Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 190432 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 930 PM PDT Sun Mar 18 2018 .SYNOPSIS...An offshore upper low will bring spotty showers to the mountains and coast through Monday, mainly during the afternoons and evenings. The interior lowlands should have dry weather. A weak ridge will be over the region Tuesday for a mostly sunny day. Rain will return Wednesday and Thursday as a system moves through, with cool showery weather Friday into the weekend. && .SHORT TERM...Radar is still picking up a few isolated showers around the area but activity will dissipate shortly with loss of diurnal heating. An upper low off the Wa/Oregon coasts will shift westward and dissipate on Monday. The region will stay between systems through Tuesday. Expect mostly dry weather, except for a few isolated diurnal driven showers, mainly over the mountains. A more significant change in the pattern develops around Wednesday. Models have not been very consistent on the timing and strength of a developing trough and frontal system. The GFS is deeper and more negative tilt while the ECMWF is further north with the low. Both solutions bring rain or showers into the region Wednesday. Most of the uncertainty is in regards to surface low development and track. The pattern favors some surface low development but the latest GFS/ECMWF do not show much development. No updates planned this evening. Mercer .LONG TERM...Previous discussion...The frontal system that moves into Western Washington from the southwest Wednesday or Wednesday night will keep rainy weather across the area on Thursday. As mentioned above and in previous discussions, the models are not in strong agreement with the details of this system, and that applies to Thursday as well. One change we`ve seen in the models is that they`re now showing a bit more surface development, and breezy to windy conditions are likely on Thursday. The upper trough following the frontal system should bring cool showery weather to the area Friday through Saturday. The snow level could fall to around 1000 to 1500 ft, so showery precipitation could fall as snow over the higher lowland areas though accumulations are really unlikely. The upper trough will probably start moving off to the east on Sunday, for decreasing showers and more sun breaks. McDonnal && .AVIATION...Upper level low pressure will shift westward and dissipate Monday. Light southerly flow aloft will become light easterly overnight. Latest satellite imagery shows some areas of mid cloud developing. This could inhibit much MVFR stratus from forming Monday morning. Mostly VFR skies by late Monday morning. KSEA...Mid clouds. A slight risk of MVFR from 12-16z. Winds will generally be light and variable. && .MARINE... Light onshore or westerly flow will continue through Tuesday. The flow will become nearly flat on Wednesday. Expect a frontal system to impact the waters on Thursday for the possibility of gale force winds over parts of the area. The weather models were not in agreement on the track and strength of Thursday`s surface Low; therefore, the forecast reflects a compromise between the model solutions. && .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.