textproduct: Eureka

SYNOPSIS

The break in wet weather and above average temperatures is coming to an end. Rain chances increase in Lake, Mendocino, and Trinity this evening and overnight. More widespread rain and gusty winds are forecast Sunday into Monday. An extended period of cold overnight lows with widespread frost and interior freezing temperatures will begin overnight Monday.

DISCUSSION

A mostly dry day is expected until this evening, when a cutoff low in southern California moves northward. Light rain is possible on the western edge of this low this evening and overnight. The highest chances are in Lake, eastern Mendocino, and Trinity Counties. Rain amounts are fairly meager for this first round, with generally less than a 1/4 inch forecast.

A stronger frontal system will increase rain chances throughout the day on Sunday. A shortwave trough and associated well defined frontal boundary will quickly move over the N CA coast Sunday afternoon/evening. A burst of heavy precipitation rates (0.3 to 0.5 inches per hour) can be expected with the frontal passage Sunday afternoon through Sunday night. Forecast rainfall totals Sunday into Monday have increase to a broad 1-2 inches, with isolated southwest facing higher terrain to receive 2.5 inches. Though these totals are relatively low, areas that experience the maximum estimated rates may see some brief minor/nuisance level hydrological issues.

There will be some elevated instability with the trough, which will likely be evolving into a closed low near or south of the Mendocino coast as it dives south past Point Arena. Wind gusts of 30 to 40 mph can be expected with the frontal passage Sunday afternoon through Sunday evening. The highest probabilities for wind gusts over 30 mph are for the coastal and areas over Cape Mendocino and southward. There will be increased instability and forcing around the developing low closer to Mendocino and Lake counties. This brings higher chances for more prolonged breezy winds. Winds sharply shift northerly behind the front (near 10 pm Sunday) as the low moves south. These northerly winds will be breezy (20-30 mph gusts) through Monday afternoon. The environment will also be sufficient for thunderstorm development for coastal areas around and south of Cape Mendocino.

Snow levels are likely to be high at 4500-5000 ft, but a dusting to an inch of snow is possible at Scott Mountain Pass on Highway 3. Lingering showers may continue through the day Monday before tapering off Monday night.

The upper-level trough behind the front will bring a much colder and drier airmass. Widespread frost and possibly freezing temperatures can be expected Monday night and Tuesday night. NBM shows high chances for temperatures below 36F in Trinity, eastern Mendocino, and Lake counties. Chances for freezing temperatures less than 32F are 50 to 60% in the coldest valleys of Trinity and Mendocino counties. Wednesday will have the coldest morning, and if offshore flow is weak enough, areas of frost will be likely over coastal areas as well. There is currently a high chance (80%) for a minimum temperature below 37F for isolated portions of the coast such as around McKinleyville. High temperatures will also be cooler, with even the warmest valleys struggling to reach 60. Overnight lows will remain chilly through next week, but values will nearly align with seasonal norms.

Most ensemble members are in agreement in an additional system arriving late next week, returning wet weather to the area. Confidence on impacts is low at this point, but rain, gusty winds, and mountain snow are all possible. Stay tuned. JB/JJW

AVIATION

LIFR conditions developed overnight across the interior valleys, including at UKI. Fog and low clouds across the interior valleys dissipated between 18Z and 20Z today. Mostly VFR conditions are then expected for all terminals through at least 3Z this evening. Conditions will most likely deteriorate at coastal aerodromes after 03Z to 06Z Sun as an upstream trough induces a push of shallow moisture onto the coast. Winds are forecast to remain light under 10kt or calm. /ZVS

MARINE

Northerly winds will increase in the lee of Cape Mendo with localized gusts to 30 kt in the afternoon and evening. Steep waves up to 6 ft are expected to build in lee of Cape Mendo by this evening. A small craft advisory has been hoisted for the southern outer waters. These conditions may briefly extend into the inner waters during the evening Sat. Otherwise calmer conditions with lighter northerlies expected north of Cape Mendo. A mix of small wave groups will also continue to subside today.

Next frontal system is forecast to rapidly approach on Sunday. Strong to near gale sustained winds are forecast to develop in the wake of the front Sunday evening into Monday. Models are now in a better agreement with gale force gust N-NW winds up to around 40 kts across the outer waters, with the strongest south of Cape Mendocino on the backside of the front. Gale Watches are out for the outer waters and the southern inner zone. A more brief period of gale conditions is likely in portions of the northern inner zones as well. A Hazardous Seas Watch is up for the zone (450) to account for the potential gale conditions, large steep waves, and confused seas state.

A large mid to short period W-NW swell will also arrive by Mon and seas will once again become hazardous. Seas could reach 18-20 ft by early Monday, especially across the southern outer waters. Another deep low will rapidly develop and approach on Wed and the risk for southerly gales with large steep waves will once again increase and may persist into Thu. DUG/JJW

BEACH HAZARDS

A larger NW swell will move through the waters Sunday evening through Monday, resulting in building surf along the beaches and increasing the potential of sneaker waves. Breaking waves will increase rapidly Sunday evening, reaching as high as 14 to 19 feet by Sunday evening along the northwest facing beaches. Sneaker waves are a unique hazard, with many minutes between waves. Pay attention and stay above the high water line and any wet sand to stay safe. If the ground is wet, waves have surged there recently. Stay safe and never turn your back on the ocean! /ZVS

EKA WATCHES/WARNINGS/ADVISORIES

CA... None. NORTHWEST CALIFORNIA COASTAL WATERS... Gale Watch from Sunday evening through Monday morning for PZZ470.

Small Craft Advisory until 2 AM PST Sunday for PZZ475.

Gale Watch from Sunday evening through late Monday night for PZZ475.