Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

957 pm PDT Wed aug 12 2020


A warming trend will continue through late week as high pressure builds toward the coast. Some tropical moisture arriving from the south will bring a chance for a few showers and thunderstorms Thursday and Friday. By Friday and moreso over the weekend hot weather returns to inland areas with triple digit heat likely continuing into early next week while a shallow marine layer persists along the coast.


As of 9:57 PM PDT Wednesday, Not planning any forecast changes for this evening. Marine layer is just around 1000 feet with low clouds from the Golden Gate southward to Big Sur. Northerly gradients around 5 mb with a shallow inversion layer and light onshore winds should keep inland penetration mainly confined to the Monterey Bay region and Salinas Valley.

Looking at satellite the massive Lake Fire has exploded this evening down on the Angeles National Forest. Farther south we then see mid and high level clouds across Baja California and spreading northward into SoCal. These clouds are remnants of former Hurricane Elida. We will be watching this moisture as it heads northward Thursday. The main concern is thunderstorm chances and as is often the case around here it remains a difficult forecast. There is a weak circulation offshore (upper low) with the strong high over the Desert Southwest. The moisture field will squeeze between these two features spreading northward on Thursday from Santa Barbara county through San Luis Obispo before finally approaching the Central Coast. The NAM and GFS models are generating some activity over SLO/Santa Barbara with some spotty qpf Thursday afternoon/early evening into Monterey county. Even our local in-house WRF shows the best chances over Monterey county. Traditional tools such as high level Total-Totals are well in excess of 30 with model cross sections showing sufficient moisture at the mid-levels down to around 600 mb. The bottom line is this remains a tough forecast. The main concern would be a lightning event could lead to several new ignitions with fuels of course dry, especially above the marine layer. Best estimate at this time is much of the area will see an abundance of mid and high clouds Thursday afternoon/evening. In addition a slight uptick in dew-point will make things feel a little more sticky/humid than normal. Believe shower/t-storm threat will first get close over SLO/Kern and Kings county as it approaches some of the hinterlands of interior Monterey county. Current forecast shows slight chance of t-storms with LAL (lightning activity level) of 2. Any storms that develop over the Los Padres Forest of Monterey county will be of concern for new ignitions. Models then want to show a more organized band of showers and t-storms developing offshore, over the ocean waters later Thursday evening. Not enough confidence to issue any Red Flag Warnings.

On Friday the offshore moisture may then try and return onshore around Pt Reyes before new convection fires farther to the north.

Once any convective threat ends all attention will turn to impending heat event. No changes to excessive heat watch. Hot temps Friday afternoon through the weekend with inland triple digit heat. The ECMWF historically does well in some of these extreme events. Right now MOS guidance shows 106 for Livermore on Friday with 113 for Paso Robles on Sunday which is often a good proxy for some of the hottest interior areas of the Central Coast. Of course any lingering sleeper fires would awake and a dangerous fire weather pattern is hot and dry weather after any lightning. Hot and dry through the weekend with a shallow fog bank along the immediate coast. Stay tuned, the weather looks changeable the next few days with possible high impact convective and heat events.

Previous Discussion

As of 2:00 PM PDT Wednesday, Fort Ord Profiler put the depth of the marine layer this afternoon around 1500 ft, with most of the low stratus hugging the coast. Of interesting note, a notable temperature gradient is evident across San Francisco County, with the west side of the county running 5-7 degrees cooler than than east side of the county, owing to the low stratus deck which has been sitting over the west side of the county all day.

The axis of the longwave trough is currently over BC, with the PAC NW on the divergent side. To our south, a shortwave trough is expected to continue moving towards our CWA. Tropical storm Elida is due west of Baja and gradually weakening as it moves to the northwest. Expecting its remnants to be absorbed by the shortwave and advected northward into our CWA Thursday through Friday. Model guidance is currently suggesting that the tropical moisture will move into our southern CWA sometime around late Thursday morning. Theta-e tongue can be observed on the NAM-12km runs for today, while Total Totals >30 are apparent on both the GFS and ECMWF runs over southern Monterey County and fairly close to the Delta. Not expecting widespread thunderstorms and showers, nor is there a significant probability for them Thursday and Friday. Nevertheless, should any convection develop, expect the most likely areas for their development to be down in Southern Monterey/San Benito counties on Thursday. Moreover, PWAT values on both the GFS and ECMWF runs today stay relatively over an inch across that area, suggesting that if any convection does develop then we should expect for it to be wet convection. Nevertheless, going to have to keep an eye for lightning. Theta-e values increase through Friday across the central part of the CWA as the tropical moisture continues tracking northward. As such, the slight chance for possible showers and thunderstorms will move north and westward on Friday. NAM- 12km runs have kept most of that risk offshore, however, so currently keeping inland chances fairly low.

Friday onwards, expecting the warm up across the region to really get going as the high over the Desert Southwest continues to build. Models suggesting heights >590dm 500hPa across most of our eastern CWA, while 850hPa temps are expected to be in the upper 20 C range. Models have continued to trend towards this setup, and if it continues to trend this way, then high confidence will exist for widespread 90s and low 100s to be expected across the interior this weekend, with some isolated locations like Pinnacles and Lake Berryessa potentially getting into the 100 to 108 degree range. Not much in the way of relief is expected through the weekend, with poor humidity recoveries and hot conditions to influence most of our CWA. Sharp temp gradients are expected between the coast and areas just a few miles inland this weekend as a result. Even along the coast, expect weekend max temps in the mid-70s and even the low 80s. All that being said and after coordinating with neighboring offices will maintain an Excessive Heat Watch beginning Friday afternoon continuing through Sunday evening for interior East Bay, Santa Clara Valley (recently added), and Southern interior of Monterey/San Benito.


.as of 5:08 PM PDT Wednesday, for 00z TAFs. Marine layer has compressed to just over 1000 feet. This will keep low clouds out of SF Bay for this evening but shallow cigs will move into Monterey Bay terminals before sunset. Given the shallow marine layer, northerly gradient and lighter onshore winds will not be expecting stratus to be as widespread Thursday morning. Should be an early mix out for any terminals that get cigs due to shallow nature. Will be expecting increasing mid and high clouds during the day Thursday, impacting the Central Coast first and then spreading northward later into Thursday evening.

Vicinity of KSFO, VFR this evening with no cigs forecast overnight. Any patchy clouds Thursday morning should mix out by 16z with some increasing high clouds by afternoon. Light afternoon seabreeze on Thursday.

SFO Bridge Approach, Similar to KSFO.

Monterey Bay Terminals, Low cigs to impact KSNS shortly and then KMRY overnight. Shallow cloud deck should mix out by 16z Thursday.


As of 09:24 PM PDT Wednesday, Expect gusty northwest winds mainly over the northern coastal waters. Northwest winds decrease later in the week. Seas will remain mixed with a short period northwest swell and a longer period southerly swell.

Watches, Warnings, Advisories

Tonight, sca, pt arena to pigeon pt 10-60 nm sca, pt arena to pt reyes 0-10 nm until 3 am

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