Scientific Forecaster Discussion
fxus66 ksew 162323
Area forecast discussion
National Weather Service Seattle Washington
323 PM PST Sat Dec 16 2017
Synopsis...rain will increase tonight and Sunday as a warm front
reaches the area. The trailing cold front will stall over the
region on Monday. A strengthening low center will pass across
the area on Tuesday, and a strong cold front will bring wind and
heavy mountain snow. Lingering showers will continue Wednesday
morning, then an upper ridge will bring dry weather on Thursday
Short term...rain will increase tonight into Sunday as a warm
front reaches the area. The trailing cold front will then stall
over the region Sunday night into Monday with strong, moist,
westerly flow over the area. The main impact is possible heavy
snow in the mountains, especially the north and central Cascades
between Mount Baker and Stevens Pass. A Winter Weather Advisory is
in effect from Sunday afternoon through Monday morning. Snow
levels are tricky and may waver between 4,000 and 5,000 feet with
generally 3 to 11" expected. Snow levels will then lower to around
3,500 feet early Monday. Snoqualmie Pass should see mainly rain or
a rain/snow mix.
In The Lowlands, cloudy and rain at times Sunday through Monday.
Total rainfall amounts will range from 1" along the coast to
0.50-1.00" in the interior. Temperatures will mainly stick in the
40s for both highs and lows, with a few low 50s possible both
Sunday and Monday afternoon.
A stronger and wetter Pacific frontal system will impact the area
on Tuesday. Heavy snow is possible in the mountains and snow
levels will be lower - dropping to around 2,000 to 3,000 feet
Tuesday night. It'll be windy too, especially with the frontal
passage Tuesday afternoon. However, the models keep waffling on
the low strength and tracking. For now it just looks like a
blustery front and winds are below Wind Advisory criteria.
Showers will taper off Tuesday night as this system exits. 33
Long term...another pattern change will take place on Wednesday
as a strong upper level ridge builds over the NE Pacific. This
gives western Washington dry, northerly flow which will help scour
out the low clouds and moisture. At this time, the ridge is
dominant through the extended forecast with dry weather through
at least Christmas day. With a drier air mass in place, expect low
temps in the 20s to lower 30s with highs in the lower 40s. 33
Aviation...notice, pwt (the Bremerton FAA observation will
be missing through Monday due to electrical work. We will maintain
the taf as best we can with no amendments.
Moderate southwesterly tonight as a warm front reaches the area,
becoming westerly on Sunday. The air mass is stable and moist at all
levels. Steady rainfall will arrive at the coast with the warm
front by early evening, with rain spreading inland the rest of the
evening. Widespread MVFR is likely as rain develops. Some local IFR
cigs below 1,000 ft and vis possibly below 3sm is possible,
especially at pwt and olm.
Ksea...MVFR 1000-1800 feet will like prevail by 00-01z through
tonight. Rain develops by late evening. Southerly wind 4-8 knots,
become south-southwest 10-14 kt W/ gusts 24 kt Sunday. Dtm
Marine...small craft winds expected all waters except the central
and western Strait as a warm front moves across the area. Winds will
ease later Sunday.
Monday will be a lull in between weather systems. A vigorous
frontal system will move through the area Monday night and Tuesday
with gale force winds possible for the coastal waters. Offshore
flow will develop Wednesday. Dtm
Hydrology...river flooding is not expected the next 7 days.
Washington...Winter Weather Advisory from 10 am Sunday to 10 am PST Monday
for Cascades of Snohomish and King counties-Cascades of
Whatcom and Skagit counties.
Pz...Small Craft Advisory until 6 am PST Sunday 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 4 PM PST Sunday for Admiralty Inlet-
East Entrance U.S. Waters Strait of Juan de Fuca-northern
inland waters including the San Juan Islands.
Small Craft Advisory from midnight tonight to 6 PM PST Sunday
for Puget Sound and Hood Canal.