Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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614 FXUS66 KSEW 210314 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 815 PM PDT Tue Mar 20 2018 .SYNOPSIS...Moist southwest flow will develop Wednesday, and a cold front will move inland early Thursday. A deep upper level low will make the air mass cool and unstable Thursday through Saturday. The snow level will fall to around 1000 feet Friday and Saturday, and isolated thunderstorms are possible Friday. && .SHORT TERM...Mostly fair skies and light winds tonight with low temps mainly in the 30s. Wet weather will return Wednesday and Thursday as a deep upper level low forms over the NE Pacific, setting up moist S/SW flow over WA. A cold front will shift inland Thursday morning, the focus for steadier precip. This timing is slightly slower compared to previous solutions. A trough axis will then swing through Thursday afternoon keeping the air mass cool and unsettled for additional showers. Temperatures will trend below normal on Thursday with highs in the 40s. Lows will be in the 30s Thursday night. The mountains will see a few inches of snow with snow levels lowering to around 1500 feet. The broad upper level low will remain over the region on Friday for more cool and showery weather. We are still under moist, diffluent flow and there`s a slight chance of thunderstorms across much of Western WA. By this point in time, 500 mb temps will drop to around -38 C, setting up steep lapse rates and a Lifted Index near 0 and showers with some graupel. .LONG TERM...The deep upper level low will finally weaken and shift inland as an open trough on Saturday. Snow levels will remain low during the morning, only a few hundred feet, but showers in the lowlands are light and spotty. Significant snowfall accumulations are not expected. Temperatures will continue to trend below normal with highs mainly in the 40s. Models are now a little wetter on Sunday as an upper level short wave trough clips the area. PoPs were increased accordingly. A warm front will reach the area on Tuesday for more wet weather. The air mass will moderate for slightly warmer conditions and rising snow levels. && .AVIATION...The air is generally dry and stable except for a few patches of clouds--one near KCLM that might finally be breaking up this evening. Elsewhere, there are some high clouds, otherwise fair skies. Moisture over Oregon will move north into the area starting around daybreak Wed and areas of rain should develop by evening as the southerly flow aloft continues to draw moisture up into the area. KSEA...Fair skies tonight and then mid level clouds increase Wednesday as moisture pushes north from Oregon. Rain should develop later in the day. Some periods of rain should arrive Wednesday night. && .MARINE...Weak high pressure will give way to a low pressure system that will drop down over the region Wednesday. Stronger southerly winds are likely to develop starting on Thursday, with small craft advisory winds possible over portions of the area waters into the weekend. && .HYDROLOGY...River flooding is not expected in the next 7 days. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...None. && $$ is the U.S. government's official web portal to all federal, state and local government web resources and services.