Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 211035 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 335 AM PDT Thu Sep 21 2017 .SYNOPSIS...
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An upper level low will maintain a chance of showers and cooler temperatures over Western Washington today, mainly over the Olympics and Cascades. Drier north flow aloft from an offshore upper level ridge will bring sunshine and a little warming Friday and Saturday. The weakening ridge will move over the area Sunday and Monday but could allow transient weak weather systems to bring some spotty light rain.
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&& .SHORT TERM...
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Current IR satellite shows a mix of high and low clouds over the area making the case for a good generalized mostly cloudy forecast. Current radar is pretty quiet with only a few showers being detected...one over the outer coastal waters...another in northern Snohomish county and a third running from NW Skagit county into SW Whatcom. All of these look to have fairly light precip. Upper level low pressure and the associated trough will dig in deeper over the western US today...but as it does so the main focus for precip does not look to be W WA. While some showers are not entirely out of the question...coverage looks pretty scattered and models show the best chances to be in the higher elevations of the Olympics and Cascades. The low makes very little headway in traveling east Friday...but the shift turns out to be enough to bring drier northerly flow over the area and eliminating any chances for precip both Friday and Saturday. As the low continues to make its slow eastward journey...upper level ridging nudges in for Saturday...helping temps to rebound into the 60s and clearing skies out to a degree. Sunday remains a bit of a question mark as the 06Z GFS suggests a shortwave disturbance within the ridge that could bring showers down from the north while the 00Z ECMWF shows no such feature...keeping a dry forecast. Given that the 00Z GFS did not have this feature...will opt for a mainly dry forecast over the area for this time frame while keeping an eye to see if this solution with the shortwave feature persists. SMR .LONG TERM...The upper level ridge persists Monday and into midweek keeping the area relatively dry...although each model shows a bit of moisture and thus the potential for showers to dip south enough to be right on the doorstep...but never really panning out. Not helping matters is that there is really not much in the way of agreement as to which systems or which days this will occur. Feels like a bit of a cop out...but best course of action is to keep the forecast painted in broad strokes with slight chance pops for much of the extended until models can gel on something resembling a singular solution. Regardless of precip...should continue to see temperatures gradually climb into the 70s for many locations for the first half of next week. SMR
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&& .AVIATION...
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Upper level trough over the area shifting east this morning with northeasterly flow aloft into Friday. Light onshore flow continuing throughout the day. Wide variety of ceilings over the area early this morning. Between KBLI and KPAE and along the coast areas of ceilings near 4000 feet with mid level clouds above. Over the central Puget Sound a 1000 foot stratus deck is forming at 10z. There is also some lifr ceilings and visibilities near KPWT. Over the Southwest Interior just some mid level clouds. South of KPAE and out to the coast expect the stratus deck near 1000 feet to becoming more widespread through 14z with the layer scattering back out again late morning/midday. The lifr conditions near KPWT will continue until around 17z. KPAE northward expect the 4000 foot cloud deck to hang around for a few hours this morning before dissipating leaving just scattered clouds. KSEA...Ceilings near 1000 feet through 17z with ceilings lifting 17z-19z. Stratus will scatter out around 19z. Southerly wind 4 to 8 knots becoming northerly around 21z. Felton
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&& .MARINE...
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Weak surface low tracking southward through the coastal waters this morning. High pressure building on Friday with high pressure remaining over the area into the early part of next week. Winds are forecast to remain below 20 knots across the waters. Felton
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&& .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...None. PZ...None.
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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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