Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 171753 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 953 AM PST Sat Feb 17 2018 .SYNOPSIS...
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Windy conditions and heavy mountain snow are expected today as a deepening surface low tracks southeast past Vancouver BC and across northern Washington. A modified arctic cold front will move southward through Western Washington early Sunday, bringing the possibility of lowland snow and strong north winds. Dry conditions are expected from late Sunday through Tuesday with cold temperatures remaining in place. Another system could slide in from the northwest around next Wednesday or Thursday.
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&& .SHORT TERM...
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Active weather is underway, with the main impacts today coming in the form of strong wind and heavy mountain snow. An elongated 998 mb low is currently located squarely over central or northern Vancouver Island, moving southeast-east. It is expected to make its closest approach to Western Washington as it passes Vancouver B.C. later today as a 995 mb low. Interesting to see a southeast moving low with a baroclinic leaf signature on satellite imagery, which is a classic signature of intensification. The PDX-BLI gradient is expected to peak near +12 mb early this afternoon, HQM-SEA around +4.5 mb, and UIL-BLI around +4.5 mb. Southwest winds are already picking up and will peak in most areas early this afternoon, though westerlies down the Strait will peak later this afternoon. Current coverage of Wind Advisories looks good, with those covered areas looking for gusts of 45+ mph. Expect scattered power outages and minor tree damage. Mountain snow is the other big deal today. Heavy snow is already underway, with snowfall rates of 1-2 inches per hour being observed over the Cascades. WSDOT reports I-90 closed currently eastbound across Snoqualmie Pass. The latest 24-hour snowfall forecasts (ending at 4 AM Sunday) based on 12z model data are for 19 inches at Snoqualmie Pass and 25 inches at Stevens. With heavy snow and strong wind in the mountains, NWAC has issued an Avalanche Warning for the Cascades today. Pack your patience and an emergency supply kit if traveling through the mountains, and stay in bounds if recreating. The fun does not end today. In the wake of today`s low center, a a continental polar air mass will quickly spread south through interior B.C. and come pouring out of the Fraser later tonight. The BLI-CYWL gradient is forecast to peak near -20 mb on Sunday morning, making this a strong Fraser Outflow event. Strong north- northeast winds will ensue, most notably near Bellingham and the San Juans, but other places will be breezy as well. The uncommonly breezy wind for the normally less breezy north direction will have greater tree impacts than a similar south wind. A modified arctic front will spread south and southwest through Western Washington on Sunday morning, with localized frontal convergence and orographic lift leading to some lowland snow accumulations. The average forecast snowfall up and down the I-5 corridor is about 1 inch. In reality, some places, especially in the Seattle metro area will see little or none, while other places could see a couple of inches. The front will be followed by the coldest air mass of this winter. A hard freeze is expected areawide on Sunday night, with Monday night likely being the coldest night of the upcoming cold spell. At least the cold will come with drier weather. The last showers will taper off over the Cascades on Sunday night, with dry weather expected Monday and Monday night. Haner .LONG TERM...From Previous Discussion: No major changes to the inherited forecast. There were some difference in the details concerning the medium range solutions. The operational ECMWF indicated an upper level low dropping into the area from the north about the middle of next week while the GFS kept us more under the influence of the offshore ridge. Beyond this period, the models began to diverge even more. At any rate, it looks, for the most part, temps and precip will be below normal during this period.
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&& .AVIATION...
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A vigorous frontal system is moving through the region this morning. The associated low pressure center is over Vancouver Island this morning. Rain and areas of fairly low cigs will give way to showers as the front moves through the I-5 corridor by midday. It will be windy with strong onshore flow that eases overnight quickly then picks back up as it turns to a north wind. KSEA...Rainy at times this morning then turning to scattered showers by midday with some clearing periods in rather blustery weather. Timing the modified arctic front and the switch to northerly winds Sunday morning was currently timed for 17z Sunday, but obviously we will look at the mesoscale models closely and that could be off by a few hours. Also, the snow level will be falling to Sea-Level Sunday morning, so we will look to the mesoscale models for guidance on that as well--it would not be unusual for an inch or two of snow to fall quickly as the modified arctic front moves through the metro area Sunday morning. Currently the TAF shows no precip, but a tempo SHSN for a couple hours seems like a good idea for the 18z TAF with that switch to north winds.
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&& .MARINE...
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A 995mb low will move to near the city of Vancouver this afternoon. As the vigorous frontal system moves inland and the low pressure falls with it, Washington state will be left with residual slowly weakening low pressure on Sunday, with cold high pressure moving south out of B.C. and forcing the residual low pressure southward. The modified arctic front will be moving south through Western Washington and the blustery southerly and westerly gales today will be giving way to Fraser outflow northeasterly gales before daybreak Sunday--and probably spreading down into the Port Townsend to Port Angeles area pretty quickly. The switch to northerly winds in Puget Sound will probably be around mid morning and Sunday will be a raw brisk day.
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&& .HYDROLOGY...River flooding is not expected the next 7 days. && .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...Wind Advisory until 6 PM PST this evening for Bellevue and Vicinity-Bremerton and Vicinity-Central Coast-Hood Canal Area-North Coast-Seattle and Vicinity-Southwest Interior- Tacoma Area. Winter Storm Warning until 6 AM PST Sunday for Cascades of Pierce and Lewis Counties-Cascades of Snohomish and King Counties-Cascades of Whatcom and Skagit Counties. Wind Advisory from midnight tonight to 6 PM PST Sunday for Western Skagit County-Western Whatcom County. Winter Weather Advisory until 10 AM PST Sunday for Olympics. Wind Advisory until 6 PM PST Sunday for San Juan County. Winter Storm Watch from this evening through Sunday morning for Eastern Strait of Juan de Fuca. Wind Advisory until 6 PM PST this evening for Admiralty Inlet Area-Eastern Strait of Juan de Fuca. PZ...Small Craft Advisory for rough bar until 4 PM PST Sunday for Grays Harbor Bar. Gale Warning until 3 AM PST Sunday 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. Gale Warning until 4 PM PST Sunday 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. Gale Warning until 4 PM PST Sunday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters Including The San Juan Islands. Gale Warning until 3 AM PST Sunday for Admiralty Inlet-Puget Sound and Hood Canal-West Entrance U.S. Waters Strait Of Juan De Fuca.
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&& $$ www.weather.gov/seattle

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