textproduct: Eureka

SYNOPSIS

Major winter weather impacts with low elevation snowfall expected through Thursday. A warmer storm with significant impacts possible next weekend into next week.

DISCUSSION

Snow levels of 2000 to 2500 feet are forecasted to spread southward through the early morning and may dip down to 1500 feet where there is locally heavy precipitation as colder air aloft gets forced downwards. Mesoscale and global spectral models continue to show an uptick in convective precipitation through Tuesday as a 500mb cold core digs offshore the northern California coast. Shallow instability and low CAPE (<500j/kg) may result in continued isolated low topped storms and small hail formation, as has already been reported. Deeper dendritic growth zones from 2000-3000 feet Tuesday will yield heavy snow for elevations above 2000-2500 feet. HREF confidence for over 4 inches in 6 hours is highest (~80%) for Humboldt, Del Norte and southern Trinity counties. Confidence for over 4 inches also increases for northern Mendocino and northern Lake, generally above 2500-3000 feet. Greater impacts are likely for lower elevations around 2000-2500 feet, particularly in northern Mendocino on highway 101 early Tuesday where 1 to 3 inches are expected. The greatest forecast uncertainty is with snow levels. Heavy precip rates will enable localized temperature variations, driving down freezing levels resulting in bursts of heavy snow and/or small hail that will stick to the roads and trees, which could possibly fall on roadways. For that reason, the winter weather advisory has been upgraded to a warning for northern Mendocino and Lake counties. Confidence in a winter storm warning amounts (5 inches or more above 1500 feet) is not high (a 30-50% chance) for locations like Cobb by 10 AM Tuesday.

Greater impacts are forecast for Berry Summit on highway 299 and Collier Tunnel on highway 199 where up to 1 foot of snow may accumulate during the next 24 to 36 hours. Lesser amounts are forecast for the Weaverville area (Hwy 299 at Oregon Mountain and Buckhorn Summit) where 2 to 5 inches are expected in 24-36 hours. A Winter Storm Warning remains in effect from this evening through Wednesday afternoon for elevations above 2000 feet in Del Norte, Humboldt and Trinity Counties.

Showers and low snow levels (1500-2000 feet) are expected to continue on Wednesday and Thursday as another cold trough drops down from the NW. Models are not in super great agreement with the timing, snow amounts, precipitation, intensity, nor snow levels. On Wednesday, ensemble guidance indicates about a 30-40% chance for 2 inches in 6 hours above 2000 feet with a spread of 500 feet either side of that. Chance for > 2 inches above 2000 feet (+/- 500 feet) in 6 hours increases to 60-80% Wed night into Thu morning. This will likely result in more winter weather products with major impacts on our highways, particularly at pass level. This is the first major snow of the season for NW California and it is best to remain off the roads and stay home if at all possible until snow levels increase.

A break in the precipitation will be possible on Friday, before a warmer and potentially windier storm takes aim at NW California this upcoming weekend. This storm system could pack a wallop with strong damaging winds and heavy rain. CW3E AR landfall tool, mainly the ECMWF ensemble, is depicting over a 70% chance for a long duration (36-48 hours) of IVT > 250 kg/m/s and a mean IVT of around 450-600 kg/m/s for that period. The GFS ensemble is not as high (50- 60% chance) with somewhat shorter duration. The ECMWF-AIFS ensemble shows rain totals of 2-4" along the North Coast Saturday through Monday for this system as of right now. The risk for flooding will increase with this system as soils will be much more saturated. As of right now, forecast confidence is low for the magnitude and exact details, yet it seems to be a large scale synoptic pattern that is conducive for hydrological concerns (flooding) for our area. Stay tuned for more details and updates through the week. DS

AVIATION

Small hail and thunderstorm activity in the vicinity of KACV and KCEC accompany the incoming winter storm overnight into Tuesday. Mixed level of cloud cover can be expected as the deep cold air mass pushes towards the PACNW. Low elevation snow is possible near the coastal terminals. Accumulation on flight deck is not out of the scope of possibilities with strong convective profiles for the area. Hail, snow, and heavy bouts of cold rain are already being reported and recorded. VFR/MVFR are expected with occasional thunderstorms. Gusty winds intermittent with heavy downpour and wintry weather through the TAF period for the northernmost terminals. KUKI is also vulnerable to the adverse weather with less magnitude and frequency. /EYS

MARINE

The onset of hazardous conditions overnight for the area waters have produced gusty gale force winds, hail, mixed wintry precipitation and thunderstorms. With the arrival of a deep cold air mass, we are expecting the threat of waterspouts as well. Combined seas around 10 feet will eventually give way to a mid period NW swell, arriving early Tuesday morning and building by the afternoon to a steep 14 to 16 ft at 13 seconds. This could bring combined seas up to 18 ft, especially in the outer waters, by Tuesday afternoon.

Winds turn southerly again Wednesday as another front approaches the area. Models diverge on the strength of the southerly winds ahead of the front, with gusts up to 25 to 30 kts possible. Winds quickly turn northerly behind the front Wednesday night into Thursday with gale force gusts possible in the outer waters. Steep wind waves are possible with these winds and could propagate into the inner waters. Northerly winds ease Friday into early Saturday as an additional storm system approaches the area. Strong southerly winds, possibly with gale or storm force gusts, are possible by Saturday. JB/EYS

COASTAL FLOODING

High astronomical tides resulted in minor coastal flooding at high tide Monday, with the tide reaching 8.82 ft above MLLW and a surge was around 1.4 feet. Applying the same anomaly for high tide today results in a tide of 8.6 to 8.7 ft MLLW at 11:30 AM at the North Spit tide gauge. Should the tide exceed 8.8 ft MLLW, minor coastal flooding is possible around Humboldt Bay, including the Arcata Bottoms and in King Salmon. JB/DS

EKA WATCHES/WARNINGS/ADVISORIES

CA... Winter Weather Advisory until 4 PM PST Wednesday for CAZ101- 103.

Winter Storm Warning until 4 PM PST Wednesday for CAZ102- 104>108-110-111-114-115.

NORTHWEST CALIFORNIA COASTAL WATERS... Small Craft Advisory until 10 PM PST Wednesday for PZZ450- 470.

Gale Warning until 10 AM PST this morning for PZZ455-475.