Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

908 pm PDT Sat may 25 2019

Synopsis

Rain showers will develop after midnight as an cold weather system moves in from the north. In addition gusty west to northwest winds will develop near the coast after midnight. Showers and isolated thunderstorms are forecast for Sunday along with unseasonably cool daytime temperatures. Dry but cool conditions are forecast for Memorial Day. A gradual warming trend is expected early next week with temperatures expected to return to near normal by midweek.

Discussion

As of 9:05 PM PDT Saturday, An unseasonably cold upper low is currently centered just off the coast of central Oregon. A frontal boundary moving out ahead of the low center is already spreading rain into northern California as far south as Mendocino County. Based on latest model data, rain in our area is forecast to begin late tonight, but extrapolation of satellite and radar data would bring rain into our area as early as midnight. In any event, expect shower activity to develop overnight as the initial boundary moves through. In addition, west to northwest winds are expected to increase around midnight and become locally gusty in the early morning hours of Sunday, mainly in coastal areas. Gusts up to 40 mph are possible near the ocean and through coastal gaps from about 2 am until mid morning. Winds are expected to subside by late Sunday morning.

Scattered showers will continue through the day Sunday as the cold upper low tracks south directly across the Bay Area. Isolated thunderstorms are a definite possibility on Sunday considering the combination of an unseasonably cold airmass aloft and strong surface heating due to the late May Sun angle. The 00Z NAM forecasts 500 mb temperatures to drop as low as minus 28 deg C by early Sunday afternoon - a temperature that would be considered quite cold for mid winter. The cold air aloft will also likely mean small hail in some of the heavier showers and isolated thunderstorms.

High temperatures on Sunday are forecast to mostly range from the mid 50s to mid 60s, with cooler readings expected in the hills. These temperatures will be as much as 20 degrees cooler than normal for late May. Snow levels are projected to drop as low as 4500 feet on Sunday and so light snowfall accumulation is possible on the higher peaks and ridges of the Santa Lucia Mountains in Monterey County. There may even be brief light snow at the summit Mount Hamilton in the Bay Area.

Precipitation is expected to taper off from north to south during the late afternoon and evening hours of Sunday as the upper low moves off to our southeast. Nearly all shower activity is expected to end by midnight Sunday night. Rainfall totals with this system are forecast to be a quarter inch or less in most locations. However, some locations in the southern half of our forecast area could see as much as a half inch, and even locally more in the southern Diablo Range or Santa Lucias.

The upper low is forecast to be centered near Las Vegas by late Sunday night and no longer pose a threat for precipitation in our area. Therefore, Memorial day is expected to be a dry day across our region. Temperatures will begin to moderate on Monday, but still be much cooler than seasonal averages.

Gradual warming is expected during the first half of next week as the longwave trough position shifts to our east. Temperatures may climb back close to normal by midweek. Longer range models maintain dry weather through the end of next week and into next weekend with temperatures remaining near, to slightly below, normal.

Aviation

As of 05:25 PM PDT Saturday, Marine stratus covers much of the coast south of San Francisco, otherwise skies are clear to partly cloudy. VFR conditions currently prevail at all SF Bay Area terminals, while MVFR ceiling onset has already occurred at KMRY and KSNS. Very unusual weather circumstances for this time of year to be developing late tonight into Sunday, as a cold winter-like upper low plunges southward into our area. It will bring gusty onshore winds, periods of -SHRA and SHRA, and even a possibility of some thunderstorms and small hail. Primary impacts look to be south of San Francisco during the day on Sunday.

Vicinity of KSFO, VFR conditions with moderately strong and gusty onshore winds are forecast to continue through the evening rush, but can't completely rule out the possibility of an earlier short-lived MVFR cloud deck spilling through the gap into the terminal area. With the approaching storm system, expect winds to actually increase significantly out of the west during the late night and morning hours, becoming strong and gusty, along with developing rain showers.

SFO Bridge Approach, Similar to KSFO, though more confident the approach will remain clear through the evening hours than the terminal itself.

Monterey Bay Terminals, MVFR ceilings are expected to prevail through the night and into Wednesday morning. Risk of lowering to IFR however is considered to be pretty low. With the approaching storm system, expect winds to strengthen out of the west during the late night and morning hours, becoming moderately strong and gusty, along with developing rain showers.

Marine

..as of 05:25 PM PDT Saturday, Marine stratus covers much of the coast south of San Francisco, otherwise skies are clear to partly cloudy. VFR conditions currently prevail at all SF Bay Area terminals, while MVFR ceiling onset has already occurred at KMRY and KSNS. Very unusual weather circumstances for this time of year to be developing late tonight into Sunday, as a cold winter-like upper low plunges southward into our area. It will bring gusty onshore winds, periods of -SHRA and SHRA, and even a possibility of some thunderstorms and small hail. Primary impacts look to be south of San Francisco during the day on Sunday.

Vicinity of KSFO, VFR conditions with moderately strong and gusty onshore winds are forecast to continue through the evening rush, but can't completely rule out the possibility of an earlier short-lived MVFR cloud deck spilling through the gap into the terminal area. With the approaching storm system, expect winds to actually increase significantly out of the west during the late night and morning hours, becoming strong and gusty, along with developing rain showers.

SFO Bridge Approach, Similar to KSFO, though more confident the approach will remain clear through the evening hours than the terminal itself.

Monterey Bay Terminals, MVFR ceilings are expected to prevail through the night and into Wednesday morning. Risk of lowering to IFR however is considered to be pretty low. With the approaching storm system, expect winds to strengthen out of the west during the late night and morning hours, becoming moderately strong and gusty, along with developing rain showers.

As of 8:54 PM PDT Saturday, Gusty northwest winds will continue through the night and into Sunday, with gale-force winds over the northern outer waters tonight. Scattered showers and isolated thunderstorms are possible beginning early Sunday morning. Moderate northwest swell will continue through the weekend with steep choppy seas generated by the gusty winds.

Watches, Warnings, Advisories

Tonight, sca, SF Bay from 9 pm sca, mry bay from 9 pm sca, pigeon pt to pt pinos 0-10 nm from 9 pm sca, pt pinos to pt piedras blancas 0-10 nm from 9 pm glw, pt arena to pigeon pt 10-60 nm until 3 am sca, pt arena to pt reyes 0-10 nm sca, pt reyes to pigeon pt 0-10 nm sca, pigeon pt to pt piedras blancas 10-60 nm sca, rough bar advisory for sf bar

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