Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
330 am PST Wednesday Nov 26 2014
Synopsis...a warm front will remain near the Canadian border today.
A cold front will move across the area tonight for more rain
and windy conditions. Rain will continue on Thanksgiving day as
another cold front approaches from the north then crosses the area
on Friday. Cool dry and breezy weather will develop by Friday
afternoon and continue through the weekend as high pressure builds
over British Columbia.
Short term...western Washington is in the warm sector as a warm
front lifted north through the region overnight. As a result...temperatures
remained mild with readings mainly in the 50s this morning. The warm
front is over southern b.C. Now and rain will mainly affect the
northern edge of the County Warning Area today...and also the mountains. Rivers are
still rising and flood watches and warning are in effect...see
hydrology discussion below. Temperatures will remain mild this afternoon
with highs in the middle to upper 50s.
Rain will increase in coverage tonight through Thursday as a cold
front moves inland. Southeast pressure gradients will tighten as this
system approaches and may see Wind Advisory speeds along the coast
and north interior zones tonight. This front will bring another shot
of heavy rain to the mountains...leading to more flood concerns.
Snow levels will remain high at 6000 to 7000 feet. Expect rain at
times in The Lowlands throughout Thanksgiving day.
Another cold front will cross from northwest to southeast through western Washington on
Friday...bringing more rain and windy conditions. This front should
exit south Friday afternoon. Models are showing gusty Fraser River
outflow winds Friday afternoon or evening as strong high pressure over
interior b.C. Sags south. This cool and dry air will filter south
into western Washington Friday night. The air mass will be cool and unstable as
a deep upper trough also clips western Washington during this period. There
is a chance of snow in the interior Friday night into Sat morning as
this trough crosses the region. Models are now trying to form a
convergence zone where cool northerly Fraser outflow meets the moist
S/SW onshore flow. Could see rain/snow in the convergence zone. But
this could occur anywhere in the interior. Tough to pin this down
exactly but definitely need to keep a close eye if this scenario
pans out. Models do show the trough exiting Sat morning with dry
weather expected. 33
Long term...long range models show a cool and dry air mass over
western Washington on Sunday. High pressure over interior b.C. Will maintain
easterly flow through the passes. Lows Sunday morning will be down
to the 20s to lower 30s once again. Highs are also below normal on
Sunday with upper 30s to lower 40s expected. Models were showing
some moisture spreading north Sun night or Monday but they are now
backing off on this idea. So the forecast is now looking drier over
this period as well. The forecast is also looking drier through
midweek now as the Euro tries to build a ridge over the Pacific northwest. 33
Hydrology...rain rates have decreased throughout the Cascades
overnight...while the warm front has continued moving slowly
northward. Rainfall amounts of 0.5 to 1.2 inches during the past 6
hours were still common from the stillaguamish basin northward...but
rates there should decrease this morning. The stillaguamish reached
flood stage early this morning...and flooding is still a short term
concern for the Nooksack river.
There has not been much rain overnight south of the stillaguamish
basin -- generally 0.5 inch or less. The Three Forks of the
Snoqualmie river appear to have crested...so the Snoqualmie at the
falls should crest early this morning with the crest moving
downstream through Carnation late this afternoon. Flooding on the
Snoqualmie will continue through tonight. The Skykomish river has
been running within a foot of flood stage...but it will probably
begin falling soon. The Snohomish...which is formed by the
confluence of the Skykomish and Snoqualmie rivers...could have some
flooding later today.
The rivers that flow off Mount Rainier are receding. The flooding
has ended for the Carbon and Nisqually rivers...but flooding will
continue for the Cowlitz and reach of the Puyallup near Orting.
Next we turn our attention to the frontal system that will bring
significant rainfall tonight through Thursday night. We are
expecting storm totals in the mountains of 3 to 5 inches during the
36 hours from 4 PM today through 4 am Friday. The Olympics and north
Cascades will receive the heaviest rainfall...but locally up to 5
inches over Mount Rainier and Skykomish-Snoqualmie-tolt-Snohomish
basin is possible too. The snow level will be 5500 to 7000 feet for
most of the event...then fall on Friday as a cold front moves
through from the northwest and precipitation decreases.
A major flooding episode is unlikely...but the rainfall could
certainly renew flooding on the rivers that flooded or almost
flooded and some other rivers could reach flood stage as well. A
Flood Watch is 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...the warm front that brought all the rain to western
Washington the past couple days has moved into British Columbia.
However a warm moist air mass remains over western Washington with
widespread low-end MVFR to IFR conditions...including some
visibilities below 3sm in fog and light precipitation.
Improvement today will be gradual and mainly limited to the Puget Sound
sound region and southwest interior. Another vigorous frontal system
will bring more rain to the coast late today...spreading into the
western Washington interior this evening. Whatever improvement
occurs today is likely to be quickly undone as a result.
Ksea...south wind mostly 10-16 knots gusting 24 knots through tonight...
except there could be a brief lull late this morning and afternoon.
The gradual improvement in ceilings should continue today...but
conditions will deteriorate tonight as the next frontal system
Marine...a cold front will approach the waters today and move
onshore tonight. Small Craft Advisory southerly winds will be common
most waters today...then gales for some waters tonight as the front
moves through. Moderate to strong onshore flow will prevail behind
the front on Thursday.
Another cold front will move through the waters on Friday. This
front will be followed by strong Fraser outflow in the northern
inland waters and moderate northerly winds over the other waters
later Friday into Saturday. Schneider
Washington...flood watches and warnings in effect for portions of
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...and northern inland waters late this afternoon and
You can see an illustrated version of this forecast discussion at