Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
330 PM PST Friday Nov 21 2014
Synopsis...a front over western Washington this afternoon will move
inland tonight. Another front will move through the area on Sunday.
A warm front will be over the area Monday. An upper ridge will build
into the area Tuesday. The ridge will move inland Wednesday and a
front may reach the area Wednesday night or Thursday.
Short term...a 990mb low was near the north coast at 3pm. The
associated frontal passage ought to be through the coast by late
this afternoon. The wind will subside with the frontal passage and the Wind
Advisory was cancelled for the coast. After the front gets through
the interior of western Washington later this evening areas of rain will turn to
showers. The windy weather in the north interior will settle down
pretty quickly too...and the advisory ends at 7pm. There will be
scattered showers at times Sat and Sat night in onshore flow with
westerly flow aloft. The next significant system will have around a 995 mb low
that tracks to the b.C. Coast north of Vancouver Island, BC while filling around
daybreak Sunday. The associated front moves through western Washington during
the day Sunday.
Long term...a warm front will move through the region on Monday and
heights rise through Tuesday and Tuesday night as the precipitation ends. A ridge
axis along the West Coast Tuesday night progresses inland to about
Idaho by late Wednesday as a front reaches the pacnw Wednesday night and Thursday
morning...with the main punch of the front staying north of western Washington.
A splitty system moves in over the West Coast later Thursday and Friday.
Hydrology...river flooding is not expected the next 7 days.
Aviation...a cold front will soon reach the coast...then pass the
I-5 corridor about 3-4 hours from now. Strong S-SW flow aloft in
advance of the front...becoming moderate westerly this evening
behind the front. Deep moisture and strong lift in the pre-frontal
environment are leading to rain with lowering ceilings and reduced
visibility. Low-level wind shear is currently an issue in the veered
pre-frontal environment...but low level wind shear will lessen following the frontal
passage. Behind the front this evening...the air mass will become
unstable with convective showers...with this weather regime
continuing through Saturday. Showers will be most heavily focused
over the mountains and in a Puget Sound sound convergence zone.
Ksea...cold frontal passage most likely around 02z-03z (6-7 pm).
Until then...rain will be moderate at times with ceilings and
visibilities coming down. Winds aloft at 4500 feet mean sea level will peak
around 50-55 knots around the time of frontal passage. Low-level wind shear will
not be mentioned in taf...but a strong easterly component to the surface
wind and south winds at 4500 feet mean sea level around 45-55 knots means a lot of
shear in the low-levels. After the cold frontal passage...surface winds
become S-SW. Heavier stratiform rain will end...turning to
convective showers. Nearest focus for showers will be in the Puget Sound
sound convergence zone...which will develop between Everett and
downtown Seattle this evening...then persist through Saturday afternoon.
Marine...a cold front will reach the coast between 4 and 5 PM...
then cross the inland waters early this evening. Frontal passage
will be accompanied by a shift from S-southeast winds to SW-west winds.
Strongest winds will be in advance of the front...with gale warnings
still in effect for the coastal waters...the East Entrance...
Admiralty Inlet...and the northern inland waters. Small craft
advisories are in effect elsewhere. A westerly push behind the front
will produce Small Craft Advisory west winds through the
Strait...though a brief gale is also possible.
Another system on Sunday is gaining strength in the models. This
could give gales on the coast and down the Strait. Swells in the
15-20 foot range are forecast to reach the coast Sunday. Haner
Washington...wind advisories are in effect for portions of the northwest
interior of western Washington until 7 PM.
Winter Weather Advisory Cascades above 3500 feet tonight.
Pz...Gale Warning coast...East Entrance...Admiralty
Inlet...and northern inland waters.
Small Craft Advisory all remaining waters.
Small Craft Advisory for rough Grays Harbor bar.
You can see an illustrated version of this forecast discussion at