Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
Issued by NWS Seattle/Tacoma, WA
206 FXUS66 KSEW 131052 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 352 AM PDT Mon May 13 2024 .SYNOPSIS...Weak upper level trough moving through the area today. Upper level ridge building tonight and Tuesday with the ridge remaining over the area until at least Thursday. Another upper level trough moving down from the north could reach Western Washington Friday through the weekend. && .SHORT TERM /TODAY THROUGH WEDNESDAY/...Satellite imagery shows weak upper level trough over Vancouver Island moving southeast. Stratus along the coast moving inland through the Lower Chehalis Valley. With the trough approaching middle level clouds are beginning to increase. Temperatures at 3 am/10z were in the upper 40s to mid 50s. Upper level trough moving through Western Washington today. Little in the way of dynamics with this feature with just increasing cloud cover this morning. Could squeeze out a shower over the Cascades but that is about it for precipitation with the trough. Cloud cover and low level onshore flow will keep high temperatures in the upper 50s to mid 60s. Trough east of the area tonight with an upper level ridge building offshore. With the ridge centered well offshore middle level cloud deck remaining over Western Washington overnight. Low level flow becoming northwesterly which will restrict the amount of stratus forming especially over the interior. Lows in the 40s. Upper level ridge continuing to build Tuesday. As the ridge begins to shift a little east more subsidence will dissipate the middle level cloud deck later in the day. Northwesterly flow in the lower levels will keep temperatures from warming up too much with highs in the 60s and lower 70s. Upper level ridge remaining over the area Tuesday night and Wednesday. Temperatures aloft warming with 850 mb temperatures going from around plus 8C Tuesday night to plus 10 to 12C Wednesday afternoon. Low level flow remaining northwesterly keeping highs in the mid 60s to mid 70s with the cooler locations near the water. Lows Tuesday night in the mid 40s to lower 50s. .LONG TERM /THURSDAY THROUGH SUNDAY/...Models still wrestling with inconsistency in the extended period especially run to run. Both the operational GFS and ECMWF have a weak shortwave moving by to the north Thursday. Ensemble solutions not in agreement with the operational runs with high temperatures just a couple of degrees cooler Thursday and the shortwave further north. Zonal flow aloft and low level onshore low cooling high temperatures a few more degrees Friday and Saturday. Operational runs bring a stronger trough into the area Sunday but once again the ensembles are not on board with this idea keeping highs a little above normal with not much cloud cover. Will add a chance of showers to the Cascades Sunday but keep the lowlands dry through the period. Highs mid 60s to lower 70s Thursday and Sunday with highs mostly in the 60s Friday and Saturday. Felton && .AVIATION...Upper trough axis over Western Washington this morning will shift eastward with upper ridge rebuilding into the region later today and tonight. Westerly flow aloft will turn northwesterly tonight. The low level flow remains onshore. IFR ceilings along the coast in stratus will extend locally inland this morning. The onshore flow will produce some weak convergence over the interior with a few pockets of MVFR ceilings into midday, but ceilings are expected to be primarily VFR in the 4000 to 6000 foot range this afternoon for interior terminals. KSEA...Upper trough passing through the region combined with some weak low level convergence may produce some tempo high end MVFR ceilings this morning, but confidence in this occurring is not high. More likely scenario for much of the day ahead will be a persistent stratocumulus deck around 4000 to 5000 feet. Surface winds expected to become light S/SW near or after 12Z this morning before gradually backing to W/NW late this afternoon and eventually light N/NE by late this evening. 27 && .MARINE...Late spring type of pattern expected with broad surface ridging remaining centered well offshore with low pressure inland throughout the week ahead for varying degrees of onshore flow. This will lead to periods of small craft advisory strength northwesterlies over the coastal waters as well as small craft advisory strength westerlies in the central/east strait each afternoon and evening. A couple of systems passing to the north of the area late in the week could increase the onshore flow enough to produce westerly gales in the central/east strait. 27 && .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 3 AM PDT Tuesday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory from 3 PM this afternoon to midnight PDT tonight for Admiralty Inlet. && $$