textproduct: Eureka

SYNOPSIS

A cutoff low is approaching the region. Rain chances increase late Sunday and chances will continue through mid next week. A period of drier weather is then expected to begin late next week.

DISCUSSION

Early Sunday morning, satellite depicts an upper level trough 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 early this morning. 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 30 mph, but there is some upper level wind support in the 925 mb level (around 2000 ft) for some locally gustier winds. The highest probabilities for gusty winds are over coastal headlands and higher ridges.

In general 1 to 2 inches of rain, with locally 3 inches over some of the ridges on the King Range and southward facing terrain, is likely from Sunday night to Tuesday night. NBM chances for over 1.5 inches of rainfall are moderate to high for the King Range and southward (60 to 80 %). 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 thunderstorm 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

Abundant high and mid layer clouds in advance of a frontal boundary will continue into Sunday. VFR conditions are expected to prevail through Sunday. MVFR ceilings in light showers or a few sprinkles will be possible (30% chance) after about 20/00Z. South to southeast winds aloft, 1500-2000 feet AGL, will also increase through the day in advance of an approaching frontal boundary. It is not very unstable or turbulent for these stronger winds to mix down and impact the forecast terminals. Laminar stable flow over the coastal ranges may yield occasional gusts to 25-30kt over the higher elevations with perhaps some turbulence for small and light weight aircraft. Chance for MVFR conditions will increase Sunday night and Monday as rain intensity and lower level moisture increases.

MARINE

Southerly to southeasterly winds will increase in advance of a slow moving frontal boundary on Sunday. Strongest wind gusts, up to 25-30 kt, will most likely occur during the afternoon and evening hours on Sunday. A secondary south-southest speed max will develop as the boundary pivots in response to an upstream trough later Sunday night into Monday morning. Strongest wind gusts during this time from 06Z-18Z appears to be around Cape Mendo and over Mendocino waters. S-SE fetch will most likely limit steep wind wave generation to 5 to 7 feet at 7 seconds.

On Monday, west to northwest swell will build to 7 to 9 ft at 12 to 13 seconds. The combination of lingering S-SE wind waves around 4 ft may yield seas around 10 feet.

The low and frontal system will begin to shift southward on Tue and NW to N winds should make a come back Tue night into Wed. Most ensemble members indicate high pressure building offshore as 500mb heights rise behind the departing low mid to late week. North winds will most likely increase toward the end the week (Thu and Fri) to at least small craft advisory strength (gusts 25-30kt). N-NW seas will also build in response the steady north winds.

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 until 3 AM PDT early this morning for CAZ103.

NORTHWEST CALIFORNIA COASTAL WATERS... Coastal Flood Advisory until 3 AM PDT early this morning for PZZ415.

Small Craft Advisory from 1 PM this afternoon to 8 AM PDT Monday for PZZ470-475.