Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

928 pm PDT Mon may 25 2020

Synopsis

Very warm to hot temperatures will persist across the interior through Thursday while coastal areas will be relatively mild by comparison. Thus, a Heat Advisory remains in effect across the interior. Cooling will first take place at the coast on Wednesday before spreading inland Thursday and moreso into Friday as onshore flow increases. Cooler temperatures and unsettled conditions return to the region this upcoming weekend.

Discussion

As of 9:20 PM PDT Monday, Upper level high continues to build over the area. Record high temperatures were recorded today at SFO Oakland and Kentfield while San Jose tied a record. Forecast highs today verified pretty well today so no changes need to be made to the forecast which calls for another 2-4 degrees warmer on Tuesday. There will be widespread 90s over inland and mountain locations with the warmest locations reaching 104. The only relief will be 70s along the immediate coast which will quickly warm up into the 80s just a few miles inland.

Latest models trending slightly cooler for Wednesday especially along the coast but we believe inland and mountain areas will be about as hot as Tuesday. Inland and mountain temperatures both days are expected to exceed 90 degrees for up to 5-7 hours which could cause heat- related illnesses to those who are outside for very long. Not much relief at night as minimum temperatures will stay mostly in the 60s with some lower to mid 70s in the hills.

FROM PREVIOUS DISCUSSION, as of 2:28 PM PDT Monday, The forecast models typically try to cool conditions too soon, yet there is good consensus among the models and ensemble members that even inland temperatures will begin to cool on Thursday. This cooling will be the result of the ridge shifting further inland and the approach of a mid/upper level cut-off low currently on the western flank of the Omega Block aloft. Thus, the current Heat Advisory's extent in time may need to be adjusted in the coming days. Regardless, temperatures will remain well above seasonal averages across the interior and to a lesser extent near the coast on Thursday.

A more robust cooling trend is likely to occur on Friday as the aforementioned mid/upper level low beings to approach the central California coast. This system will not only cool conditions region- wide but also bring the potential for unsettled weather as it pushes into northern California late Friday into Saturday. Thus, there will be a slight chance of rain showers and even the potential for an isolated thunderstorm or two. While confidence remains low at this time regarding the details, we will need to monitor for thunderstorms as dry lightning and potential fire starts would be of concern given the prolonged period of hot and dry conditions. Stay tuned.

Aviation

As of 4:55 PM PDT Monday, For 00z TAFs. Skies will remain clear this evening and overnight. Strong 7 mb northerly gradient in place with high pressure overhead. Local onshore gradients will produce some moderate west winds this evening.

Vicinity of KSFO, Clear skies overnight into Tuesday. West winds to around 15 kt this evening and again Tuesday afternoon.

SFO Bridge Approach, Similar to KSFO.

Monterey Bay Terminals, Clear skies this evening and overnight. Evening northwest winds then light overnight with westerly seabreeze developing around 20z Tuesday.

Marine

As of 08:22 PM PDT Monday, Weakening high pressure over the eastern Pacific will keep northwesterly winds light to moderate across much of the coastal waters. Locally breezy conditions will persist over the northern waters through much of the week. These winds will generate steep fresh swell resulting in hazardous seas conditions for smaller vessels. Mixed seas will continue with a short period northwest swell and a longer period southwest swell. A longer period northwest swell will arrive later this week.

Watches, Warnings, Advisories

Tonight, heat advisory, caz506-507-510>513-516>518-528 sca, pt arena to pt reyes 0-10 nm sca, pt arena to pigeon pt 10-60 nm

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