Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

927 pm PDT Sun aug 14 2022


Gradual warming and drying continues as high pressure builds into the area from the east. Excessive heat concerns for the more interior locations in the mid week. Cooler conditions expected into next weekend.


As of 08:44 PM PDT Sunday, Another day of interior warmth and coastal cool is in the books, with the more inland areas peaking in the 90s and a few 100s, while the coast stayed breezy and cool in the 60 and 70s. Smoke has found its way into the Bay Area from the Six Rivers Lightning Complex fires, causing hazy conditions. The night is setting up to be mild again, mostly 50s and 60s.

The evening forecasting team did not see fit to adjust the afternoon forecast package, meaning the concerns listed in the previous discussion below remain. The main focus of the overnight forecast package will be with the warming interior areas and the cooling trend during the second half of the work week. The Excessive Heat Watch is still in effect from late Tuesday morning into that night.

Previous Discussion

As of 01:45 PM PDT Sunday, Afternoon satellite imagery shows predominately clear skies across the region aside from wisps of marine stratus along the immediate coast. 1PM temperatures are running a few degrees warmer today except throughout the Monterey Bay where the marine layer is having a cooling influence.

The warming and drying trend that has been advertised for much of the state is well underway with interior reaches of the Bay Area and Central Coast forecast to approach the mid to upper 90s this afternoon as the coastal trough weakens and the interior heat dome expands westward. The warming trend will peak on Tuesday, resulting in temperatures up to 15 degrees above normal across the interior, or into upper 90s to low 100s. Coastal and nearshore areas will remain nearer to seasonal normals due to the mitigation factors from the marine layer where the forecast is only for afternoon temperatures around 3 to 6 degrees above normal. Monsoonal moisture will return in the mid levels through the upcoming week which should moisten up the currently bone dry atmosphere while the marine layer remains banked along the coast, making pushes nearshore valleys into the afternoon and evening.

Latest model data continues to tip the scales towards higher temperatures Monday-Tuesday-Wednesday across the interior and the latest batch of data has pushed several California weather forecast offices, including ours, to include mention of an excessive heat watch for portions of their forecast areas. For us, our eastern most zones adjacent to the Central Valley will experience potentially dangerous heat on Tuesday afternoon and so we have issued an excessive heat watch from 11am to 8pm Tuesday to cover these risks. This product will be reevaluated over the next 24 hours and converted either to a heat advisory (most likely) if data remains on track, upgraded to a heat warning if even more heating is expected over a broader area (possible), or cancelled if the forecast changes significantly (unlikely).

One other item to watch: There is one particular swath of monsoonal moisture that arrives Tuesday PM into Wednesday AM into the Central Coast and SE San Francisco Bay Area that has a greater than zero chance of generating higher based convection. CAM derived output indicates Modified Total Totals of 32 to 35 (>30 is sufficient) and MUCAPE of 100-200 J/Kg (>100 is sufficient) as a weak vort wave passes near the area. There has been quite a bit of model to model and run to run consistency regarding this risk, though timing seems to vary +12/-18 hours from Tuesday PM. As such, will continue to monitor this risk and potentially need to adjust forecast to include slight chance mentions going forward. For now, have slightly boosted chance of precipitation to 5 to 10 percent range pending further confirmation from model data of this risk.


As of 5:10 PM Sunday, For the 00Z TAFs. It's VFR except for a few areas of stratus and fog /IFR/ along the immediate coastline. The marine layer depth is 1,000 feet at the Bodega Bay and Fort Ord profilers; not much change in depth is forecast tonight and Monday since steady lower level thermal ridging is forecast to remain in place. Robust northerly pressure gradient 6.3 mb ACV-SFO is per usual stronger than recent (18z) NAM output by ~ 1 mb. A compressed marine layer and ongoing northerly pressure gradient and wind should help limit the inland intrusion of stratus and fog, at least initially this evening. Gusty winds getting through the San Bruno Gap, peak gusts KSFO 30 to 33 knots so far. As the diurnal land temps vs sea temps contrast diminishes later in the evening so will the onshore gradient and wind. Gusty west to northwest winds redeveloping Monday afternoon and evening.

Vicinity of KSFO, VFR, strong ACV-SFO gradient and wind plus a compressed marine layer will help keep VFR going at least through early to mid evening. VFR likely to continue tonight and Monday morning mainly if the marine layer continues to remain compressed; little change in depth is forecast since the lower level thermal ridge is forecast to remain similar in strength as to present strength.

KSFO Bridge Approach, Similar to SFO.

Monterey Bay, VFR except VLIFR-IFR conditions increasingly likely by mid to late evening with northwesterly winds advecting stratus and fog toward the coast and into nearby valleys. Onshore winds easing by mid or late evening then gusty winds redeveloping Monday afternoon and evening.


As of 09:26 PM PDT Sunday, Strong northwesterly winds continuing across the coastal waters, with gale force gusts possible in the northern outer waters. Steep, hazardous short period seas will build as a result. Winds diminish Tuesday afternoon through mid week. Light southwest swell from 14 to 16 seconds continues through the week.

Watches, Warnings, Advisories

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

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