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 continuing through much of the week.

DISCUSSION

A shortwave high pressure continues to build into the area today, promoting a period of dry weather through Friday. However, some isolated light showers are possible to developed inland this afternoon due to a lingering low level moisture and diurnal heating, especially in Mendocino and Lake counties. Tonight, areas of fog and patchy dense fog are expected to developed along the river valleys and around Humboldt Bay into Friday. Near-normal daytime temperatures are expected, but with a cooling trend continuing through mid next week.

Wet and unsettled weather pattern return this weekend and continue through next week, as a series of cold trough and associated frontal system moves across the area. An elongated H5 longwave positively tilted trough will form off the Northern California coast on Saturday, before evolve into a closed low Saturday night/Sunday. Resulting in the beginning of wetter and colder pattern through the extended forecast. An associated potent surface low will form well off the coast over the weekend. Pressure gradients tighten up on Saturday and winds should increase for coastal areas and across the interior mountains. This have the potential to bring periods of breezy to windy south winds 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. Pre-frontal showers are expected as early as Saturday morning, with widespread precipitation on Sunday. Moderate to locally heavy rain amounts expected. Early through mid next-week, a potent cold upper-level trough will dig south- southeastward from the Gulf of Alaska toward the West Coast. These incoming troughs with kick the previously mentioned low south and then eastward as it weakens. Winds form this system will still bear watching if less weakening occurs with it as it pushes ashore. 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. NBM probabilities are predicting a high likelihood for 1.5" to 3" in 72 hours from early Sunday to early Wednesday morning for most of the CWA, with 3 to 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 around 5,000 feet Saturday and gradually drop to around 2,000 to 3,000 feet MSL by early next week. 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 50-70% chance for snow levels below 2000 feet across the northern portions of the forecast area, and 25-40% chance for less than 1500 feet. Heavy snow across the higher ridges above 4000 feet could accumulate from 1 to 3 feet from Saturday evening through mid next week. Travel impacts are anticipated from Sunday through most of the next week. Stay tuned! ZVS

AVIATION...(18Z TAFs)

Stratus has started to lift from both the interior valleys and the coastal areas. While stratus has lifted near the coast, it hasn't scattered and conditions have remained MVFR for much of the afternoon. Coastal stratus is likely to return/remain tonight, with IFR to MVFR ceilings likely. Interior valleys could see fog again tonight as moisture remains available. UKI has around a 40% chance to see fog again overnight. Any stratus and fog is likely lift and scatter by late morning Friday. JB

MARINE

Northerly winds increase this afternoon, especially south of Cape Mendocino. Gusts of 20 to 25 kts are possible in the immediate lee of the Cape, with 10 to 20 kt gusts possible elsewhere. Steep northerly wind waves will build bringing hazardous conditions to small crafts. A large long period northwest swell will also build peaking early Friday at around 12 ft at 16 seconds. Combined seas up to 14 ft are possible Friday.

North winds start to subside and turn southerly by Friday 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. NBM shows around a 50% chance for gale force gusts in the outer waters Saturday and a 40% chance on Sunday. Winds are likely to remain elevated through the week as the pattern remains active. There is at least a 30% chance for gales in the outer waters each day through Friday of next week. JB

BEACH HAZARDS

A steep, northwest long period swell arrives this evening, bringing a sneaker wave threat tonight through Friday. This swell will peak around 12 to 13 feet at 16 seconds during the daytime Friday. 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 from 10 PM PST this evening through late Friday night for CAZ101-103-104-109.

NORTHWEST CALIFORNIA COASTAL WATERS... Small Craft Advisory from 4 AM Friday to 3 AM PST Saturday for PZZ450-455.

Small Craft Advisory until 3 AM PST Saturday for PZZ470-475.