textproduct: Eureka

SYNOPSIS

High pressure dominates the weekend with unseasonably warm afternoons and coastal fog. A pattern shift begins Monday as a Pacific frontal system introduces a wet and breezy period through mid-week.

DISCUSSION

KEY MESSAGES:

* Warm and Calm Weekend: Unseasonably warm temperatures and dry conditions will persist through Sunday, though coastal areas and valleys will see night and morning dense fog. * Pattern Change Monday: A weak-to-moderate atmospheric river will impact the North Coast Monday through Wednesday, bringing beneficial rain and breezy southerly winds; snow levels will remain very high (above 6,000 feet). * Stronger Storm Potential Late Week: Long-range ensembles signal the potential for a more significant storm system late next week (Dec 19-21), with forecast confidence regarding impacts increasing, though some model ensemble member spread still exists.

Scientific Rationale & Synoptic Setup:

Broad upper-level ridging currently anchored over the Great Basin is promoting gentle offshore flow and subsidence, suppressing the marine layer to a very shallow depth. This setup is driving 500mb heights well above climatological means, resulting in the unseasonably warm temperatures observed across the interior. However, global models are in excellent agreement regarding the breakdown of this ridge by late Sunday as a longwave trough over the Northeast Pacific begins to approach the coast. This transition marks the onset of a progressive, wet pattern.

Probabilistically-Informed Details:

Fog/Clouds:

Visible satellite imagery shows a classic "southerly surge" of stratus hugging the immediate coast. Rapid Ensemble Forecast System (REFS) suggests a 40-70% probability of visibilities dropping below 1/2 mile along the Redwood Coast and in the Eel River Valley overnight into Sunday morning.

Rainfall (Monday - Wednesday):

Confidence is high (80%) that the ridge will be displaced by a moisture plume characterized by Integrated Vapor Transport (IVT) values consistent with a weak-to-moderate Atmospheric River. Kept low chance of thunder for parts of the area Monday as the NBM v4.0 and to some extent, the NBM v5.0, indicate this, and perhaps into Tuesday night as well, although this was held back from the current deterministic forecast.

* Timing: Precipitation onset is expected late Monday afternoon for Del Norte County, spreading south to Cape Mendocino by Monday evening. * Amounts: The National Blend of Models (NBM) shows a tight clustering of solutions for the North Coast, forecasting 1.50 to 3 inches of rain for the Smith and Klamath basins through Wednesday. For the immediate Eureka/Arcata area, the probability of exceeding 1.00 inch of rain in 48 hours is approximately 75%. * Southern Extent: There is a sharp gradient south of Cape Mendocino. Lake and Southern Mendocino counties have only a 50-70% chance of exceeding 0.50 inches through Wednesday, as the best dynamics lift north. * Impacts: Given the antecedent dry conditions, the WPC Excessive Rainfall Outlook (ERO) remains low, with 12Z Tuesday through 12Z Wednesday Excessive Rainfall Outlook in the Marginal Category (at least 5%)and less for Wednesday. The flash flood threshold probabilities are currently low to near zero.

Wind:

As the cold front approaches, the pressure gradient will tighten. NBM probabilistic guidance highlights a 40-70% chance of south wind gusts (using 24 hr wind gust probabilities) exceeding 35 mph along the exposed coastal headlands and ridges of Del Norte and Humboldt counties Monday night and more so on Tuesday night. Thursday evening will probably be the windiest with a 70-90% chance of wind gusts greater than 35 mph. However, the probability of damaging wind gusts (>58 mph) remains low, generally under 15%.

Snow Levels:

This is a warm system. Determination of thermal profiles indicates 500mb heights and 850mb temps that will keep snow levels generally well above pass levels, likely hovering between 6,000 and 7,000 feet, dipping to 4,500 to 6,000 feet Wednesday night. Colder air moves in Thursday night into Friday night and Saturday next week with forecast snow levels of 3,500 to 4,500 feet. Snow accumulation chances are low (20%) to near zero Wednesday night at highway pass levels (Berry Summit, Collier Tunnel, Ridgewood Summit), but by Thursday night, a near 40% chance of an inch of snowfall at Scotts Mountain Pass on Highway 3.

Long Term (Thursday - Weekend):

Looking toward the end of next week, the 500mb cluster analysis still reveals some divergence of solutions, but overall, confidence is increasing for next weekend to be rather wet and windy.

* Approx. 80% of ensemble members: Shows a deep trough digging offshore with somewhat varying degrees of eastward progression, which would direct a strong AR with heavy rain (2-6 inches) and potential high winds into NW California. * Approx. 20% of ensemble members (mainly from the ECMWF members): Maintains a flatter flow or broadly ridging, keeping the heaviest precip into Oregon/Washington. /Heinlein

AVIATION...(18z TAFs)

After the patchy yet dense ground fog dissipated this morning, mostly VFR conditions by 17z at KACV but this could change this afternoon with southerly winds pushing clouds into the terminals. Broken to Overcasts cloud groups and ceilings below 500ft as of 2130z for both KCEC and KACV respectively, possibly signaling the conditions to come. Model guidance suggest ceilings below 500ft until Sunday at 15z-16z for KCEC, with a less pessimistic outlook for KACV until later tonight at 06-07z with LIFR low ceilings setting in. This could mean that KACV will be in and out of LIFR/IFR with observations already in LIFR and satellites showing gaps in optical depth with swaths of less dense stratus breaking up the OVC skies. Otherwise KUKI is likely to have prevailing VFR conditions but with the southerly push, there could be advected stratus late tonight despite no development presently as of 22z. /EYS

MARINE

Southerly winds have taken over the prevailing cardinal direction today, with the northern waters a few knots higher at 10- 12kts and are expected to build by Sunday late afternoon. Conditions will be just under SC.Y, small craft advisory, through the rest of the day and become a bit more hazardous Sunday evening as southerlies strengthen just under Gale force conditions. Moderate to strong southerly winds are likely north of Cape Mendocino, with the strongest over the northern outer waters. Lighter winds are expected south of Cape Mendocino. Winds will gradually diminish in the wake of the front Monday evening. A mid-period westerly swell around 8 to 10 feet is expected to build in Monday night into early Tuesday morning. /ZVS/EYS

EKA WATCHES/WARNINGS/ADVISORIES

CA... None. NORTHWEST CALIFORNIA COASTAL WATERS... None.