428 FXUS66 KSEW 201109 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 409 AM PDT Sat Oct 20 2018 .SYNOPSIS...Upper level ridge will remain over the area through Monday but weaken with time. Low level flow will remain light through Monday as well. Dissipating system moving through Western Washington Tuesday night with a stronger system arriving Wednesday night and Thursday. && .SHORT TERM...Satellite imagery shows fog has already formed over much of the coast, lower Chehalis Valley, Southwest Interior, Hood Canal and San Juan Islands early this morning with areas of visibilities down to a half mile or less. Clear skies for now over most of the rest of the area. Temperatures at 3 am/10z were in the mid 30s to mid 40s. No change in the conditions this morning with an upper level ridge over the area, light surface gradients and light low level flow ( Sandpoint profiler has winds less than 10 knots up to 2500 feet ). Plenty of low level moisture around combining with light low level flow to produce widespread fog over Western Washington this morning. Visibilities of a half mile or less will be common through the morning hours. Sandpoint profiler also shows low level inversion slightly weaker this morning versus Friday. The layer is shallow with tops 1000 feet or less so expect another midday/early afternoon breakout across the area. High temperatures will be in the 50s and lower 60s. Upper level ridge remaining over the area tonight into Sunday. No changes in the lower levels with light flow and light surface gradients. Low level inversion redeveloping overnight with fog forming once again in the late night hours. Lows will be in the mid 30s to mid 40s. Upper level ridge still over the area Sunday morning but beginning to show signs of weakening. Conditions in the lower levels unchanged with light surface gradients and light low level flow. Expect another round of morning fog with afternoon sunshine. Visibilities below a half mile will be common again Sunday morning. Highs in the 50s and lower 60s. Upper level ridge continuing to weaken Sunday night with the ridge axis moving east of the area. Still no changes in the lower levels resulting in another round of late night fog formation. Lows will be in the mid 30s to mid 40s. Western Washington on the backside of the upper level ridge on Monday with southwesterly flow aloft. Low level flow still light and with the approaching front well offshore surface gradients are light as well. End result, you guessed it, another morning with fog in the area and reduced visibilities followed by sunshine in the afternoon. Highs for the third day in a row in the 50s and lower 60s. .LONG TERM...00z extended model runs continuing the trend of a slower and weaker front with what is left of the system moving through Western Washington Tuesday night. With the trend of slower eastward movement of the front have dry weather in the forecast Tuesday from about the Seattle area eastward and just chance pops over the area for Tuesday night. Some differences in the solutions start showing up on Wednesday with the GFS a few hours faster with the onset of precipitation ahead of the next approaching front. The GFS has precipitation into Western Washington by Wednesday afternoon while the ECMWF gives the area a little longer break with the precipitation not starting until Wednesday night. The flow aloft is consolidated so can't rule out the GFS solution at this point. Wednesday forecast has a chance of rain. Stronger and wetter system arriving by Thursday with rain across the entire area. Still the possibility of some windy conditions along the coast Wednesday night into Thursday. Another system trying to get organized quickly behind the front for Friday. Main message of the extended is we are going back to our regular fall pattern next week. Felton && .AVIATION...Strong upper level high pressure centered over eastern Oregon and central Idaho early this morning, with moderate W-SW flow aloft over Western Washington. Upper ridging over the Pac NW will flatten and start to weaken through Sunday morning, but not enough to break the low-level temperature inversions. Therefore, the air mass will remain very stable, with dry air above a moist, shallow boundary layer. This setup will be conducive for development of late night and morning dense fog both this morning and Sunday morning. On both days, weakness of low-level flow combined with low-level temperature inversions, and somewhat anemic late October insolation will amount to late burnoff of fog, in some cases waiting until after noon. KSEA...Dense fog (<=1/4SM) appears likely for a couple of hours this morning and again on Sunday morning, especially in the hours immediately following sunrise. Slow burnoff expected in the 18z- 20z time frame. North wind will be light, especially during the more critical morning hours (5 knots or less). Haner && .MARINE...Light to moderate onshore flow will continue today. Westerly winds in the strait have subsided allowing for the inherited SCA to expire. Winds are expected to turn northerly and offshore Sunday into Monday. An approaching front on Monday will turn the low level flow southeasterly. Advisory level winds are expected over the coastal waters for a time Tuesday, but the system is forecast to dissipate rapidly. A stronger system will arrive around Wednesday, with advisory level southerly wind expected to focus once again over the coastal waters. SMR && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...None. && $$ www.weather.gov/seattle |