textproduct: Eureka

SYNOPSIS

Light to moderate rain will continue through the day today, 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 into the weekend and likely early 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. Up to 0.75" has been recorded in parts of Mendocino and Lake counties as of this writing with much less rain off to the north in northern Humboldt and Del Norte. Looking at the model guidance, this rain is on the wetter side of the model envelope through 06Z. Additional rainfall is expected through the day on Thursday as the 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.

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 may also be needed into Lake County but confidence is not yet high enough to expand into that area at this time. 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.

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. If these trends continue it will be highly likely that flood advisories and/or warnings will be needed this weekend into Monday. /RPA

AVIATION

A shield of rain has spread over Northwest California with light to moderate rain rates at times. Rain will persist through much of Thursday. Southerly winds aloft will begin to increase on Thursday resulting in some wind shear at Ukiah, but otherwise mostly light winds are anticipated through the period. Mostly MVFR ceilings and visibilities are expected at the TAF sites however periods of IFR are likely in periods of moderate rain and where fog attempts to form in the interior valleys. /RPA

MARINE

Southerly winds are now beginning to increase over the coastal waters as an area of low pressure south of the area moves north. This system will move inland on Thursday but another front will quickly approach the waters behind the departing system. Southerly winds will ramp up across the outer and southern inner water zones late Thursday night into Friday morning with gale force winds expected and a few storm force gusts possible as the front approaches Friday afternoon/evening. There is some uncertainty as to how much of this wind makes it within 10 nm of the coast north of Humboldt Bay, so this zone has not been included in the gale warning, but wind gusts to gale are certainly possible. 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. /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 a 9.07 ft high tide Wednesday 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 9.3 to 9.6 ft Thursday. A Coastal Flood Warning has been issued for Humboldt Bay. The storm surge model has been consistently modeling a high tide up to 10 ft at North Spit Friday and again Saturday. This scenario would bring 1 to 1.5 feet of saltwater onto normally dry ground around Humboldt Bay, 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 Watch from 11 AM PST this morning through Saturday afternoon for CAZ101-103-104-109.

Wind Advisory from 1 PM Friday to 10 PM PST Saturday for CAZ101>103-105-106-109-110-112.

Coastal Flood Warning from 7 AM to 11 AM PST this morning for CAZ103.

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

High Wind Watch from Friday morning through late Saturday night for CAZ104.

NORTHWEST CALIFORNIA COASTAL WATERS... Coastal Flood Warning from 7 AM to 11 AM PST this morning for PZZ415.

Small Craft Advisory from 4 PM this afternoon to 2 AM PST Saturday for PZZ450.

Gale Watch from late Friday night through Saturday evening for PZZ450.

Small Craft Advisory from 4 PM this afternoon to 6 AM PST Friday for PZZ455.

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

Small Craft Advisory from 11 AM this morning to 2 AM PST Friday for PZZ470-475.

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