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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
685 FXUS66 KSEW 191550 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 850 AM PDT Mon Mar 19 2018 .SYNOPSIS...Today will be partly to mostly sunny with isolated showers in the mountains. A weak ridge over the region Tuesday will bring a mostly sunny day. Rain will return Wednesday and Thursday as a system moves through, with cool showery weather Friday into the weekend. && .SHORT TERM...An upper level low will spin offshore today then weaken and merge with a broader cutoff low well off the CA coast. Across Western Washington, skies will be partly sunny with patchy morning fog in the south sound and in the San Juan Islands. Models are showing isolated showers over the mountains this afternoon and evening, otherwise expect dry conditions over the lowlands. High temperatures will be close to normal with temperatures in the low to mid 50s. The pattern remain rather benign on Tuesday with weak onshore flow. Isolated showers are expected again, mainly in the Olympics and north Cascades. Wet weather will return on Wednesday as a deep upper low forms over the NE Pacific, setting up moist S/SW flow over WA. A cold front will shift inland Wednesday night. Model consensus looks good so far. 33 .LONG TERM...Previous discussion...Models have recently backed off on the strength of the surface gradient over the region as a surface trough moves toward the coast on Thursday. Now it appears conditions will be no more than breezy. Rain will continue at times on Thursday. The upper trough following the frontal system will bring cool showery weather to the area Friday through Saturday. The snow level will fall to around 1000 ft, so precipitation could fall as snow over the higher lowland areas. Despite being past the equinox on Wednesday, this is a wintry pattern. Highs will be mostly in the 40s, and snow will fall in the mountains. The upper trough will move off to the east on Sunday, for decreasing showers and more sun breaks. Burke && .AVIATION...An upper level low over the Oregon offshore waters is dropping slowly southward while a ridge of high pressure aloft along the Canadian border moves into the area. Light NE flow aloft today will become northerly tonight and northwesterly on Tuesday. Light pressure gradients will prevail at lower levels. The air mass is somewhat moist. The stable air mass this morning will become weakly unstable over the mountains late this afternoon and evening resulting in the development of isolated light showers there. The air mass will stabilize again overnight tonight. Patches of MVFR and IFR conditions are seen across the area this morning with fog and low stratus concentrated over the San Juan Islands and in the southwest interior. Patches of stratus are producing ceilings at some of the central Puget Sound terminals at times this morning as well. Stratus and fog will lift by about 18Z. Expect scattered CU this afternoon and evening with isolated showers confined to the mountains during the late afternoon and evening hours. Conditions later tonight and Tuesday morning will be similar to this morning with mainly VFR conditions and patches of stratus and fog. Albrecht KSEA...A patch of stratus has developed around KBFI and may nose into KSEA for an hour or so this morning before 18Z, but with a light southeast wind expect the stratus to likely remain north of the terminal. VFR conditions are expected this afternoon through Tuesday morning. Light winds will become north at less than 8 kt this afternoon and evening. Albrecht && .MARINE...A ridge of high pressure extends from the offshore waters into the coastal waters while another ridge sits over the Cascades. expect light northerly flow over the coastal waters and light to moderate afternoon and evening sea breezes near the coastline and in the inland waters through Tuesday. A developing frontal system will slowly move into the coastal waters Wednesday then inland later Wednesday night or early Thursday. Expect small craft advisory conditions do develop over most of the waters early Thursday. Some forecast guidance suggests that gales could occur over the coastal waters and from Puget Sound northward into the Northern Inland Waters. Model guidance has shown quite a bit of variability in the strength of the flow for Thursday morning so confidence in the wind speed forecast is low. Weaker frontal systems will affect the waters Friday through the weekend. Albrecht && .HYDROLOGY...River flooding is not expected in the next 7 days. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...None. && $$ is the U.S. government's official web portal to all federal, state and local government web resources and services.