textproduct: Eureka

SYNOPSIS

Light to moderate rain will continue through the tonight, followed by a frontal system with moderate to heavy rain, high mountain snow, and gusty winds on Friday into Friday night. Chances for moderate to perhaps locally major coastal flooding increase Friday through Saturday. Bouts of rain and gusty winds are forecast to continue Sunday and likely next week.

DISCUSSION

The first in a series of storm systems is now moving through northwest California producing light to moderate rainfall throughout the region. Generally 0.5 to 1 inch has been recorded over the region as an area of low pressure moves northeast across California. This rain will help to prime the environment for what is to come Friday and into the weekend. The passing low is generating some gusty ridgetop winds.

A more potent front will approach the region on Friday, first bringing increasing southerly winds across the entire region. Ridgetop winds could reach advisory levels by late Friday morning across Humboldt and Del Norte counties, expanding inland through the day. The front will then rapidly move through the region during the evening or overnight hours Friday night with a narrow band of potentially very strong winds which could mix down to the coastal plain. Wind advisories have been issued for all coastal areas and coastal ridge tops. Wind advisories have been expanded through Mendocino and Lake Counties. There may be a quick hit of stronger winds to warning levels on the ridgetops near Cape Mendocino and a high wind watch is in effect for that area. Behind this front a secondary low will approach the northwest California coast within a more unstable, mixed atmosphere. It is highly likely that this low will bring another round of strong southerlies on Saturday, especially in Humboldt and Del Norte counties, and the winds could mix better down to the coastal plain and again bring strong winds to the ridgetops. A High Wind Watch has been issued for Del Norte County for this threat. Some expansion further south along the North Coast may be considered as the exact track in strong, compact lows is difficult for models to resolve.

In addition to the wind from these two systems, heavy rain is also expected. Guidance is now in very good agreement on a band of rain with heavy rain rates rapidly moving through the region in advance of the front. Total rainfall is not expected to be exceptionally heavy, around 1-3 inches across the region, but this rain should allow for many small streams to begin to rise. Quickly behind this front the rain will re-enhance as the secondary low approaches and an atmospheric river may aim toward Humboldt, Trinity, Mendocino, and Lake counties. Guidance continues to increase the rainfall potential and Saturday into Sunday. Overall rainfall during the Thursday through Monday time period could range from 3-6 inches across the region with locally higher amounts up to 8 inches.

This heavy rain could easily result in small stream flooding and even main stem rivers reaching critical flood levels. Thus, a flood watch has been issued for all of Humboldt, Mendocino, and Lake counties. An expansion of the Flood Watch to Del Norte County may be considered based on trends in rainfall mounts farther north, and the potential for persistent or training convective showers or thunderstorms. The Storm Prediction Center has placed our coastal regions in the Marginal Risk for strong thunderstorms Saturday. Though it is uncertain how much thunderstorms activity materializes, this inclusion to Marginal Risk highlights the amount of available instability to enhance precipitation rates. If these trends continue it will be highly likely that flood advisories and/or warnings will be needed this weekend into Monday.

Snow levels will lower to 4000 to 3500 ft through the weekend and early next week. Snow impacts currently look mostly confined to Scott Mountain Pass in Northeast Trinity County. Total snowfall amounts have flirted at times with Winter Storm criteria, but currently is looks like an Advisory would cover the threat if totals do not trend lower. RPA/JJW

AVIATION

A passing area of low pressure is brining light to moderate rainfall Thursday. Abundant inland valley fog is likely and forecast to remain into the night. VIS and CIGS have been quite variable Thursday, moving into IFR/LIFR at times before improvement quickly returns. Southerly winds have increased with the passing low, and stronger south winds of up 40 kts through 2000 ft AGL are creating a threat for low level wind shear. Southeast winds will strengthen tonight and will work to lift ceilings and disrupt the inland fog. Winds and rainfall will trend much higher Friday with the arrival of a second much stronger front. Southerly winds and rain will not ease much before even stronger and heavier wind/rain moves in Friday.

MARINE

Fresh south winds today will ramp up later tonight across the outer coastal waters and nearshore along the Mendocino County coast. Gale warnings remain in effect for these locations very early Friday morning into Saturday. Additionally, a gale warning was issued for the northern inner waters beginning late Friday night. As the low pressure offshore digs and travels eastward, it will whip some gale force gusts across the northern inner waters. A few storm force gusts will possible, especially across the outer waters.

Winds will decrease slightly early Saturday morning to possibly just below gale but then another low will approach, potentially stronger then the last, allowing winds to increase to above gale for most of the waters and potentially exceeding storm in the northern outer zone Saturday afternoon and evening. All of these strong southerly winds will allow seas to build and become very steep. Seas could exceed 20 feet by Saturday afternoon in the outer waters zone and some of these steep seas will propagate toward the coast. Winds will decrease overnight into Sunday morning as the low moves off to the north and moderate southerlies will then persist into Monday as stormy conditions continue. KZ/RPA

HYDROLOGY

A series of atmospheric rivers will bring an increased risk for urban and small stream flooding late this week, through the weekend and likely into next week. Mainstem rivers will also likely rise sharply and will need to be monitored for possible flood stage exceedance. Be on the lookout for watches, advisories, and warnings as this next heavy rain event unfolds. Stay alert and remember to never attempt to drive through water covered roads. Water may be moving much faster than you think. /RPA

COASTAL FLOODING

A period of very high tides will continue through the 4th. The north spit tide gauge already recorded an impressive 9.74 ft high tide Thursday morning. The storm surge models continue to show over foot of surge, now verified at the north spit tide gauge as of 11 pm on the 31st, but higher amounts are possible with strong southerly winds expected to line up with the highest tides. High tides will likely reach 9.8 to 10.02 ft Friday at the North Spit gauge. A Coastal Flood Warning has been issued for the entire coastline from Southern Mendocino to Del Norte for likely historic high tides. The storm surge model is now modeling a high tide up to 10.3 ft Saturday, and 9.8 ft Sunday. The persitent and strong southerly winds may even nudge these values higher. This scenario would bring 1 to 1.5 feet of saltwater onto normally dry ground, and this is plausible if the strong winds, heavy rainfall and building high surf align. Outside of Humboldt Bay, the storm surge model is also showing over a foot of storm surge with at least advisory thresholds being met along the outer coast during high tide. Strong winds and large surf could cause waves to wash onto roads exposed to the surf including Highway 101 south of Crescent City, Pebble Beach in Crescent City, Moonstone Beach parking lot, Centerville Beach road and parking lot, low spots along Highway 1 on the Mendocino coast, and other near coast infrastructure. At this time surf isn't expected to be exceptionally large, so elevated locations like the homes in Shelter Cove are less likely to be impacted but it isn't out of the question as a possibility both Friday and Saturday at high tide if larger seas materialize. Because of all of these possibilities, a Coastal Flood Watch remains Thursday through Saturday for the entire coastline, and further warning issuance should be expected each day. Though the watch is in effect for the entire day, the flood threat is only 1-2 hours before and after the morning high tides. /RPA

EKA WATCHES/WARNINGS/ADVISORIES

CA... Coastal Flood Warning from 7 AM to noon PST Friday for CAZ101-103-104-109.

Coastal Flood Watch from Saturday morning through Sunday afternoon for CAZ101-103-104-109.

High Wind Watch from Saturday morning through Sunday morning for CAZ101-102.

Wind Advisory from 1 PM Friday to 10 PM PST Saturday for CAZ103-105-106-109-110-112.

Flood Watch from Friday afternoon through Sunday evening for CAZ103>106-108>115.

High Wind Warning from 7 AM Friday to 7 AM PST Sunday for CAZ104.

Wind Advisory from 1 PM Friday to 4 PM PST Saturday for CAZ113>115.

NORTHWEST CALIFORNIA COASTAL WATERS... Coastal Flood Warning from 7 AM to noon PST Friday for PZZ415.

Small Craft Advisory until 10 PM PST Friday for PZZ450.

Gale Warning from 10 PM Friday to 11 PM PST Saturday for PZZ450.

Small Craft Advisory until 6 AM PST Friday for PZZ455.

Gale Warning from 6 AM Friday to 4 AM PST Saturday for PZZ455.

Small Craft Advisory until 2 AM PST Friday for PZZ470-475.

Gale Warning from 2 AM Friday to 11 PM PST Saturday for PZZ470-475.