FXUS66 KEKA 232154 AFDEKA AREA FORECAST DISCUSSION National Weather Service Eureka CA 254 PM PDT Wed Oct 23 2024 .SYNOPSIS...Dry weather and mild daytime temperatures are expected through the week. Wetter and cooler weather is expected this weekend into early next week as a series of fronts move through. && .DISCUSSION...Surface high pressure will build tonight after a fast-moving shortwave trough and an associated weak cold front exits to the east this afternoon. A cooler and drier airmass will settle over the region tonight. Clearer skies, light winds and lower dew points in the valleys will be an ideal set up for longwave cooling during the night. Frost advisories and a freeze warning remains in effect for late tonight into early Thursday morning in areas where there are still growing season impacts. In particular, subfreezing temperatures are expected for northern interior Mendocino by early Thu morning. It will be even colder in the valleys of Trinity and portions of Humboldt, however the growing season has ended as of October 15th and a season ending killing freeze has already occured in Trinity. A drier airmass is expected on Thursday into Friday, though temperatures aloft will warm considerably and overnight lows will probably increase Thursday night for higher elevations with breezy easterlies through the night. Daytime highs will warm above normal. With greater drying and light winds in the valleys, potential for early morning frost even out over the coast and adjacent coastal river valleys will continue. Ensemble clusters and deterministic guidance continues to trend toward higher chances for rainy and cooler weather this weekend and early next week with a broad and cold upper low dropping across the area from the NW. Deterministic guidance continues to indicate an IVT plume intersecting the coastal terrain late Sat into early Sun with high NBM and ECWMF ensemble probabilities for 1 to 2 inches in 24 hours. Duration of the influx of moisture and heavy rain rates appears short. Another good soaking is expected for the northern portion of the forecast area. We are still well below normal for rain this month. Southern zones will get much less if any this weekend. Instability and periods of moderate/heavy rain will follow later on Sunday into Monday as the frontal boundary weakens and lifts out the northeast. Multi-day precip amounts will vary drastically from over 2 inches in Del Norte to very little or nothing in southern Lake. Greater uncertainty arises for early next week as the 500mb trough traverses the area and heads into the northern Rockies or Great Basin. This could end up evolving into another inside slider event with blustery northerly winds and very low humidity. We are still in high season and the southern portion of the forecast area remains critically dry with below normal October rain. Fire weather concerns are probable early next week with this inside slider. To what extent remains uncertain. DB && .AVIATION...A weak cold front moved by to the north of the area. This some mid to higher clouds and along the coast there were some local areas of stratus and a few sprinkles. This has passed by and skies have cleared. Breezy winds are expected along the coast and over the higher terrain, especially this evening. Tonight winds are expected to turn more northeast bringing offshore flow to the coast. This will likely keep skies clear for the night. Lighter winds are expected on Thursday. MKK && .MARINE...A weak frontal boundary has moved past the area and has been bringing light winds. Behind this front as high pressure builds in there will be a brief period of stronger northwest winds. These are expected to peak around 20 to 30 kt with the strongest winds south of Cape Mendocino. Thursday winds will start to diminish again as another front approaches the area. Thursday night into Friday morning winds will become southerly and increase once again. Friday night into Saturday morning these winds are expected to peak with some gale force gusts possible in the northern areas well off the coast. Also on Saturday a larger long period swell is expected to build in. The GFS wave model currently shows this peaking around 12 feet at 12 seconds. This is expected to gradually diminish later on Saturday. A second swell is now expected on Sunday afternoon with heights around 12 feet and a period of around 14 seconds. MKK && .FIRE WEATHER...RH's plummeted overnight into the single digits and teens at some of the highest mountain peaks of zones 203, 264, 276, 277 and 283. Winds were not very strong with peak gusts from 8 to 14 mph coincident with these very low RH's. Very dry conditions are forecast to continue tonight through Friday with a general downward trend of daytime minimum RH's through Thursday for the remainder of the area. E-NE peak gusts around 20 to 30 mph are possible over the ridges tonight into Thu AM and a headline for elevated fire weather conditions has been added to the planning forecast. Easterly breezes are forecast to persist over higher elevations of zones 201, 203, 211 and 212 Thu night into early Fri as the airmass warms aloft and humidity recoveries continue to remain poor for the upper elevations. Winds are forecast to shift around to the southeasterly and southerly during the latter portion of the day on Friday and increase during the evening. Timing and duration of the strongest southerly winds is not 100% certain. Gusty south to southeast winds with low RH's below 30% could last into the early morning hours of Saturday before any meaningful rain arrives. Greeter concern for red flag conditions crops up again early next week with ECMWF EFI and NBM already narrowing down the potential outcomes. Another inside slider event and the potential for strong offshore winds and diminishing humidity is certainly possible based on multiple 24-hr ensemble cluster means of ridging offshore and a trough over the Great Basin. Greatest concern is across our southern forecast zones early to mid next week, particularly Lake County where October rainfall has been well below normal and fuels remain critically dry. The evolution of this next inside slider is still not clear and we will need to keep a close eye on it in terms of the duration and magnitude of dry offshore flow. Gusty W-NW winds could develop as early as Sunday night and Monday, though humidity and rainfall forecast remains uncertain during this time period. Current deterministic guidance and official QPF has multi-day storm totals of only a few hundredths or zero for southern Lake and southeast Mendocino through early next week. DB && .HYDROLOGY...No significant concerns for the next 7 days. NBM indicates multi-day probabilities for over 2 inches in 24 hours for windward facing aspects over Del Norte and SW Humboldt over the weekend as a weak atmospheric river storm approaches the coast and increases the potential for a period of heavy rain. Highest chance for this occurrence is the 24-hour period from 5 PM Saturday to 5 PM Sunday. A slight 6 hour shift either way could skew this if the boundary stalls or slow offshore. Granted heavy rain will most likely be in a narrower time period than 24 hours. Higher instability and bouts of locally heavy rain will follow behind the front into early next week. Dry antecedent conditions will preclude any major concerns. DB && .BEACH HAZARDS...Sneaker wave index indicates no or very low threat for the remainder of the week. Threat grows to moderate late this weekend, specifically on Sunday, based on the GFS-Wave model. ECMWF deterministic surface cyclone evolution was similar to the GFS and large WNW swell will most likely arrive Sunday. Index appears to key in on the forerunners. Also, GFS-Wave model may be too low with the magnitude of the swell, in other words, it may come in higher than predicted. Weather conditions are not exactly forecast to be pleasant on Sunday for most people. DB && .EKA WATCHES/WARNINGS/ADVISORIES... CA... Frost Advisory from 3 AM to 9 AM PDT Thursday for CAZ102- 104. Frost Advisory from 2 AM to 9 AM PDT Thursday for CAZ103- 105-106-112>114. Freeze Warning from 2 AM to 9 AM PDT Thursday for CAZ110- 111. NORTHWEST CALIFORNIA COASTAL WATERS... Small Craft Advisory from 3 PM this afternoon to 3 AM PDT Thursday for PZZ455. Small Craft Advisory from 3 PM this afternoon to 3 PM PDT Thursday for PZZ470. Small Craft Advisory until 3 PM PDT Thursday for PZZ475. && $$ 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