Area Forecast Discussion
Issued by NWS Eureka, CA
Issued by NWS Eureka, CA
657 FXUS66 KEKA 291138 AFDEKA AREA FORECAST DISCUSSION National Weather Service Eureka CA 438 AM PDT Fri Aug 29 2025 .UPDATE...(12Z TAFs)...MVFR conditions along the coast early today. Overcast ceilings forecast to scatter out late this morning (18-20Z) as the influence of a shortwave trof deepens the marine layer, increasing daytime mixing. Marine layer expected again tonight with a 50% probability of ceilings below 1kft AGL at the coast and in adjacent river valleys. Light, variable winds expected this morning everywhere. VFR expected inland with terrain driven winds. && .PREV DISCUSSION... /issued 1221 AM PDT Fri Aug 29 2025/ SYNOPSIS...Temperatures remain around seasonal norms into early next week. Dry conditions will persist with no precipitation expected. High pressure will build in next week to increase inland temperatures. DISCUSSION...A deepened marine layer has limited the duration and coverage of daytime clearing for portions of the North Coast, with better clearing south of Humboldt Bay. Stratus coverage observed on satellite early Friday morning shows very sparse coverage over the coast, but it is modeled to fill back in. Model guidance steers smoke from the interior wildfires northeast, with far northeast Trinity County potentially experiencing lowered air quality and visibility through the weekend with southwest flow aloft. Steering flow will turn more southerly early next week. Apart from some bouts of light coastal drizzle, no precipitation is expected through early next week. An upper low will very slowly graze the coast late in the week and through the weekend on its way northeast. There will be instability and shower/thunderstorm activity associated with it, but models keep it well offshore and into Oregon as it passes. The low will then undergo some unusual steering flow, retrograding back west early next week. This will mean a rebound of upper heights and a warmup. There is high confidence in synoptic pattern for the building heat next week, but some questions in the details. By Tuesday, 57% percent of the ensemble members build 500 mb height 590 dm or higher. 81% of the ensemble members build 850 mb temperatures at 26C to 30C, with notably 13% of the members ranging from 30C to 32C. There is similar confidence going into late next week. These values support high temperatures over 100 for the interior valleys. NBM hold a high chance (70-85%) for temperatures to exceed 100 in the warmer valleys. There is a big question on the strength of a potential shortwave trough undercutting the ridge Tuesday and Wednesday. ECMWF deterministic model runs have more persistently resoled a stronger shortwave, and GFS has been leaning that way in recent runs. This scenario also includes advection of monsoonal moisture with the potential for interior thunderstorms and perhaps suppression of some inland high temperatures. NBM holds a 45% chance for 500 RH over 45% Tuesday. There is a large discrepancy in the ensemble members with only up to 30% bringing in the mid level moisture, but the forecast bears watching as thundertorms that form would be dry. JJW AVIATION...Stratus is fairly limited this morning along the coast with some weak offshore flow. There is some stratus lingering and this may expand to the coastal airports around 12Z. The HREF indicates it will be MVFR or briefly IFR. Another possible scenario is skies remain clear and radiational fog forms bringing very low conditions. At this point this scenario looks less likely, but remains a possibility. If any stratus does form, it will likely clear fairly quickly in the morning. Generally 5 to 10 kt winds are expected in the afternoon at KACV and KCEC. At Ukiah the winds are expected to be around 10 kt in the afternoon, but are expected to be north to northwest instead of the south they have been. The HREF has low probabilities of stratus returning in the evening at KACV, but shows it returning more quickly at KCEC. It is eventually expected to come back at KACV as well. MKK MARINE...Northerly winds are starting to move into the southern waters although they remain around 5 to 10 kt. Farther north light southerly winds persist. By Friday afternoon winds are expected to be northerly across the area and increase to 5 to 15 kt with the strongest winds in the south. These winds are expected to continue to increase in the southern waters Saturday afternoon with an area of 15 to 20 kt winds, especially downwind of Cape Mendocino. Models are in good agreement that the northern waters will remain around 5 to 10 kt. Sunday the winds may increase slightly more, but there is starting to be more model uncertainty, especially in the northern waters. Some models suggest they may reach close to 20 kt while others remain around 10 kt. Winds are expected to continue to increase next week as well, but there is still fairly good agreement on them remaining below gale force. Waves remain low, around 2 to 3 feet. These waves are a combination of a northwest wave around 8 seconds and a southerly wave around 17 seconds. These waves are generally expected to continue through the weekend with the addition of short period wind driven waves that rise as the winds start to pick up. There is a small threat for some sneaker waves, but with the southerly swell expected to be only around 2 foot this is not expected to be very significant. MKK FIRE WEATHER...The marine layer is relatively deep, and this has allowed for further inland push of marine influenced air through the overnight and morning hours. Another westerly to southwesterly wind surge will occur Friday afternoon. Winds gusts over 20 mph are probable 50-70% Friday. There are higher chances Saturday (75-90%) for Lake and eastern Mendocino counties. Locally critical fire weather conditions will likely be reached, particularly in Lake County. The marine influence will work to slightly improve minimum RH values, with general improvement through the weekend, though they will still lower to the mid to low teens and possible some isolated single digits through Saturday. Recoveries will remain poor over the ridgetops above the inversion, currently around 2500 ft. There is a low chance for monsoonal moisture and thunderstorm potential early next week while interior temperatures warmup with a building ridge. Inland valley temperatures are forecast to reach or exceed 100 by early next week. JJW && .EKA WATCHES/WARNINGS/ADVISORIES... CA... None. NORTHWEST CALIFORNIA COASTAL WATERS... None. && $$ NOTE: The full forecast discussion is now regularly issued between 10 am/pm and 1 pm/am. Additional updates will be issued as needed. Visit us at https://www.weather.gov/eureka Follow us on Facebook and Twitter at: https://www.facebook.com/NWSEureka https://www.twitter.com/NWSEureka For forecast zone information see the forecast zone map online: https://www.weather.gov/images/eka/zonemap.png