Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

917 pm PDT Fri Sep 30 2022

Synopsis

Look for a slight cooling trend this weekend as onshore flow increases and the marine layer deepens. Temperatures gradually warm across the interior for next week as high pressure builds over the region.

Discussion

As of 09:14 PM PDT Friday, Relatively quiet weather on the last day of September. A slight warmup was observed this afternoon due to sunny skies and more importantly warmer airmass with weak offshore flow. High temperatures topped out in the 80s to low 90s across the interior/bay shoreline with 70s along the coast.

There will be a slight change in the upper level pattern tonight and over the weekend with weakening high pressure. The weak offshore flow noted earlier is also easing as lower level onshore flow returns. Satellite fog product shows the low clouds lurking along the coast for most of the day are returning inland late this evening. Some patchy fog was seen here at the office (Monterey) earlier when the clouds returned, but has since lifted to a low cig. Nearby Monterey Airport has a cig of about 400 feet. Monterey Bay coastline and Salinas Valley already have a decent amount of cloud cover. Fort Ord Profiler puts the marine push at roughly 900 feet. Expect the marine layer to hold around 1,000 feet tonight, but spread into more inland valleys.

For the weekend, night and morning low clouds will continue with onshore flow. The weakening upper level high pressure will allow for a subtle cooling trend to develop. Highs will be mainly in the upper 60s to 80s or a few below normal. Despite the "cool-down" it will still be pleasant each afternoon.

Previous Discussion

As of 01:56 PM PDT Friday, Conditions across the region continue to be defined by a shallow marine layer with low clouds and/or fog near the coast, mid/upper level low over the Intermountain West and building high pressure out over the eastern Pacific. The system inland has resulted in weak offshore flow across the higher terrain of the Interior North Bay Mountains and the East Bay. However, winds are not overly strong and humidity is not critically low, limiting fire weather concerns. That said, temperatures across the interior have already warmed into the mid 80s with a few degrees of additional warming through the afternoon. At the coast, 60s are being reported within the low level clouds and/or fog while locations with sunshine have warmed into the 70s.

The mid/upper level low will weaken and pull away from the region through the weekend. This will allow for a stronger push of onshore flow resulting in a cooling trend. Additionally, the marine layer will likely deepen keeping low clouds and cooler conditions near the coast while the interior warms into the 70s to lower 80s.

The aforementioned mid/upper level ridge over the eastern Pacific will then shift to the east and strengthen through much of next week. This will bring a gradual warming trend to the interior with more widespread 80s by midweek and a few lower 90s in the region's warmest locations by Friday. Along with the warming trend, dry weather conditions will prevail through the forecast period.

Aviation

As of 6:03 PM Friday, For the 00Z TAFs. The marine layer depth varies from 500 feet at the Bodega Bay profiler to 1,000 feet at the Fort Ord Profiler. Fog and stratus /VLIFR-IFR/ along the immediate coast locally extending inland on onshore breezes this evening and overnight. Otherwise VFR during the period, best chances for VFR far inland. The SFO-SAC pressure gradient is 1.5 mb and the SMX-SFO pressure gradient is 2.6 mb. VLIFR-IFR Saturday morning until vertical mixing developing later in the morning and afternoon helping to lift ceilings to MVFR-VFR. HREF output forecasts coastal fog and stratus /VLIFR-IFR/ returning Saturday evening and night.

Vicinity of SFO, VFR, west to northwest wind near 15 knots through the evening. Wind decreasing to 5 to 10 knots tonight and Saturday morning with MVFR ceiling 12z-17z Saturday. Conditions lifting to VFR late Saturday morning and afternoon with a west wind 10 to 20 knots in the afternoon and early evening.

KSFO Bridge Approach, Similar to SFO.

Monterey Bay, Fog and stratus /VLIFR-IFR/ tonight and Saturday morning, onshore winds 5 to 15 knots in the evening easing to near 10 knots overnight. Conditions lifting to MVFR-VFR by late Saturday morning and afternoon.

Marine

As of 05:45 PM PDT Friday, High pressure will center east of the area resulting in diminishing winds and seas through the weekend. Low pressure will evolve over the eastern Pacific towards the mid to late portion of next week with an increase in winds and seas anticipated.

Watches, Warnings, Advisories

Tonight, none.

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