602 FXUS66 KEKA 151223 AFDEKA AREA FORECAST DISCUSSION National Weather Service Eureka CA 523 AM PDT Sun Sep 15 2019 .SYNOPSIS...As a cold front approaches, increasing clouds and rain will begin to spread across northwest California this afternoon and evening. Scattered showers will linger into Monday, with noticeably cooler temperatures inland. Another storm system will bring a second dose of rain later Tuesday into Wednesday. Drier and warmer weather will return for the end of the week. && .DISCUSSION...As the first widespread rain event of the season approaches, the initial forecast challenge before the rain arrives will be sky cover. The GOES-17 night fog product is depicting a fairly deep layer of marine clouds extending well inland up the river valleys, however this layer has scattered along portions of the coast, with partially clear conditions in some areas. This is likely due to some offshore flow induced as an arm of Pacific high pressure is shoved inland in advance of the cold front approaching offshore. Meanwhile, some patches of lower clouds and even some fog have redeveloped near the immediate coast. The best way to sum it up would probably be 'mixed sun and clouds' for most of the coastal plain to start the day, while the river valleys will probably be tougher to scour out until the late morning hours. In any case, the more impactful weather will roll in later today and tonight with the cold front, in the form of a period of beneficial rain. Showers should reach the Del Norte coast before the morning is out, while the rest of the coast down to Eureka will likely have to wait until the afternoon hours. Low-level southerly flow ahead of the front may keep the Humboldt Bay vicinity from seeing more than a few showers until more substantial rain pushes in this evening. The main frontal rain band may only last a few hours at any one location as it crosses northwest California from northwest to southeast this evening. Scattered showers will follow for Monday, mainly north of Mendocino county, however there will likely be a relative lull with even partial clearing later tonight into Monday morning. The main upper trough axis will then cross our area, but will begin to fill as it does so. Conditions will be unstable enough on Monday for downpours with the showers, and even a rumble or two of thunder, but am not too excited about the strength of any convection given the filling trough and marginal CAPE. The trough axis will shift east of our area in the afternoon as well, which will tend to result in diminishing shower activity especially toward the coast, and likely some partial clearing there by day's end. Total rainfall from the latest high-resolution guidance are looking a bit more meager over eastern Trinity and Mendocino, but still around a tenth of an in inch in most valleys, with higher amounts in the mountains. A quarter to three-quarters of an inch is expected for most of Humboldt and Del Norte, locally higher in the mountains of Del Norte county. Winds will not be an issue with this front, with even the more wind prone spots struggling to gust to 20 mph (exposed mountain ridges aside). Tuesday will start off drier as high pressure temporarily builds in, with plenty of valley fog but also some sunshine. Clouds will increase over Humboldt and Del Norte ahead of the next front. Model guidance has come into good agreement with this next system, with a cold front pushing southward across our area Tuesday afternoon and Tuesday night, followed by an area of low pressure tracking southward along the Pacific Northwest coast and enhancing shower activity Wednesday. The latter may result in some heavier rainfall amounts of one to two inches in the higher terrain of Humboldt and Del Norte, otherwise expect a similar shot of rain compared to the first system. There will be some brief rises on area creeks and streams with these rains, but very limited response is forecast on the main stem rivers by the California- Nevada River Forecast Center. Overall the rain will be very beneficial, and will help quell our wildfire season. The main impacts will be to travel, with possible slick roads from this summer's oil patches, as well as some localized rock fall near steep slopes. At this point, the last couple of days of the upcoming week into next weekend are looking drier and sunnier, with temperatures warming back closer to seasonal averages. /AAD && .AVIATION...Broken patches of stratus lingering along the North Coast overnight have made for a difficult forecast this morning. ACV has been mainly VFR as the stratus bank remains just offshore, however CEC has fluctuated between categories from LIFR to VFR as clouds and patchy fog drift in and out of the terminal intermittently. Thinking conditions will settle in around MVFR with some variations during the morning as a more extensive area of stratus approaches from offshore. There should then be enough sunshine in advance of the trough to scatter out this initial cloud layer and produce a few hours of VFR conditions before rain moves in later in the afternoon with the arrival of the front. This should bring IFR conditions to the coastal terminals between 21-00z continuing into the evening, with conditions becoming more showery in nature during the overnight hours. Conditions will also deteriorate across the interior from this evening into the overnight as the front pushes inland. /CB && .MARINE...Small Craft Advisories have come to an end this morning as northerly winds and short period seas continue to trend downward south of Cape Mendocino with the approach of the upper level trough. A brief period of moderate to fresh southerlies will sweep across the northern waters in the vicinity of Point Saint George later this afternoon as the surface front reaches the coast. The trough will also bring a showery regime which could result in localized gusty winds in the vicinity of the most vigorous showers. Otherwise, expect generally light winds over the next couple days as seas are dominated by a building NW swell on the order of 7-8 feet at 12 seconds. A smaller, longer period SW swell will also remain present through much of the coming week. A stronger push of southerly winds looks to accompany the next trough on Tuesday, with the possibility for some eventual headlines north of Cape Mendocino in that time frame. A northerly regime may then return late in the week behind the last in the coming series of troughs. /CB && .EKA WATCHES/WARNINGS/ADVISORIES... CA...None. NORTHWEST CALIFORNIA COASTAL WATERS...None. && $$ 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