Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 211138 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 338 AM PST Wed Feb 21 2018 .SYNOPSIS...
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Another weak system moving down the British Columbia coast will reach Western Washington tonight. Light snow out ahead of the system will begin along the North Coast this afternoon spreading to some locations in the interior this evening. A break in the weather is expected between systems on Thursday. A stronger system will give wet weather to the area Friday into Saturday with low snow levels.
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&& .SHORT TERM...
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Satellite imagery shows the next system to reach Western Washington up near haida Gwaii early this morning. Closer to home skies are mostly clear over Western Washington at 3 am/11z. With the clear skies temperatures have dropped into the 20s for the most part with the colder places in the upper teens. Forecast problem for the day is the track of the low moving down the British Columbia coast today. The GFS solution is amount 100 miles further west with the low center versus the NAM solutions. The GFS solution keeps most of the precipitation confined to the Coast this afternoon into the evening hours. The 00z NAM with the further east track spreads light precipitation, which will be in the form of snow, over the entire area beginning late this afternoon in the north with the precipitation spreading over the remainder of the interior in the evening hours. With the further east track on the NAM the moisture will less than the more over water GFS solution. So...for the morning package will go ahead and issue a snow advisory for the Coast because with either solution there is the potential for a couple of inches of snow this afternoon into the evening hours. Will start and end the advisory 3 hours sooner along the North Coast ( start 21z/1pm end 03z/7 pm ) compared to the Central Coast ( 00z/4 pm start end 06z/10 pm ). Now what model to go with this morning for the remainder of the forecast area. Current satellite trends at 11z favor the NAM solution. Because of this have likely pops over the northern portion of the interior tonight with chance pops over most of the remainder of the interior, higher than the GFS pops but not as high as the NAM leaving some wiggle room for the day shift. Precipitation amounts in the NAM are limited, a tenth of an inch or less, so have not issued an advisory for the northern interior with snow accumulations forecast to be an inch or less. For the metro area current NAM timing brings the light snow into the area at the tail end of the evening commute ( start time around 7 pm ) with just flurries after midnight. Skies will begin clearing from the north early Thursday morning and by 12z the chance for light snow will be confined to the Southwest Interior. Before all the snow action clear skies this morning will give way to cloudy skies this afternoon. With the increasing cloud cover highs will only be in the 30s with overnight lows in the mid 20s to lower 30s. Sunny skies return on Thursday in the morning hours with the upper level trough continuing to dig south and dry northeasterly flow aloft over the area. Still a chance for some light snow over the Southwest Interior but that threat will be gone by 18z as the system moves south of the area. Even with the sunshine highs will have a hard time getting out of the 30s. Clouds on the increase Thursday night as yet another system starts to make its way down the British Columbia coast but not in time to prevent temperatures from dropping below freezing once again with lows in the 20s and lower 30s. Tough call on the precipitation type for the Friday system. Surface gradients go southerly ahead of the system but model 1000-850 mb thickness values stay below 1300 meters over all but the Coast during the day. For now will go with low snow levels, less than 500 feet, but not on the surface. Northwesterly flow aloft will help rain shadow the Central Puget Sound area a little but precipitation amounts north and south in the interior are enough that another round of winter weather advisories is possible. Highs on Friday will be near 40 except on the Coast where afternoon temperatures will be a couple of degrees warmer. .LONG TERM...Honestly did not spend a lot of time on the extended forecast this morning with two possible snow producers coming down the pike and 00z ECMWF model not getting ingested into the computer this morning. GFS brings another system down from the north on Saturday with stronger low level onshore flow ahead of the system. This should lift the snow levels off the surface but the air mass aloft is still relatively cool keeping snow levels low, around 1000 feet. Another possible system on Sunday with another cool upper level trough settling in over the area for the first part of next week keeping the below normal temperature trend of the last half of February intact. Felton
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&& .AVIATION...
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Flow aloft is northwesterly across Western Washington today. The air mass is stable with increasing high and mid level moisture across the region ahead of an approaching weak upper level trough moving southward along the British Columbia coast. MVFR ceilings will develop along the coast this afternoon along with areas of light snow. A period of MVFR ceilings with light snow is possible over portions of interior western Washington late this evening. The northwest flow aloft could lead to some shadowing over parts of the Puget Sound area. VFR conditions will return by early Thursday along with northerly flow aloft as the trough moves southward to the northern Oregon coast. KSEA...VFR conditions through this afternoon with increasing high and mid level moisture ahead of a weak upper level trough sliding down the British Columbia coast. A period of MVFR ceilings in light snow is possible in the 04Z-09Z time frame. Recent model runs have shifted ever so slightly in favor of a very light snow accumulation tonight. There`s a 30 to 40 percent probability of up to an inch of accumulation in the late evening. The approaching system does not have a lot of moisture with it and the northwest flow aloft favors some shadowing of the Puget Sound. VFR conditions should return by 10Z-12Z Thu as the system departs to our south. Light east wind this morning...becoming light and variable midday...then becoming NE 5-10 KT late tonight. 27 .MARINE...Northeast pressure gradients will weaken this morning as a low near Haida Gwaii drops southeastward. Small craft advisory conditions have ended except in the Northern Inland Waters where they will persist until about 7 AM. The low to the north of the area will move SSE through the Washington coastal waters this afternoon and onto the northern Oregon coast this evening. Northeasterly flow will increase somewhat behind the low later tonight into early Thursday. At this time, winds are expected to remain below small craft advisory criteria as this rather weak low moves through the waters. A ridge of high pressure will move SE across the waters Thursday afternoon. Another low, this one somewhat stronger, will move SE through the waters on Friday. Widespread small craft advisory conditions are expected on Friday. Stronger systems will affect the region Saturday and Sunday and may give gales to some of the waters over the weekend. Albrecht
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&& .HYDROLOGY...River flooding is not expected the next 7 days. && .SEW WATCHES/WARNINGS/ADVISORIES...
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WA... Winter Weather Advisory from 1 PM this afternoon to 7 PM PST this evening for North Coast. Winter Weather Advisory from 4 PM this afternoon to 10 PM PST this evening for Central Coast. PZ...Small Craft Advisory until 7 AM PST this morning for Northern Inland Waters Including The San Juan Islands.
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