Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
930 PM PST Wednesday Nov 26 2014
Synopsis...a front will move east across western Washington tonight
for more rain and windy conditions. Rain will continue on
Thanksgiving day as a cold front approaches from the north then
crosses the area on Friday. A cold upper level trough will move
over the area Friday night and Saturday for a chance of
showers...possibly mixed with snow. Cooler and dry northwest flow
aloft and a surface high over British Columbia will combine for cool
dry weather and more sunshine Sunday through early next week.
Short term...current satellite imagery still shows impending front
off the coast with timing on frontal passage remaining consistent...hitting
the coast around or just before midnight and over Puget Sound sound before
12z/4 am. Observed winds continuing to ramp up in advance of
approaching front...though not quite hitting anticipated low end
advisory speeds just yet. Some locations on the north coast and
around bli are getting close. No change in expectation for winds to
ease once the front passes late tonight. Current radar shows
activity along the Cascades and in advance of the front...leaving
Puget Sound sound and east Puget Sound lowlands mostly dry for now. Will expect
that hole to fill in as front draws closer.
Picture for Holiday weekend remains consistently wet. Following
frontal passage tonight...may catch a slight break before second front moves
slowly over the area Thursday and into Friday...bringing continued
wet conditions. Rainfall amounts expected over the next 48 hours in
a period stretching from this evening through Friday afternoon range
from around 1 to 2 inches of rainfall in The Lowlands and 3 to 6
inches of rainfall in the higher elevations of the Cascades and
Olympics. The exception being up to 7 inches of rainfall around Montana.
Baker. Needless to say...this amount of rainfall will impact rivers
that have already hit flood stage today. Even though these rivers
are trending downward with regard to flow/levels...this additional
rainfall will likely bring another peak in flow/levels Friday. For
further details...refer to the hydrology section below.
Mild temperatures are expected to remain in place for Thanksgiving
before conditions begin to cool. An interesting forecast conundrum
arises Friday night into Saturday. A dipping upper level trough will
bring colder air to western Washington Friday night...just at the
tail end of precipitation event. GFS and European model (ecmwf) indicate that things may
line up just enough for mixed lowland precipitation Friday night with
lowland snow possible Saturday. However...latest run of the WRF GFS
4km does not look as convincing as the other two models. Inherited
forecast has included mentions of mixed precipitation and snow for this
time frame and will allow for further reexamination with future
model runs. Smr
Long term...from 410 PM discussion...a strong surface high settling
over b.C. On Friday will allow cold Fraser River valley outflow to
develop...bringing more cold air dry S over West Washington. The air mass will
dry out Saturday night with temperatures dropping into the 20s
Sunday morning. Skies will be mostly sunny. The Fraser valley
outflow will probably generate advisory level NE winds over Whatcom
and San Juan County Friday night into Saturday morning.
The European model (ecmwf) has leaned more consistently than the GFS in keeping a
cool dry pattern over West Washington into early next week. The latest European model (ecmwf)
now keeps the pattern dry through Wednesday although there are
system that get close. The GFS had been more aggressive in bringing
rain up over the are from the S...but has backed away from this on
the 12z and 18z runs. It may have been premature...but I have dried
out early next week. Confidence is shaky but hope it stays that way.
Hydrology...from 410 PM discussion...a warm front held heavier
rainfall north of the Washington/British Columbia border today. This is
allowing most rivers to slowly recede across western Washington. Rain amounts up
to 5 inches since Monday in places like Mount Rainier and the
Skykomish-Snoqualmie-tolt-Snohomish basin will still cause some
residual flooding...mainly through this evening.
Another front will bring renewed rainfall to western Washington tonight and
Thursday...locally heavy in the mountains. The Olympics and north
Cascades will receive the heaviest rainfall with 2 to 4 inches of
rain expected. 1 to 2 inches may fall in the central Cascades. The
snow level will be near 7000 feet.
A secondary surge of moisture will arrive Thursday night into
Friday. Models vary much more on the distribution and potential
rainfall amounts with this second frontal system. In
general...heavier rain amounts Thursday night and Friday should
occur along S/SW facing slopes of the Olympics and north
Cascades...although the central Cascades could also see heavy
precipitation by Friday. Additional amounts of 2 to 3 inches are
possible. This means the 36-48 hour rainfall totals for the Olympics
and Cascades may reach 4 to 7 inches with locally higher
amounts...especially in the north Cascades.
These amounts will likely drive several rivers to flood...although
widespread major flooding appears unlikely. At highest risk for
flooding is the Skokomish River in the Olympics...the Nooksack and
Skagit rivers in the north...and the
stillaguamish...Snoqualmie...and tolt across central areas. If the
heavier rain shifts far enough south then flooding could be possible
on the Deschutes river and The Reach of the Skookumchuck river near
Bucoda in Thurston County.
A Flood Watch remains in effect through Friday...which includes all
the counties with rivers flowing off the west slopes of the Cascades
and Mason County for the possibility of a flood on the skokomish.
Colder and drier weather is likely this weekend...so rivers should
recede and the flood threat will end for the foreseeable future.
Aviation...rain in increase in coverage across western Washington tonight
as a cold front moves inland. The flow aloft is westerly. Ceilings are
mainly VFR this evening but should lower into MVFR range overnight.
Gusty south winds are also expected with the front...especially
along the coast and north interior. Moist westerly flow will
continue on Thursday with showers mainly affecting the coast and
Ksea...expect rain at times tonight with ceilings lowering to MVFR
range. Gusty south winds 20015g25kt expected at the terminal
throughout the night. 33
Marine...a cold front will shift inland tonight with increasing
south winds over the waters. Gale warnings remain in effect for the
coastal waters...entrances to the Strait of Juan Delaware fuca and the
northern inland waters. A Gale Warning was also issued for Admiralty
Inlet. Winds will ease overnight but will still see Small Craft
Advisory winds across the waters. Pressure gradients will remain
tight Thursday night through Friday as another cold front pushes south from
b.C. Strong high pressure over interior b.C. Will bring north/NE Fraser
River outflow winds late Friday night into Sat. Offshore flow will
prevail sun and Monday with high pressure east of the Cascades. 33
Washington...flood watches and warnings in effect for portions of
Wind Advisory north coast...western Whatcom...San
Juans...Admiralty Inlet area.
Pz...Small Craft Advisory all waters.
.Small Craft Advisory Grays Harbor bar for rough bar.
.Gale Warning coastal waters...entrances Strait of Juan Delaware
fuca...northern inland waters...Admiralty Inlet.
You can see an illustrated version of this forecast discussion at