Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 170006 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 406 PM PST Thu Nov 16 2017 .SYNOPSIS...
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An upper level trough offshore will move inland tonight, bringing a continuation of showery weather with a chance of evening thunderstorms along the coast. A flat upper ridge will build over the area on Friday and Friday night, decreasing shower intensity and coverage. A wet frontal system will bring rain to the area Saturday night through Sunday night. The rain threat will decrease Monday as another ridge moves across the area. Rainy and possibly stormy weather is expected at time from Tuesday through Thanksgiving Day.
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&& .SHORT TERM...
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A broad upper trough axis is now located between the coast and 130W. There is very cold air aloft, with 500 mb temps of -32C to -34C. Until the trough axis has completed its trek across Western Washington late tonight, the resulting instability and lift in advance of the trough will lead to loosely organized bands of showers. So the weather through late this evening will be characterized by short-lived rounds of heavy showers, separated by an hour or more of dry weather, perhaps with a glimpse of stars. The Winter Weather Advisory for the Mt. Rainier area still looks valid with another 3 to 8 inches of snow forecast tonight. Will need to continue monitoring radar trends for any persistent areas of showers that possibly dump more snow than forecast on other mountain zones. Snow level will be 2000-2500 feet tonight. Once the trough axis passes east overnight, large-scale lift will lessen, and the depth of the unstable layer will become more and more shallow through Friday. So there will be an overall decreasing trend in showers on Friday. The remaining showers will be weaker. The combination of onshore flow but weaker large-scale lift means the remaining showers will be more anchored to terrain features, namely the windward side of our mountain ranges. Parts of the lowlands, especially Olympic rain shadow, could remain dry on Friday. By Saturday, a flat upper ridge axis will develop over the Pac NW. Especially on Sat afternoon, dry weather appears likely over the entire forecast area, except perhaps the northwest Olympic Peninsula. Have removed even a slight chance of rain south and southeast of Seattle and Olympia on Saturday. Looking further north, a wet frontal zone will develop Saturday further up the B.C. coast on the periphery of the upper ridge. As the flat ridge axis moves east to the Rockies and beyond, the front will start to steadily sink south down the B.C. coast. This front has a juicy moisture tap from the central North Pacific. As the front spreads in from the north, rain will overspread the northern half of the area late Saturday night, then spread across the rest of Western Washington on Sunday. Hourly rainfall rates could become intense. However thankfully, the front will be progressive and moving steadily without any hang-ups, so the somewhat short duration of intense rainfall will limit the risk of river flooding for all but the Skokomish. Haner .LONG TERM...Models are in much better agreement today in carving out a dry period centered on Monday PM thanks to a sharp and skinny shortwave ridge axis following Sunday`s front. Was able to lower PoP a little more during the Monday/Monday night time frames. A strong and deep upper trough will dig and amplify around 140W on Tuesday, directing strong southwest flow aloft toward the area. At first, this will push a strong, wet warm front across Western Washington. There are still timing differences in guidance, with the GFS showing early Tuesday rain arrival and the more consistent ECMWF holding off until Tuesday evening. As the warm front approaches could get windy on Tuesday in east-wind prone locations such as the Cascade foothills. At the least, it`s safe to say that the Wednesday pre-Thanksgiving travel day will be sloppy, though with high snow levels. All models show copious rain. Will need to watch for flood and landslide potential. To add to the mix, the 12z ECMWF was the second consecutive run to show a deepening low passing N-NE off the coast on Tuesday night or Wednesday. The GFS has not caught on to this potential feature yet. If the EC is right, this could also bring windy conditions. Check back in the days ahead for the latest as model guidance hopefully develops more consensus in the details. Thanksgiving Day could be relatively benign compared with Tue and Wed, but shower and breezy weather could still linger. Of course, Thanksgiving week is climatologically the wettest week of the year. Haner
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&& .AVIATION...
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Moist westerly flow aloft will turn northwesterly tonight as an upper trough passes through the area. The air mass will remain slightly unstable as this trough moves through...generally stabilizing overnight although some instability will remain in place over the coastal waters. While some breezy conditions remain at the surface, the southeasterly pressure gradient is expected to gradually weaken this evening and tonight allowing for winds to ease. Cigs over the area remain a mix of VFR to MVFR conditions this afternoon...with the potential for a little more improvement but not much. Tonight cigs are expected to lower with TAF sites generally expected to be generally MVFR although more IFR prone locations such as OLM and PWT will likely see lower cigs with the prospect of some fog given available low level moisture. Speaking of which...although a weak upper level ridge starts to nudge into the area Friday...said moisture will reduce any improvement in cigs for Friday afternoon keeping conditions generally MVFR although some locations may reach low-end VFR criteria. KSEA...Cigs generally VFR this afternoon and evening before dropping down into MVFR tonight. There will be some improvement Friday afternoon but cigs only expected to reach high end MVFR. Surface winds will remain southerly at 6-10 kt. SMR
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&& .MARINE...
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A surface low will move toward northwest Washington and dissipate tonight. Inherited small craft advisories look good to be pulled down for afternoon forecast package as winds steadily diminish with the bulk of obs well under SCA criteria. A weak ridge will cross the region Friday. A strong front will slowly approach the area late Saturday then move inland Sunday morning. Will start the ball rolling by putting a Gale Watch up for the coastal waters for this time frame. Gales do not look to impact the eastern strait or the northern inland waters until a little bit later...so while they will not be included in the afternoon issuance...should forecast solutions remain consistent can easily foresee their inclusion in future forecasts. As the front moves through the area quickly...expect winds to diminish just as fast with winds subsiding by Sunday evening. SMR
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&& .HYDROLOGY...
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The Skokomish River at Potlatch fell below flood stage earlier this afternoon. Can still take around 6 more hours for floodwaters to flush out of the river`s mouth and for residual floodwaters to disperse. Heavy rain is expected Sunday through Sunday evening. In the midst of the heaviest rain, snow levels will run 4000-5000 feet. Rivers will rise, but the progressive nature of the front will limit duration. As a result, the only river that appears at risk of flooding is the normally flood-prone Skokomish River in Mason County. The heaviest 6-hour block of rain in the Skokomish basin appears to be 18z-24z (10 am - 4 pm) on Sunday. Around next Tuesday or Wednesday, Western Washington may see a prolonged period of heavy rainfall. If this occurs, all rivers will see rises with some approaching or exceeding bankfull levels. There will have been enough rain in previous weeks to make landslides a point of concern too. Timing details and quantitative details on mid-week rainfall are still not yet resolved, so check back in the days ahead for a more detailed threat assessment. Haner
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&& .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...Winter Weather Advisory until 4 AM PST Friday for Cascades of Pierce and Lewis Counties. PZ...Gale Watch from Saturday evening through Sunday afternoon 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.
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&& $$ 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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