Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
Issued by NWS Seattle/Tacoma, WA
411 FXUS66 KSEW 210341 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 841 PM PDT Mon May 20 2024 .SYNOPSIS...Upper-level ridging has produced warmer and drier conditions today. But, troughing will arrive on Tuesday and Wednesday with a return of cooler and wetter weather. A brief dry break is possible on Thursday, before cool and unsettled conditions return Friday and may persist into the start of the weekend. && .SHORT TERM /TONIGHT THROUGH THURSDAY/...Forecast remains on track this evening with dry conditions but increasing mid clouds ahead of the next weather system. See the remainder of the previous short term discussion below. Clouds will increase ahead of the next disturbance bringing widespread rain and cooler temperatures into Tuesday. As mentioned, conditions are expected to deteriorate as early as the wee hours of the morning on Tuesday for the coast before spreading inland. By the morning rush hour, an upper-trough is expected to bring widespread rain across much of western Washington. This disturbance appears to be rather impressive for mid-late May. QPF forecasts indicate a half inch or more for much of the lowlands and 1 to 2 inches for both the Cascades and Olympics (including the coast) through Tuesday night. High temps are forecasted to be upwards of 10-12 F below average with values in the mid to upper 50s. Lingering showers with 4,000 ft snow levels are in the forecast for Wednesday as guidance hints at the upper-low centered directly over south- central WA and north- central OR. We`ll warm up slightly but still running cooler than normal with highs in the upper 50s to lower 60s. Overnight lows are to fall into the 40s for both nights. Thursday will be a bit of a transition day with transient ridging sliding overhead. We`ll see a brief return of drier conditions along with near average temperatures. However, it won`t be long lasting as unsettled conditions return soon after. .LONG TERM /FRIDAY THROUGH MONDAY/...The next upper-low looks to arrive on Friday with another round of precipitation of cooler conditions. Chance PoPs and below normal temperatures continue into the weekend as the region remains under the influence of troughing aloft. Ensemble guidance hinting at a possible shift in the pattern early next week with upper ridging rebounding over the Intermountain West and upper troughing positioned over the Gulf of Alaska and NE Pacific. McMillian && .AVIATION...North-northwesterly flow aloft as an upper level trough exits the region and a weak positively tilted ridge builds in its wake. VFR conditions across the region with broken cloud cover along the coast and over the Cascades. Expect VFR conditions to continue through the evening. A frontal system will approach from the northwest, bringing with it rain showers and MVFR ceilings area- wide. Widespread rain will approach the coast early this evening, spreading inland through the overnight hours. Some local IFR conditions may be possible, in particular along the coast. Onshore/westerly flow will continue through this evening, becoming more south/southwesterly tonight as the system approaches. Winds will pick up tomorrow, with gusty winds possible, in particular through the interior terminals. KSEA...VFR conditions this afternoon with scattered cloud cover. VFR conditions continue through early this evening with increasing clouds. MVFR conditions and rain showers beginning around 12Z Tuesday and persisting through the day tomorrow. Westerly winds becoming southerly this evening around 4 to 8 kt, increasing 10 to 15 kt tomorrow, with gusts to 25 kt possible as the system passes through the region. LH && .AVIATION...North-northwesterly flow aloft as a weak positively tilted ridge will be pushed out by an incoming shortwave trough. VFR conditions across the region with increasing clouds from the northwest ahead of the next frontal system. A few radar echos are already visible near Vancouver Island. Expect VFR conditions to continue through the evening. A frontal system will approach from the northwest, bringing with it rain showers and MVFR ceilings area- wide. Widespread rain will approach the coast early tonight, spreading inland through the overnight hours. Some local IFR conditions may be possible, particularly along the coast. Some visibility restrictions will be possible at times in heavier showers. Onshore/westerly flow will continue through this evening, becoming more south/southwesterly tonight as the system approaches. Winds will pick up tomorrow, with gusty winds possible, in particular through the interior terminals. KSEA...VFR conditions this afternoon with scattered cloud cover. VFR conditions continue through early this evening with increasing clouds. MVFR conditions and rain showers beginning around 12Z Tuesday and persisting through the day tomorrow. Westerly winds becoming southerly this evening around 4 to 8 kt, increasing 10 to 15 kt tomorrow, with gusts to 25 kt possible as the system passes through the region. LH && .MARINE...High pressure over the NE Pacific will maintain onshore flow across western WA this week. Very typical to the pattern, the strongest winds will continue through the Strait of Juan de Fuca this evening; the Small Craft Advisories will continue through early tomorrow morning. A frontal system will cross the waters on Tuesday which will boost winds and seas, likely warranting additional headlines. Combined seas over the coastal waters are expected to rise to near 15 feet by Wednesday especially over the northern coastal zones. Waves will be steep with short period waves dominating. Another frontal system may pass through area waters on Friday. KC/LH && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Small Craft Advisory until 5 AM PDT Tuesday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca. && $$