Marine Weather and Tide Forecast for Millbrae, CA

SHORT TERM
(This evening through Saturday)
Issued at 207 PM PDT Fri Apr 18 2025

While not quite as deep as yesterday, the marine layer was deep enough to support a fairly impressive layer of cloud cover across the Bay Area and Central Coast. Cloud cover is rapidly dissipating with most sites expected to clear by early this afternoon. As of 1PM most sites are in the mid to upper 50s but temperatures should start to rise rapidly as cloud cover clears out. High temperatures today are forecast to peak in the upper 60s to mid 70s. As we head into the evening/overnight hours, the marine layer will compress to around 500 ft as high pressure builds. This will help to keep any stratus that is able to develop confined to coastal regions, to the Monterey Bay, and to the San Francisco Bay shoreline. With clearer skies forecast overnight across the interior the question becomes, what about fog? Well it all comes down to moisture. High resolution guidance is showing much lower PWAT values across the region (generally less than 0.5") tonight in comparison to the past few nights. The combination of lower PWAT values and a compressed marine layer suggests that any moisture will be confined to the coast and not extend into the interior Bay Area/Central Coast. The two main factors contributing to drier interior overnight conditions would be our building high pressure/ridging and continued offshore flow Friday evening into early Saturday morning. This offshore flow will be light, but, it should serve to transport drier, continental air into the Bay Area overnight thus decreasing moisture across the interior and inhibiting widespread fog development.

Diving a little deeper into offshore flow, generally light offshore winds have been observed across the region. The exceptions are the highest peaks of the North Bay Interior Mountains where Mt.
St. Helena West (elevation 4340 ft) reported a peak gust of 57 mph early this morning. Elsewhere, winds have generally been calmer with peak winds between 20-35 mph observed. The synoptic setup remains progressive with the positively tilted trough set to weaken and eject eastward while high pressure continues over the West Coast. Winds will then transition from offshore back to onshore again by early to mid Saturday morning (remaining light)
as ridging becomes more zonal/less positively tilted over the West Coast.

LONG TERM
(Saturday night through next Thursday)
Issued at 207 PM PDT Fri Apr 18 2025

Saturday night into Sunday, a weak shortwave trough will move through the region before zonal flow and ridging redevelop Sunday through Tuesday. The main impact of this shortwave will be to deepen the marine layer Saturday into Sunday and bring more widespread overcast conditions and some potential for fog to both the coast and interior regions. By Sunday night, zonal flow will again dominate with the marine layer expected to compress below 500 ft.
Temperatures will remain fairly seasonal through Tuesday with highs in the mid to upper 70s across the interior and upper 50s to mid 60s along the coast with overnight temperatures in the 40s to low 50s.
Minor HeatRisk, primarily impacting those incredibly sensitive to heat, is forecast across urban areas through mid next week. If you are sensitive to heat, remember to take breaks while outdoors and drink plenty of water.

The pattern changes mid week when an upper level trough is set to deepen and move into the West Coast. Cluster guidance has come into slightly better agreement (in comparison to yesterday) that troughing will develop mid week but there is still variance as to how deep the trough will be. Temperatures are expected to drop, becoming below seasonal normals, with highs in the upper 50s to low 60s, as this trough moves inland Wednesday and Thursday. Ensemble guidance indicates some potential for light rain as this system arrives but confidence is low as to the exact amounts. The CPC has below normal temperatures and leaning above normal precipitation continuing from the middle of next week through the end of April.

AVIATION
(18Z TAFS)
Issued at 1034 AM PDT Fri Apr 18 2025

Another morning with a pesky marine layer. Obs, satellite, and ACARS show depth still 2k feet deep. As such, clearing/thinning has been slow going. Therefore, many TAF sites were pushed back 30-60 min on clearing times or closer toward 19Z. Some stratus lurking this afternoon, but trending toward VFR. Tonight is low confidence as offshore flow develops at 2500 ft, but doesn't mix down until Saturday morning. Some compression of the marine layer is expected with impacts closer to the coast and immediate bay shoreline. Conf for tafs is medium today and low tonight.

Vicinity of SFO...Cigs will linger through 19z. Some thinning on cams/satellite, but delayed from prev taf. VFR this afternoon with onshore flow. Tonight, low cigs sneak through the GG and in the Gap, but conf is low on impacting SFO. Therefore, SFO will remain VFR. The same can be said across the Bay at OAK.

SFO Bridge Approach...Thinning quicker over approach and clearing earlier is expected 1830-19z.

Monterey Bay Terminals...Cigs lingering through 19z. VFR this afternoon and early return tonight.

MARINE
(Today through Wednesday)
Issued at 859 AM PDT Fri Apr 18 2025

Weak high pressure off the California coast will maintain light to moderate winds today with decreasing seas. Breezy winds build in the northern outer waters for Friday night with lighter winds expected for the inner waters. Seas and winds become more elevated into the weekend into the next work week.

MTR WATCHES/WARNINGS/ADVISORIES
CA...None.
PZ...Small Craft Advisory from 9 PM this evening to 3 AM PDT Sunday for Pt Arena to Pt Reyes 10-60 NM.