textproduct: Eureka

SYNOPSIS

Light rain showers and sprinkles is forecast for mostly Del Norte and Humboldt Counties late tonight into Sunday. Otherwise, dry and stable weather is forecast to continue through most of next week.

DISCUSSION

Visible satellite imagery depict high level clouds streaming across Northern California this afternoon. Surface observations reported high temperatures generally in the mid 50's to mid 60's, with the exception of valleys in NE Trinity where prevailing low clouds remain highs in the mid 40s to low 50s.

A shortwave trough embedded to a broad upper-level trough over the NEPAC will swing northeastward toward Northern California and the Pacific Northwest tonight into Sunday. A push of moist air with an associated progressive decaying frontal system will bring a chance for light rain showers or sprinkles to primarily Del Norte and Humboldt Counties late tonight or after 2-4 AM Sunday. The highest chance for rain greater than 0.25 inches in 12 hours ending 10 AM Sunday is forecast for Del Norte County, around 40-50%. Much lower chance is forecast for most of Humboldt, 10-20%. Rain forecast amounts range from 0.25 to 0.5 inch for Del Norte County, while few hundredths to a quarter of an inch are expected for Humboldt County. Light sprinkles or drizzle are possible Trinity and northern Mendocino counties. Dry weather will likely persist for southern Mendocino and Lake counties. Chances of precipitation diminish quickly throughout the day on Sunday as dry air mass filter over the area in the wake of the front, promoting a gradual clearing skies.

High pressure aloft will spring back up by Monday and then dominate most all next week resulting in multiple days of dry and stable weather. A shallow moist air push behind the front and N-NW surface flow may result in fog and low clouds overnight Sunday into Monday morning in the interior valleys. Otherwise, a general warming trend is forecast for much of the area early to mid next week. Highs are forecast to warm up into the lower to mid 70's in the interior next week. South facing exposures and higher terrain stand the best chance for highs in the 70's, while valleys trapped under the nocturnal inversion with night and morning fog may not warm up even into the lower 60s, especially in Trinity and northern Humboldt. Overnight low temperatures will remain on the chilly side with subfreezing minimums highly probable for valleys in Trinity and perhaps portions Humboldt, Lake and northern Mendocino. Global Ensembles and WPC cluster guidances are suggesting a break-through in the westerlies or undercutting of the blocking ridge late next week and next weekend. This have the potential to bring increasing chances of precipitation and a cooling trend. Stand by for more in this on coming next week. /ZVS

AVIATION

VFR with variable mid and high cloud cover is forecast to continue for tonight as a decaying frontal boundary approaches. Chance for MVFR conditions and rain increases after 12Z Sunday for coastal terminals. Chance for IFR will increase after frontal passage as onshore winds develops, after 18-20Z Sunday. KUKI will likely remain VFR with only high or some mid clouds. Influx of shallow moist air may bring better chances for IFR Sunday night or early Monday to KUKI. Otherwise, chance of for fog and low clouds continue tonight for valleys in Trinity, Humboldt and northern Mendocino. Slightly stronger mixing with prefrontal southerlies or southeasterlies may limit the extent and duration of fog tonight. Chance for low level wind shear and shallow turbulence will increase across the mountains of Del Norte and Humboldt tonight and persist into Sunday morning with passage of a front. The shallow turbulence could have an impact small light-weight aircraft.

MARINE

Southerly windS increase a notch tonight as a decaying frontal boundary moves across the waters. Chance for gusts > 20 kt increases to 60-80% from about 11 PM tonight to 6 AM Sunday north of Cape Mendo. Chance for gusts > 25 kt are lower, around 20-40% for that same frame north of Cape Mendo. Winds should shift around to SW and NW after frontal passage in the late morning and afternoon Sunday and remain fairly light, under 10 kt. Primary hazard will be seas around 10 ft and will warrant continuation of the advisory for small craft. There are multiple wave groups coming out of the NWPS. The advisory for hazardous seas hinges largely on steep southerly waves around 5 to 7 feet. These wind waves may be too high for winds under 15 kt and a limited fetch and duration. Short period waves combined with westerly swells around 7 to 9 ft will result in seas around 10 feet. Confidence is lowest for the inner waters where there is much more spatial variability. A larger west swell will steadily build through the day on Sunday and eventually reach 11 to 13 feet at 16 seconds by Sunday evening. This swell will remain sufficiently elevated to warrant extension of the advisory for hazardous seas into Monday. Westerly swell will subside below criteria by Tue as a fairly tranquil wind regime persists for NW California coastal waters.

BEACH HAZARDS

Two long period (>16 seconds) westerly swell groups will spread into the coastal this weekend. The first one is expected to arrive late this afternoon and build to around 3 to 4 ft. Spectral density plots so far showed all the energy in the 11 second band. Another long period westerly swell train arrives on Sunday with heights eventually building to 12 to 14 feet with periods near 16 seconds by Sunday. This swell may be too large for sneaker waves, though it could still pose a threat to beachgoers. Breakers up to 16 feet for west and northwest facing beaches are forecast based on average beach slope profiles. Beachgoers venturing near or in the surf zone Sunday night or early Monday morning should remain vigilant. Avoid rocks, jetties and steep beaches.

COASTAL FLOODING

High astronomical tides are predicted to continue on Sunday. High tides are forecast to be at 8.22 feet at the North Spit tide gauge at around 10:42 AM local time on Sunday. High tides combine with surge and tidal anomaly will result in water level at or near 8.8 feet. Minor coastal flooding around Humboldt Bay, including King Salmon and Jackson Ranch Road in the Arcata Bottoms, will be likely between 9AM and 12PM. A Coastal Flooding Advisory will be in effect from 9AM to 2PM Monday. High tides are predicted to be at 7.98 feet @ 11:31 PM on Monday. At this moment, combine with surge and anomaly is expected to remain below advisory for MOnday. /ZVS

EKA WATCHES/WARNINGS/ADVISORIES

CA... Coastal Flood Advisory from 9 AM to noon PST Sunday for CAZ103.

NORTHWEST CALIFORNIA COASTAL WATERS... Small Craft Advisory from 8 AM Sunday to 4 PM PST Monday for PZZ450-455.

Small Craft Advisory from 4 PM this afternoon to 4 PM PST Monday for PZZ470-475.