Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 080318 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 818 PM PDT Fri May 7 2021 .UPDATE...
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The only change to the ongoing forecast this evening was to update rain chances to better match current and projected radar trends. Water vapor imagery this evening shows broad cyclonic flow over the Pacific Northwest and into the Northern Rockies, with a closed area of low pressure located within from roughly West-Central BC down into Eastern Washington. Just upstream from this cyclonic flow resides a shortwave ridge of high pressure over the offshore waters of Washington and British Columbia. Latest surface analysis from WPC this evening shows a weak trough of low pressure over Eastern Oregon, with high pressure extending from the Pacific into Western Washington and a warm frontal boundary well offshore the Washington and British Columbia Coasts. Recent MRMS radar imagery continues to pick up scattered showers along the Coast and across the Olympic Peninsula, along with a lingering convergence zone over the southern half of King County. Latest SPC mesoanalysis shows surface based instability and mixed layer instability decreasing rapidly over the last hour or so, and this makes sense given both the loss of daytime heating, and also the departure of the coldest mid level air to the east of the Cascade Crest as the aforementioned closed low slides away from the area. The result will be a diminished threat for isolated lightning through the remainder of the evening. High-res model guidance continues to show the CZ surviving through about midnight before it noticeably subsides into early Saturday. That will leave most areas with partly to mostly cloudy skies through this evening and overall dry. Although heights will rise noticeably as the shortwave ridge begins to move in overnight, plenty of moisture in the vicinity of the approaching warm frontal boundary will lead to overcast skies by daybreak Saturday. Widespread, light stratiform rain will also spread into the area early Saturday morning, mainly affecting the Olympic Peninsula. It should spread towards and east of Puget Sound by late-morning, but many areas may be spared the showery weather as the warm front becomes disorganized. Therefore, mainly expect an overcast day with generally a 20-40% chance of light rain showers (although rain chances will be higher near the Coast). High temperatures will be similar to today in the mid- upper 50s. Previous discussion can be referenced below with an updated marine and aviation section. Kovacik
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&& .PREV DISCUSSION...
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.SYNOPSIS...An upper trough will move inland overnight with scattered showers and isolated thunderstorms tapering off this evening. A weak weather system will bring a little rain to the coast on Saturday. That system will break up and dissipate as it moves inland Saturday night. An upper ridge will build into the region early next week for sunnier warmer weather. The ridge will break down in the second half of next week.
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&& .SHORT TERM /TODAY THROUGH MONDAY/...
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The showers and isolated thunderstorms will die down as the heating of the day gives out this evening. As the upper trough moves inland a weak warm front will move to the coast on Saturday for a little light rain there. That system should mostly peter out as it moves inland Saturday night and Sunday ought to see some sunshine and temps warming up a bit. Monday will see an upper ridge begin to build in the area, although it isn`t a very strong ridge and temps will recover only slowly and gradually into the beginning of net week. 19 .LONG TERM /TUESDAY THROUGH FRIDAY/...The modest upper ridge over the region on Tuesday will weaken and give way to weak fronts and a trend back to developing an upper trough over the region late next week. Temps on Tuesday look pretty nice with highs ranging from the lower 60s on the coast to the lower 70s over the interior of southwest Washington. After that, temps fade through the end of the week and marine layer clouds should be increasing. There is a chance that there will be coastal marine layer stratus each night and morning for most of the days next week but the chance for any significant precip probably wont develop til the following weekend. 19
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&& .AVIATION...
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Showers and thunderstorm activity has largely ceased over the area this evening and will continue to taper off overnight. Isolated showers may linger in places, but will be short lived. Surface wind speeds will swing back around to southerly overnight. VFR into early tonight may transition to MVFR late tonight into early Saturday, especially along the coast. Winds will mainly be west-southwesterly through tonight. KSEA...VFR cigs tonight Winds will mainly be westerly, with northerly component at times into this evening. Winds will be southerly later tonight into Saturday, generally between 5-8 knots. Kristell
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&& .MARINE...
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Onshore flow through this weekend with high pressure over the coastal waters. Increasing onshore flow into this evening with gales possible for the central and eastern portions of the Strait of Juan de Fuca through 09Z, small craft advisory winds for the Admiralty Inlet and northern inland waters into tonight. Winds will briefly increasing Saturday morning for the coastal waters with south winds, though winds will be below SCA. Mainly lighter winds over the weekend with increasing onshore flow Sunday into early Monday morning Seas of 7 to 8 feet tonight with 4 to 6 foot seas by Saturday evening. JD/Kristell
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&& .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...None. PZ...Gale Warning until 6 AM PDT Saturday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory until 2 AM PDT Saturday for Northern Inland Waters Including The San Juan Islands. Small Craft Advisory until 6 AM PDT Saturday for Admiralty Inlet.
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&& $$ www.weather.gov/seattle

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