FXUS66 KSEW 122248

Area Forecast Discussion
National Weather Service Seattle WA
248 PM PST Thu Dec 12 2019

SYNOPSIS...A frontal system will move east of the area tonight
with cool and unsettled weather Friday and Saturday. A weak warm
front will result in some areas of light rain on Sunday. Upper
level ridging will likely support a generally dry start to next
week followed by a return to potentially active weather later in
the week.


associated with the front is working into the interior and coast
as of 22Z. This will shift north and east through the afternoon
and evening hours. The main impact will be a period of heavier
snowfall with the front the mountains this afternoon and evening
with significant accumulations above 3500 feet. Snow showers will
continue behind the front through Friday morning tapering off
through the afternoon hours. Total snow accumulations of 12 to 18
inches above 3500 feet through Friday morning with a few higher
amounts in some locations. The winter storm warning remains in
effect through 10 AM.

There continues to be enough instability for the potential for
thunderstorms, mainly along the coast through the evening hours.
Otherwise expect scattered showers and limited rainfall over the
lowlands into Friday morning behind the front. Shower activity
will decrease further Friday afternoon with highs in the mid to
upper 40s. The upper level trough trailing the front will linger
over the area Friday night into Saturday keeping showers over the
area. Lows in the upper 30s and 40s. Highs in the mid and upper

continue to show an upper level ridge building over the eastern
Pacific Sunday with some warm advection over Western Washington
which may result in some light rain, mainly during the day.
Guidance continues to show the upper level ridge shifting onshore
Sunday night into Monday and lingering at least through the early
part of next week. This is pretty solidly reinforced by the
ensembles with uncertainty regarding the timing of the transition
and the strength of the ridge as it shifts onshore. The
uncertainty associated with the strength of the ridge is
significant since it highlights the potential for a few, weak
systems passing over the ridge to bring at least a chance of
precipitation to Western Washington at times in the extended
period. If this occurs it is more likely to be little more than
these systems brushing the area as they pass to the north.
Ensembles do show the large scale pattern shifting later in the
week as the upper level ridge shifting eastward possibly setting
the stage for a bit more active weather.


.AVIATION...Westerly flow aloft continues with a broad upper trough
over the northeast Pacific. The air mass is moist and somewhat
unstable. A frontal boundary reached the coast around 21Z
and will be east of the Cascades by late this evening. A mix of VFR
and MVFR in scattered showers will continue into the overnight
hours. Gusty S-SW surface winds through the Puget Sound area
terminals will persist through the evening hours then gradually
ease overnight. With lighter surface winds and low level moisture
still in place, MVFR ceilings could be more prevalent across the
region by daybreak Friday.

KSEA...MVFR possible in and around heavier showers this evening.
MVFR ceilings could become more persistent by Friday morning as
the air mass stabilizes and surface winds ease. Surface winds S-SW
10 to 15 knots gusting 20-25 knots this evening then easing to 10
knots or less by 09Z-12Z.  27


.MARINE...Small craft advisory winds easing overnight over the
coastal waters in the wake of today`s frontal passage. Coastal seas
will remain in the double digits into Friday and Saturday for a
continuation of small craft advisory hazardous seas criteria as well
as high surf at the coastal beaches. Gusty post-frontal
southwesterlies will necessitate the small craft advisory for Puget
Sound and Hood Canal to extend until around midnight.

A weak surface ridge will build over the coastal waters and persist
into the weekend for a transition to quieter conditions across area
waters. Low level offshore flow is expected to develop early next
week as surface ridging builds east of the Cascades.    27


HYDROLOGY...There are no flooding concerns at this
time. Relatively low snow levels and limited precipitation
amounts will result in little or no impacts on area rivers.


WA...Winter Storm Warning until 10 AM PST Friday for Cascades of
     Pierce and Lewis Counties-Cascades of Snohomish and King
     Counties-Cascades of Whatcom and Skagit Counties.

     High Surf Advisory until 10 AM PST Friday for Central Coast-
     North Coast.

     Winter Weather Advisory until 9 PM PST this evening for Olympics.

PZ...Small Craft Advisory until 10 AM PST Saturday 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-Grays Harbor Bar-West Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory until midnight PST tonight for Puget Sound
     and Hood Canal.




NWS SEW Office Area Forecast Discussion

Discussion Script by Ken True at Saratoga Weather - WFO Script by SE Lincoln Weather