Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

237 pm PDT Wed Mar 22 2017

Synopsis

Scattered showers along with a few thunderstorms can be expected into the early evening hours. The next storm system is projected to impact the region late week. Widespread rainfall and stronger winds return to the northern portion of the region late Thursday and will spread southward during the day Friday. Dry conditions return for most of Saturday with another weak system forecast to push through starting Saturday night. Dry weather and warmer temperatures are then likely next week.

Discussion

As of 3:00 PM PDT Wednesday, KMUX radar shows scattered showers continuing across our region -- especially for the southern third of the CWA. Returns indicate a few moderate cells so would expect a few are dropping small hail. Showers will greatly diminish once the Sun goes down, and additional accumulations will mostly be under a tenth.

Dry weather will return for tonight through the majority of Thursday before another system from the north moves through and brings widespread rainfall back into the picture. Models have been advertising this system for nearly a week and the latest guidance has only slightly wavered from previous runs. Now looks like rain will move into the North Bay Thursday night, down to SF Bay Friday morning and to Monterey Bay starting around noon. However, the main story will be the moderate to heavy band of rain associated with a surface front that will go through later than the initial round of rain. That band could produce high rain rates for a brief period possibly leading to some urban and small stream issues. At the same time southerly winds will be on the increase as the front approaches. Gusts in many urban locations of 25 to 35 mph can be expected. Values to 50 mph are possible above 1,500 feet. Guidance has slightly trended downward so at this time do not expect to issue wind advisories.

Rainfall amounts have only slightly changed from previous forecast packages. Still looks that the best IVT numbers (and longest duration) will focus on the North Bay with widespread 1.5-3" expected (locally 4+" for the North Bay Mountains). Similar values can be expected for the SC Mountains plus the Santa Lucias. Around SF Bay generally 2/3" to 1.5" is forecast with mostly less than 2/3" from the Santa Clara Valley southward.

Behind the front scattered showers are possible into early Saturday. This will be followed quickly by another system that will again move north-to-south late Saturday night through Sunday. This feature will be the weakest of the week and rainfall totals should be generally under 1/2" for most urban locations.

A ridge of high pressure will build behind that system and bring warmer and drier conditions back to our region for most of next week and possibly through the following week.

Aviation

As of 10:50 AM PDT Wednesday, Expect to see a trend towards gradually improving ceilings, visibilities, and fewer rain showers today. The bulk of the rain activity has now shifted eastward into the Central Valley, however, isolated to scattered rain showers could continue into the afternoon (primarily for east Bay and Monterey Bay regions). Winds will become breezy this afternoon, especially around the San Francisco Bay. Model data indicating potential for 2000-3000 ft cigs overnight.

Vicinity of KSFO, VFR. Onshore winds 10-20kt. Occasional gust to 25-30kt possible this afternoon.

SFO Bridge Approach, Same as KSFO.

Monterey Bay Terminals, Patchwork VFR/MVFR due to lowered ceilings. Some rain showers will move across the Monterey Bay terminals through late morning into the early afternoon.

Marine

As of 02:24 PM PDT Wednesday, Increasing northwest winds through Thursday morning. winds will weaken then turn southerly ahead of the next system through the day Thursday. Strong south to southwest winds will then prevail as a storm system moves across the coastal waters. High winds and seas can be expected Friday as the front moves through.

Watches, Warnings, Advisories

Tonight, sca, pigeon pt to pt pinos 0-10 nm from 9 pm until 3 am sca, pt arena to pt reyes 0-10 nm until 3 am sca, pt reyes to pigeon pt 0-10 nm until 3 am sca, pt arena to pigeon pt 10-60 nm until 3 am sca, pt pinos to pt piedras blancas 0-10 nm

Share this forecast!