textproduct: Eureka

SYNOPSIS

An extended period of wet and colder weather that will span through next week will begin over the weekend. Significant mountain travel impacts begin from these systems by early next week and continue through much of the week.

DISCUSSION

Shortwave ridging continue to shift eastward this afternoon, while an upper level trough deepen from the Gulf of Alaska. An approaching cold front is spreading high- level clouds across the forecast area this afternoon. Meanwhile, coastal stratus and low clouds persist along the river valleys. Onshore flow and some isentropic lifting focusing across the interior will take advantage of this moisture, and may produced some light rain late this afternoon and evening.

Unsettled weather pattern return this weekend and continue through mid next week, as a series of cold trough and associated frontal system moves across the area. The first system is expected to evolve into a surface is expected to continue to approaches the area tonight and through the weekend. Pre- frontal showers are expected to increase tonight into Saturday, followed by a widespread precipitation on Sunday. Moderate to locally heavy rain amounts expected, with the heaviest focusing on areas south of Cape Mendocino.

In addition, breezy to windy south to southeast winds is expected to developed over the coastal headlands and exposed ridges Saturday night into Sunday as a potent surface low will form well off the coast. However, uncertainties remains regarding of how strong those winds could be as models continues struggling with the placement of these surface low. At this point, breezy to windy south winds is most likely from Saturday into Sunday. NBM 90th percentile (worst case scenario) shows widespread 30 to 40 mph, with locally strongest over the more exposed ridges and coast headlands.

A stronger and much colder trough will then dive down from the north Monday. The approaching trough will kick a previously mentioned surface low eastward into the California coast on Monday. An associated compact surface low is expected to track toward the Bay Area on Monday morning, and then shifting inland. Even so, enhanced wind will likely span through Mendocino and Lake counties where wind gusts up to 45 mph will be possible over the ridges and some coastal headlands Monday afternoon and evening. This will followed by a potent and broad surface low over the NEPAC tracking southward along the West Coast Monday night through Tuesday. These have the potential to bring gusty west-to Northwest winds Monday night through Tuesday, focusing in Humboldt, Mendocino and Lake counties. In addition, an enhanced in precipitation is expected on Monday, with widespread moderate to locally heavy rainfall.

A colder air mass will subsequently spread across the West Coast Monday night through Tuesday, resulting in increasing instability and steep lapse rates. These will promote heavy precipitation, along with isolated thunderstorms (10-20% chance) and the potential for accumulating small hail along the coast by Tuesday. Most likely rainfall total amounts range from 1.5" to 3" in 72 hours from early Sunday to early Wednesday morning for most of the CWA, with up to 5.5" in the windward facing terrain. Minor flooding of smaller river, creeks and streams, urban areas with poor drainage will be possible.

The main concerns with this system are the potential for heavy mountain snow, which would impact highway passes, along with the potential for lower elevation snow. Snow levels are expected to start about 5,000 feet Saturday and gradually drop to around 2,000 to 3,000 feet MSL by Monday night. There is uncertainty on just how low snow levels could get due to the probability of convective showers, which has the potential to lower snow levels for localized areas. There is a 35-45% chance for snow level less than 1500 feet for the northern portion of the area, and a 25-45% chance for below 2000 feet for Mendocino and Lake counties. A Winter Storm Watch is in effect starting first Sunday morning for elevation above 4500 feet in Trinity County, and then expanded for much of the interior forecast area Monday evening through Wednesday evening for elevations above 2000 feet for interior Del Norte, interior Humboldt and Trinity counties. A Winter Storm Watch is also in effect from Monday evening through Wednesday morning for elevations above 2500 feet in portions of interior Mendocino and Lake counties. Heavy snow, with storm total accumulations at 1 to 2.5 feet for elevations above 3000 feet possible in Trinity County, with locally up to 3.5 feet across the higher ridges in the Trinity Alps. 6 to 18 inches of snow possible for elevations above 2000 feet for interior Del Norte and interior Humboldt counties. Up to 5 inches of snow possible for elevations above 2500 feet in portions of interior Mendocino and Lake counties. Travel impacts are expected across much of highway 3 north of Weaverville and Hayfork Summit, Hwy junction 36 and 3 at South Fork Mountain, Hwy 299 at Berry Summit, Buckhorn Summit and Oregon Mountain, Hwy 101 in Rattlesnake Summit, and Hwy 175 in Cobb. Stay tuned for the latest updates! /ZVS

AVIATION...(18Z TAFs)

Widespread stratus is being observed across the area, both over the interior valleys and the coast. Conditions are MVFR at all terminals. Ceilings will likely lift, perhaps to VFR conditions by this afternoon, and scatter out for the interior valleys. Coastal stratus is likely to remain this evening and overnight, with MVFR to VFR ceilings expected. Interior valleys, including UKI, could see stratus again tonight, with LIFR to IFR ceilings possible. Light rain showers are possible for Del Norte County by mid Saturday morning. JB

MARINE

Northerly winds and steep wind waves continue to diminish as winds ease. A large long period northwest swell has built into the waters and is forecast to peak at around 12 ft at 16 seconds. Combined seas up to 14 ft are possible today.

North winds continue to subside and turn southerly by this afternoon as low pressure approaches the area. The center of the low pressure is likely to be off the northern California coast, and the exact location of the strongest winds is currently uncertain. Strong northerlies are possible on the western edge of this low with moderate to strong southerlies possible on the eastern edge. High resolution models are showing some potential for gale force gusts Saturday afternoon and evening. As of now these winds look brief and isolated, but winds have been trending upwards in recent model runs. NBM still shows about 50% to 60% chances for gale force gusts in the outer waters. This will need to be watched for any potential upgrades to Gale Warnings. Steep southerly wind waves of 6 to 10 feet are also possible with these winds. Winds are likely to remain elevated through the week as the pattern remains active. There is at least a 30% chance for gale force gusts in the outer waters each day through Friday of next week. JB

BEACH HAZARDS

A steep, northwest long period swell is building in, bringing a sneaker wave threat. This swell will peak around 12 to 13 feet at 16 seconds. This swell will align with beaches in Humboldt, Del Norte, and Mendocino counties, increasing the threat of sneaker waves. The wave is expected to produce set behavior with breakers leading to rapid and unexpected beach run ups of over 18 feet. Calmer winds and few short period seas along the coastline Friday will further enhance this danger making waves more sneaky, especially on steeper beaches. Take care if going to the beach late in the week. Choose flatter beaches if possible. Stay off of rocks, logs, and jetties. Never turn your back to the ocean! DS

EKA WATCHES/WARNINGS/ADVISORIES

CA... Beach Hazards Statement through late tonight for CAZ101-103- 104-109.

Winter Storm Watch from Monday evening through Wednesday morning for CAZ102-105-106-110-111-114-115.

Winter Storm Watch from Sunday morning through Wednesday morning for CAZ107-108.

NORTHWEST CALIFORNIA COASTAL WATERS... Small Craft Advisory until 10 AM PST Saturday for PZZ450.

Small Craft Advisory until 3 PM PST Sunday for PZZ455-470- 475.