textproduct: Eureka

SYNOPSIS

A cutoff low is approaching the region. Rain and some thunderstorms, periods of breezy to strong southerly winds, and mountain snow will impact the region through Tuesday. After some additional rainfall Wednesday, a stretch of drier weather is then expected to begin late in the week.

DISCUSSION

Satellite indicates broad cyclonic flow progressing east over the NE Pacific early this morning. Small disturbances are also observed orbiting the cutoff low as it proceeds towards California. An occluding frontal boundary will setup along or just off the N CA coast starting very early Monday. There is still some uncertainty as the low appears to stall very early this morning.

There is high agreement on one of the smaller disturbances rounding over the coast early Monday morning. As this occurs, a narrow jet of strong southerly winds, likely gusting up to 35 or 40 mph, will form on the eastern periphery of the feature as it quickly accelerates northward. Latest high resolution models have trended down with the strength of these winds, but exposed ridges and coastal headlands could still see gusts potentially up to 45 mph. The North Coast will be gusty this morning, but the strongest gusts will remain more south and east of northern Humboldt and Del Norte County. Additional bands of stronger southerly winds may brush the coast Monday while the ridges remain breezy. Monitor the latest forecasts for developments on this threat.

Generally moderate rainfall rates are forecast, with some more localized heavy rates from convective activity and terrain enhancement. A broad 1 to 2 inches of rain are forecast for most of the area (local totals up to 3 inches around the King Range and windward facing slopes) Monday through late Tuesday. Snow levels drop to 4500 ft Monday, and 4-7 inches of snow are forecast for Highway 3 over Scott Mountain Pass. A Winter Weather Advisory has been issued for NE Trinity for this reason.

The cold core of the upper low will begin nudging eastward Tuesday as it weakens and begins to open up some. Thunderstorm chances increase from the strong forcing, steep lapse rates and modest instability. There are still some questions as to how far the low pushes ESE as it makes landfall, and this will determine how much instability is realized. There will be 250-350, up to 500+ J/kg of surface CAPE. Instability will be more focused south through Mendocino and Lake County where the higher chance for thunderstorms are. Thunderstorms will be capable of small hail. A secondary weak shortwave will push through Wednesday to bring additional light rainfall over mainly areas around and south of the Cape. Much cooler overnight lows will arrive over the interior with this system, and especially Thursday morning when frost and potentially subfreezing temperatures develop. JJW

AVIATION

Conditions are forecast to deteriorate on Monday as rain with an incoming front pushes into the area. Generally, MVFR ceilings and visibilities are anticipated for all forecast terminals through the day. Potential for low level wind shear and shallow turbulence over the coastal ranges will increase late Sunday night and persist into early Monday morning as a S-SE speed max aloft develops in advance of the offshore boundary.

MARINE

A frontal boundary will continue to edge eastward toward across the coastal water Sunday night through Monday morning. A secondary south-southest speed max will develop late Sunday night as the boundary pivots in response to an upstream trough. Strongest average wind gusts (25-30kt) with this secondary speed max appears to be around Cape Mendo and over Mendocino waters. Mesoscale scale ensemble (HREF) indicates about a 60% chance for gusts near 35 kt for Mendo waters from 1 AM to 5 AM. The coverage is in a narrow corridor along the inner and outer waters. These stronger gusts should diminish quickly. Another ramp up with gusts to 25-30kt is expected in the afternoon north of Cape Mendo as yet meso-low develops on the boundary.

During the day on Monday, west to northwest swell will build to 7 to 9 ft at 12 to 13 seconds. The combination of lingering S-SE wind waves around 4-6 ft may yield seas around 10 feet. Advisories for small craft have been extended.

A broad and complex surface low will remain over the waters through at least Tue and then slowly meander southward on Wed. Overall, wind fields appear lighter on Wed as temperature gradients diminish. More unstable conditions associated with colder air aloft will be favorable for convective activity, showers and isolated tstms.

COASTAL FLOODING

A new moon phase is bringing higher than usual tidal swings. An 8.8 ft tides was observed at the North Spit tidal gauge at 1 am Sunday morning. A similar value is anticipated around the 1:45 am high tide this morning. Increasing southerly winds may nudge the value slightly higher. A coastal flood advisory has been re-issued to cover the threat.

EKA WATCHES/WARNINGS/ADVISORIES

CA... Coastal Flood Advisory until 4 AM PDT early this morning for CAZ103.

Winter Weather Advisory from 1 PM this afternoon to 11 AM PDT Tuesday for CAZ117.

NORTHWEST CALIFORNIA COASTAL WATERS... Coastal Flood Advisory until 4 AM PDT early this morning for PZZ415.

Small Craft Advisory until 2 PM PDT this afternoon for PZZ455-470-475.