Scientific Forecaster Discussion
000 FXUS66 KSEW 220930 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 230 AM PDT Mon Jul 22 2019 .SYNOPSIS...Dry weather will continue into Monday with temperatures slightly above normal. A cold front and upper level trough will bring showers and a chance of thunderstorms in the Cascades on Tuesday. Strong onshore flow will prevail on Wednesday then weaken on Thursday. Mild weather will continue into the weekend. && .SHORT TERM /TODAY THROUGH WEDNESDAY/...Upper level ridging will continue to weaken today as an upper trough over the Northeast Pacific gradually shifts closer to the region. Low level onshore flow has increased over the past 24 hours. This has brought some shallow marine air inland. This combined with lowering heights will knock around 10 degrees off of high temperatures over the interior compared to yesterday. The main area of marine stratus is still offshore early this morning, but should spread onto the immediate coastline later this morning. For the remainder of the area, it will be another sunny day ahead. Onshore flow increases tonight and Tuesday as the upper trough offshore edges closer. This should initiate a solid marine push tonight with stratus reaching most of the lowlands by daybreak Tuesday. Temperatures will cool further on Tuesday with the stubborn marine layer likely only yielding some partial afternoon clearing over the interior lowlands. Increasingly moist and unstable southerly flow aloft ahead of trough will produce a slight chance of thunderstorms over the Cascades Tuesday afternoon and evening. The upper trough will quickly exit to the east on Wednesday with a flat upper ridge moving over the area. Any shower threat should end by Wednesday morning. Highs will be near normal on Wednesday. .LONG TERM /THURSDAY THROUGH SUNDAY/...Low amplitude ridging will produce dry conditions and slightly above average temperatures Thursday into Friday with persistent low level onshore flow. The models are advertising a weak trough passing mainly to our north on Saturday. Apart from a chance of showers in the North Cascades, it looks to do little more than deepen the marine layer and cool temperatures a bit. Ridging weakly rebuilds for the later half of the weekend for perhaps a slight warmup and continued dry weather. 27 && .AVIATION...An upper trough over the Northeast Pacific will slowly shift east today with southwest flow aloft for W WA. At the surface, onshore flow will continue to increase through the day. The air mass remains dry and stable. Current IR satellite shows some high clouds over portions of the area. With onshore flow expected to increase...low level clouds are expected to push inland at least as far as coastal obs sites this morning...perhaps a bit further...but given that these clouds have failed to push inland in as far as said coastal sites...suspect this push may not be as far inland as previous shift thought. VFR conditions in place for most of the area and that will likely persist with coastal locations such as HQM likely seeing MVFR to IFR conditions develop at some point with the aforementioned low clouds. Any cloud cover that sets up there will likely recede by noon. Stronger onshore flow tonight should bring about better penetration of marine low level clouds by Tuesday morning. KSEA...Mostly clear today. Light and variable winds this morning will become more southwesterly this afternoon with speeds 5-10 knots before becoming more northerly tonight. SMR && .MARINE...Onshore flow will increase today and then continue for the remainder of the week with high pressure offshore and lower pressure inland. Small craft advisory west winds are likely each day for the Central and Eastern Strait of Juan de Fuca with winds peaking each evening. In accordance with this expectation...have hoisted another SCA for this area...as well as the adjacent northern inland waters and Admiralty Inlet...starting this afternoon and extending into Tuesday. Models remain consistent that gales will be possible in the strait Tuesday..but timing falls just outside third forecast period. THus...will leave any needed headlines for that time frame to future shifts. SMR && .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... WA...None. PZ...Small Craft Advisory until 5 AM PDT early this morning for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory from noon today to 5 PM 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 5 PM this afternoon to 11 PM PDT this evening for Admiralty Inlet-Northern Inland Waters Including The San Juan Islands.