Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
000 FXUS66 KSEW 202144 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 244 PM PDT Tue Mar 20 2018 .SYNOPSIS...Expect mainly dry conditions tonight with increasing clouds late. Moist southwest flow will develop on 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...Isolated showers may develop along the coast, Olympics and north Cascades tonight as an upper level low starts to dig offshore. Elsewhere, dry conditions are expected with low temperatures 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. Small hail is possible too. 33 .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. 33 && .AVIATION...Broad upper level ridge slowly making its way over the area this afternoon and evening keeping light northwest flow aloft in place over western Washington into early this evening before shifting to the southwest tonight as an upper level trough approaches from the northwest. The air mass is generally dry and stable except for some mid level moisture that may result in some scattered flat cumulus this afternoon and evening. High and mid level moisture will gradually increase over the next 24 hours as the region comes under the influence of the approaching trough. Cigs generally VFR area-wide...with some isolated exceptions that continue to burn off low level moisture from the morning. Although high and mid level moisture is expected to increase this evening and tonight...conditions are expected to remain widespread VFR into Wednesday. SMR KSEA...FEW to SCT mid and high level clouds this afternoon and evening before increasing this evening in advance of approaching upper level low. Cigs still expected to be VFR. N-NW winds this evening and tonight before becoming more easterly early Wednesday morning. Speeds expected to remain less than 8 knots for the TAF forecast period. SMR && .MARINE...Weak high pressure continues to influence the coastal waters as it makes its way eastward this afternoon and evening. This 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. Overall weather pattern will transition to a more blustery weather pattern typical of spring mid-week and beyond. && .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.