Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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865 FXUS66 KSEW 152219 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 219 PM PST Sat Dec 15 2018 .SYNOPSIS...A front will move through the area late tonight and Sunday bringing rain, locally windy conditions, and high mountain snowfall. A stronger front will move across the area Monday night through Tuesday giving locally windy conditions with and heavy rainfall. Another strong front will move across the region Thursday or Thursday night. && .SHORT TERM /TONIGHT THROUGH TUESDAY/...Conditions are mainly dry across the region this afternoon as we sit between weather systems. A strong frontal system that is now seen over the Washington and Oregon offshore waters is moving slowly eastward and will cross the area late Sunday afternoon. While the front will be stretching and weakening somewhat as it moves into the area, pressure gradients will be strong enough to produce southeast winds 20-35 mph with gusts to 45 to 50 mph over areas near the water in Western Whatcom County, the San Juan Islands, and the area around Admiralty Inlet and the eastern Strait of Juan de Fuca. A wind advisory has been issued for these zones tonight through Sunday morning. Precipitation with the front is not expected to be exceptionally heavy, but it will be heavy enough over the southern slopes of the Olympics to potentially cause the Skokomish River in Mason County to go back above flood stage late Sunday. This river is running just below flood stage at this time and it would not take too much rain to cause it to flood again. A flood watch is out for Mason County. Snowfall will be heavy, especially Sunday morning through early afternoon, in the higher Cascades of Skagit and Whatcom counties. The Mount Baker Ski Area will likely see 12-18 inches of new snow before precipitation turns to rain and becomes lighter Sunday afternoon. A Winter Storm Warning has been issued for the Cascades of Whatcom and Skagit county for tonight into Sunday afternoon as a result. Sunday night through early Monday will see post frontal showers gradually diminish across the area. There may be a brief break in the action between systems Monday afternoon. From Monday night through Tuesday, a strong frontal system associated with a deep low moving into the SE Alaska Panhandle will move SE across the area. This front will bring more wind and rain to the area. The air mass ahead of this front will be quite moist, and SSW winds 60-75 mph at 5000 ft will produce good orographic uplift over the southwestward facing Olympics and Cascades. Models have been advertising heavy precipitation amounts with this front for several days now, but the more recent model solutions have picked up on a more progressive frontal system for less of a flood threat over the northern half of Western Washington. Snow levels ahead of the front will only be modestly high -around 4500-6000 ft. Flooding may be a threat to rivers south of the Seattle area, but it is too early to tell at this time. Albrecht .LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...Extended models have moved toward a solution where there is a break in the heavier rainfall between Tuesday`s front and the next progressive frontal system Thursday night. Some models show the potential for Thursday night`s front to be quite windy with the deterministic ECMWF and some GFS ensemble members showing a 990ish mb low moving eastward along the Canadian border. Another front may move through the area next weekend. A blended model solution was used as a first guess forecast for the Tuesday night through Saturday period. Albrecht && .AVIATION...A strong frontal system will reach the area tonight and move inland on Sunday. Southerly flow aloft. At the surface, easterly flow tonight will become southerly late Sunday as the front moves through. The air mass is moist and stable. Mostly mid and high clouds currently although there are some 5000-7000 foot ceilings floating around. The trend will be for gradual lowering ceilings this evening as the front nears and rain develops. Low clouds should become widespread later tonight but easterly winds ahead of the front will likely keep things VFR for the interior. There should be some MVFR ceilings Sunday, mainly west of Puget Sound. KSEA...Mid and high clouds will lower to 5000-6000 feet this evening as rain develops. Easterly wind 5-10 knots will increase a little tonight and continue into Sunday, then turn southerly late in the day as the front passes. Schneider && .MARINE...A strong frontal system will bring another round of gale force southerly winds to most waters tonight and Sunday morning. There will be a relative lull in between fronts later Sunday through Monday morning. A second strong frontal system will bring more gales later Monday and Monday night. Moderate onshore flow behind the second front will ease later Tuesday and Tuesday night. A third frontal system will approach from the west on Wednesday and likely bring another period of gales on Thursday. The latest wave model shows 20-22 foot west swell reaching the coast late Sunday afternoon and Sunday night. Schneider && .HYDROLOGY...The flood prone Skokomish river is below flood stage for now but the rain on Sunday will probably drive the river back above flood stage: A flood watch is in effect for Mason county. The weather pattern will remain active through next week and we will have to keep an eye on rainfall amounts, snow levels, and river forecasts. Overall, there seems to be enough of a gap between the heavier rain in each system, and looking at the plumes of moisture-- the so called atmospheric river of moisture does not seem to stall over Western Washington long enough to put us in an obvious flood pattern. We will need to watch rivers over the southern portion of the forecast area closely if the front slows in its southeastward movement. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...Winter Storm Warning from 6 PM this evening to 4 PM PST Sunday for Cascades of Whatcom and Skagit Counties. Flood Watch through Wednesday morning for Mason County. Wind Advisory from 6 PM this evening to noon PST Sunday for Admiralty Inlet Area-San Juan County-Western Whatcom County. PZ...Small Craft Advisory for rough bar until 4 PM PST Sunday for Grays Harbor Bar. Gale Warning until 10 AM PST Sunday for Coastal Waters From Cape Flattery To James Island 10 To 60 Nm-Coastal Waters From Cape Flattery To James Island Out 10 Nm-Coastal Waters From James Island To Point Grenville 10 To 60 Nm-Coastal Waters From James Island To Point Grenville Out 10 Nm-Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm- Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm. Gale Warning until 4 PM PST Sunday for East Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters Including The San Juan Islands. Gale Warning until 4 AM PST Sunday for West Entrance U.S. Waters Strait Of Juan De Fuca. Gale Warning until 4 PM PST Sunday for Central U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory until 10 AM PST Sunday for Admiralty Inlet. Small Craft Advisory until 4 PM PST Sunday for Puget Sound and Hood Canal. && $$ is the U.S. government's official web portal to all federal, state and local government web resources and services.