textproduct: Eureka

SYNOPSIS

Light to moderate rain returns from south to north Wednesday into Thursday, 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 Thursday through Saturday. Bouts of rain and gusty winds are forecast to continue into the weekend and likely next week.

DISCUSSION

High clouds are beginning to spread over the region as an area of low pressure develops off the southern California coast. This area of low pressure will move north toward our region Wednesday with a shield of light rain continuing to spread across Northwest California from south to north Wednesday night. On and off showers across the region will persist through the day Thursday into Thursday night as the low fills and moves east. Rainfall from this first system isn't expected to be particularly heavy with 0.5-1.5 inches across the region, locally up to 2 inches in favored locations. Since streams have all receded over the last 5 dry days, flooding is unlikely from this first period of rainfall. Additionally, wind fields are expected to remain not particularly strong from this first low with no wind concerns. Depending on how quickly the first system moves out of the area, many areas, especially along the coast, may see a mostly dry 6- 12 hours Friday morning.

Conditions will deteriorate through the day Friday into Friday evening as the next storm system approaches. Guidance is now in agreement that this system will be stronger and contain more moisture flux than the Thursday system. A quick passing bout of heavy rain and strong southerly winds is anticipated Friday. Guidance is indicating 925mb winds exceeding 60 kt just off the immediate coast and 850mb winds over 65-70kt Friday into Friday evening. Though the front will move past quickly, strong wind gusts will be likely for at least several hours over the near coastal mountain ranges and coastal headlands. These winds will likely be strongest from southern Humboldt and north, but there are some indications that the wind field could extend south through Mendocino and Lake counties.

A secondary strong shortwave and potential closed surface low will pivot by the coast Saturday, generating likely stronger winds than Friday. Wind Advisories have been issued For Humboldt, Del Norte, and Coastal Mendocino, with a High Wind Watch out over Cape Mendocino. Further expansion of the the advisories upgrades to High Wind Watches, particularly Saturday may be necessary based on quickly evolving model guidance and uncertainty in the exact track of the low.

The rain with the primary front Friday into Friday night will be brief but heavy with another 1-2.5 inches across the region. Behind this front, the secondary area of low pressure will re- enhance rain across the entire region. At the same time, another area of low pressure will be deepening and diving southeast well off to the west. This should cause any eastward progress of the entire moisture flow into northern California to become stalled, allowing moisture flux to remain targeted at the northern half of the state. An additional 1-4 inches is forecast Saturday through Sunday. There is a lot of uncertainty as to the details during the 2nd half of the weekend into early next week, but if the moisture flux remains aimed into our region, flooding will likely become more and more of a concern through the weekend. Latest guidance is increasing confidence on this rainier scenario, and a Flood Watch has been issued from Southern Humboldt/Trinity southward over Mendocino and Lake counties. RPA/JJW

AVIATION

Light rainfall is spread northward across the region from a weak frontal system. VFR to MVFR ceilings are initially expected across the region from the light showers and rainfall. CIGS and VIS will lower Thursday afternoon as rainfall rates increase. Chances for IFR then increase after 21Z Thursday. Higher chances for IFR to LIFR increase going into Friday evening for the interior valleys, including UKI.

MARINE

Light winds and a 2 to 5 foot swell is bringing quiet conditions to the waters this morning. These are expected to continue through Wednesday night. Thursday southerly winds are expected to slowly increase to around 15 to 20 kt Thursday morning. These may diminish slightly in the afternoon.

Thursday night the winds are expected to start to increase more rapidly. Gales are expected by early Friday morning. Confidence is highest on these in the outer waters, but they will likely make it into the inner waters later in the day as well. Friday afternoon the southerly wind driven waves are expected to build and then peak around 12 to 14 feet at 8 to 10 seconds Friday night. Saturday and Saturday night these transition to a fresh swell out of the southwest. Current GFS Wave model shows them building to around 12 to 14 feet at 12 seconds. Confidence remains lower than normal on this swell.

Periods of strong winds are expected again Saturday and Monday, although confidence the details of these systems is low. MKK

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 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.

COASTAL FLOODING

Another round of very high tides will continue through 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, 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/JJW

EKA WATCHES/WARNINGS/ADVISORIES

CA... Coastal Flood Watch from Thursday 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.

Coastal Flood Warning from 7 AM to 11 AM PST Thursday 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 Thursday for PZZ415.

Small Craft Advisory from 4 PM Thursday to 2 AM PST Saturday for PZZ450-455.

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

Gale Watch from late Thursday night through late Friday night for PZZ470-475.