textproduct: Eureka

SYNOPSIS

Major winter weather impacts (heavy wet snow, rain & gusty winds) expected through Thursday. A break in the precip is expected Thursday afternoon through Friday before another storm system moves in for the weekend bringing rain, snow & strong winds.

DISCUSSION

Widespread rain, heavy wet snow, and gusty winds are expected through Thursday as an occluded cold front drops down from the Pacific NW. Snow levels are expected to lower to around 1,000 to 2,000 feet, with levels as low as 500-1,000ft for Trinity, interior Del Norte, Trinity, and northern Humboldt counties. Down south, a warmer air mass will slightly raise the snow levels to around 2,000- 3,000 feet at Mendocino and Lake counties Thursday. The heaviest wet snow rates are expected Thursday morning. Additional accumulations of 3 to 6 inches are forecast for elevations above 2,000 feet in Del Norte, Humboldt and Trinity counties; lighter amounts up to 1 inches for elevations above 1,000 feet to 2,000 feet. Meanwhile, 4 to 6 inches of wet snow is expected for elevations above 2,500 feet in Mendocino and Lake counties. Major travel impacts will continue on highways 199, 299, 3, 36 and 101 through Thursday. There is the potential for accumulating small hail at lower elevations with the heavy showers tonight through Thursday morning, which can create slippery "ice rink" conditions on the roads in a matter of seconds.

Showers are expected to diminish Thursday evening as northerly winds aloft bring a drier air mass across the area. This airmass will be very cold, yet how much this is felt at surface will depend on the extend of cloud coverage. Overnight low temperatures may end up warmer than expected if there are more low clouds. Either way, temperatures are forecasted to be cold enough to cause a cold weather advisory for Thursday morning and Friday morning for SW Humboldt, coastal Mendocino, the Russian River Valley, and Southern Lake COunty. On Friday, a brief break of precipitation is expected allowing even colder overnight lows if clouds do not inhibit nighttime cooling.

This weekend and into early next week, the active weather pattern continues with the approach of a broad closed upper- level trough and associated frontal system. The WPC 500 mb cluster analysis continues to show a decent agreement that a broad closed upper level trough will settle over the Northeastern Pacific. This system is expected to bring heavy rainfall, heavy mountain snow and strong southerly winds. The ensemble means for both the GEFS and EPS are showing a high probabilities (90-100% chance) of at a weak atmospheric river (IVT's greater than 250 kg/m/s), however there is an increasing chance (35-60% chance) for over 500 kg/m/s. While this is not particularly strong, it looks like there will be multiple impulses that may over whelm the rivers and streams. There still uncertainties on exact timing of the rain pulses, which will be a big factor in the level of impacts. Strong, southerly winds are also expected with the strongest through Saturday night. The mean NBM 24 hour max gust shows around 40 to 55 mph at the lower elevations and the higher elevation wind gusts are showing over 70 mph. /ds

AVIATION

VFR conditions and light surface winds have prevailed at the terminals with periods of random rain showers, intense and mixed with small hail at times, has occasionally resulted in MVFR ceilings from 1500-3000 feet and IFR vsby around 1 to 2 miles. Expect VFR to predominate through Thursday. There is about a 50% chance for MVFR ceilings and a 20% chance for IFR vsby in more intense convection or showers at any given time. Winds will remain predominately light, 10 kt or less, with a 25% chance for gusts to 30 kt in showers.

MARINE

A large, steep, northwest swell will diminish into Thursday. A west wind surge is forecast to develop again after frontal passage tonight, followed by periodic gale force N wind gusts for mostly the outer waters on Thu. Gales are most likely for the southern outer waters. Conditions settle down very briefly on Fri before a series of powerful frontal systems bring a moderate to high risk for gale or storm force winds Fri night through Sun night. More specific updates and details for the hazardous weekend will become available soon as high resolution model data becomes available.

HYDROLOGY

An atmospheric river event is expected to impact NW California this weekend and into early next week. Moderate to heavy rainfall will increase the potential for rapid rises on rivers, streams and creeks across the region. The Weather Prediction Center (WPC) included Del Norte County, Humboldt, Mendocino and Lake counties in the Marginal Risk (at least 5%) of excessive rainfall for Sunday. There is a 55 to 80% chance for 24-hr probability of precipitation >1" from 4 PM Saturday through 4 PM Sunday, with up to 95% chance for the exposed ridges in Del Norte and Humboldt counties. There is a 50-75% prob. for >2" & a 30-50% for >3" for elevated coast terrain Sat to Sun.

A rapid rise of the main stem river is expected on Sunday. At this point, the Russian River at Hopland has around a 20% chance of exceeding flood stage Sunday evening. Many other rivers such as the Navarro, Mad, and Eel are expected to reach action/monitor stage. These will need to be watched as it gets closer. The rest of the main stem river are not expected to reach/exceed flood stage as of yet./ds

EKA WATCHES/WARNINGS/ADVISORIES

CA... Winter Weather Advisory until 10 AM PST this morning for CAZ101-103.

Winter Storm Warning until 10 AM PST this morning for CAZ102-105>108.

Winter Storm Warning until 6 AM PST early this morning for CAZ104-110-111-114-115.

Cold Weather Advisory until 10 AM PST this morning for CAZ104-109-112-113-115.

NORTHWEST CALIFORNIA COASTAL WATERS... Small Craft Advisory until 3 AM PST Friday for PZZ450.

Small Craft Advisory until 8 AM PST this morning for PZZ455.

Gale Warning from 8 AM this morning to 7 PM PST this evening for PZZ455.

Small Craft Advisory until 3 AM PST early this morning for PZZ470-475.

Gale Warning until 4 PM PST this afternoon for PZZ470.

Gale Warning until 11 PM PST this evening for PZZ475.