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. 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.

KEY MESSAGES

-Minor coastal flooding is expected Wednesday around Humboldt Bay.

-Persistent southerly winds, heavy rainfall, and high surf may contribute to moderate to locally major coastal flooding Thursday through Saturday if southerly winds and heavy rainfall align.

-Strong southerly winds, widespread moderate to locally heavy rainfall from an Atmospheric River brings an increased risk of urban and small stream flooding late Thursday and into Saturday.

-Differences in timing of heaviest rainfall onset remain, and forecast rainfall totals have trended slightly higher.

-Pacific storm track to remain open and active, bringing a risk for additional strong storm systems over the weekend into early next week.

DISCUSSION

High pressure remains over the area today, bringing mainly clear skies except for fog and low overcast in some interior valleys. Tonight into Wed multi-layer clouds are forecast to spread over the area from the south in advance of an upper level low approaching the area from the south. This is expected to keep overnight temperatures warmer. Shallow humid layer for coastal areas will likely keep temps warmer and may even result in thicker fog and/or low clouds.

An area of low pressure will push into Southern to Central CA Wednesday through Thursday. Light to locally moderate rainfall will spread in from the south Wednesday afternoon through Thursday. Given the multiple days of drying, impacts from this first system are expected to be little to none. PWATS do increase to over 250% of normal, and some of the ridges from Cape Mendocino and southward will receive 1-1.5 inches of rainfall, possible up to 2 inches locally. Southerly winds will become breezy on the ridgetops Wednesday afternoon, particularly south through southern Mendocino and Lake County. Snow levels are expected to be fairly high with this first system, likely over 6,000 feet.

A strong, negatively tilted trough with a deepening surface low will take shape over the NE Pacific and push east to northeast. The storm system will deliver a strong cold front with an associated IVT plume. Ensembles have better agreement on the magnitude, with ensemble mean just over 500 kg/m/s, with higher end range of 625 and a moderate Atmospheric River. This will bring widespread moderate rainfall, with areas of heavy rainfall rates. The modeled potential duration of category Atmospheric River remains at Moderate criteria through late Saturday. The moderate to heavy rainfall will be shorter in duration, but this reflects big differences in the onset timing, which could be as early as Friday morning or later Friday night.

The GFS is hinting at a secondary area of low pressure developing along the front and closer to the coast. This may bring some higher winds to the coastal ridges, the GFS shows 60 kt winds at 925mb just off Cape Mendocino. There is still quite a bit of uncertainty on how much stronger winds will mix down to the low elevations around Humboldt Bay and other areas along the coast. Ridges will gust to 60 mph or more for sure with 65+ kt at 925mb. Rainfall rates are expected to be higher with this front as well, and the threat for urban and small stream flooding will increase. Granted we have had multiple days of no rain and receding water levels which may mitigate tee risk for rapid run- off. The current forecast has trended higher to around 1 to 1.5 inch in the valley and rain shadow locations with 2 to 3.5 inches in the higher terrain and windward slopes over 36 hours.

High tides and rainfall run-off may result in flooding around Humboldt Bay and along the coast. Snow levels are expected to stay mainly over 5,000 feet through Friday evening and then start to fall late Friday or Saturday depending on the timing of the system. It looks like the colder air will move in after the heavier precip, but there may be some light snow down to 3,000 or 4,000 feet. GFS and ECMWF models are showing cold 500mb temps around -25C, instability and steep lapse rates. Isolated low topped thunderstorms and some small hail will be possible in the colder air behind the front late Friday into Saturday. Snow levels plunge on Saturday and some light accumulations are possible down to 3500 ft. NBM snow levels will need to evaluated.

The Pacific storm track is to remain open and active bringing a risk for additional strong storm systems over the weekend into early next week. JJW

AVIATION

Light winds and VFR conditions prevailed Tuesday. A frontal system to the south will begins to increase southerly flow and surface moisture tonight. This will bring a low confidence chance for fog around Humboldt Bay and perhaps north along the North Coast. Chances for fog and low clouds quickly decrease after 12Z. UKI received some brief fog Tuesday morning and chances were lower for Wednesday morning. Light precipitation will then begin spreading in from the south Wednesday afternoon. JJW

MARINE

Light winds and relatively low seas will persist through at least Wednesday before conditions deteriorate later into the week. Fresh southerly winds will develop on Thursday and approach near gale by Friday as a surface trough from the Gulf of Alaska digs towards the U.S. West Coast. National Blend of Models is highlighting a 40-70% chance of wind gusts exceeding 34 kt across the northwest California coastal waters, especially around Cape Mendocino and the Mendocino County coast. Additionally, 925mb level winds are predicted be 50 to 70 kt with this system. Any instability along with the frontal passage could potentially mix down some storm force gusts. Thus, we will be monitoring the potential and timing for Small Craft Advisories leading up to a potential Gale Warning later in the week. Large, steep seas will develop with the approach of this storm system with 10 to 15 foot seas possible in the waters over the weekend. KZ

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 warning 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 is coming up the 31st through 4th. The storm surge models are already showing nearly a foot of surge, but higher amounts are possible if strong southerly winds line up with the highest tides. The high tide plus anomaly will likely yield a high tide over 8.8 ft at the North Spit gauge Wednesday. High tides will likely exceed 9 ft Thursday through Saturday. The storm surge model has been consistently modeling a high tide up to 10 ft at North Spit Friday. 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. JJW

EKA WATCHES/WARNINGS/ADVISORIES

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

Coastal Flood Advisory from 7 AM to 9 AM PST Wednesday for CAZ103.

NORTHWEST CALIFORNIA COASTAL WATERS... Coastal Flood Advisory from 7 AM to 9 AM PST Wednesday for PZZ415.