Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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952 FXUS66 KSEW 182129 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 230 PM PDT Sun Apr 18 2021 .SYNOPSIS...Onshore flow will trigger a gradual cooling trend starting Monday. As an upper level ridge remains in place, dry conditions will persist although systems passing near the area will allow for an increase in clouds over the first half of the week. A pattern change is still expected for the end of the week with a return to cooler and wetter conditions. && .SHORT TERM /TONIGHT THROUGH WEDNESDAY/...Clear skies remain in place over much of W WA this afternoon. Temps around the area not deviating too much from where they were 24 hours ago with some locations running a degree or two cooler, others a degree or two warmer. Coastal locations proving to be an exception though running anywhere from 9 to 18 degrees cooler thanks to the change to onshore flow. Not seeing much of an eastward push just yet, but that will be discussed in a bit. Upper level ridging remains the main headline for the short term period, keeping dry conditions in place. The aforementioned transition to onshore flow at the surface will result in a more widespread impact on temperatures Monday as temps fall to around 60 along the coast and into the upper 60s to around 70 for the interior lowlands. A shortwave trough evolves into a small closed low Monday night into Tuesday but is not expected to make much of an impact to W WA given both its sharp SE trajectory taking it south of the area as well as the fact there is very little moisture associated with it. The largest expected impact will be an increase in cloud cover over the western half of the CWA into Tuesday morning. Upper level ridging rebuilds over the nearshore coastal waters, clearing these clouds out Tuesday afternoon although the gradual cooling will remain in place as temps will be be a degree or two cooler than Monday. This ridge stays in place Wednesday, however deterministic models show an interesting feature...although the details differ. The GFS points to a weak upper level low coming down the Pacific ridge just off the WA coast but no real moisture with it while the ECMWF shows a similar feature but this passing to the north of the area with potential impacts east of the Cascades. Ensembles register a bit of a blip to represent the prospects of these solutions however, proving them to be more outliers and not really warranting inclusion in the forecast at this time. 18 .LONG TERM /THURSDAY THROUGH SATURDAY/...Models seem to have backed off on the timing for the general pattern change over W WA a little bit. Deterministic models hinted at this 24 hours ago and ensembles look to be following that trend for now, keeping upper level riding in place for Thursday. The ridge finally starts to break down Thursday evening as a system moving south over the ridge may bring some showers to the northern two-thirds of the Cascades. The ridge continues to diminish and move eastward throughout the day Friday, setting W WA up for an upper level low to bring rain back to the area for the upcoming weekend. Temps during this time will continue to cool, with mid 60s expected for the interior lowlands...upper 50s along the coast...for both Thursday and Friday and further cooling on Saturday...mid 50s along the coast, only around 60 for the interior lowlands. 18 && .AVIATION...Increasing northwesterly flow this afternoon, persisting through the period. VFR through the period at all TAF sites, with the exception of KHQM where MVFR to IFR stratus will build in over the terminal later this evening through Monday morning. CIGs generally between 500 and 1000 feet here in this time period. Prevailing winds generally AOB 10 kts at all sites during the day Monday. KSEA...VFR through the period. Northwesterly 4 to 7 kts through the remainder of the afternoon, becoming northerly this evening. Winds will become southeasterly between 12 and 15Z Monday morning, with speeds remaining below 10 kts. Pullin && .MARINE...With the exception of spotty advisory level winds across the Northern Inland waters and in the west entrance and adjacent near-shore waters this evening through Monday morning, quiet weather and calm seas will prevail through much of the week. Predominantly offshore flow will transition to onshore late Monday into Tuesday and will persist through the end of the period. Pullin && .HYDROLOGY...No river flooding expected over the next 7 days. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Small Craft Advisory from 11 PM this evening to 11 AM PDT Monday for Coastal Waters From Cape Flattery To James Island Out 10 Nm-West Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory from 8 PM this evening to 11 AM PDT Monday for Northern Inland Waters Including The San Juan Islands. && $$ www.weather.gov/seattle

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