Scientific Forecaster Discussion
fxus66 ksew 171128
Area forecast discussion
National Weather Service Seattle Washington
330 am PST Mon Dec 17 2018
Synopsis...lingering showers are still present over portions of
western Washington in the wake of an exiting front this morning.
After a brief break this afternoon, a strong front will move slowly
southeast across the area tonight through Tuesday bringing locally
heavy rain and windy conditions, with snow in the mountains.
Additional strong fronts will move through the area on Thursday and
late next weekend.
Short term...showers continue over portions of the area this early
morning thanks to lingering shortwaves behind the front that passed
through yesterday. This will keep some precip in the forecast this
morning. These showers are expected to dissipate somewhat by this
afternoon...just as the next frontal system makes its way to the
As this front makes its way into West Washington a fair amount of headlines
come with it. Pressure gradients with this front will result in
strong and gusty winds along the coast by afternoon and then in the
north interior this evening. As such...a High Wind Warning will go
out with the morning forecast package to cover this. Precip with
this system is likely to exacerbate flooding along the
skokomish...but may also begin to trigger other rivers in the area
to rise to near flood stages as well. This is covered further in the
hydrology section below as well as in the esf/hydrologic outlook.
Then...plodding slowly eastward...the front will likely result in
some heavy snowfall for the north Cascades of Whatcom and Skagit
counties...so feel good about upgrading the inherited Winter Storm
Watch into a warning. Model precip output is also putting 11 to 17
inches in the vicinity of Stevens Pass for the period from tonight
through the day Tuesday and into Tuesday evening...Landing it right
in Snow Advisory criteria. Those headlines will also go out in the
morning package. One Point of interest will be how snow levels will
rise throughout the progression of the system...starting out around
3000-3500 feet and then lifting to above 4000 Tuesday morning and
Hinted at earlier in this discussion...this system is a slow mover
with effects lingering through most of the day Tuesday before very
gradually tapering off Tuesday night into Wednesday. Models continue
to suggest some degree of upper level ridging over the pac northwest for
Wednesday...but for West Washington...the feature looks pretty dirty...with the
GFS retaining its shortwaves keeping the Prospect for some showers
in place while the European model (ecmwf) keeps the ridge progressive enough for the
next system to overrun the ridge and spill into the area.
Thus...pretty hesitant to advertise any sort of break for midweek.
The pattern remains active as yet another system is on tap for
Long term...some model disagreement on finer details of Thursday
system with the GFS a little bit faster with it. Both models do
agree that precip will be present most of Thursday...but the GFS is
again a little faster...pushing the system out by Friday morning
while the European model (ecmwf) lets it linger. Both models do agree in dry
conditions by midday Friday as an upper level ridge builds into the
area. The GFS keeps dry conditions into Saturday while the Euro
dirties the ridge up with some precip starting to creep into the
area through the day. Another system looks to sweep through the area
for Sunday. Smr
Aviation...increasing southwest flow aloft today as another strong
frontal system over the offshore waters moves into the region later
today. The air mass is moist and generally stable as scattered Post-
frontal showers continue to move onshore over the area. Ceilings are
generally VFR away from the coast early this morning. This trend
should continue through late morning with shower activity generally
winding down. By midday, high and mid level moisture will already be
rapidly thickening with rain spreading inland from 21z Onward. MVFR
ceilings in rain will develop across most of the interior by 02z or
03z and continue through the night.
Winds aloft will become quite strong again with this system and
southerly winds above 2000 feet could reach 40 to 50 knots after
00z...with 60 knots or greater at 5000 feet. Forecast models show a
mesolow forming on the leeward side of the Olympics this
evening...and this could mean some particularly strong surface winds
for the kbli area this evening. Elsewhere...increasingly gusty S-southeast
winds can be expected tonight.
Ksea...generally VFR this morning...bkn040-050 with passing showers.
Should see a decreasing trend toward later this morning. Steady rain
spreading back into the area by 00z with ceilings dropping back to
MVFR by 03z or so and persisting through the night. As mentioned,
winds aloft becoming rather strong...especially 03z-06z. Surface
winds southerly around 10 knots becoming southeast late afternoon and
rising to 15 gusting 25 knots this evening. 27
Marine...a strong front will approach the coast today and move
inland tonight. High end gales are likely on the coast with this
system. There will also be gales in the Strait and over the northern
inland waters, with at least Small Craft Advisory strength winds
Moderate to strong onshore flow will follow on Tuesday and then ease
on Wednesday. Another strong frontal system will bring more gales
Wednesday night and Thursday.
Long period west swell 20-22 feet will affect the coast through this
morning. A high surf advisory is in effect for the coast. Swells
will fall below 20 feet later this morning and diminish somewhat
over the next few days. The storm on Thursday could bring 20-22 feet
swells back to the area. Chb
Hydrology...the Skokomish River remains above flood stage. It
will slowly fall today but rise again tonight in response to more
rain. The river is unlikely to fall below flood stage for at least a
few days, and could reach the moderate flood stage late tonight.
Rainfall from the system tonight will be focused on the Olympics and
southwest interior. There is a possibility of flooding on some
rivers in those areas on Tuesday. A hydrologic outlook covers that
Washington...Winter Weather Advisory from 6 PM this evening to 6 PM PST
Tuesday for Cascades of Snohomish and King counties.
Winter Storm Warning from 6 PM this evening to 6 PM PST Tuesday
for Cascades of Whatcom and Skagit counties.
High Wind Warning from 4 PM this afternoon to 1 am PST Tuesday
for Admiralty Inlet area-San Juan County-western Skagit
County-western Whatcom County.
High Wind Warning from 1 PM this afternoon to 10 PM PST this
evening for central coast-north coast.
High surf advisory until 10 am PST this morning for central
Pz...Small Craft Advisory for rough bar until 4 PM PST Tuesday for
Grays Harbor bar.
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.
Gale Warning from 10 am this morning to 10 PM PST this evening
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.
Small Craft Advisory until 1 PM PST this afternoon for East
Entrance U.S. Waters Strait of Juan de Fuca-northern inland
waters including the San Juan Islands.
Gale Warning from 1 PM this afternoon to 4 am PST Tuesday 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 Tuesday for Admiralty Inlet-
Puget Sound and Hood Canal-West Entrance U.S. Waters Strait
of Juan de Fuca.