436 FXUS66 KEKA 281238 AFDEKA AREA FORECAST DISCUSSION National Weather Service Eureka CA 538 AM PDT Sun Apr 28 2024 .SYNOPSIS...Light rain or drizzle will be possible for mostly coastal areas of Del Norte and northern Humboldt Counties through Monday. Otherwise, dry weather with slightly below normal temperatures are expected into early next week. Offshore flow around mid week will result in a warming and drying trend for the interior. && .DISCUSSION...A weak frontal boundary passes through the northern areas of the CWA this morning, bringing light southerly winds and light rain or drizzle north of Humboldt Bay. Temperatures this morning have been insulated by cloud cover, although frost products have been issued for Monday morning. Cloud cover will scatter out later this evening, allowing radiative cooling and lower temperatures overnight into Monday. A frost advisory may not be necessary for coastal areas due to the potential for stratiform rain. Zonal flow Monday morning will condense low clouds into light precipitation with the help of anticyclonic flow from the offshore high pressure system, pushing subsidence into the area. 700mb pressure maps show drier air, yet model soundings show moisture at the boundary with dry air aloft. Accumulation will be minimal but beneficial to native Redwoods absorbing the moisture in the canopy. Forecast uncertainty increases Monday night and Tuesday regarding light precip with global models trending drier as another shortwave trough progresses into the PAC NW. Surface ridge and northerlies will likely strengthen after trough passage and dry weather will most likely prevail except for perhaps a few showers near the OR/CA border. Overnight/early morning temperatures are forecast to remain below late April normals with nightly occurrences of mid-upper 30's at the typical colder valleys that do not cloud up during the night, mostly Trinity County and Mendocino. The coldest morning looks to be Tuesday (based on the NBM) when probabilities for subfreezing temperatures are highest (over 70% in some of the coldest valleys). A Freeze Watch may need to be hoisted for Tuesday morning which the next shift may issue as higher resolution data comes in. The upshot this time of year is solar insolation will yield mild afternoon temperatures; upper 60s to lower 70s. Northerly winds will strengthen Tue night into Wed, followed by robust offshore or E-NE flow as an inverted surface trough develops along or offshore the coast. Expect temperatures to cool down in the valleys under a drier air mass after winds decouple. Upper elevations will warm in response to the offshore winds and large scale subsidence (adiabatic warming). Daytime highs will warm above normal mid to late week. Probabilities for >80F are highest for Mendocino and southern Lake (40-60%). Mid 80's are in the realm of possibilities, 40% or less with the current set of NBM ensemble members. Chance for 90F or more is no more than 10% and highly unlikely based on the latest NBM set. There are still subtle variations and differences between the ensemble systems and that is why the forecast still has a slight chance (20%) for rain Wed-Thu north of Cape Mendo. About 50% of 500mb cluster members by day 7 (Saturday May 4th) indicate yet another trough dropping down from the NW for the start of a cooling trend and perhaps more rain. The overall "cool" pattern with below normal temperatures so far has been consistent. /DB /EYS && .Aviation...A weak warm front is bringing is bring some lower ceilings and some occasional light rain or drizzle to the coastal areas of Humboldt and Del Norte counties including KACV and KCEC. The HREF shows an 80 to 90 percent probability of ceilings below 3,000 feet, but only around a 30 percent probability of ceilings below 1000 feet. Sometime around 16 to 18Z low ceiling probabilities rapidly decrease along the coast and then inland a few hours later suggesting VFR conditions will move in and persist through the afternoon. Tonight a dry cold front is expected to move through in the evening. Behind this front there may be some showers and some more low ceilings. Probabilities are lower and less consistent indicative of a more showery environment. MKK && .Marine...A weak warm front is bringing a small area of 5 to 10 kt southerly winds from Pt St George to Cape Mendocino. Elsewhere west to northwest winds persist, but remain in the 10 to 15 kt range. Tonight the northwest winds are expected to increase slightly south of Cape Mendocino. For now it looks like the increase will be mainly just in the expansion fan south of Cape Mendocino. These will increase to around 20 kt with lighter winds elsewhere. There are two main swells moving into the waters this morning. One is around 3 feet at 8 seconds and the other is around 3 feet at 14 seconds. These will generally merge this afternoon and tonight to around 6 feet at 10 to 12 seconds Tonight and Monday northwesterly winds will persist around 15 to 25 kt, strongest in the south. These winds will continue to increase on Tuesday as high pressure builds into the area. Currently winds are expected to peak Tuesday afternoon or evening and some of the models are showing gale force winds in the southern waters. Wednesday through late in the week models are showings winds diminish some, although there is some uncertainty on how much and how quickly these will diminish. The northwest swell is generally expected to persist into Monday. Tuesday it gets reinforced and builds to around 8 feet at 12 seconds. It is then expected gradually diminish late in the week. MKK The main swell moving into the waters this morning is around 5 feet at 9 seconds. A new swell hast started to move into the waters and currently it is around 2 feet at 14 seconds. MKK && .EKA WATCHES/WARNINGS/ADVISORIES... CA... Frost Advisory from 3 AM to 9 AM PDT Monday for CAZ107-108- 110-111. 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