textproduct: Eureka

KEY MESSAGES

* Convective potential remains low (up to 15% chance) and limited to northern Trinity and far interior Del Norte through early evening.

* Expanding high pressure aloft and compressing marine air will drive a sharp warming trend Tuesday through Thursday, bringing increased HeatRisk potential and temperatures 10-15 degrees above early September normals across interior valleys.

SYNOPSIS

A mid-level disturbance exits the region. Atmospheric high pressure centered over the Desert Southwest will build westward starting Monday, initiating a pronounced warming trend through mid-week. Offshore flow on Wednesday will clear coastal low clouds temporarily before marine air moderates conditions late in the week.

DISCUSSION

A trough exits the region with mid level heights quickly increasing across the region from a strong Desert Southwest area of high pressure. The ridge expands NW through midweek, with 500 heights rising to 591 to 594 dam. An anomalously warm airmass will accompany the ridge, with 850 mb temperatures increasing to 24 to 26C. Deep daytime mixing of this warm airmass supports widespread upper 90s to lower 100s across N CA interior. Additionally, a developing thermal trough and offshore component to the low level flow should limit marine influence and enhance warming, especially Wednesday.

The hot conditions will reach the coast, with a high probability of temperatures reaching the 70, and perhaps upper 70s to even low 80s if the offshore winds are fully realized. Interior valleys such as Garberville and Ukiah have over a 70 percent chance of exceeding 100 Wednesday, and 850 temperatures support these values. There is also a lesser 34 to 45 percent chance of a maximum high temperature of 105 in the described locations, including the Hoopa through Willow Creek valleys. If this high value occurs, it should be very localized. This heat in combination with warm overnight lows in the 60s will yield Moderate HeatRisk (2/4), with localized Major (3/4) category possible.

While temperatures may remain below advisory criteria, some caution is warranted for those spending extended periods outdoors. The rapid transition from the recent cool and troughy patter will provide little opportunity to acclimate to the quickly building heat. This may be particularly noticeable in normally cooler coastal and near-coastal areas, where temperatures could reach levels not experienced so far this season.

A complication to the high temperature forecast will develop as a slug of tropical moisture moves through the region Wednesday off passing tropical system Marie. Models and NBM probabilities show very little to no instability coinciding with the moisture, and the environment will be strongly capped. This will limit thunderstorm chances to less than 5 percent, but associated cloud cover could suppress the high temperature potential of the thickness is sufficient. High temperatures will quickly lower again Friday and through the weekend as troughing returns.

AVIATION

Marine low clouds will return along the coast after 07Z, with a decreased probability of 40% for LIFR conditions into Monday morning for mainly around Humboldt Bay and up the Eel River delta. What does form will quickly dissipate Monday morning. Light offshore flow and drying conditions will maintain VFR elsewhere. Regional terminal KUKI will remain VFR throughout the period with light daytime west northwest winds Monday afternoon.

MARINE

An offshore high-pressure system interacting with the coastal thermal low continues to drive northerly winds, strongest across the southern marine zones.

For the Northern Inner Waters, north winds of 10 to 15 knots with gusts to 20 knots will persist through Monday. Combined seas will average 4 to 6 feet, composed of a dominant northwest wind wave of 4 feet at 8 seconds and a long-period southerly swell of 2 to 3 feet at 15 seconds.

For the Northern Outer Waters, northerly winds will range from 15 to 20 knots. Wave steepness remains marginal with dominant periods around 9 seconds. This decision is deferred to the upcoming midnight shift as sea state criteria remain marginal. For the Southern Inner Waters, a Small Craft Advisory remains active through 5 AM Monday for north-northwest winds of 15 to 25 knots and wave heights of 6 to 8 feet at 8 seconds.

For the Southern Outer Waters, a Small Craft Advisory continues through 5 PM Monday for north winds of 20 to 25 knots, gusts to 30 knots, and steep seas of 7 to 9 feet at 9 seconds.

Additionally, a south-southwest swell of 3 to 5 feet at 15-16 seconds from Hurricane Marie will decay across all waters through Monday night. A thermal trough positioning over the coastline Wednesday will briefly reduce wind speeds across all waters before northerly flow re-strengthens Thursday evening.

FIRE WEATHER

High pressure building over the region Monday through mid-week will bring much warmer and drier conditions. High temperatures in interior valleys will climb into the 98 to 102 degree range Tuesday through Thursday, with afternoon humidity levels dropping between 12 and 22 percent. Winds will follow standard daily patterns, blowing uphill during the day and down the valleys at night. Brief offshore winds Wednesday morning will spread warm temperatures and lower humidity closer to the coast before fog and low clouds rebuild late in the week. Offshore winds may be locally breezy and will need to be monitored given the expected heat and low RH.

BEACH HAZARDS

A long period southerly continues in waters, only slowly losing energy through the week, but 15 to 16 second periods are still being recorded. Thus, there is an ongoing risk of sneaker waves Monday for south facing beaches in Mendocino and Southern Humboldt counties, including the Lost Coast Trail.

Sneaker waves are a unique hazard, with up to 30 minutes between waves. Large, unexpected waves can sweep across the beach without warning, sweeping people into the sea from rocks, jetties, and beaches. These sneaker waves can also move large objects such as logs, crushing anyone caught underneath. Even ankle to knee deep water can knock you down and drag you out when a sneaker wave hits. Pay attention and stay above the high water line and any wet sand to stay safe. If the ground is wet, waves have surged there recently. Stay safe and never turn your back on the ocean!

EKA WATCHES/WARNINGS/ADVISORIES

CA... Beach Hazards Statement through Monday evening for CAZ104- 109.

NORTHWEST CALIFORNIA COASTAL WATERS... Small Craft Advisory until 5 AM PDT Monday for PZZ455.

Small Craft Advisory until 5 PM PDT Monday for PZZ475.