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fxus66 ksew 171029 
afdsew

Area forecast discussion
National Weather Service Seattle Washington
229 am PST Fri Jan 17 2020

Synopsis...active weather will continue over the next week. The
next system will arrive late tonight with lowland rain and
mountain snow. Cold air trapped in Whatcom County could hang on
long enough for the precipitation to start as snow. A more typical
January pattern with periods of lowland rain and mountain snow
will continue well into next week.

&&

Short term /today through Sunday/...upper trough axis moving
onshore over western Washington early this morning with a few snow
showers. The showers should dissipate later this morning as weak
shortwave ridging moves over the area in advance of the next
approaching frontal system. High and mid level clouds will be
increasing across the area by later this morning, but steady rain
with the next system should generally hold off until late
afternoon along the coast. The rain will spread inland across the
interior tonight. Stubborn cold air remains entrenched over
western Whatcom and this could lead to the precipitation briefly
starting as snow before southeast winds arrive to boost
temperatures.

As the surface low associated with this system lifts northeastward
across the offshore waters late tonight, it should tighten the
gradient for windy conditions with southeast exposure in the north
interior, Admiralty Inlet, and the San Juans. The high wind watch
has been converted to a windy advisory for those area where gusts
of 40 to 50 mph are possible. A Winter Storm Watch remains in
effect for the Olympics and Cascades tonight and Saturday where 8
to 12 inches of snow could fall at the passes with higher amounts
in upper elevations. The front will dissipate over western
Washington midday Saturday with showers gradually decreasing as
weak upper ridging builds south of the region.

The ridge gains amplitude as it builds east of the Cascades and
a warm front brushes western Washington on Sunday. This will give
temperatures a further boost with some spots breaking into the
lower 50s. The building ridge to our east on Sunday slows the
forward progression of the next approaching system and may well
give areas east of the olympic peninsula a short breather from
precipitation for much of Sunday into Sunday night.

Long term /Monday through Thursday/...another frontal system
arrives on Monday, but loses steam as it bumps up against the
ridge over the intermountain west. This should generally produce
only light amounts of precipitation over western Washington. A
potentially more potent system looks to arrive in the Monday night
or Tuesday time frame. Operational runs of both the GFS and Euro
as well as their ensembles continue to lend low confidence in the
forecast details with respect to timing and strength of additional
systems Wednesday and beyond. 27



&&

Aviation...northwesterly flow aloft this morning becoming more
westerly by this afternoon as a frontal system approaches the area.
Air mass stable as weak upper level ridge quickly passes through West
Washington this morning and afternoon in advance of aforementioned front.

Cigs over the area this early am are generally VFR to MVFR for most
of the area...the exceptions being the usually fog prone areas
within the SW interior and some low cigs being reported at Port
Townsend. Incoming ridge is anything but clean...with plenty of
lingering moisture around. As such...these VFR to MVFR conditions
likely to remain in place...although some periods of brief
improvement will certainly be possible throughout the morning hours.
Mid and high level clouds this afternoon with cigs around 5000 ft
ahead of the front arriving this evening. Current models suggest
cigs with the front generally being VFR but would not rule out
precip bringing periods of MVFR conditions. Of course...terminals
more prone to lower cigs will likely dip down into IFR during this
time.

Ksea...MVFR conditions in place...but some hints in both obs and
satellite trends that improvement to VFR might be possible by mid-
morning. Prev discussion suggested possibility of snow showers...but
looking at current trends...now leaning toward pops below tempo
criteria and may merit removing in 12z taf issuance. Mid and high
level clouds this afternoon with ceilings lowering this evening.
Southerly winds 5-10 kts becoming light southeasterly this
afternoon and evening. 18

&&

Marine...a strong frontal system will move through the
area Friday night and Saturday bringing the potential for another
round of gales to most area waters. Models have remained consistent
with this over the past 24 hours...so will go ahead and upgrade the
current gale watch into a Gale Warning for most waters with the
morning forecast package. The only waters not meeting gale
criteria...Puget Sound...does look to meet Small Craft Advisory criteria...and as
such have that headline in place as well. Rough bar conditions look
to set up for Saturday...but will leave that for next shift to
consider. East to southeast flow will linger into early next week
with low pressure offshore. 18

&&

Hydrology...an organized frontal system will affect the olympic
peninsula Friday night and much of Saturday. With estimated
rainfall totals between 2 and 3 inches, rivers in the area are
expected to rise, particularly the skokomish. The snow level
will be marginal for flooding as it will start very low but rise
as high as 4000 feet by the end of the storm. Depending on the
rainfall total, increase in snow level, and potential melting of
low elevation snow in the valleys, minor flooding is possible on
the skokomish. Continue to monitor the latest forecast
information as the details of the event evolve.

&&

Sew watches/warnings/advisories...
Washington...Winter Storm Watch from this evening through Saturday afternoon
for Cascades of Pierce and Lewis counties-Cascades of
Snohomish and King counties-Cascades of Whatcom and Skagit
counties-Olympics.

Wind Advisory from 10 PM this evening to 10 am PST Saturday for
Admiralty Inlet area-San Juan County-western Skagit County-
western Whatcom County.

Pz...Small Craft Advisory from 6 am this morning to 4 PM PST this
afternoon 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.

Gale Warning from 4 PM this afternoon to 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.

Gale Warning from 10 PM this evening to 1 PM PST Saturday for
Admiralty Inlet-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-West Entrance
U.S. Waters Strait of Juan de Fuca.

Small Craft Advisory from 10 PM this evening to 4 PM PST
Saturday for Puget Sound and Hood Canal.

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