textproduct: Eureka

SYNOPSIS

Cold temperatures are expected this morning. A mostly dry day is expected today until later this evening. An atmospheric river is expected to bring heavy rainfall, higher mountain snow and strong to very strong south winds this weekend and continuing into mid next week.

DISCUSSION

Temperatures are dropping rapidly across the coastal areas and interior valleys. Cold air and light winds will lead to very cold temperatures and low wind chills tonight through Friday morning. Temperatures as low as 27 degrees are expected for the coastal areas, while lows between 15 to 25 degrees are expected for the interior valleys. A Cold Weather Advisory is in effect through 9 AM for the coastal areas and interior valleys.

Today is expected to be mainly dry with below-normal daytime temperatures. However, a gradual warming trend is expected to begin Friday and continue through the weekend. High temperatures are forecast to be in the 50s along the coastal areas, while generally remain in the 40s across the interior; except for Trinity County with highs in the mid 30s into low 40s.

More wet and unsettled weather returns this weekend as the next of a series of frontal system along IVT plume impacts the area with heavy rain, higher mountain snow and strong south winds. The WPC 500 mb cluster analysis continues to show a good agreement that a broad closed upper level trough settle over the NEPAC, while the ridge rises and shifts eastward this weekend. A warm front will be driven by a compact surface low swinging northeastward toward the Oregon Coast on Friday night. Strong to very strong southerly winds are expected Saturday through Sunday as the surface pressure gradient tightens and a long-duration low level jet at 925mb from 65 to 75 kts develops along the coast. It will be more unstable and strong winds aloft Saturday into Sunday most likely mix down to the surface, especially near or along the surface front. Southerly winds are expected to gradually increase this Friday night, and peak up Saturday evening/night. The strong winds area expected to develop from north to south, impacting first Del Norte and Humboldt counties Friday night and spreading southward into Mendocino and Lake counties Saturday night. There is an increasing chance (55-80%) for wind gusts exceeding 45 mph along the coast on Saturday evening into Sunday. For highest elevations, there is a 60-85% chance for max wing gusts exceeding 70 mph in Del Norte and Humboldt counties Saturday into Sunday. ECMWF Extreme Forecast Index (EFI) highlight Saturday night into Sunday in the 80th-95th percentile above normal wind gusts, specifically for Del Norte and Humboldt counties, while in the 50th-80th percentile for Mendocino and Lake counties. A Wind Advisory has been issued for Del Norte County and higher elevations of Humboldt County. These may need to be expanded and/or upgraded to High Wind Warnings, especially as winds appear likelier to mix down to the lower elevations later Saturday into Sunday.

The ensemble mean for both the GEFS and ECMWF EPS are now showing an increasing probabilities (50-70% chance) of a moderate atmospheric river (IVT's greater than 500 kg/m/s) impacting the area Sunday through Tuesday next week. There still uncertainties on exact timing of the breaks in rain will be a big factor in the level of impacts. 72 HR NBM probability indicates there is a 50-80% chance of precipitation exceeding 3 inches across Del Norte, Humboldt, western Trinity and northern Mendocino counties from Sunday through Tuesday, and a 20-35% chance for around Humboldt Bay and lowest elevations in southern Mendocino and Lake counties. Stay tuned! /ZVS

AVIATION

Showers continue to diminish early this morning. Offshore winds will aid in keeping coastal terminals mostly free of low clouds;however, as a system approaches, cloud cover will increase along with southerly winds through the next TAF period. Interior valleys have the possibility to develop low clouds before sunrise.

MARINE

Northerly gusts continue to decrease throughout the waters. Steep, short period seas will diminish but still remain around 12 feet at 12 seconds. Short period seas should diminish to below 6 feet by mid morning. Calm conditions will be short lived as a strong storm system approaches the area on Saturday. There is high confidence that southerly winds will rapidly increase during the day Saturday, especially in the northern waters. Gale conditions are most likely in all waters by sunrise on Saturday. Winds will build to storm force (gusts in excess of 55 kts) for the outer waters in the afternoon. Strong gale force gusts with some more isolated storm conditions will push into the inner waters by Saturday night and through Sunday. Short period seas will grow as steep as 19 feet at 13 seconds by Saturday night. Very steep and dangerous sea conditions will persist into at least Monday even as winds gradually decline. /JHW

HYDROLOGY

An Atmospheric River event is expected to impact NW California this weekend and into mid next week. Moderate to heavy rainfall will bring the potential for rapid rises on rivers, streams and creeks across the region. The Weather Prediction Center (WPC) included northwest California in the Marginal Risk (at least 5%) of excessive rainfall for Sunday and Monday. There is a 50 to 80% chance for 24-hr precipitation exceeding 1.5 inch from 10 PM Saturday through 10 PM Sunday, with up to 90% chance for the exposed ridges in Del Norte and Humboldt counties. The chances lower slightly on Monday, however moderate to high chances (50-80%) for daily precipitation exceeding 1 inch remain across the area with a prolonged moderate to heavy rainfall through Tuesday.

A rapid rise of the main stem river is expected on Sunday. At this point, the Russian River at Hopland currently has around a 30 percent chance of exceeding flood stage Sunday evening. There is an increasing chance for many other main stem rivers to reach or exceed flood stage early to mid next week such as Eel River at Fernbridge, Mad River at Arcata, and Navarro River at Navarro. These will need to be watched as it gets closer. The rest of the main stem river are not expected to reach/exceed flood stage as of yet. /ZVS

EKA WATCHES/WARNINGS/ADVISORIES

CA... Cold Weather Advisory until 9 AM PST Friday for CAZ101>115.

Wind Advisory from 10 PM Friday to 10 AM PST Sunday for CAZ101-102-104>106.

Winter Storm Watch from Saturday evening through Monday morning for CAZ107.

NORTHWEST CALIFORNIA COASTAL WATERS... Small Craft Advisory until 3 AM PST Friday for PZZ450-470.

Gale Watch from late Friday night through late Saturday night for PZZ450-455.

Small Craft Advisory until 6 AM PST Friday for PZZ455-475.

Gale Watch from Friday evening through Saturday morning for PZZ470.

Storm Watch from Saturday morning through late Saturday night for PZZ470.

Gale Watch from Friday evening through Saturday afternoon for PZZ475.

Storm Watch from Saturday afternoon through late Saturday night for PZZ475.