Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
Issued by NWS Seattle/Tacoma, WA
697 FXUS66 KSEW 152138 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 238 PM PDT Wed May 15 2024 .SYNOPSIS...High pressure remains offshore of Washington, providing one more day of mostly clear skies and above average temperatures across Western Washington today. By Thursday, an upper level trough will dig down into the region from Canada. This will return a chance of showers for most of the outlook area Thursday. A cool and damp pattern is expected to persist from this weekend into next week, with below average temperatures and several chances for showers. && .SHORT TERM /TONIGHT THROUGH SATURDAY/...One more mostly sunny day is panning out across the outlook area today. An upper level ridge/high pressured centered off the coast still remains through today. Satellite imagery shows some high cirrus across the entire area, with a few cumulus over the Cascades and Olympics. A cumulus cloud band remains off the coast, thanks to the northwest flow aloft and near the surface. Temperatures have climbed into upper 60s and low 70s in most of the lowlands this afternoon (helped by light winds and maximum daytime heating). This part of the forecast remains largely on track, and a few areas will likely peak into the mid 70s this afternoon. By Thursday, an upper level trough swings around a low deep in northern Canada. This will dip a jet streak/vorticity max down into Washington for Thursday, as well as breaking down the high over the Pacific. Down at the surface, a cold front will swing through with this shortwave during the day. Due to limited moisture available with this front, precipitation is expected to remain limited with shower activity across the area. The heaviest of the showers are expected to be in the northern Cascades. QPF amounts there range from around a quarter to a half an inch, with just several hundredths in the lowlands. With the cooler air coming through, snow levels will drop to around 3500 feet Thursday afternoon. Additionally, the onshore flow behind the front may result in a convergence zone over western Snohomish/Skagit counties. Breezy southwest/west winds at 10 to 15 mph are possible post-front in the lowlands, with gusts to 20 mph possible. The Thursday trough will depart the region Friday, leaving the remainder of the day dry with high temperatures only peaking in the low to mid 60s. Clouds will break up some for the afternoon. Another shortwave trough appears to dip into Washington on Saturday. This will return the chance of showers (with best chance again being east of I-5 in the Cascades). NBM was giving a brief slight chance of thunder in the northern Cascades for the afternoon, with the cooler air aloft. Temperatures for Saturday slightly will be slightly cooler, with upper 50s to low 60s. Winds will diminish going into Friday/Saturday. .LONG TERM /SUNDAY THROUGH WEDNESDAY/...There is better agreement now between the ensemble clusters and deterministic models for early next week. It appears the pattern will be influenced by cool and unsettled weather via a trough over the northwest. Flow will be northwesterly, which will keep temperatures at or below average for the first part of the week. Shower chances will be possible via several shortwaves first part of Sunday, and Monday through Wednesday. None of these days appear to be washouts at this point, with coverage being spotty and confined to primarily the mountains. The NBM again mentioned thunder for the northern Cascades on Monday. HPR && .AVIATION...Northwesterly flow aloft through Thursday morning with upper-level ridging aloft. Flow aloft will then shift more westerly by Thursday evening as a shortwave trough rounds the area. VFR across western Washington as high clouds stream over. VFR conditions expected to remain in place for most terminals for the remainder of the day, however will see cigs gradually lower from west to east throughout the night in response to an upcoming system on Thursday. By 12z Thursday, stratus will be widespread across area terminals with MVFR in the cards. Can`t rule out localized IFR and LIFR for terminals such as HQM. Some brief improvement to low-end VFR is possible into Thursday afternoon but rain showers and convergence zone activity is likely to hang around. KSEA...VFR with high cirrus currently. But as mentioned, MVFR is possible going into early Thursday morning as stratus fills in around 09-12z. Rain showers and convergence zone activity is possible into Thursday also. McMillian && .MARINE...A round of SCA level winds expected in the Strait of Juan de Fuca starting this afternoon and look to continue into Thursday as a front passes. A stronger push is expected to accompany an incoming frontal system Thursday night and Friday which may need yet another set of headlines. A SCA has also been issued for the coastal waters, associated with the aforementioned front Winds expected to ease for most areas, save perhaps the outer coastal waters this weekend into early next week. Seas will hold around 4 to 6 ft through Thursday before increasing 8 to 12 ft from Friday into the weekend, reducing to 6 to 8 ft early next week. McMillian && .HYDROLOGY...The daily hydrology discussion has ended until the start of the next rainy season; it will only be updated as needed. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Small Craft Advisory until 5 AM PDT Thursday for Central U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory from 5 PM this afternoon to 5 AM PDT Thursday for East Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters Including The San Juan Islands. && $$