UPDATE
Issued at 817 PM PDT Thu Oct 3 2024

Warm conditions continue tonight across the interior Bay Area and Central Coast, with current temperatures in the 80s and lower 90s across the thermal belts. The Excessive Heat Warning for the North Bay Interior Mountains does expire at 11 PM tonight, but a Heat Advisory will be issued at that time to last through Saturday. Elsewhere as stated in the previous discussion, heat products (Excessive Heat Warnings and Heat Advisories) have been extended into Saturday as the forecast has trended warmer. The operations floor considered letting the Heat Advisory expire tonight for the coastal regions, as temperatures will start to cool slightly. However, based on the warming trend for the weekend, and the cumulative effects of heat on the human body, we ultimately elected to continue the Heat Advisory through Saturday.
An extension into Sunday is still possible, particularly for the interior Bay Area where widespread Major HeatRisk (heat that impacts anyone without effective cooling or adequate hydration)
continues.

The Red Flag Warning in the interior Central Coast was allowed to expire, but near critical fire weather continues into the weekend.
For more information, see the updated Fire Weather discussion.

DialH

SHORT TERM
Issued at 240 PM PDT Thu Oct 3 2024 (This evening through Friday)

Key Points: * Updated forecast trending warmer into the weekend * Some heat hazard products have been extended into Saturday * Red Flag Warning may expire this evening, but NEAR CRITICAL conditions persist in the weekend * Check latest SFONPWMTR for Heat Product updates

The Bay Area and Central Coast continue to be gripped by an impressive round of heat. While 24 hour trends do show cooler temperatures (slightly cooler airmass/shifting of upper level high pressure)that is not the whole story. Current observations around the region still show many locations well into the 90s and lower 100s. Pretty impressive to see Big Sur sitting at 104 degrees. Additionally, San Jose has a chance of breaking 100 degrees again. If that happens, it would be the first time San Jose is 100 deg or more three days in a row for October. I have been be forecasting CA weather for nearly 20 years and this event is one of the more notable heat events for October that I can remember. Not just daytime temps, but overnight lows staying in the 80s even with longer nights.

Tonight into Friday: The ridge overhead will provide another night of mild temperatures and mostly clear skies. Thermal belts will remain active with lows staying in the 70s/80s hills and mid 50s to lower 60s valleys/coast. The one thing that could be a plus is a southerly surge sneaking up the coast. Not looking promising at the moment, but low cigs remain around Pt Conception and the HRRR does show some cloud sneaking up the coast to Monterey Bay (20-30%). This may bring some shallow/immediate coastal relief, but most of the forecast area won't feel any impacts.

To round out the workweek...a slight pause or downward dip in the heat. A shortwave trough (currently seen on satellite off the PacNW) will move through flattening of the ridge. Temperatures will cool 3-5 degrees. Despite the cooling HeatRisk still remain elevated and temperatures will still be near record levels. Highs on Friday will be in the 70s to near 90 coast and upper 80s to lower 100s inland/mts. Current Heat Advisory and Excessive Heat Warning remain for Friday. The exception will be N Bay Mts where conditions meet Heat Advisory. This will likely be issued by the night shift.

LONG TERM
Issued at 250 PM PDT Thu Oct 3 2024 (Friday night through next Wednesday)

The brief dip with the longwave pattern goes away as the upper level ridge surges northward over the forecast area again. For a few days models showed a gradual step down temperature wise for Saturday and Sunday, but latest forecast has trended warmer.
Still no real sign of a big marine push and the ridge doesn't go away. Therefore, temperatures have been raised both day and night for Saturday and Sunday. HeatRisk still highlights a large portion of the Bay Area for Major on Saturday with even a few pockets of Extreme. Have extended the Excessive Heat Warning for South and East Bay, and Santa Cruz Mts. Elsewhere will be a Heat Advisory for Saturday. The good news is subtle cooling Sunday will allow for many locations to drop categories HeatRisk. Will likely only need advisories on Sunday.

Next week, the cooldown has been delayed and Monday now looks to hold onto some impactful heat. Will have to evaluate over time if any advisories are needed on Monday. Cooler weather finally returns Tuesday as the ridge breaks down and a trough approaches form the northwest. Onshore flow and a possible marine layer return.

AVIATION
(00Z TAFS)
Issued at 422 PM PDT Thu Oct 3 2024

Currently VFR at all terminals as high pressure continues to dominate. High confidence in VFR through the TAF period at all terminals. Winds will become light overnight with a sea breeze tomorrow afternoon. High pressure and light winds may also result in reduced slant range visibilities due to haze.

Vicinity of SFO...Currently VFR with northwesterly flow. High confidence in VFR through the TAF period. Winds will become light overnight with a sea breeze tomorrow afternoon.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Currently VFR with northwesterly flow at both terminals. High confidence in VFR through the TAF period at both terminals.

MARINE
(Tonight through next Wednesday)
Issued at 817 PM PDT Thu Oct 3 2024

Gentle to moderate northerly winds will prevail through Sunday as high pressure remains in place. As the pressure gradient tightens Monday, northwesterly breezes become moderate to fresh and seas become rough in the outer waters.

FIRE WEATHER
Issued at 817 PM PDT Thu Oct 3 2024

Although the Red Flag Warning for the interior Central Coast has been allowed to expire, near critical fire weather conditions continue through the weekend across the district, due to low relative humidities and low fuel moistures. Pulses of offshore winds could develop in the higher elevations each night and morning, especially in the North Bay and Central Coast, but these are not expected to reach Red Flag criteria.

From the previous discussion: One potential area of concern are winds picking up across the higher terrain of the N Bay due to a passing disturbance to the north. Not strong enough to need a Red Flag warning, but some gusts of 20-25 mph will be possible.

Additionally, the Central Coast will continue to get bursts of offshore flow through Saturday keeping fire weather concerns elevated.

Still valid: Other concerns from a fire weather perspective are the fuels. Latest observations suggest critically dry fuels.
Current 100 and 1000 hr fuels are near or slightly below avg, but trending drying Current 10 hr fuels are critically low as well.
Additionally, latest intel with Live Fuel Moisture shows values trending toward the 60% threshold. Simply put, fuels are receptive to fire starts.

CLIMATE
Issued at 832 PM PDT Thu Oct 3 2024

Record high temperatures and years of occurrence at the long term climate stations October 4 through October 5.

Station Oct 4 Oct 5

Santa Rosa 101 2020 103 1933 Kentfield 99 1987 100 1987 Napa 100 1987 101 1987 Richmond 99 1987 99 1987 Livermore 106 1980 100 1980 San Francisco 100 1987 102 1987 SFO Airport 95 1987 99 1987 Redwood City 101 1980 99 1987 Half Moon Bay 87 1953 90 2023 Oakland museum 92 2014 96 1987 San Jose 96 1987 101 1987 Salinas Airport 100 1987 105 1987 King City 106 1933 105 1933

MTR WATCHES/WARNINGS/ADVISORIES
CA...Heat Advisory until 11 PM PDT Saturday for CAZ006-506-509- 528>530.

Excessive Heat Warning until 11 PM PDT this evening for CAZ502>504.

Excessive Heat Warning until 11 PM PDT Saturday for CAZ508-510- 512>515.

Excessive Heat Warning until 11 PM PDT Friday for CAZ516>518.

PZ...None.