textproduct: Eureka

SYNOPSIS

Overnight temperatures moderate as high pressure begins giving way to a trough. Rain chances increase late Sunday and chances will maintain through mid next week. A period of drier weather is then expected to begin late next week.

DISCUSSION

An upper trough is evident on satellite Saturday afternoon over the NE Pacific. A cold core upper low will cut off from the flow and proceed toward the California coast late Sunday. Rain chances begin to increase late Sunday, along with southerly winds. An increase in moist, southerly flow and some associated upglide instability may coax out some light precipitation tonight. Moderate to some instances of heavy rainfall will arrive Monday, particularly with some smaller scale disturbances which will rotate in around the periphery of the cutoff low. Precipitable water values will be high, rising to 200% of normal, and this increases the chances for some heavier rainfall rates.

Southerly winds will turn gusty at times Sunday and Monday, mainly for the ridges. The isolated disturbances that pass through may locally increase winds at times. NBM is struggling to resolve meaningful probabilities for gusts over 25 mph, but there is some upper level wind support in the 925 mb level (around 2000 ft) for some locally gustier winds. A general 1 to 2 inches of rain, with locally 3 inches over some of the ridges on the King Range and southward is likely. Lighter amounts can be expected north of the King Range. NBM chances for over 1.5 inches of rainfall are moderate to high for the King Range and southward (40 to 65 %). Snow levels will drop to around 5000 ft, and some light snow is possible over Scott Mountain pass in far northern Trinity County.

The cold core of the low will cross over N CA Tuesday. The steepened lapse rates, broad ascent, and increased instability will bring a chance for thunderstorms (15 to 25%). Soundings are resolving some relatively high CAPE for the region of 300 to over 500 J/kg for the interior, mainly focused over and south of Cape Mendcino. The increased winds around the low will generate effective shear to 30 kts. These conditions will bring a more favorable chance for some stronger storms capable of hail over the southern portion of the region through Mendocino and Lake counties, but the exact placement of the low may nudge the greater threat north or even south out of the area over Sonoma County or the Sacramento Valley. Soundings show more of "skinny" CAPE profiles for the coastal areas north of Cape Mendocino, and thunderstorms coverage for those areas may be more limited to what wraps in from the southeast. Veering flow is also noted south of Cape Mendocino along the Mendocino coast, and this will have to be watched for any potential for weak rotation in any storms.

Chances for at least additional showers may linger into Thursday when a weak shortwave moves through behind the departing low. Generally dry weather is likely thereafter, with perhaps a weak disturbance bringing a chance for a shower or two at times. JJW

AVIATION

VFR conditions are in place this afternoon once the morning stratus cleared out. A frontal boundary is approaching the area and winds are gradually becoming south to southeast. The southeast winds are expected to increase tonight and Sunday morning. The main forecast challenge is when/if the winds will pick up enough to keep the stratus off the coast. Most of the HiRes models are keeping the stratus off the coast. However, it is still possible some low clouds will form at KACV and KCEC this evening before the winds pick up. For now have some MVFR conditions in the TAFS highlighting this. The rain is expected to remain offshore through Sunday evening and finally move over the terminals very early Monday and this will likely bring lower CIGS in the rain. MKK

MARINE

A frontal boundary ahead of an area of low pressure is approaching the area this afternoon. Winds are starting to turn southerly. These will be completely southerly and start to increase shortly after midnight tonight. Winds are expected to be around 15 to 20 by Sunday afternoon and remain elevated Sunday night.

Monday southerly winds are expected to continue, but slowly diminish through the day as the low approaches and weakens. A westerly swell building in is expected to become the dominant wave as it quickly builds to 8 to 10 feet at 12 seconds through the day Monday. Eventually on Tuesday the low will start to move over the area and off to the east, but confidence is low on how quickly this will happen. Once the low moves past the waters winds begin to turn northerly once again. Winds will likely remain fairly light on Wednesday. Thursday and Friday high pressure builds back in and northerly winds are expected to increase again. MKK

COASTAL FLOODING

A new moon cycle is bringing higher than usual tidal swings. A near 1 foot positive tidal anomaly is also contributing to higher tides. Slackening northerly winds and increasing southerly winds may further increase the anomaly. The high tide plus anomaly may yield a high tide near 8.8 ft at the North Spit gauge near 1 AM Sunday morning. A coastal flood advisory has been issued to highlight this. High tides will likely near this value again near 2 AM Monday morning.

EKA WATCHES/WARNINGS/ADVISORIES

CA... Coastal Flood Advisory from midnight tonight to 3 AM PDT Sunday for CAZ103.

NORTHWEST CALIFORNIA COASTAL WATERS... Coastal Flood Advisory from midnight tonight to 3 AM PDT Sunday for PZZ415.