Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 201550 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 850 AM PDT Tue Mar 20 2018 .SYNOPSIS...Today will be mostly dry, with weak high pressure over the area. Moist southwest flow will develop on Wednesday, and rain will develop by Wednesday night. 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...Models show isolated showers developing today, mainly at the north coast, Olympics and north Cascades from a weak system over B.C. Elsewhere, dry and mild conditions are expected with high temperatures in the low to mid 50s - near normal. 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 swing through on 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. 33 .LONG TERM...Previous discussion...The upper trough gets really close on Friday, and 500 mb heights will fall into the 520s. That is cold for early spring. The snow level will fall to around 1000 feet Friday and remain there through Saturday. Showers are likely both days, and there could be snow flurries at times. The cold air aloft will make the atmosphere unstable, so have introduced a slight chance of thunderstorms into the forecast for Friday. Highs on both Friday and Saturday will be in the mid to upper 40s. The upper trough will move east of the area Saturday night, and by Sunday heights will build a bit. Weak impulses moving through the northwest flow aloft will keep a chance of showers in the forecast. High temperatures will rebound a bit, into the low 50s. Burke && .AVIATION...Broad upper level ridge well offshore with light northwest flow aloft over western Washington for the remainder of this morning and into the afternoon. The flow aloft will back to southwesterly by this evening as an upper level trough approaches from the northwest. The air mass is generally dry and stable except for some mid level moisture across the northern half of the area and some areas of low stratus around Puget Sound. The stratus will clear later this morning, but high and level moisture will gradually increase over the next 24 hours as the region comes under the influence of the approaching trough. Cigs are a mixed bag this morning thanks to the aforementioned low stratus around Puget Sound resulting in MVFR to IFR conditions. Of particular note is some fog around OLM reducing visibilities to less than 1/2 SM. Most of the area is expected to recover into VFR conditions by noon today and will see those conditions persist into tonight. SMR KSEA...Low level clouds have moved away from the terminal...but mid to high level clouds are expected for much of the day today...increasing this evening in advance of approaching upper level low. Light and variable wind for the remainder of this morning becoming N-NW less than 8 knots this afternoon. 27/SMR && .MARINE...Weak high pressure will give way to a low pressure system that will drop down over the region after the middle of the week. Stronger southerly winds are likely to develop starting on Thursday, with small craft advisory winds likely at times into the weekend. Models still show a degree of uncertainty in the details...but the quiet weather of the past several days is pretty likely to give way to a more blustery weather pattern typical of spring toward the end of the week. && .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.