textproduct: Eureka

SYNOPSIS

The intense interior heat will continue to subside today as high pressure weakens. A passing upper-level disturbance will bring temperatures down to near seasonal averages by Friday, along with a low probability of lightning strikes over dry mountain fuels.

Key Messages:

* High temperatures decrease through Friday, lowering heat risk.

* Potential for dry thunderstorms over interior mountains late Thursday and Friday.

* Marine stratus will persist near the coast, lifting gradually late in the week.

DISCUSSION

The heatwave has peaked, with valley locations cooling down into the upper 80s to mid-90s under reduced HeatRisk categories. Along the coast, enhanced northerly winds should disrupt the marine inversion enough to clear coastal skies Wednesday afternoon, though a shallow stratus deck is expected to redevelop Wednesday night into Thursday morning.

A distinct cooling trend takes place Thursday as an upper-level trough moves toward the area. This feature will introduce increased cloud cover and strengthen onshore flow. Consequently, interior highs will drop into the 80s by Thursday, while the coastal marine layer becomes deeper.

Mid-level moisture advecting northward from the Sacramento Valley will interface with the cool pool aloft associated with the approaching trough. This setup introduces a 10 to 20 percent chance of dry thunderstorms across higher terrain in Trinity and Mendocino Counties during the late afternoon and evening hours for both Thursday and Friday. Temperatures will reach their lowest points on Friday before a minor warming trend initiates over the weekend.

AVIATION

Early morning LIFR ceilings and perhaps even LIFR visibility from time to time will become VFR by afternoon Wednesday due to increased boundary layer mixing and increasing northwest winds. Current confidence (80% chance) favors return of low stratus and reduced visibility Wednesday night. The marine layer will become less compressed on Thursday, allowing for quicker afternoon clearing and stronger coastal sea breezes.

MARINE

A tight pressure gradient between offshore high pressure and an inland thermal trough is generating a strong northerly winds across the outer waters, with gusts reaching gale force. This persistent fetch has generated steep, short period waves as well. Nearshore waters remain significantly calmer regarding wind speed, through the swell will maintain wave heights up to 10 feet through Wednesday. Conditions will gradually improve Thursday into Friday as the core of the strongest winds shifts farther offshore.

FIRE WEATHER

Extremely hot and dry conditions over the past several days have driven Energy Release Component (ERC) values toward the 90th percentile across interior zones. While ambient temperatures will cool into the 80s and minimum relative humidity values will recover above 25 percent by Thursday, fine fuels will remain highly receptive to ignition.

The primary operational concern is the risk of dry lightning Thursday and Friday evenings. Model guidance contains uncertainty regarding the depth of moisture, but instability probabilities support a 10 to 20 percent chance of thunderstorm development over the mountain ridges of Trinity and Mendocino Counties. Any cloud-to- ground strikes will carry a significant risk of fire initiation due to parched state of the vegetation.

COASTAL FLOODING

Perigean spring tides will peak Wednesday night near 9 feet shortly after midnight entering Thursday. Minor tidal overflow is expected to impact low-lying areas surrounding Humboldt Bay, including local roadways near Arcata Bottoms and some properties in King Salmon.

EKA WATCHES/WARNINGS/ADVISORIES

CA... Coastal Flood Advisory until 3 AM PDT Wednesday for CAZ103.

NORTHWEST CALIFORNIA COASTAL WATERS... Coastal Flood Advisory until 3 AM PDT Wednesday for PZZ415.

Small Craft Advisory until 9 PM PDT Wednesday for PZZ450.

Gale Warning until 9 PM PDT Wednesday for PZZ470.

Small Craft Advisory until 3 PM PDT Wednesday for PZZ475.