Marine Weather and Tide Forecast for Mill Valley, CA

SHORT TERM
(This afternoon through Friday Night)
Issued at 1217 PM PDT Thu Apr 24 2025

Short Term Key Messages -Cool and cloudy conditions persist -Rain chances increase late Friday

The marine layer has established itself across the region and it's quite a stark contrast from 24 hours ago. Widespread cloud cover has been noted on the latest satellite presentation, though inland areas and regions of terrain are clearing very gradually.

With broad mid-level troughing encouraging more in the way of onshore flow, I anticipate clearing will be short lived and for some regions, especially along the coast, mostly cloudy skies will prevail. We'll be locked into this onshore flow regime through at least Saturday, and the forecast will remain on the cool side as a result. As it pertains to rain chances, it appears that a bulk of the "heavier" precipitation won't arrive until after 00 UTC Saturday or 5pm Friday. Prior to that, pockets of drizzle are forecast with perhaps a few areas measuring a few hundredths of an inch. If the forward speed of the upper system accelerates, then more widespread rainfall will be on the table earlier in the evening on Friday.

LONG TERM
(Saturday through next Wednesday)
Issued at 1217 PM PDT Thu Apr 24 2025

Long Term Key messages -Rain and isolated thunderstorms forecast -High elevation snow (Santa Lucia and Diablo Ranges)
-Warmer and largely rain-free Sunday onward

The main upper low will pinwheel just west of the Southern California Bight on Saturday. Forecast 700-500mb lapse rates should approach 7 to 7.5 C/km across the marine areas and southern Monterey County beneath the cold pocket associated with the upper low. This translates to a few hundred joules/kg of CAPE, though the max CAPE resides across SLO and points southward. The positioning of the low does modulate the thunderstorm risk. If the low slides more to the south, then the pocket of cooler air aloft will be farther displaced from our area, thereby lowering the isolated thunderstorm risk. If the upper low remains a bit farther north, then a greater portion of the Central Coast will be at a greater risk for isolated storms. Currently the primary hazard would be isolated lightning strikes. Brief accumulating hail cannot be discounted, especially with any of the more vigorous showers and/or storms.

Overall, rain amounts will be beneficial, especially across the Central Coast. This area largely missed out on some of the more robust AR events from this past year and this rain will at least mitigate the fire weather threat initially. Rain amounts will be quite respectable, given the time of year and amounts will average between a few tenths of an inch to perhaps as great as 1/2 inch.
In fact there's around a 50% chance that some parts along the Big Sur Coast/Santa Lucia Range may see between 1/2 to 3/4 of an inch of rain.

But wait...there's a little bit more. Some of the higher peaks of the Santa Lucia and Diablo Ranges may even get a bit more than "novelty" snowflakes as blended guidance suggests around a 50% chance for 2" of snow. At this time, no plans for any winter headlines as snow will be across a very confined to the highest peaks, but if snow levels come in a little lower, we may need to consider stronger messaging/wording.

Saturday evening through the remainder of the extended forecast appears that it'll remain quiet. This is supported with agreement among the various model solutions with ridging taking shape.
Moreover, longer term model predictions support odds leaning toward warmer and drier than normal conditions precip.

AVIATION
(18Z TAFS)
Issued at 1057 AM PDT Thu Apr 24 2025

MVFR cigs are expected to persist through most of the forecast period with increased chances of VFR late in the period by mid- morning Friday with an increase in mixing as the upper level low continues to approach from the north. Expect periods of drizzle along coastal sites tonight through early Friday morning as well.
Breezy to gusty onshore flow will also increase as the upper level system approaches.

Vicinity of SFO.. MVFR is expected to prevail through the forecast, however there is a chance of enough mixing for a window of VFR conditions this afternoon into early evening. Onshore winds will increase this evening becoming breezy as the upper level low approaches from the north. VFR chances increase toward the end of the TAF period late Friday morning.

SFO Bridge Approach...Similar to SFO

Monterey Bay Terminals...Expecting MVFR cigs to persist through the forecast period. There is a chance for VFR conditions this afternoon into the early evening, however current trends show a steady fetch of moisture rich onshore flow limiting that possibility. Patchy fog and drizzle are possible later tonight through Friday morning as well.

MARINE
(Tonight through next Tuesday)
Issued at 323 PM PDT Thu Apr 24 2025

Expect quiet weather for the rest of today. Light rain may enter our waters late Friday morning, with chances increasing by the late evening hours or early overnight hours on Saturday. There is a slight chance for thunderstorms for areas along and south of Pigeon Point. Rain chances look to linger through Saturday evening, especially for areas south of Pigeon Point. Strong northwesterly breezes and rough seas return late Saturday.

MTR WATCHES/WARNINGS/ADVISORIES
CA...None.
PZ...None.