Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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737 FXUS66 KSEW 230523 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 900 PM PST Wed Nov 22 2017 .SYNOPSIS...A cold front will bring more rain to Western Washington tonight through Thanksgiving. Scattered showers will continue through Thursday night as an upper level trough follows the front. A weak warm front will bring a little light rain Friday night. The next system to bring widespread light rain should arrive Saturday afternoon. This active weather pattern should continue into early next week. && .SHORT TERM...Radar indicates rainfall increasing in coverage and intensity across Wrn Washington this evening. Satellite imagery shows the tap of moist tropical air continues with another surge of moisture expected to bring more heavy mountain rain tonight. The leading edge of the cold front is probably near the north coast but not moving inland much if at all. The Forks sounding shows temps around 1K feet up around 12C while the Sand Point profiler indicates 20C at that level. The warmer air is clearly still in place over interior Wrn Washington. Record highs were reached at several location including Seattle at 68F, breaking the old record of 58F in 1959. That`s some unusually warm air but can see why looking at the source region way out just east of Hawaii. The flow is very strong from the low levels all the way up on the 00z sounding. Southwesterlies are about 40 kt down to 1k ft and 60-70 kt at a few thousand feet up. As the next wave arrives, this should hold the front up until Thursday morning. Very warm moist air and strong flow at all levels spells heavy orographic rainfall overnight. The 00z 4km WRF came in surprisingly low on QPF, with up to 2-2.5 inches only along favored southwest slopes, especially the Olympics. Models trended toward somewhat less QPF tonight than had been expected. Looking at satellite, soundings/profiler, and flow, my guess is that rain amounts could be quite a bit heavier than the WRF. Forecast QPF was only lowered slightly which was probably a good call. See the Hydro discussion below and flood bulletins for further details. Rain will be slow to taper off. Models disagree on how quickly the front will finally push through. The new 00z NAM brings another wave up during the day Thursday. Other models are not as wet in the afternoon. But some adjustment to raise pops for the afternoon hours might be needed with the next forecast update. Friday and Friday night appear to show a lull in rainfall. Another strong wave with possible surface low development will approach on Saturday. There are differences on timing, track, and strength but additional rain looks certain. Windy conditions could develop with the lower later on Saturday depending on the track and depth. Will need to keep an eye on this, there is a great deal of uncertainty at this point. Mercer .LONG TERM...Previous discussion...The GFS and ECMWF show the offshore longwave trough weakening on Saturday then moving E across W WA Sunday night. The GFS is a little faster than the ECMWF. This will keep the weather active with rain at times. The GFS has stuck with bringing a warm front E across the area Monday night, but the ECMWF, after showing the same solution for the past 2 days now significantly weakens the warm front offshore. The models diverge even more for Tuesday and Wednesday. The GFS shows a dry period with a moderate amplitude upper level ridge over the area Tuesday night and Wednesday, but this will probably end up being a discarded solution by next week. Kam && .AVIATION...Moist, SW flow will continue over Western WA tonight and Thursday for rain across the area. Expect mainly MVFR ceilings with patchy fog possible. A trough will push inland Thursday afternoon for strong onshore flow and showers with ceilings lifting. Showers will mainly affect the mountains Thursday night with strong westerly flow aloft. 33 KSEA...MVFR tonight, improving to VFR late Thursday afternoon. South winds 8-12 kt tonight, becoming southwest and increasing to 15g25 kt Thursday afternoon. 33 && .MARINE...Gales over the waters will ease by midnight but will still see Small Craft Advisory winds over most zones. Strong onshore flow will continue through Thursday as a trough moves inland. The next front will stall over southern B.C. on Friday then dissipate. Another vigorous system will arrive around Saturday night. Strong southerly gradients are possible over the entire area Sunday. Gales are possible over most waters with Small Craft Advisory conditions over Puget Sound and Hood Canal. 33 && .HYDROLOGY...Just waiting for the next round of precip to kick in and assess where all the rivers are at the time. Some rivers have crested, some have fallen below flood stage, some are still rising. Warnings and watches change as needed. For the current list, please check: www.wrh.noaa.gov/sew/get.php?pil=RVA&sid=SEW The next front will move through tonight and Thanksgiving Day, with a forecast of generally 1 to 3.5 inches over the mountains. This is slightly less than previously forecast as the models are all trending down in amounts. River forecast updates will likely show some lover crests with the new QPF numbers. The snow level will be around 7500 feet as this system arrives, falling to 4000 feet late Thursday as precipitation decreases. With this second round of rainfall tonight and Thursday, there is a possibility that flooding will become more widespread and/or prolonged. There is at least a minor threat that any flood-prone river in our area reaching flood stage. In addition, there is a threat of landslides across the lowlands of Western Washington. The USGS landslide guidance shows soil wetness above the threshold of concern, and additional rain through Thursday will drive those numbers higher and act as the trigger for landslides. JBB/McDonnal && .SEW WATCHES/WARNINGS/ADVISORIES... WA...Flood Watch in effect for Lewis Grays Harbor Jefferson Clallam Pierce King Snohomish Whatcom. Other flood statements in effect. PZ...Small Craft Advisory for rough bar until 4 PM PST Thursday for Grays Harbor Bar. Small Craft Advisory until 4 PM PST Thursday for Admiralty Inlet- 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 from midnight tonight to 4 PM PST Thursday for East Entrance U.S. Waters Strait Of Juan De Fuca- Northern Inland Waters Including The San Juan Islands. Gale Warning until midnight PST tonight for East Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters Including The San Juan Islands. Small Craft Advisory until midnight PST tonight for Central U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory until 4 AM PST Friday for Puget Sound and Hood Canal. && $$ www.weather.gov/seattle You can see an illustrated version of this discussion at www.wrh.noaa.gov/sew/gafd/latest_webafd.html

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