Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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491 FXUS66 KSEW 221059 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 359 AM PDT Thu Mar 22 2018 .SYNOPSIS...A cold front will move through the area this morning. A large cold upper level low offshore will slowly move south tonight and Friday with the low off Vancouver Island Friday afternoon. The air mass over western Washington will become unstable with the snow level lowering to near 1000 feet on Friday. The low will continue to move south ending up off the southern Oregon coast Saturday afternoon. An upper level ridge will build off the coast the first part of next week for drier and warmer weather. && .SHORT TERM...Satellite imagery shows cold front offshore now within a hundred miles of the coast at 10z/3am. Doppler radar has rain over most of the area from about the Puget Sound westward. Little spread in the temperatures with readings in the mid 40s to near 50. Cold front parallel to the upper level flow this morning which will slow down the eastward movement. Frontal passage not expected until later this morning with the front reaching the Cascades midday. Rain at times forecast ahead of and with the front. Behind the front the air mass will become unstable but the coldest air aloft will continue wrapping around the upper level low offshore and not move inland today. Lifted indexes are near 0 along the coast with model 500 mb temperatures a cold -37C. Will confine the slight chance of thunderstorms to the coast. High temperatures will occur this morning, in the mid 40s to near 50, with temperatures remaining steady or falling a couple of degrees this afternoon. Southwesterly flow aloft tonight with the low off Vancouver Island by 12z Friday. Colder air aloft filtering into the interior but without any daytime heating to mix things up will stay with the idea of just showers for the interior. Models do keep lifted indexes near 0 along the coast so will keep a slight chance of thunderstorms in the forecast for the coastal areas. With the cold air loft any break in the cloud cover will allow temperatures to drop quickly. By early Friday morning the coldest locations will be near freezing with the remainder of the area in the 30s. Upper level low off Vancouver Island not moving much on Friday with southwesterly flow aloft continuing over the area. Air mass aloft remaining cold with model 500 mb temperatures in the -35 to -38C range Friday afternoon. Snow levels will only be in the 1000 to 1500 foot range. Nothing organized spinning out of the upper low into the area but with the air mass unstable expect showers to develop especially in the afternoon hours. Model lifted indexes 0 or less over most of the area Friday afternoon so will have a slight chance of thunderstorms over the entire lowlands. Ice pellet/small hail showers certainly possible in this scenario. High temperatures will have a hard time getting out of the 40s. Upper level low on the move offshore Friday night and Saturday ending up off the southern Oregon coast Saturday afternoon. As the low drifts further south the pops will decrease over western Washington. Air mass not moderating much at all even with the low to the south with model 500 mb temperatures still around -35C Saturday. Air mass still unstable Friday night into Saturday but with the low moving south will show a decreasing trend in the pops with only chance pops by Saturday afternoon. A little less cloud cover Friday night with more locations in the Southwest Interior and Lower Chehalis Valley ending up below freezing Saturday morning. After lows in the 30s highs will once again struggle to get out of the 40s Saturday afternoon. Snow levels will remain low, around 1000 feet on Saturday. Once again ice pellet/small hail showers are possible in the afternoon. .LONG TERM...Extended models in good agreement Sunday with the low moving inland and a weak system embedded in the northwesterly flow aloft over western Washington moving into the area. Will keep the chance pops going into Sunday. Some slight differences for Monday and Tuesday with the GFS a little stronger with the building upper level ridge offshore which will help push the next system in the northwest flow aloft north and east of the area versus the weaker ridge ECMWF solution. Both the GFS and ECMWF have the upper level ridge over the area on Wednesday with 500 mb heights in the lower to mid 570 dms. Current forecast leans toward the GFS solution with a dry forecast beginning Monday night. Nothing in the 00z run to change this idea with the early morning package. Felton && .AVIATION...Moderate southerly flow aloft continues over western Washington this morning with a moist air mass in place. A cold front will move through the region late this morning ushering in a colder, more unstable air mass. Rain will turn over to showers with the passage of the front this afternoon. Ceilings will lower to MVFR as the front moves through this morning then areas of MVFR can be expected in and around the shower activity in the afternoon. Increasing low level onshore flow behind the front will bring some gusty south to southwest surface winds to the area from mid morning into the afternoon. KSEA...Expecting ceilings to lower to MVFR with rain this morning ahead of a cold front...then periods of MVFR in showers from midday into the evening hours. Fairly strong post-frontal onshore flow will lead to some gusty S-SW winds of 20-25 knots at times from 15Z through 00Z. 27 && .MARINE...A low pressure system and cold front will affect Western Washington today. Stronger southerly winds will develop this morning as the front moves ashore. The low pressure area will linger offshore Friday and then weaken and move ashore over northern California over the weekend. High pressure will build into the area next week, but weather systems will still brush the area. && .HYDROLOGY...River flooding is not expected in the next 7 days. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Small Craft Advisory until 5 AM PDT Friday 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-West Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory until 6 PM PDT this evening for Admiralty Inlet-Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters Including The San Juan Islands-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.