SHORT TERM
(This evening through Monday)
Issued at 158 PM PST Sun Dec 8 2024

Widespread hazy conditions were observed this morning throughout the Bay Area with several airports reporting visibility reductions as late as 10 AM this morning. Wondering why this happened? The 4AM sounding can help us figure it out. At 4AM, the sounding observed a temperature inversion, also known as "capping inversion", in the lowest levels of the atmosphere. Under normal conditions the ambient air temperature cools as you go up, however, when an inversion is present the ambient air temperature actually warms as you go up. If you were to take an air parcel from the surface and launch it into the atmosphere under non-inversion conditions, the parcel will cool as it rises and will continue rising until the ambient air temperature is warmer than the air inside the air parcel upon which the parcel will sink back down to the surface. Under normal conditions this allows the atmosphere to be relatively well mixed and can help prevent prolonged periods of hazy conditions at the surface. Comparatively, if you attempted to lift a surface air parcel when an inversion is present, the parcel would not be able to rise as the air above it would already be warmer than the air at the surface. This prevents the atmosphere from being well mixed and can keep pollutants trapped at the surface, leading to more widespread hazy conditions as was observed this morning.

Hazy conditions should improve this afternoon into the evening as surface temperatures continue to warm, weakening the low level temperature inversion. Surface winds will also pick up slightly (peaking between 10 to 15 mph today) which will help to increase mixing in the lower levels of the atmosphere and help any lingering patches of haze to dissipate. Unfortunately, model guidance shows the low level temperature inversion strengthening again overnight and is expected to persist into tomorrow. This may allow for a return of hazy conditions overnight through tomorrow morning.
Outside of haze concerns, satellite shows a few high level clouds moving out of the Monterey/San Benito region with clear skies to prevail everywhere afterwards. Temperatures will peak in the mid to upper 60s across the interior while closer to the cost highs will largely be in the upper 50s to low 60s. With clear skies expected overnight, low temperatures will drop between 3 to 6 degrees from last night. For most locations this keeps lows in the low to mid 40s while portions of the interior North Bay, East Bay, and Central Coast will cool into the mid to upper 30s. Across the southern Salinas Valley a Frost Advisory has been issued from 12 AM to 9 AM Monday for temperatures as low as 32 degrees. The coldest spots will be in the Fort Hunter Liggett area and around the town of Bradley where lows will be between 32 to 34 degrees. Spots closer to and north of King City within the southern Salinas Valley will be slightly warmer with lows between 36 to 38 degrees expected.

LONG TERM
(Monday night through next Saturday)
Issued at 158 PM PST Sun Dec 8 2024

As we head into Monday, an upper level trough will deepen and dig into the Intermountain West while upper level ridging and high pressure builds back in over the West Coast. This will result in a setup conducive for gusty offshore (north to northeast) winds across our CWA with the interior North Bay Mountains, East Bay Hills, and Santa Cruz Mountains seeing the strongest gusts. Gusts between 30 to 40 mph are expected across the interior North Bay Mountains, particularly in Napa County along the borders of Lake and Yolo Counties. Gusts between 20 to 30 mph are expected across the East Bay Hills and Santa Cruz Mountains. While gusts will be stronger in the higher elevations, residents of the North Bay, East Bay, and near the Santa Cruz Mountains should secure loose outdoors objects in the case of isolated stronger gusts mixing down to the valleys/lower elevations. Dry conditions continue across the interior Central Coast but widespread fire weather concerns are not expected.

Highs will peak in the mid to upper 60s on Monday before a gradual cooling trend begins Tuesday with highs peaking in the low to mid 60s through Thursday. Friday into the weekend, highs will drop into the upper 50s to low 60s as our next potential rain system arrives.
Widespread cold temperatures in the low to upper 30s are expected Monday morning through Wednesday morning. Confidence continues to increase that a widespread Frost Advisory will be needed across the interior regions Tuesday morning which is shaping up to be the coldest day of the week. We will start to see morning temperatures warm into the upper 30s to low 40s Thursday into the weekend for the interior, excluding the North Bay Valleys and southern interior Monterey County where lows will remain in the mid to upper 30s.
Early risers should remember to check the temperature before heading outside this week and bundle up as needed.

Guidance continues to indicate the pattern will become more active mid to late next week with a shortwave trough arriving midweek and a stronger low pressure system arriving Friday into the weekend.
Precipitation is not expected with the initial shortwave trough (Wednesday into Thursday) but guidance continues to indicate a low (15-20%) chance of drizzle along the coastline. The low pressure system arriving Friday into next weekend continues to be the more impressive potential rain maker but totals have continued to fluctuate compared to last night and yesterday. The overall trend is still the same with the highest totals (around half an inch to an inch) in the North Bay and generally less than half an inch across the rest of the Bay Area and Central Coast. Totals are likely to continue fluctuating as we get closer to Friday so continue to stay tuned and ready for the latest forecasts!

AVIATION
(00Z TAFS)
Issued at 400 PM PST Sun Dec 8 2024

VFR conditions with mostly high clouds over the terminals but some terminals are experiencing lower visibility from haze but still remain in the VFR thresholds. Due to the temperature inversion, expect haze to return this evening as winds become very light. This will bring a mix of MVFR/IFR/LIFR conditions tonight into early Monday morning. VFR is expected to return by 17-18Z, as winds start to increase. Overall winds will remain light to moderate through the TAF period.

Vicinity of SFO...Currently VFR but confidence is low to moderate on timing of haze development. Models suggest near 09Z but there is a chance for a later development near 12Z and lasting through 17Z.
Wind direction will be predominately offshore but light through the TAF period.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR with lower visibility near KMRY but still remains VFR. Expect VFR to prevail through the TAF period but if haze continues to develop there is a chance for MVFR conditions overnight.

MARINE
(Tonight through next Saturday)
Issued at 400 PM PST Sun Dec 8 2024

Northerly winds hold for the rest of today and will quickly decrease overnight. This will allow for seas to abate by mid- Monday. For the rest of the week, look for light, shifting wind and a moderate NW swell. The next chance for rain, increased winds, and building seas could arrive late in the week.

MTR WATCHES/WARNINGS/ADVISORIES
CA...Frost Advisory from midnight tonight to 9 AM PST Monday for CAZ516.

PZ...Small Craft Advisory until 9 AM PST Monday for Pigeon Pt to Pt Pinos 10-60 NM-Pt Arena to Pt Reyes 10-60 NM.