textproduct: Eureka

SYNOPSIS

The risk of thunderstorms over the the interior will peak Sunday afternoon and evening. Cooler conditions will briefly build Monday with warmer and mostly calm conditions for most of the week.

KEY MESSAGES

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

-Cooler and cloudy conditions Monday followed by warmer and generally calm conditions by midweek.

DISCUSSION

A weak shortwave continues to influence the area today. Marine influence has lifted higher but remains robust along the coast, particularly north of Cape Mendocino and into the Eel River Valley. Despite persistent instability across the interior, moisture has so far remained too low and surface forcing to weak to create much more than cumulus clouds. some isolated thunderstorms remain possible (10% chance) this evening with some potential for storms into Del Norte County.

The shortwave will continue to morph into a cutoff low which will pass and begin to travel south of the area on Sunday, generally sticking to the shore. As the low pushes south, it will help pull more tropical moisture around and up the Sacramento Valley. Precipitable water increasing above 1.0 inch combined with persistent moderate instability and some marginal shear will increase the risk of scattered thunderstorms (20% chance over the northern interior) with a some potential for isolated severe storms in Trinity county. Storm potential will peak late Sunday afternoon and into the early evening. Easterly steering flow will help push any storms that do form closer to shore, though any storms will weaken as they move to the west off the high terrain. Stronger storms could create 0.3 to 0.6 inches of heavy rain, strong outflow winds, and some damaging hail. The strongest storms will be in Trinity and far eastern Humboldt and Del Norte Counties. Due to the track of the low, storm chances are much lower in Mendocino and Lake Counties. Meanwhile, an elevated, gloomy marine layer will most likely persist along the coast.

Drier conditions will quickly return by Monday and Tuesday. Colder air being pulled from the northeast aided by lingering clouds will briefly cool interior temperatures into the 60s Tuesday. Clouds and valley moisture will limit any frost concerns. The east wind will likely help erode and shallow the marine layer, but most models show stratus persisting through the early week. Benign and weak high pressure will build late in the week, bringing slightly warmer than average temperatures but otherwise dry, seasonable, and calm weather. /JHW

AVIATION

Coastal stratus continues along the North Coast again today. Strong northerly surface winds have moved off shore allowing variable to southerly winds to develop. THis southerly surge increases the likelihood of stratus returning to KCEC if any clearing happens this afternoon. KACV is expected to be socked in with MVFR categories through the day with overnight conditions dropping down into IFR as ceilings lower slightly and increased mist forms. Inland, daytime heating is mixing away the valley stratus giving way to VFR conditions. There is a slight (10-20%) thunderstorm threat for interior NW CA, yet observed high cloud coverage could keep a cap on any formation of these cells. Valley fog is likely to return tonight to interior valleys with conditions dropping down to LIFR in the early morning.

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 continue through the day before decreasing after sunset. A Small Craft advisory has been issued for the outer waters due to the continued steep, short period seas.

This evening, a cold air mass will approach the western seaboard, this cold air will disrupt the surface pressure gradient allowing the northerly winds to weaken down to gentle breezy conditions. This will begin an extended period of lighter winds and smaller seas through next week as relatively light winds flow over the coastal waters. From Sunday through the week, the GFS wave model shows two main swells impacting the coastal waters, the first is a long period, westerly swell building up to 6ft@15s and another is a NW swell of 4ft@8s. Minimal impact is expected next week from fresh, short period waves.

BEACH HAZARDS

Sunday morning, a long period northwest swell with periods between 17-20 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 11 PM PDT this evening through Sunday evening for CAZ101-103-104-109.

NORTHWEST CALIFORNIA COASTAL WATERS... Small Craft Advisory until 9 PM PDT this evening for PZZ470.

Small Craft Advisory until 4 PM PDT this afternoon for PZZ475.