Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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032 FXUS66 KSEW 212128 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 228 PM PDT Wed Mar 21 2018 .SYNOPSIS...Moist southwest flow aloft will continue tonight, 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. A ridge will build inland early next week for warmer and drier weather. && .SHORT TERM...A pattern change will bring wetter and cooler weather to the region tonight through the weekend. A deep upper level low off the B.C. coast will continue to dig offshore tonight, setting up moist S/SW flow over Western WA. Rain will increase late tonight into Thursday morning as a cold front moves inland. Strong onshore flow will develop behind the front. Showers will continue through Thursday afternoon as an upper level trough axis swings through. The mountains will see a few inches of snow with snow levels lowering to around 1500 feet Thursday night. 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 -37 C, setting up steep lapse rates and a Lifted Index near 0. Small hail is possible too. 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. 33 .LONG TERM...An upper level ridge will develop over the NE Pacific on Sunday for NW flow over Western WA. A weak passing upper level disturbance may trigger scattered showers. The upper level ridge will start to drift east and inland on Monday then stall over the region through early next week. This sets up a drying trend with temperatures rising a few degrees above normal. The ridge may flatten toward the end of next week from a passing trough over western Canada. 33 && .AVIATION...High and mid level moisture continues to increase this afternoon as an upper level trough approaches the region from the northwest. Rain is still expected to spread into the area from the southwest later this evening...with current radar putting the leading edge of this activity along the WA/OR border at the time of this writing. Once started...rain is expected to continue well into Thursday as a front starts to move through the area. VFR conditions in place over the entire CWA and will remain there into this evening. Although cigs will gradually lower due to increasing cloud cover...MVFR conditions may hold off until early Thursday morning. As the aforementioned front starts to move through the area Thursday...expect winds to increase with some gusts...especially Thursday afternoon and evening...and generally MVFR conditions. SMR KSEA...Above discussion applies. Cigs likely to remain VFR into early to mid morning Thursday...falling to MVFR between 12-15Z. Surface winds light and variable for the remainder of the afternoon with speeds generally below 6 kt. Developing offshore gradients ahead of approaching system will likely bring some east winds 5 to 10 knots after 03Z tonight. Winds expected to turn southerly 10-15 kt by noon Thursday with gusts possible. 27/SMR && .MARINE...Exiting high pressure will give way to a low pressure system that will drop down over the region through Thursday. Stronger southerly winds are likely to develop Thursday and small craft advisory winds expected over the coastal waters and west strait starting Thursday morning and will have headline in place for afternoon package. Small craft winds are also possible over portions of the area waters...such as the central and eastern strait as well as the northern inland waters Thursday evening...but any potential headlines can wait for additional analysis of new model data. && .HYDROLOGY...River flooding is not expected in the next 7 days. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Small Craft Advisory from 5 AM Thursday to 11 AM PDT Friday for Coastal Waters From Cape Flattery To James Island 10 To 60 Nm-Coastal Waters From James Island To Point Grenville 10 To 60 Nm-Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm. Small Craft Advisory from 11 AM Thursday to 11 AM PDT Friday for Coastal Waters From Cape Flattery To James Island Out 10 Nm- Coastal Waters From James Island To Point Grenville Out 10 Nm-Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm. Small Craft Advisory from 11 AM to 11 PM PDT Thursday for West Entrance U.S. Waters Strait Of Juan De Fuca. && $$ is the U.S. government's official web portal to all federal, state and local government web resources and services.