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 next week.

DISCUSSION

Broad cyclonic flow was evident on mid-level water vapor imagery Sunday afternoon over the NE Pacific. 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 late Sunday. There are still questions with precipitation timing due to the complexity of modeling the exact placement of nearing cutoff low and associated stalling front.

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 45 mph, will form on the eastern periphery of the feature as it quickly accelerates northward. Latest high resolution model runs have trended more aggressively with these winds and a closer bush with the north coast. A Wind Advisory was closely considered for areas over the King Range and south through far southern Humboldt and coastal northern Mendocino County. Stronger winds may also extend over Point Arena. At this time, confidence on instances of 45 mph gusts outside of the unpopulated higher coastal terrain along the Mendocino coast and southern Humboldt coast over the King Range was not high. Latest HRRR guidance wraps some stronger winds up to Humboldt coast as well, and this model usually has a better grasp on these type of storm evolutions. 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 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 trough Mendocino and Lake County where the higher chance for thunderstorms is. 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 potentially subfreezing temperatures develop. JJW

AVIATION

VFR conditions persist across the area this afternoon. Southerly winds have started to increase ahead of an area of low pressure approaching. Later tonight a front ahead of the low is expected to bring some rain to the area. Winds on the coast will continue to be south to southeast bringing which will be downslope for KACV and to some extent KCEC. This will keep the ceilings up through much of the night. Eventually though MVFR CIGS are expected in the heavier rain. Winds aloft are expected to increase more quickly than at the surface overnight so have added some wind shear to KACV and KCEC to highlight this. MKK

MARINE

Southerly winds have increased across the waters this afternoon to around 10 to 20 kt with gusts to 25 kt. These are expected to generally continue over the next 24 hours. Wind driven waves are around 4 6 to ft and may increase to 5 to 7 ft. The exact timing and location of the winds and waves is a bit more uncertain than usual due to the upper low right off the waters and the uncertainty in forecasting a closed upper level low. Tonight and Monday a west to northwest swell is expected to build into the waters. This is expected to be 6 to 8 feet at 14 seconds by Monday morning. This will then gradually diminish through Tuesday to around 6 feet at 11 seconds.

Late Monday afternoon and Monday night these winds are expected to slowly diminish as the low moves overhead. Tuesday afternoon it now looks like the low will finally move off to the east and the winds will become northwesterly behind it. A few of the models show a brief surge in these northwesterly winds to 15 or 20 kt, but most of them keep the winds lighter. Northerly winds of 10 to 15 kt are expected through most of Wednesday and Wednesday night. Stronger winds are expected for the end of the week, although at this point it looks like they will remain below gale force.

COASTAL FLOODING

A new moon phase is bringing higher than usual tidal swings. An 8.8 ft tides was observed the North Spit tide gauge at 1 am Sunday morning. A similar value is expected around the 1:45 am high tide Monday 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 from midnight tonight to 4 AM PDT Monday for CAZ103.

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

NORTHWEST CALIFORNIA COASTAL WATERS... Coastal Flood Advisory from midnight tonight to 4 AM PDT Monday for PZZ415.

Small Craft Advisory until midnight PDT tonight for PZZ450.

Small Craft Advisory until 11 AM PDT Monday for PZZ455.

Small Craft Advisory until 8 AM PDT Monday for PZZ470-475.