textproduct: Eureka

SYNOPSIS

Isolated showers and thunderstorms are possible in the interior each afternoon through Monday. Drier weather returns mid next week.

DISCUSSION

High pressure continues to weaken today as a deep upper level trough, positioned over the Northern Great Plains, will continue to pull a weaker upper level trough off the Central California Coast into its rotation over the weekend (according to the GFS and ECMWF models). As this happens, moist air will filter into Northwest California over the weekend with cooler temperatures aloft that will support instability over the interior, to some degree, each afternoon through Monday. The main uncertainty for today is if there will be enough moisture to support thunderstorm development. However, CAMs still show a few showers developing over eastern Trinity and the Yolla Bolly's late this afternoon and early evening. As the weaker upper level trough moves onshore in Central California tonight and into tomorrow, more moisture is expected to move up the Central Valley and encounter the eastern side of the Coastal Range in Trinity, extreme eastern Mendocino, and Lake counties. This would support more shower and thunderstorm development over this region, with less of a chance west of this region. The chance for wetting rain (>0.1") over a 72 hour period (through Monday evening) is around 45% to 55% over much of Trinity and NE Mendocino, with a 25% to 35% chance in northern Lake and extreme eastern Humboldt. Chances for wetting rain declines rapidly further west, north and south of the specified area. Additional chances for showers and isolated interior thunderstorms are possible Sunday and Monday afternoons, depending on the positioning of the low. At the coast, any precipitation would likely be in the form of drizzle or a stray light shower. Drier and warmer weather is likely to return by mid next week as ridging redevelops. JB&JLW

AVIATION...18Z TAFs

A passing area of low pressure deepened the marine layer, leaving well embedded MVFR CIGS over the North Coast terminals. Light westerly winds are driving the marine stratus over Humboldt Bay and up the Eel River delta. These areas will struggle to clear out through the evening hours. A pronounced surface eddy will form near 6Z, ushering a stratus surge back up the North Coast tonight. Subsidence will then enter N CA tonight on the periphery of a broad area of low pressure to the north. The sinking air will likely act to compress the marine layer, but to what extent is uncertain.

Ensemble and statistical guidance continues to increase probability for IFR CIGS in the 700 ft AGL area. There is a low but noteworthy chance for LIFR CIGS at CEC (20%), with a lesser chance at ACV (10%). Introduced a FEW to SCT layer of 400 ft to highlight the potential.

For Ukiah, stratus is expected to advect into the Russian River Valley from southerly flow tonight. MVFR ceilings have a high probability (40 to 60%) chance of occurrence. There is some chance for IFR (25%) before improvement after 17Z Saturday.

MARINE

Stronger northerlies are pushing west over the outer waters Friday as a small surface low weakens the pressure gradient. Steep seas have propagated into the inners, but both winds and seas will ease through Saturday morning. Northerly winds will become reinvigorated Saturday afternoon, and a quick round of 6 to 8 ft short period seas will result. This will include the northern inner zone late Saturday, but there is not high enough confidence on the duration and coverage of up to 7 ft seas for issuance of an Advisory at this time. Northerly winds drop significantly Sunday and most of Monday while a small, long period WNW swell moves through. Steeper northerly seas of 6 to 8 ft will continue to periodically enter the outer waters, and the Advisories have been expected to cover this hazard to small crafts. The northerlies are forecast to restrengthen through next week.

EKA WATCHES/WARNINGS/ADVISORIES

CA... None. NORTHWEST CALIFORNIA COASTAL WATERS... Small Craft Advisory until 5 PM PDT this afternoon for PZZ450.

Small Craft Advisory until 1 PM PDT Sunday for PZZ470-475.