Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 191120 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 320 AM PST Mon Feb 19 2018 .SYNOPSIS...
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Dry and cold northerly flow aloft will prevail over the region through tonight. A weak disturbance will bring some light precipitation to mainly the southwest part of the state on Tuesday. Another system will follow late Wednesday into Thursday. Temperatures will remain below normal.
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&& .SHORT TERM...
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A few light snow showers or flurries lingered over the interior early this morning. This activity should end by midmorning. The presence of clouds and/or wind kept temperatures from falling as much as they would have if there were clear skies and light winds. Temps at this time were in the lower 20s to lower 30s across the lowland and mainly in the single digits and lower teens in the mountains. Meanwhile, look for strong, dry, and cold northerly flow aloft to prevail over the Pacific Northwest through tonight in the wake of the strong system that moved across the region over the weekend. Overall, highs today should be about the same or a few degrees colder than yesterday. Min temps Tuesday morning should be colder at most places than this morning due to less wind and clouds. Have undercut model guidance temps in those areas where significant snowfall fell and still remains on the ground. Expect a weak disturbance to bring a threat of light precipitation to mainly the southern part of the CWA on Tuesday. The GFS had a farther west track with this system than the ECMWF, so the forecast reflects a compromise between the two solutions. Not expecting any significant snowfall from this feature. Amounts will be less than an inch in most areas. The exception is Lewis County, where amounts of up to 3 inches are possible. Another low pressure system will approach from the north during the day Wednesday before moving over the CWA late Wednesday night. Have chosen to follow the ECMWF solution in regards to this feature since it has been the most consistent of the models. Essentially, this model is a little stronger than the others and developed a closed 532 dam low over southern mainland British Columbia Wednesday night. At this time, it looks like precip amounts will be light. .LONG TERM... The Wednesday night upper level low will move southwest across the area on Thursday. Will need to keep an eye on this system because there is a chance that the precip amounts could end up higher than forecast, especially if the low`s trajectory ends up slightly farther west. Beyond Thursday, it looks like the region will remain under northerly flow aloft due to the ridge axis remaining more or less near 150W. This will keep the door open for systems in western Canada to drop in the CWA during this period. Will need to keep an eye on the end of the week system as it could bring the potential for significant snowfall to parts of the lowlands. Otherwise, the main message for this period is below normal temps and near or slightly below normal precipitation.
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&& .AVIATION...
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Northerly flow aloft continues across western Washington today as a broad upper level trough advances into the Intermountain West. Air mass is generally dry except for some areas of stratocumulus generated by northerly flow creating some upslope flow on terrain. This will gradually erode from the north with time. VFR conditions are expected to continue. Some high level moisture will arrive late today as a weak system arrives over the offshore waters. Northerly surface gradients will ease through the day bringing gusty northerly winds around the region to an end. KSEA...Stubborn mid level deck of clouds created by north- northwesterly flow upsloping on terrain around the region shows signs of eroding from the north through the morning. Gusty northerly surface winds will ease through the today as gradients relax. Surface winds will switch to light southeasterly tonight as a weak system approaching offshore spreads some high clouds into the area. VFR conditions to persist through the forecast period. 27
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&& .MARINE...
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Fraser outflow winds are slowly starting to ease. Gales still lingering over portions of the Central and Eastern Strait as well as the Northern Inland Waters...but only just barely. Timing of inherited warning looks good with an expiration of 4 AM PST. Have introduced a Small Craft Advisory as a follow-up in those areas as conditions will be slow to calm this morning...but generally looking to be headline free in these areas at some point in the afternoon. Coastal waters are similar in that have extended SCAs there into the late morning again with the expectation that conditions will fall below criteria by noon today. Also seeing wind speeds above SCA criteria lingering around Puget Sound this morning...warranting an extension there as well. Lastly...bar conditions are finally expected to fall into moderate conditions...thus allowing for the SCA there to expire soon after the early morning forecast package is sent. A series of weather systems will drop down out of the northwest Tue- Sat so there will probably be about three periods where fairly light southerly winds develop, and then a pretty decent push of NW winds behind each front comes through the region--especially later in the week when the systems seem be be successively stronger. SMR
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&& .HYDROLOGY... River flooding is not expected the next 7 days. && .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...None. PZ...Small Craft Advisory until 10 AM PST this morning for Coastal Waters From Cape Flattery To James Island 10 To 60 Nm- Coastal Waters From Cape Flattery To James Island Out 10 Nm- Coastal Waters From James Island To Point Grenville 10 To 60 Nm-Coastal Waters From James Island To Point Grenville Out 10 Nm-Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm-Puget Sound and Hood Canal. Small Craft Advisory until 4 PM PST this afternoon for East Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters Including The San Juan Islands. Small Craft Advisory until 10 AM PST this morning for Central U.S. Waters Strait Of Juan De Fuca.
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&& $$ www.weather.gov/seattle

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