08/103 AM.
A warming and drying trend will develop through the weekend as offshore flow establishes beneath building high pressure aloft. Temperatures above seasonal levels are likely Saturday through Monday with 80s and 90s being common across most coastal and valley locations. A storm system will likely move over the region the latter half of next week and could bring moderate to heavy rainfall.
(tdy-Mon), 08/720 AM.
***UPDATE***
Latest satellite and surface observations indicate stratus and patchy dense fog across the coastal plain south of Point Conception as well as interior sections of SLO county with clear skies elsewhere. Current sounding data indicates a very shallow marine inversion AOB 700 feet. As for winds, many reports of north to northeast winds, gusting 25-40 MPH, and even some local gusts up to 55 MPH across the Santa Lucia Mountains.
For the immediate short term, no significant issues are expected. Gradients will exhibit a weak diurnal trend today (offshore this morning and onshore this afternoon). So, current N-NE winds will peak after sunrise this morning, but should diminish into the afternoon hours. So, at this time, no concerns about any widespread advisory-level winds today. Current stratus/fog along the coastal plain should dissipate by late morning with sunny skies for all areas this afternoon. With weak offshore gradients this morning and a warmer boundary layer, per TEMP STUDY, today will be a few degrees warmer for all areas although the marine layer presence will limit coastal warming south of Point Conception.
Overall, current forecast has good handle on the immediate short term. So, no significant updates are expected. For the afternoon forecast suite, attention will be on offshore flow and warm temperatures through Monday and rain chances later next week.
***From Previous Discussion***
Temperatures and wind are the main characters of the weather story for Southwest California in the short term. An upper level ridge of high pressure will continue to build over the area today, and peak in strength on Sunday and Monday before weakening and moving eastward starting Tuesday.
At the surface, offshore flow will mostly persist through Monday, but the north-south gradients will weaken each day, while the east-west gradients strengthen. This means that winds will shift from the north to more northeast this weekend. N-NE wind gusts of 20-30 mph are currently blowing across the typical Santa Ana wind Corridor, while northeast wind gusts of 25-35 mph, locally up to 45 mph, are prevalent over The Santa Lucia Mountains on the Central Coast. These winds are expected to continue through mid- morning and then taper off into the afternoon. Another round of gusty winds will occur across the Santa Ynez and I-5 corridor tonight into the early hours of Sunday, though likely under advisory levels for the Santa Ynez Range especially as the gradients will be 2.5 to 3.5 mb weaker than Friday night into this morning. As for Sunday and Monday, winds will focus over the usual Santa Ana wind areas of Ventura and Los Angeles counties, as well as the Santa Lucia Range. However, the upper level support isn't especially strong, and the LAX-DAG gradients look to reach around -4 to -5 mb, so this event will be on the weak to barely moderate side of things. Current thinking is that maybe 1 or 2 advisories may be needed at most, mainly for the mountain areas.
As for temperatures, the high pressure strengthening (with associated 500mb heights increasing) paired with offshore gradients strengthening will result in warming across the region through Monday. By Monday, high temperatures will be in the mid 80s to lower 90s on the coastal side of the interior mountains, with the coasts spanning the 70s, and the interior areas in the 80s. These temperatures will be around 10-15 degrees above normal for this time of year. Additionally, overnight low temperatures will be elevated, with most foothills and breezy areas only dropping into the 60s to lower 70s. Between the highs and warm overnight lows, heat advisories are not out of the question, but the overnight lows will be the determining factor on if advisories are needed.
Last to be talked about in the short term, are the marine layer clouds. The low clouds are expected to be less expansive as the 500mb heights rise and the offshore gradients strengthen this weekend, but if the offshore winds are slow to kick in tonight, and/or are weaker than expected, clouds will be able to sneak back in the immediate coastal locations tonight, and dense fog will be a possibility. The NBM suggest this outcome, and given how the gradients are slightly weaker than models previously suggested, allowed the official forecast to follow the NBM for tonight into Sunday morning. By Sunday night into Monday morning, however, offshore flow and heat should be established enough to keep marine layer clouds away.
(Tue-Fri), 08/354 AM.
In the extended period of the forecast, models are in agreement Tuesday, but starting Wednesday things start to differ. What they do agree on, however, is that rain is coming to SoCal sometime during the latter half of the upcoming week.
Tuesday into Wednesday, the upper level ridge will weaken and shift east of the region as a low pressure system approaches from the northwest. This pattern will result in a cooling trend with a return of onshore winds and increasing marine layer clouds and fog.
Thursday and Friday all models are showing a wet and unsettled pattern with an upper level low pressure system affecting the coast. Both the deterministic runs of the EC and GFS now show a low pressure system dropping into SoCal from the north, though the details of placement of the low, the timing, and amounts still differ. While the majority of the models agree that Wednesday will be as previously mentioned, the NBM starts to bring rain in starting early Wednesday morning, while the EC and GFS and their respective ensemble members mainly start the rain late Wednesday night/early Thursday morning. The EC deterministic suggests that rain will have moved through the region by Saturday evening, and another blip of rain will be possible early in the following week. Meanwhile, the GFS deterministic and ensembles hint at rain being more spread out and carrying on through the weekend. As for amounts, there still is quite a bit of variation as well, as the GFS brings the low more over the ocean, and the EC moving more over land. The ensembles also remain to have widespread outcomes from each member to another. So, current NBM POPs look good (though maybe 12 hours too early), with likely/chance POPs for the entire area. However, amounts through Friday are very much in questions, ranging from minor to significant. Over the coming days, the models should come to a better consensus and details can be greatly fined-tuned. Essentially, be prepared for some rain beginning Thursday.
08/1823z.
At 1759Z at KLAX, the marine layer was 1100 feet deep. The top of the inversion was 1900 feet with a temperature of 24 C.
High confidence in TAFs for KBUR, KVNY, KPMD and KWJF. 10% chance for VLIFR conds at KBUR and KVNY from 07Z-17Z.
Moderate confidence in TAFs for KPRB, KSBP, KSMX. There is a 20-30% chance for VLIFR to LIFR conds at KSBP/KSMX from 02Z-18Z, and KPRB from 12Z-17Z. Highest chances at KSMX.
Low confidence in TAFs for KSBA, KOXR, KCMA, KSMO, KLAX, and KLGB. 15% chance for no clearing at KOXR, KCMA, KSBA. There is a 20% chance for coastal sites to remain VFR through the period once cigs clear. Otherwise, clearing and arrival times may be off +/- 4 hours (highest uncertainty for arrival this afternoon and evening. 20-40% chance vsbys of 1/4SM and cigs VV001, highest chance KSBA, KOXR, KCMA.
LLWS and turbulence is possible over and near mountainous terrain after 00Z focused across interior LA and Ventura Counties. KLAX, Low confidence in TAF. 10% chance for no return of low clouds. 20% chance for vsbys less than 1/2SM and cigs less than VV002. Best chances between 10Z and 17Z. Timing of arrival may be as early as 00Z or as late as 07Z. No significant east wind component expected.
KBUR, Low confidence in TAF. 10% chance for VV001-VV002 cigs and vsbys less than 1/2SM from 07Z-17Z.
08/746 AM.
High confidence in large seas adjacent to the Central Coast subsiding through today. 10-15 knot E to NE winds will be common early Sunday morning through Monday afternoon from Ventura to Point Dume. Otherwise, relatively mild conditions will continue into at least Tuesday.
Moderate to high confidence that a storm system will approach the Coastal Waters as early as Wednesday night, bringing rain and at least Small Craft Advisory level winds and large steep and choppy seas to the region, including nearshore. There is a potential for Gale Force Winds. Confidence is low in timing and magnitude of impacts.
Ca, High Surf Advisory in effect until 9 PM PST this evening for zones 340-346. (See LAXCFWLOX). PZ, Small Craft Advisory in effect until 4 PM PST this afternoon for zones 645-670-673. (See LAXMWWLOX).
