Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
000 FXUS66 KSEW 211744 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 944 AM PST Wed Feb 21 2018 .SYNOPSIS...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. && .SHORT TERM...
-- Changed Discussion --
Lowland snow will be the main forecast challenge for the next 24 hours and beyond. However, amounts do not look heavy. Cool north-northwest flow aloft will continue for the next week. In the short term, satellite imagery shows the next shortwave in this flow pattern diving S-SE down the B.C. coast. The shortwave trough axis will move southeast across Western Washington late this evening, around 06z or 10 pm. In advance of the trough axis, 850 mb progs show some westerly flow off the Pacific and a warm advection pattern, which leads to lift. Meanwhile, the surface reflection will be a weak 1020 mb low which will move S-SE between the coast and 130W, so surface flow will never turn strongly onshore. This will help the low-level air mass stay cold enough for precip type to remain snow, except perhaps at the immediate coast this evening. 12z models seem to agree in bringing heavier precip southward across the Olympic Peninsula this evening and into Southwest Washington overnight. Just updated the gridded forecast with 12z model blends, and 1-2 inch snow amounts are being forecast for the lowlands of the entire Olympic Peninsula and the I-5 corridor from about Lacey on south. Light snow is expected elsewhere too, but snowfall amounts are currently forecast to be about 1/2" or less. Based on this expectation, may need to expand the Winter Weather Advisory as far inland as the San Juans, Hood Canal, the Southwest Interior, and possibly the Bremerton area. Haner Rest of short term from previous discussion: 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...From Previous Discussion: 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
-- End Changed Discussion --
&& .AVIATION...
-- Changed Discussion --
Northwesterly flow aloft with increasing mid and upper level moisture. The air mass is generally stable ahead of an approaching upper level trough that will shift south along the British Columbia coast today. The approaching weak upper level trough will help MVFR ceilings develop along the coast this afternoon with areas of light snow. Over the interior, a period of MVFR ceilings and light snow is also possible this evening. VFR conditions will return early Thursday along with northerly flow aloft as the trough moves south of the area. KSEA...Increasing mid and high clouds with VFR conditions through this afternoon. A period of MVFR ceilings in light snow is possible in the 04Z-09Z time frame with a chance of up to an inch of accumulation. VFR conditions should return by 10Z-12Z Thu as the system departs to our south. Light east winds will become light and variable midday...then NE 5-10 KT late tonight.
-- End Changed Discussion --
&& .MARINE...
-- Changed Discussion --
Northeast pressure gradients will continue to ease this morning as a surface low moves south along the British Columbia Coast and onto the northern Oregon coast this evening. Northeasterly flow will increase again behind the low later tonight into early Thursday with winds expected to remain below small craft advisory criteria. High pressure will move SE across the waters Thursday afternoon followed by another surface low expected to move SE across area waters on Friday. This system is expected to bring widespread small craft advisory conditions on Friday. Stronger systems continue to be expected Saturday and Sunday and may result in gale conditions over some of the waters over the weekend.
-- End Changed Discussion --
&& .HYDROLOGY...River flooding is not expected the next 7 days. && .SEW WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
WA... Winter Weather Advisory from 4 PM this afternoon to 10 PM PST this evening for Central Coast. Winter Weather Advisory from 1 PM this afternoon to 7 PM PST this evening for North Coast. PZ...None.
-- End Changed Discussion --
&& $$ www.weather.gov/seattle

USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.