textproduct: Eureka

SYNOPSIS

Thunderstorms are possible each afternoon through Monday with the greatest strength and coverage of storms on Sunday. Drier weather is likely by mid next week.

KEY MESSAGES

-Isolated thunderstorms (10-20% chance) over Del Norte, Humboldt, and Trinity Counties this afternoon and evening.

-Scattered thunderstorms (15-25% chance) over the the interior Sunday afternoon with some storms pushing closer to shore.

-Strong to severe storms are possible Sunday with the potential for strong outflow winds, damaging hail, and localized downpours.

DISCUSSION

An upper low off the coast of northern California will continue to bring unsettled conditions to the interior through the weekend. Increased marine influence and cloud cover will slightly cool interior temperatures with highs mostly in the mid 70s. Lower pressure has helped lift the marine layer onshore with most models showing stratus sticking around even into the afternoon near the coast.

The greatest impact of the shortwave will be the potential for thunderstorms over the interior. While thunder activity remained out of the area Friday, chances increase today as CAPE increases to values peaking around 1000 to 1500 J/kg or higher. The main activity looks focused in Trinity, interior Humboldt, and interior Del Norte. Despite very high CAPE, moisture is still a limiting factor. Upper- level RHs are slightly higher than on Friday, however, so the environment is slightly more favorable for thunderstorms. High-res models are showing limited activity because of the limited moisture, but moisture and CAPE has generally been trending upwards in recent model runs. Bulk shear approaches 20-30 kts in interior Humboldt and Del Norte, so should storms develop (which is still fairly uncertain), they could be organized and strong. Stronger storms could bring winds up to 40-50 mph, hail, and localized heavy downpours. Northeast flow could steer storms over interior Humboldt and Del Norte over coastal Del Norte.

The shortwave will morph into a cutoff low and travel down along the California coast Saturday night into Sunday. Tropical air wrapping up the Sacramento valley will bring much more moisture to the area with precipitable water over 1.2 inches. Combined with increasing wind shear and still solid CAPE, the chances for thunderstorms will be much greater on Sunday. Storm chances over the interior are around 25% with greater potential for storms to push east to west into the coast (10 to 15% chance of storms along the Humboldt and Del Norte Coast). Some of the storms over the interior hold the potential to be severe with strong outflow winds and isolated damaging hail. Storms are also likely to be wet, with wetting rain (>0.1") likely from about Fort Bragg and northward. Isolated areas of heavy downpours from stronger storms could bring pockets of over 1" of rain with the highest chances in interior Del Norte. This will need to be watched for potential flash flooding, especially over the burn scars from recent years.

Storms will likely leave behind cooler and cloudier conditions for next Monday, though moisture and clouds will generally limit any frost or freeze concerns. Generally calm and warm conditions will most likely build for the remainder of next week. /JB&JHW

AVIATION

A heavy blanket of coastal stratus stretched from Del Norte to the King Range and Cape Mendocino but began to dissipate towards 01z. Improved conditions for KCEC are already in play after gusty northerlies helped dissipate the marine layer. Southerly winds are now the prevailing direction and will continue into Saturday morning. Flight categories for the overnight TAF period at KCEC will be mostly VFR though a few hours of IFR are possible with ceilings below 1000ft. Similar conditions at KACV with a greater probability of IFR categories at longer intervals. KUKI could see fog/stratus advection as southerly winds pick up early Saturday morning. /EYS

MARINE

Low pressure over the interior interacting with a skinny ridge of high pressure in the NE Pacific is reinforcing the strong surface pressure gradient over the coastal waters. This synoptic scale setup is resulting in continued northerly winds over the waters as well as large, steep seas. While the largest waves (up to sig. heights of 13-15ft) are modeled for the outer waters, steep seas of 7 to 9 ft are propagating into the inners. Near-gale conditions with localized areas of gale force gusts will develop during the afternoon Saturday. Gale strength gusts will be more persistent around and south of Cape Mendocino, yet coverage is too low to warrant a widespread gale warning for the southern waters. Hazardous Seas Warnings persist over the outer zones to account for the periods of light gales, but more importantly, the continued large, steep seas. After these warnings expire, expect small craft advisories to be issued for the outer waters. Conditions hazardous to small crafts are being observed for the inner zones due continued moderately breezy winds and steep seas. Saturday evening, an area of low pressure will approach the PacNW from the Gulf of Alaska, allowing the northerly winds to weaken down to gentle breezy conditions. This will begin an extended period of lighter winds and smaller seas which may carry into next week. The GFS global water shows a long period swell filling into the waters Saturday afternoon. As it arrives, the swell is forecasted to be around 2ft@20s before building up to 6ft@15s by Sunday evening.

BEACH HAZARDS

On Sunday, a long period northwest swell with periods between 16-19 seconds will arrive. This swell continues to be monitored as it may pose a moderate risk of sneaker waves for coastal areas. A Beach Hazards Statement has been issued for the coast Sunday. Sneaker waves are anomalously large, unexpected waves that sweep across beaches without warning. There can be up to 30 minutes of smaller waves before a sneaker wave arrives to the shoreline. Choose your recreation wisely by avoiding steep beaches and jetties as well as staying much farther back from the ocean than normal. Never turn your back on the ocean!

EKA WATCHES/WARNINGS/ADVISORIES

CA... Beach Hazards Statement from this evening through Sunday evening for CAZ101-103-104-109.

NORTHWEST CALIFORNIA COASTAL WATERS... Small Craft Advisory until 9 AM PDT this morning for PZZ450- 455.

Hazardous Seas Warning until 9 AM PDT this morning for PZZ470-475.