Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

938 am PST Sun Dec 9 2018

Synopsis

Patchy dense fog will continue in some of the inland valleys through late morning. Otherwise, expect increasing clouds and today along with cooler temperatures. A weak front will bring the possibility of light rain tonight and Monday morning, primarily to the North Bay but also locally south of the Golden Gate. Another weak system may bring light rain to the far northern portion of our region around midweek. There is a potential for more substantial and widespread rainfall late in the week and into next weekend.

Discussion

As of 8:55 AM PST Sunday, Dense fog persists in the East Bay Interior Valleys this morning. The Livermore Airport is currently reporting a visibility of less than one quarter mile. A Dense Fog Advisory remains in effect for the East Bay Interior Valleys until 11 am. The advisory also covers the San Francisco Bay north of the Bay Bridge, San Pablo Bay, Suisun Bay, and the West Delta. Visibilities are expected to improve in all areas by late morning.

High clouds have been spreading across our area since a few hours before dawn. These high clouds, associated with a weak disturbance just ahead of an incoming Pacific frontal system, will result in cooler temperatures across our area today compared to yesterday.

Satellite currently shows a weak cold front about 400 miles west of San Francisco. This front is forecast to move into northern California late tonight and Monday morning and bring light rain primarily to areas north of I-80. The 12Z NAM forecasts light rain will spread across most of the North Bay tonight and as far south as Santa Cruz County on Monday morning. The latest HRRR, however, forecasts only spotty light rain in the North Bay tonight, as well as the SF Peninsula on Monday morning. Rainfall amounts through Monday morning are only a tenth of an inch or less as forecast by the HRRR, while the local WRF model is somewhat more optimistic in forecasting locally up to 0.40" in the North Bay. Dry conditions will likely return to most areas by Monday afternoon.

With widespread cloud cover and possible precipitation late tonight, it's unlikely we will see much in the way of dense fog. After the weak cold front moves through tomorrow, models indicate that drier air will mix into the boundary layer which will reduce dense fog chances by Monday night.

Another weak cold front may bring light rain to the far northern fringes of our forecast area late Tuesday night. Otherwise, expect dry weather through most of the remainder of the work week. Rain chances will return by Friday and into next weekend, when longer range models indicate the potential for more significant and widespread rainfall.

Aviation

.as of 9:38 AM PST Sunday, for 18Z TAFs. Tule Fog in the Central Valley continues to affect the East Bay with KLVK reporting 1/4 mile vis and fog. Fog also remains in areas of the North Bay affecting KSTS. Expect fog to continue for maybe the next hour or two. Once the fog clears, VFR conditions to prevail through tonight. Chances of fog returning again late tonight/early tomorrow morning for KSTS and KLVK. High clouds currently overhead between 15,000 and 25,000 feet with spotty lower cigs due to fog. Cigs will gradually lower overnight as the cold front approaches, but thus far expecting cigs to remain generally above 3,000 feet. Latest high res models have backed off a bit on precip for the region. Kept VCSH in for the North Bay and Bay Area taf sites from late tonight through late tomorrow morning. Winds will remain generally light through the period.

Vicinity of KSFO, MVFR/VFR conditions through the period. Expecting improvement of vis to VFR by the afternoon. High clouds will gradually lower tonight, but are expected to remain above 4,000 feet outside of passing showers. Shower chances begin tonight lasting through the early afternoon tomorrow. Generally light winds through the period.

SFO Bridge Approach, Similar to KSFO.

Monterey Bay Terminals, VFR conditions are forecast to prevail at both SNS and MRY through the period. Generally light winds through the period as well, locally higher southerly winds this morning at KSNS. Latest high res models continue to suggest less precip for the Monterey Bay region. Regardless, shower chances do not begin until after this current taf period.

Marine

As of 08:07 AM PST Sunday, Locally dense fog will continue to impact the northern San Francisco Bay through at least late this morning. Southerly winds will increase today ahead of a weak cold front set to arrive into tomorrow morning. Winds will then increase and become northwesterly in the wake of the frontal passage tomorrow. Moderate northwest swell will continue before a larger northwest swell arrives towards the end of the coming week.

Watches, Warnings, Advisories

Today, dense fog advisory, caz510 sca, pt arena to pigeon pt 10-60 nm marine fog advisory, SF Bay until 11 am

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