textproduct: Eureka

KEY MESSAGES

* Convective potential remains low (up to 15% chance) and limited to northern Trinity and far interior Del Norte through early evening.

* Expanding high pressure aloft and compressing marine air will drive a sharp warming trend Tuesday through Thursday, bringing increased HeatRisk potential and temperatures 10-15 degrees above early September normals across interior valleys.

SYNOPSIS

An exiting mid-level disturbance moving into Siskiyou County is keeping isolated shower and thunderstorm chances near the Oregon border through this evening. Atmospheric high pressure centered over the Desert Southwest will build westward starting Monday, initiating a pronounced warming trend through mid-week. Offshore flow on Wednesday will clear coastal low clouds temporarily before marine air moderates conditions late in the week.

DISCUSSION

Water vapor imagery shows a shortwave trough axis shifting east of the forecast area. Mid-level lapse rates around 7.5 C/km across northeastern Trinity and interior Del Norte are providing modest mid-level instability. The NBM maintains a low probability (under 15%) for rainfall up to 0.05 inches, with moderate west-southwesterly steering flow carrying isolated convective cores rapidly into adjacent forecast areas.

As the trough exits tonight, upper-level heights build across northern California under the expanding ridge. Subsidence under the ridge will compress the coastal inversion while interior surface pressure falls steepen the zonal onshore pressure gradient between the eastern Pacific surface high and the interior thermal low. Inland valley maximum temperatures will rise into the 98 to 102 degree range Tuesday through Thursday.

Following operational coordination with the Weather Prediction Center, interior maximum temperature grids for Tuesday through Thursday were lowered 3-5 degrees relative to raw NBM guidance to align with ensemble bias trends. By Wednesday, a thermal trough positioning near the coast turns low-level flow offshore, bringing an 80% probability of coastal highs exceeding 70 degrees around Humboldt Bay before the marine layer re-establishes late Thursday.

AVIATION

Coastal terminals KCEC and KACV are experiencing temporary clearing this afternoon following morning MVFR/IFR stratus. Marine low clouds will return along the coast after 04Z, with a 60-70% probability of LIFR conditions late tonight into Monday morning. Light onshore surface winds will shift to weak northeasterly drainage flow overnight. Regional terminal KUKI will remain VFR throughout the period with daytime northwest wind gusts around 15 knots diminishing after sunset.

MARINE

An offshore high-pressure system interacting with the coastal thermal low continues to drive northerly winds, strongest across the southern marine zones.

For the Northern Inner Waters, north winds of 10 to 15 knots with gusts to 20 knots will persist through Monday. Combined seas will average 4 to 6 feet, composed of a dominant northwest wind wave of 4 feet at 8 seconds and a long-period southerly swell of 2 to 3 feet at 15 seconds.

For the Northern Outer Waters, northerly winds will range from 15 to 20 knots. Wave steepness remains marginal with dominant periods around 9 seconds. This decision is deferred to the upcoming midnight shift as sea state criteria remain marginal. For the Southern Inner Waters, a Small Craft Advisory remains active through 5 AM Monday for north-northwest winds of 15 to 25 knots and wave heights of 6 to 8 feet at 8 seconds.

For the Southern Outer Waters, a Small Craft Advisory continues through 5 PM Monday for north winds of 20 to 25 knots, gusts to 30 knots, and steep seas of 7 to 9 feet at 9 seconds.

Additionally, a south-southwest swell of 3 to 5 feet at 15-16 seconds from Hurricane Marie will decay across all waters through Monday night. A thermal trough positioning over the coastline Wednesday will briefly reduce wind speeds across all waters before northerly flow re-strengthens Thursday evening.

EKA WATCHES/WARNINGS/ADVISORIES

CA... Beach Hazards Statement through Monday evening for CAZ104- 109.

NORTHWEST CALIFORNIA COASTAL WATERS... Small Craft Advisory until 5 AM PDT Monday for PZZ455.

Small Craft Advisory until 5 PM PDT Monday for PZZ475.