Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 270212 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 712 PM PDT Wed Jun 26 2019 .SYNOPSIS...
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An upper level low will be in the vicinity of the Pacific Northwest through Saturday bringing unsettled weather to the region. The low weaken as it slides northeast on Saturday. A weak upper level trough will move over the area through early next week.
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&& .NEAR TERM /THROUGH TONIGHT/...
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Convection is weakening this evening, but unlike last night, shower activity is likely to persist overnight. As of 700 PM, showers and isolated thunderstorms are noted over the eastern portions of the Olympic Peninsula as well as from the Skagit County border north into Canada. Both of these areas will gradually weaken over the next 1-2 hours. Looking forward, there are two areas to note. One is an area of convection that is rapidly moving north along the eastern slopes of the Cascades associated with a robust and rather deep vorticity lobe. This lobe is expected to wrap NE and eventually E across the North Cascades and northern lowlands tonight into Thursday morning. Once it crosses the Cascades, instability will be very limited and therefore mainly showers are expected. The second area to watch will be the far southern border of the forecast area. Active convection is ongoing across Cowlitz and Skamania counties and is slowly pushing north associated with a much weaker vorticity lobe. Since this is moving into an area that was worked over pretty well this afternoon, I suspect that convection to weaken as it progresses northward. There is some indication that once it moves north of Olympia as we approach midnight, that shower activity will redevelop/increase and move north through the central Puget Sound region. Again, instability is marginal at best and with the weak nature of this lobe, I would expect mainly shower activity. We will need to monitor both of these areas for the potential for thunderstorms overnight. The forecast has been updated to reflect the latest trends and short range guidance. -Wolcott-
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&& .SHORT TERM /THURSDAY THROUGH SATURDAY/...
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The upper level low located offshore near 43N/130W will slide northeastward tonight into Thursday and be positioned along the WA/OR coast by Thursday afternoon. A few showers will be around through the morning with more numerous showers developing into the afternoon. The air mass will become unstable again Thursday afternoon. Lifted indices on Thursday from -1 to -4 with CAPE values from 500-1000 J/kg will at least be sufficient for the risk of a few isolated t-storms across the Cascades into southeast portions of the CWA Thursday afternoon into the evening. Though thunderstorm activity should be less numerous than today given a bit more stable environment. Temperatures on Thursday will be cooler in the low to mid 60s across the area. The upper low will slide northeastward Friday into Saturday as it progresses north of the area into eastern British Columbia. Showers will be around on Friday as the upper level lifts northeast of the area with high temperatures in the mid to upper 60s across the area. A weak upper trough will develop over W WA on Saturday. This will keep at least a chance of POPs on Saturday with some clouds around. Temperatures on Saturday will likely be a bit warmer with highs in the mid 60s to low 70s. JD .LONG TERM /SUNDAY THROUGH WEDNESDAY/...Guidance is in general agreement of an upper level trough over the Pac NW into next week. GFS suggests a wetter solution for W WA on Sunday and Monday with the upper level low overhead while the ECMWF is a drier. At least a chance of POPs in forecast in places for Sunday and Monday, especially in the Olympics and Cascades. Upper level low moves over the area on the ECMWF solution on Tuesday with showers around for Wednesday, as the GFS slides the upper level low east of the area on Wednesday. Both solutions show at least a slight chance for POPs in places. JD
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&& .AVIATION...
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Thunderstorm activity is decreasing this evening, however isolated storms will remain possible, mainly south KPWT as well as along the Cascade Crest. Winds have been variable across the area due to modest outflow from afternoon showers/storms. Expect winds to settle to a more typical S/SW flow heading into this evening. Mainly VFR conditions outside of shower/thunderstorm activity. MVFR conditions likely to develop in many areas overnight as additional shower activity impacts the region. Another round of showers and thunderstorms possible Thursday afternoon/evening. KSEA...Mainly VFR through roughly 09z-10z with variable winds due to outflow from earlier showers. CIGS then lowering to MVFR continuing through Thursday AM with showers in the area. Winds will become S/SW 5-10 kt as showers move in overnight. Another round of showers and an outside chance of thunderstorms possible Thursday afternoon/evening. -Wolcott-
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&& .MARINE...An upper level low will linger over the offshore coastal waters for the next few days and will maintain onshore flow across western Washington through the week. Small craft westerlies will be likely along the central and eastern Strait of Juan de Fuca the next couple of afternoons and evenings, while other waters will remain mostly below advisory criteria. 27 && .HYDROLOGY...The daily hydrology discussion has ended until the start of the next rainy season; it will only be updated as needed until then. && .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...None. PZ...Small Craft Advisory until 5 AM PDT Thursday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.
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