Scientific Forecaster Discussion
fxus66 ksew 161127
Area forecast discussion
National Weather Service Seattle Washington
330 am PST sun Dec 16 2018
Synopsis...a front will move through the area today bringing rain,
locally windy conditions, and High Mountain snowfall. A stronger
front will move across the area Monday night through Tuesday
resulting in another round of locally windy conditions and heavy
rain. Yet another strong front will move across the region Thursday.
Short term...wet conditions persist as current radar shows plenty
of echoes over the area. Linking this to current observations...the
overwhelming majority of sites are reporting rainfall as the front
continues to push into West Washington.
This system will slowly push through West Washington throughout the day today
and while the front will move east of the Cascades this
evening...the main upper level trough remains off the coast before
moving inland very late tonight/Monday morning...keeping wet
conditions overhead for the next 24 hours at least.
This leads to the first headline...a Winter Storm Warning remains in
effect for the western slopes of the north Cascades through late
this afternoon. An additional 4-12 inches of snowfall is expected
there today on top of the 3-6 inches that were forecast to have
already fallen there...making for a storm total 12-18 inches.
Similar to the previous system...tight pressure gradients with the
front will once again make for breezy to windy conditions for much
of the area...however...the strongest winds will not be as
widespread as they were with the previous system. This leads to the
second headline of the morning...as a Wind Advisory remains up for
the San Juans...western Whatcom...and the Admiralty Inlet area with
wind speeds of 20-35 mph with gusts to 45-50 mph expected to
continue through the morning before tapering off around noon today.
While speeds here will fall below advisory criteria...winds look to
remain elevated area-wide...only trending downward very gradually
through the afternoon hours and into Monday.
As stated already...the upper level trough associated with all this
activity will pass through the area during the day
Monday...weakening as it moves eastward. This will allow for precip
to start to taper off possibly leading to a very brief dry period
Monday afternoon. The next system is hot on its heels
though...making it to the coast by mid-afternoon Monday and pushing
inland Monday evening and into Tuesday. This follow-up system will
have the potential for some moderate to heavy rain along with the
possibility of yet another round of breezy conditions. This leads to
the next headline...as a Flood Watch remains in effect for Mason
County. Given precip amounts already seen...in addition to what is
expected in the Monday-Tuesday time frame...flooding is certainly a
possibility on the Skokomish River. Wet conditions will linger over
the area for the majority of Tuesday and into Wednesday. Smr
Long term...models continue to suggest some form of ridging for
Wednesday but depending on the model the feature could be either
dirty at best or so riddled with shortwaves at any sort of break in
the precip would be more to it being showery in nature than any true
drying impact that is typically associated with an upper level
ridge. So...typical pac northwest autumn/winter. The European model (ecmwf) is drier with
this ridge...but is also more progressive with it...moving it
eastward by Thursday. The GFS keeps conditions wet for West Washington with the
feature...but lets it hang around and even build a bit for the first
half of Thursday before the next system moves in with gusto. This
rainmaker moves on quickly though...ejecting by noon Friday thus
allowing for rain to taper to showers quickly and the potential for
a dry period from Friday afternoon into Saturday. Smr
Aviation...strong southerly flow aloft over western Washington
today with a moist air mass in place ahead of broad upper level
trough and associated occluded front that is moving through the
offshore waters. The front will reach the coast around 00z then
slowly push eastward through Puget Sound late this evening. Low
level flow is strong easterly this morning and this is keeping
ceilings mostly VFR. As the front pushes inland later today, the low
level gradient will turn more southerly and ceilings will
deteriorate first near the coast and then over the interior late
this afternoon or evening. Areas of MVFR are likely across the
region from this evening into Monday morning.
Ksea...fairly strong low level easterly flow likely to keep ceilings
up around 4000 to 5000 feet into later this afternoon, then lower to
MVFR at times this evening as the front pushes slowly inland. Winds
aloft are rather strong...reaching southerly 50 knots at 5000 feet
this morning. Surface winds east-southeast 8 to 12 knots turning southerly
after 00z this evening. 27
Marine...a strong front is just off the coast, moving slowly. It
is forecast to move into western Washington today and weaken. Strong
gale force southerly winds, 30-45 kt, are occurring over all waters
except Admiralty Inlet and Puget Sound, where 15-30 kt winds are
forecast. Wind should diminish below gale on the coast this morning,
but inland the gales will last into the afternoon.
There will be a relative lull in between fronts later today through
Monday morning. A second strong frontal system will bring more gales
later Monday and Monday night. Moderate onshore flow behind the
second front will ease later Tuesday and Tuesday night. A third
frontal system will approach from the west on Wednesday and likely
bring another period of gales on Thursday.
Wave models have consistently shown 19-22 foot west swells reaching
the coast tonight and continuing into Monday morning. A high surf
advisory is now in effect. Chb
Hydrology...the flood prone Skokomish River is below flood stage
for now but hydrographs are already starting to trend
upward...indicating that the current round of precipitation will
likely drive the river back above flood stage: a Flood Watch remains
in effect for Mason County.
The weather pattern will remain active through next week and we will
have to keep an eye on rainfall amounts, snow levels, and river
forecasts. Overall, there seems to be enough of a gap between the
heavier rain in each system, and looking at the plumes of moisture--
the so called atmospheric river of moisture does not seem to stall
over western Washington long enough to put US in an obvious flood
pattern. We will need to watch rivers over the southern portion of
the forecast area closely if the front slows in its southeastward
Washington...Winter Storm Warning until 4 PM PST this afternoon for Cascades
of Whatcom and Skagit counties.
Flood Watch through Wednesday morning for Hood Canal area-lower
Chehalis valley area-Olympics.
Wind Advisory until noon PST today for Admiralty Inlet area-San
Juan County-western Whatcom County.
High surf advisory from 4 PM this afternoon to 10 am PST Monday
for central coast-north coast.
Pz...Small Craft Advisory for rough bar until 4 PM PST Monday for
Grays Harbor bar.
Small Craft Advisory from 10 am this morning to 4 am PST Monday
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 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-West Entrance U.S. Waters Strait of
Juan de Fuca.
Gale Warning until 4 PM PST this afternoon 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.
Small Craft Advisory until 4 am PST Monday for Admiralty Inlet.
Small Craft Advisory until 4 PM PST this afternoon for Puget
Sound and Hood Canal.