Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

430 am PDT Mon Oct 15 2018

Synopsis

Offshore flow will result in warmer and drier conditions region-wide today, along with critical fire weather conditions in the hills of the North and East Bay through midday. Gusty winds in the hills are forecast to diminish by this afternoon. Dry weather is forecast to persist through the end of the week, along with mild to warm daytime temperatures.

Discussion

As of 3:20 AM PDT Monday, North to northeast winds have been locally strong and gusty in the North Bay Mountains since last evening. Wind sensors near the summit of Mount Saint Helena in the far North Bay (4300' elevation) have been consistently reporting sustained winds of 40 to 60 mph, along with gusts of 50 to 70 mph, since Sunday evening. Elsewhere in the North and East Bay Hills, winds have generally been gusting between 25 and 45 mph. Mesoscale models have indicated that winds would peak between late last evening and about now, and then gradually subside. Already seeing some evidence that wind speeds are starting to come down, but the rate of decrease is expected to be slow. A Wind Advisory remains in effect for the North Bay Mountains through 10 am this morning.

Gusty offshore winds in the hills have brought a very dry airmass into the North and East Bay, where relative humidity values did not recover at all overnight, and mostly remained in the range of 15 to 25 percent. Widespread Red Flag conditions have been occurring across the higher elevations of the North and East Bay since yesterday afternoon. The Red Flag Warning is set to end at Noon today when winds finally subside sufficiently. For more details on critical fire weather conditions, see the Fire Weather Discussion below.

The dry airmass that has arrived across most of our area, and continued light offshore flow through the day today, will mean warmer temperatures today. Highs are expected to be in the 70s in most coastal areas, and 80s across inland areas. A shallow marine layer has managed to persist in coastal areas from San Mateo County southward, where offshore flow has not been strong enough to mix into the boundary layer. Thus, our southern coastal areas may not warm as much today as anticipated today, with temperatures perhaps remaining in the 60s.

The current synoptic scale pattern consists of an upper ridge centered to our north along the coast of the Pacific Northwest and a trough to our south-southeast over the Desert Southwest and far southern California. This Rex Block pattern is forecast to undergo only minor changes as the week progresses. What this means for our area is continued light offshore flow, mild to warm daytime temperatures, and persistently dry air across most of our area. The low confidence part of the forecast for the rest of the week concerns the marine layer. The magnitude of offshore flow may be too weak to prevent a shallow marine layer from persisting in coastal areas (as is currently the case along our southern coast). So, coastal temperatures will be difficult to predict this week. Also, dry air and relatively long mid-October nights will mean rather chilly nights for the inland valleys starting tonight, with some of the coldest valleys expected to see overnight lows in the upper 30s and lower 40s.

Longer range models indicate the blocking pattern will finally break down by early next week, and we could see rain in the northern portion of our area by next Tuesday (the 23rd).

Aviation

As of 04:30 AM PDT Monday, For 12z tafs. Offshore flow has kept much of the area under clear skies overnight. Stratus has developed along the San Mateo coast and southward to Monterey and down the Salinas Valley. VFR conditions will prevail through the period for all sites with the exception of KMRY and KSNS. LLWS is forecast to continue through the morning when winds aloft subside.

Vicinity of KSFO, VFR. N to NE winds around 10 kt will continue with LLWS through around 18Z with NE winds 25-30 kt above 2000 ft. Winds aloft will subside throughout the morning.

SFO Bridge Approach, similar to KSFO.

Monterey Bay Terminals, IFR cigs occasionally scattering out through around 15Z-17Z. VFR conditions will then prevail. Generally light winds through the period.

Marine

As of 02:13 AM PDT Monday, Gusty offshore winds can be expected through late this morning in the Suisun Bay and the West Delta as well as over the waters near coastal gaps. Winds will diminish this afternoon and then remain generally light to moderate through the forecast period. Mixed swell will continue with a long period southerly swell through mid week and a more moderate period northwest swell. A longer period northwest swell will arrive during the second half of the week.

Watches, Warnings, Advisories

Today, wind advisory, north bay mountains until 10 am red flag warning, north bay mountains and east bay hills until noon sca, pt reyes to pigeon pt 0-10 nm until 2 pm sca, SF Bay until 11 am

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