Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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892 FXUS66 KSEW 171527 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 827 AM PDT Sun Sep 17 2017 .SYNOPSIS...
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A front will bring rain later today. An upper trough and additional weather systems will keep the weather cool and showery into the middle of the week. Drier weather is likely late in the week.
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&& .SHORT TERM...
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A front will move through the area today, bringing rain to the coast this morning and to the interior this afternoon. Highs will be in the 60s which is 5 to 10 degrees cooler than yesterday. An upper trough will move over the area Monday with snow levels lowering to 5000 to 5500 feet. It will be a showery day and the higher locations in the mountain could pick up a few inches of snow. There will also be a slight chance of thunderstorms Monday with cold unstable air aloft over the area. Highs will be about 10 degrees below normal. Models show an upper short wave moving through the area Monday night causing an uptick in showers, especially over the south. This could bring 6+ inches of snow to Paradise which would meet winter storm criteria for the first event of the season. If the 12Z models continue to show this, a winter storm watch will likely be issued for the Cascades of Pierce and Lewis counties later today. Showers will linger into Tuesday but probably not as prevalent as Monday as the upper trough weakens over the area. Highs will be about 10 degrees below normal again, similar to Monday. Schneider .LONG TERM...A vigorous upper short wave will move onshore Tuesday night, clipping the south part of the forecast area. This should bring steadier precipitation with heavier amounts south of Seattle. An associated surface low will track inland just to the south of the area. This should keep windy conditions south of the area but it is close enough that it needs to be monitored. An upper trough will be over the area Wednesday for more showers. Drier northerly flow aloft will develop over Western Washington Thursday. It looks like most of the area will be completely dry by Friday and Saturday. Schneider
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&& .AVIATION...
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A cold front will move through Western Washington today. The radar shows some rain just off the coast at 8am. There is some low stratus on the coast this morning at Hoquiam but that will lift up and merge with the increasing frontal clouds. The low clouds made it to shelton this morning but most of the interior of Western Washington just has mid and high clouds so far--but cigs will come down this afternoon and the rain pushes in. Showers are on tap tonight in onshore flow and a PSCZ will probably form for a few hours. The smoke will go away, replaced with showers and clouds obscuring the mountains at times for the next several days as an upper trough moves slowly across the region with cool unstable air. KSEA...The current TAF shows the first front of fall, a little rain later today, southerly winds. Sometime this evening, probably around 04z, a PSCZ will probably develop and could cause the winds at Sea- Tac to become VRB05kt for several hours
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&& .MARINE...
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The first real front of fall should give many areas breezy weather for awhile today. The cold frontal passage and wind shift from 20-30kt southerlies to more moderate westerlies over the coastal waters should occur midday to early afternoon, reaching Grays Harbor around 3pm. Westerlies will come down the Strait of Juan de Fuca mid to late afternoon. For the inland waters southerly winds 15-25kts will turn to a convergence zone wind pattern and ease this evening. Looking at the wrfgfs, small craft advisories might have to stay up for awhile this evening in the north and Strait for winds behind the front. Looking at the 06z GFS...a low pressure trough moves through the area Monday night, and then another system with a deeper low moves through the area Tuesday night--with a 992mb low moving ashore over NW Oregon, so those are systems to watch for some blustery weather.
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&& .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...None. PZ...Small Craft Advisory until 10 PM PDT this evening 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. Small Craft Advisory until 11 PM PDT this evening for 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. Small Craft Advisory until 6 PM PDT this evening for West Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory until 6 PM PDT this evening for Admiralty Inlet-Puget Sound and Hood Canal.
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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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