wednesday and sits along the coast of California for some time.
The threat of monsoonal convection will continue through at least Wednesday as upper level flow remains south to southeasterly and PWs sit around 1.5 inches at KLAX. Over the northern Ventura County mountains as well as Eastern LA County mountains and Antelope Valley, a 10-15% chance of showers/thunderstorms exists this afternoon. Chances increase to 30-50% for similar areas plus interior Santa Barbara County tomorrow and Wednesday. The upper level trough along the coast will try to siphon in monsoonal moisture on Wednesday, and this moisture combined with elevated instability brings a 10-15% chance for thunderstorms in the afternoon through late evening across much of SLO and Santa Barbara Counties, highest in northwestern SLO County. While the trough hangs out along the coast, there still exists a 5% chance for convective storms to develop anywhere Thursday through Friday, with lowering PW's helping to keep any thunderstorms chances very low.
Outside of potential convection, we can expect cooling each day through Thursday thanks to the influence of this weak trough along the coast. As the troughing pattern sets up, southwest flow aloft will help aid in the slight increase in onshore flow, helping bring cooler temperatures and more widespread marine layer cloud coverage to the coasts coastal valleys this week. However, mid to upper level clouds will try to inhibit the formation of marine layer clouds. But with a fairly solid marine layer already in place, we're forecasting it to move into the coasts each day through Thursday. We also expect it to deepen and reach most coastal valleys by early Thursday morning. Regarding the increase in onshore flow, wind advisories are not likely, but there is a low to moderate chance for advisories to be issued for Wednesday and Thursday across the Antelope Valley.
(Fri-Mon), 10/206 PM.
Weak upper level troughing will continue into the weekend. However, 500 mb heights are expected to steadily climb Saturday through Monday. The EC and GFS are in fairly good agreement that 500 mb heights will rise to around 590-592 dam by Sunday. Beyond Sunday is where a significant disagreement occurs. Yesterday, the EC favored another troughing pattern and the GFS favored the ridging building back in. Today the two models have completely flipped. There is even quite a range of outcomes starting Monday within the EC Ensemble members. To say there is considerable uncertainty for Monday and beyond would be an understatement.
What does seem likely is much lower chances of any sort of convection as much of the monsoonal mositure moves back to the south and east. However, there still remains a very low chance (5%) for thunderstorms to develop virtually anywhere into the weekend and the trough along the coastal will try to pull in some remnant monsoonal mositure. Any sort of forcing from the trough will attempt to create a build up. With generally limited moisture, we would expect storm threats to generally be dry lightning or gusty outflow winds rather than downpours.
Slightly increasing temperatures into the weekend also appear likely. The good news for folks tired of the hotter than normal temperatures is that current forecast high temps will still remain below mid-August normals. One wrinkle is that onshore flow is expected to steadily weaken into the weekend and the cooling effects of nature's air conditioning will decrease. The magnitude of the decrease in onshore flow will certainly influence maximum temperatures for this weekend into Monday.
11/0057z.
At 2338Z at KLAX, the marine layer was 800 feet deep. The top of the inversion was at 2400 feet with a temperature of 27 Celsius.
High confidence in TAFs for the valleys and deserts with VFR conds expected. Lower confidence along the coast with LIFR to IFR conds along the Central Coast, with a 30% chance that FG does not affect KSMX. Further south, IFR conds at KSBA, KOXR and KCMA, but with a chance of periodic or short duration CIGs occurring. IFR to MVFR conds for the LA Coastal sites, where CIGs could also occur periodically or for a short time.
There is a chance of -TSRA at KWJF KPMD through 03Z (10%) and on Tuesday (30%). All other sites have a 5% chance of of -TSRA on Tuesday.
KLAX, Low confidence in the 00Z TAF for the overnight period, with bkn008-012 CIGs from 07Z to 17Z. There is a 40% chance that CIGs do not occur, or they form and dissipate through that time frame. There is also a 5% chance of -TSRA on Tuesday. High confidence in minimal easterly winds.
KBUR, High confidence in VFR TAF with typical winds through Tuesday. 5% chance of -TSRA Tuesday.
10/152 PM.
High confidence that conditions will remain below Small Craft Advisory (SCA) across all of the southwestern California coastal waters through at least Friday night. Patches of dense fog are possible on the Central Coast each morning. There is also a 5-10% chance of a thunderstorm over the coastal waters Tuesday through Wednesday.
Ca, Coastal Flood Advisory in effect until midnight PDT Wednesday night for zones 349-350-354-362-366. (See LAXCFWLOX). PZ, NONE.
