Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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345 FXUS66 KSEW 191051 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 351 AM PDT Fri Jul 19 2024 .SYNOPSIS...
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Western Washington on the back side of an upper level ridge centered over the Rockies. An approaching upper level trough will build the ridge over the area Saturday. The trough will nick the northwest portion of Western Washington Sunday. A change in the weather pattern beginning Monday with an upper level trough moving into the area. The trough will remain over Western Washington through Thursday resulting in cooler weather next week.
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&& .SHORT TERM /TODAY THROUGH SUNDAY/...
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Satellite imagery shows stratus along the coast, moving down the Strait of Juan de Fuca and moving through the Lower Chehalis Valley into the southern portion of the Hood Canal. Clear skies over the remainder of the area with a little smoke in the air. Temperatures at 10z/3am were in the mid 50s to mid 60s. Stratus will make a run at the Puget Sound area this morning. Surface gradients are onshore but weaker than Thursday morning when the stratus did not make it across the Puget Sound. Small finger of stratus over Kitsap county could make it into the Seattle area for a couple of hours after sunrise. Outside of this feature forecasting stratus to remain west of Puget Sound. Light onshore flow continuing today. Ridge building slightly over the area with upper level trough digging offshore. 500 mb heights this morning in the lower 580 dms rising to the upper 580 dms this afternoon. With the light onshore flow highs today just a couple of degrees warmer than Thursday with mid 70s to mid 80s inland and mid 60s to mid 70s along the coast. Upper level ridge continuing to build tonight into Saturday with 500 mb heights in the lower to mid 590 dms and the flow aloft becoming southerly by Saturday afternoon. Temperatures aloft warming with 850 mb temperatures rising from plus 14C to 16C tonight to plus 16C to 20C Saturday afternoon. Low level flow remaining light onshore. The warming temperatures aloft combined with the marine layer being pretty shallow will result in less morning cloud cover and warmer high temperatures. Highs Saturday in the upper 70s to near 90 inland and in the 70s along the coast. Lows tonight in the 50s and lower 60s. Upper level ridge slowly shifting east Saturday night into Sunday as an upper level trough approaches the area. Onshore flow increasing Sunday with the marine push overnight into Sunday morning resulting in cooler high temperatures by 5 degrees or so Sunday afternoon for the interior and 10 degrees or so along the coast. Weak shortwave moving by to the west could produce a shower or two along the coast Sunday morning. Highs in the mid 70s to mid 80s for the interior and mid 60s along the coast. Lows Sunday morning in the 50s and lower 60s. .LONG TERM /MONDAY THROUGH THURSDAY/...Models in good agreement with a pattern change beginning Monday with an upper level trough moving over the area. The trough will remain over Western Washington through the week with stronger onshore flow in the lower levels. This combination will push high temperatures down near normal. The last time Seattle had back to back days with below normal high temperatures was June 26 and 27. Morning clouds with afternoon sunshine will be the rule through the period with the possibility of the clouds hanging in all day along the coast. Highs for the interior in the upper 60s to upper 70s with mostly 60s along the coast. Felton
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&& .AVIATION...
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Western Washington is positioned between an upper trough offshore and ridging aloft over the interior West for west to southwest flow aloft. The low level flow is weak onshore. IFR stratus will spread locally inland from the coast through the Chehalis Gap and Strait of Juan de Fuca before burning back to the coast by around 18Z. LIFR/IFR ceilings in stratus and fog will return to the coast tonight, but will likely make little progress inland as onshore flow will be weaker. KSEA...Much like yesterday morning, stratus is making slow progress toward Puget Sound. Current thinking is to scale back coverage to tempo IFR ceilings between 13Z and 16Z then VFR for the remainder of the day. Surface winds southwesterly 4 to 8 knots veering to northwest this afternoon then northerly by late this evening. 27
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&& .MARINE...
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Surface high pressure offshore and lower pressure inland will continue to produce varying degrees of onshore into the weekend. Onshore flow will increase Saturday night into Sunday as thermally induced low pressure over the interior shifts eastward. This will likely result in gales for the central/east strait and small craft advisory winds for adjacent waters and portions of the coastal waters. A broad upper trough will settle over British Columbia early next week as a series of weather systems rotate around it. This will keep onshore flow in place with additional headlines expected...especially in the Strait of Juan de Fuca and across portions of the coastal waters. 27
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&& .FIRE WEATHER...
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Warm and dry conditions through Saturday. Afternoon minimum RHs again dipping close to critical thresholds both today and Saturday for the lowlands and today through Sunday for the Cascades. Instability may increase enough to keep a mention of elevated concern for the Cascades especially Saturday afternoon and evening. Another disturbance may bring a renewed threat for thunderstorms Sunday across some portion of the mountains, but confidence is low in this scenario. A cooler more humid pattern develops Monday as the pattern switches to be dominated by an upper level trough over the region. Cullen/Felton
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&& .CLIMATE...
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Seattle tied the record for consecutive days with highs 80 degree plus with Thursday being the 15th day in a row. Forecast highs continue the streak through the weekend before it comes to an end Monday. Moving down a notch, the record for consecutive days with highs 75 degrees plus in Seattle is 23 days from June 17th through July 9th in 2015. There have only been 3 streaks of 20 plus days in 80 years of records. The other two 20 day plus streaks, 21 days July 29th through August 18th, 1977 and 20 days July 18th through August 6th, 1994. Felton
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&& .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...None. PZ...Small Craft Advisory from 5 PM this afternoon to 3 AM PDT Saturday for Central U.S. Waters Strait Of Juan De Fuca- East Entrance U.S. Waters Strait Of Juan De Fuca.
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&& $$