textproduct: Eureka

SYNOPSIS

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

DISCUSSION

Cold frontal boundary moved across the North Coast this afternoon. A colder air mass will follow behind a secondary trough passage later this evening and overnight. Snow levels are forecast to plummet to 2000 to 2500 feet later tonight and may dip down to 1500 feet in locally heavy precipitation. Mesoscale and global spectral models continue to show an uptick in convective precipitation tonight through Tuesday as a 500mb cold core digs offshore the northern California coast. Shallow instability and low CAPE (<500j/kg) may result in isolated low topped storms and small hail. Deeper dendritic growth zone from 2000-3000 feet tonight and Tuesday will yield heavy snow for elevations above 2000-2500 feet. Confidence for over 4 inches in 6 hours is highest for Humboldt, Del Norte and southern Trinity Counties where HREF continues to indicate over an 80% chance. Chance 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 1500-2500 feet, particularly in northern Mendocino on high 101 later tonight or early Tuesday where 1 to 3 inches are expected. The greatest uncertainty is with snow levels. Heavy precip rates will drive down freezing levels and result in bursts of heavy snow and/or small hail that will stick to the roads. For that reason the winter weather advisory has been upgraded to a warning for northern Mendocino and Lake. Confidence in 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 (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/intensity and 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 precipitations will be possible on Friday, before a warmer and potentially windier storm takes aim on 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. GFS ensemble is not as high (50-60% chance) with somewhat shorter duration. The risk for flooding will increase. To what magnitude and exact details remain elusive at this point, but it is large scale pattern that is conducive for hydro concerns (flooding) for our area. Stay tuned for more details.

AVIATION...(18Z TAFs)

VFR to MVFR conditions are being observed at the terminals as rain showers continue. Interior valleys, including UKI saw low stratus and fog this morning with moisture from the recent rains. Mainly MVFR to VFR conditions are expected, except during periods of heavy showers, where IFR ceilings and visibilities are possible. Gusty northwest winds are possible this afternoon and evening, exceeding 20 kts at times. Small hail showers are possible overnight into Tuesday at the coastal terminals. Isolated thunderstorms along with gusty and erratic winds of 30 kts or higher are possible with these showers. There is a possible low level wind shear threat as well with these winds, especially in sheltered interior valleys, including UKI, where winds at 2 kft AGL are likely to be much higher than the surface winds. JB

MARINE

Northerly winds have increased behind a cold frontal passage, with gusts of 30 kts or higher being observed on the buoys. Gale force gusts are possible this afternoon and evening, especially in the southern waters. Steep seas of 7 to 9 ft will be associated with these winds. Combined seas of 9 to 12 ft are possible when factoring in a decaying northwest swell at around 7 ft. This evening and overnight, winds will turn westerly but remain elevated with widespread gusts of 20 to 30 kts. Greater instability will bring heavy rain showers tonight, the strongest of which could bring isolated thunderstorms, gale force gusts, small hail, and isolated waterspouts.

West winds ease slightly Tuesday, especially by the afternoon in areas close to shore from about Trinidad northward, where gusts may subside to 10 to 15 kts. Elsewhere, gusts of 15 to 20 kts are likely through the day Tuesday. The bigger impact will be the mid period northwest swell building Tuesday morning, which is expected to peak at around 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

COASTAL FLOODING

High astronomical tides resulted in minor coastal flooding at high tide late this morning, with the tide reaching 8.82 ft above MLLW. The surge was around 1.4 feet at high tide this morning. The same anomaly at high tide tomorrow, which is at 11:30 AM at the North Spit tide gauge, would result in a tide of 8.6 to 8.7 ft MLLW. 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

EKA WATCHES/WARNINGS/ADVISORIES

CA... Winter Weather Advisory from 10 PM this evening to 4 PM PST Wednesday for CAZ101-103.

Winter Storm Warning from 7 PM this evening to 4 PM PST Wednesday for CAZ102-105-106.

Winter Storm Warning from 10 PM this evening to 4 PM PST Wednesday for CAZ104.

Winter Storm Warning until 4 PM PST Wednesday for CAZ107- 108.

Winter Storm Warning from 10 PM Tuesday to 4 PM PST Wednesday for CAZ110-111-114-115.

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

Small Craft Advisory until 1 PM PST this afternoon for PZZ455.

Gale Warning until 10 AM PST Tuesday for PZZ455.

Gale Warning until 10 AM PST Tuesday for PZZ475.