19/307 AM.
Mostly clear skies will cover the area today and Saturday save for some morning low clouds. It will be cooler each day but high temperatures will remain above normal. There will be increasing clouds, cooler temperatures and a slight chance of rain over San Luis Obispo county Sunday and Monday. A strong storm will bring many inches of rain to the entire area Tuesday through Wednesday.
(tdy-Sun), 19/759 AM.
***UPDATE***
Latest satellite and surface observations some stratus/dense fog along the Central Coast, LA coast and the Salinas Valley with scattered high clouds elsewhere. Current sounding data indicates marine inversion around 700 feet deep. Some northerly winds, gusting 30-40 MPH, are currently observed across the western Santa Ynez Range.
Forecast-wise for the immediate short term, no significant issues are expected. Current stratus and dense fog will dissipate by mid-morning. So, will let the DENSE FOG ADVISORIES expire at 1000 AM or even end it sooner. So, mostly sunny skies will prevail this afternoon with scattered high clouds overhead. Current winds across the western Santa Ynez Range should diminish through the day. As for temperatures, the return of onshore pressure gradients today along with a cooler boundary layer will bring cooler conditions to most areas. The only exception will be the deserts where some slight warming is anticipated.
Overall, current forecast has good handle on the immediate short term. So, no significant updates are expected. For the afternoon forecast suite, attention will remain focused on precipitation chances next week.
***From Previous Discussion***
The strong 592 dam upper high that brought all of the warm weather this week will be pushed to the south today. The flow aloft will flatten out into the W to E configuration and hgts will fall to 582 dam. The offshore flow from the E will turn onshore and then increase through the day. Offshore flow from the N will remain but will be weaker than ydy. Look for low clouds and patchy dense fog to develop across the LA cst, western SBA county and the Paso Robles area. Skies, otherwise will be mostly sunny. The change in the sfc flow as well as the falling hgts will lead to 3 to 6 degrees of cooling across the csts/vlys. The lack of cool air advection from the San Joaquin Vly will lead to 2 to 4 degrees of warming across the interior.
Flat westerly flow will continue over the area on Saturday. Hgts will dip down to 579 dam. Onshore flow to the east will increase from 1 mb in the morning to about 3 mb in the afternoon, the offshore flow from the north will turn weakly onshore in the afternoon. The onshore flow and falling hgts will bring plenty of low clouds to most of the csts and lower vlys in the morning. The increasing onshore flow in the afternoon will slow clearing and more than a few beaches may be cloudy all day. The southern progress of the atmospheric river (AR) affecting the northern portion of the state is no forecast to remain north of the area which will eliminate the rain threat for SLO county and reduce the amount of mid and high clouds over the area. The cooling trend will continue as falling hgts and better onshore flow combine to lower most temps another 3 to 6 degrees. The Central Coast will be the exception with little change in temps. Max temps, however, will remain above normal (3 to 6 degrees interior csts, 5 to 10 degrees vlys and 10 to 15 degrees mtns and far interior).
(Mon-Thu), 19/300 AM.
Sunday and Monday will be fairly similar days. The upper flow pattern will slowly tilt more and more into a SW to NE direction on Sunday as the AR system sags a little more to the south. A slight chc or chc (20 to 30 percent) of rain will develop over northwestern SLO county. Other areas will just see increasing clouds Sunday and mostly cloudy skies Monday. Rainfall totals during the two day period will most likely be under a tenth of an inch. Max temps will fall 2 to 4 degrees across almost all of the area Sunday and then will change little on Monday. Max temps will remain above normal (1 to 2 degrees across the csts; 4 to 8 degrees vlys and 8 to 16 degrees mtns and interior).
Deterministic, ensemble based and AI based mdls have been in very good agreement up until now. Annoyingly quite a bit of disagreement has now shown up. The following forecast is based on a broad mixture of all available mdls, but now the confidence is lower esp for Tue and Thu.
On Tuesday the flow will tilt to the SW. The southern end of the AR will be swept up by a trof and will also assume a SW to NE orientation. The trof will translate eastward and will bring rain to the area. Rain will become likely (60 to 70 percent chc) across the Central Coast in the morning, while a chc (20 to 40 percent chc) develops south of Pt Conception. Rain is a near certainty across SLO and SBA counties in the afternoon. Ventura County will see a 50 percent chc and LA county a 40 percent chc. By Tuesday evening the entire 4 county area will be engulfed by rain. Due to the southerly flow associated with this event the south facing slopes will see the highest totals and rates. Right now from dawn Tuesday to dawn Wednesday it looks like most areas will see about an inch of rain with the south facing slopes receiving about two inches. Snow levels during this time will be near 8000 ft. All this said there are several solutions (notably the GFS and many of its ensembles) that are slower with the arrival of the AR - this outcome would lead to a much drier day.
The best confidence remains on Wednesday when the AR should move across the entire area. Wednesday should feature the most rainfall and the highest rainfall rate. It should rain all day across all of the area. It is likely that 1 to 2 inches of rain will fall across the csts/vlys and 2 to 4 inches across the foothills and mtns. The interior as usual will see less rain likely around an inch to an inch and half. Due to the strong south flow snow levels will remain near 8000 ft.
Pretty low confidence for the Christmas forecast. Many mdls move the bulk of the rain out of the area in the morning with much lighter rain in the afternoon. There is still the potential for 1 to 2 inches of additional rain, but this might be on the high side of potential outcomes. Will have to wait and see which way the mdls take this days forecast. Colder air will be ushered in behind the AR and snow levels will fall through the day ending up around 7000 ft late in the afternoon and then falling to 6000 ft overnight. It will be a coolish day with max temps only in the lower to mid 60s for most of the csts/vlys.
Model agreement falls off pretty rapidly after Thursday, but there is a potential for more rain Fri Sat and Sun.
19/1804z.
Around 09Z, the marine layer depth was 700 feet. The inversion up to 4600 feet with a temperature of 20 degrees C.
High confidence in VFR TAFs for KPMD and KWJF. Winds may gusts up to 10 kt higher than forecast.
Moderate confidence in TAFs for KLAX, KLGB, and KSMO. Timing of flight cat changes may be off +/- 3 hours. 20% chance for vsbys lower than 1/2SM from 06Z-16Z. Low chance for some IFR to MVFR conds through the afternoon as the fog bank will linger along the coast.
High confidence in remaining TAFs through 00Z, then low confidence thereafter. Timing of flight cat changes may be off +/- 4 hours. Chance for VFR conds to prevail at KSBP (50%), KSMX (30%), KSBA (30%), KOXR (20%), KCMA (25%), KBUR (30%), and KVNY (30%). Low confidence in minimum flight cat, but VLIFR- IFR is likely.
KLAX, Moderate confidence in TAF. Low chance for some IFR to MVFR conds through the afternoon as the fog bank will linger along the coast. Otherwise flight cat changes may be off +/- 3 hours. 20% chance for vsbys under 1/2SM from 06Z-16Z. Moderate confidence in any east wind component remaining below 8 knots.
KBUR, High confidence in TAF through 06Z, then low confidence thereafter. If cigs arrive, 50% chance for vsbys less than 1/2SM. However, 30% chance VFR conds prevail. Relatively mild winds expected.
19/753 AM.
Moderate confidence in the current forecast. Moderate confidence exists in the sea forecast through tonight, then higher confidence thereafter.
Low clouds and fog will continue across portions of the coastal waters through later this morning due to the shallow marine inversion over the coastal waters.
For the waters southwest through northwest of the Channel Islands and the nearshore waters along the Central Coast, Small Craft Advisory (SCA) level winds will likely continue through tonight, highest during the afternoon and evening hours and from near Point Concpetion south to San Nicolas Island. There is a low-to- moderate (20-40 percent) chance of SCA conditions over the weekend and into Monday, then there is a likely (60-80 percent) chance of widespread SCA conditions on Tuesday and Tuesday night with a moderate-to-high (30-50 percent) chance of GALES.
Inside the southern California bight, there is a moderate-to-high (30-50 percent) chance of SCA level winds this afternoon and evening, highest across the western portion of the Santa Barbara Channel. Winds and seas should drop below SCA levels over the weekend and into Monday, there is a high-to-likely (50-60 percent) chance of widespread SCA conditions on Tuesday and Tuesday night.
Ca, NONE. PZ, Small Craft Advisory in effect until 9 PM PST this evening for zones 645-670. (See LAXMWWLOX). Small Craft Advisory in effect from 2 PM this afternoon to 9 PM PST this evening for zone 650. (See LAXMWWLOX). Small Craft Advisory in effect until 9 AM PST Saturday for zones 673-676. (See LAXMWWLOX).
