Tacoma, Virginia 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

921
FXUS66 KSEW 301008
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
208 AM PST Thu Jan 30 2025

.SYNOPSIS...A frontal system will bring lowland rain and heavier
mountain snow tonight through Friday night. An upper low sliding
southwards across British Columbia will usher in a colder pattern
along with the potential for lowland snow Saturday into early
next week.

&&

.SHORT TERM /TODAY THROUGH SATURDAY/...Today begins what will be a
significant shift in the weather pattern from recent weeks. Deep
upper troughing digging southward from the Alaska panhandle will
merge with some energy offshore to carve out a broad, cold upper
trough over the region by the weekend as strong upper ridging
builds upstream over the Aleutians. This is shaping up to be an
impactful situation in terms of winter weather. So, let`s break
down the general sequence of events that are expected as well as
some of the probabilities and uncertainties involved.

What`s left of an upper level ridge today will give way to
an approaching frontal system. Rain associated with this front
will reach coastal areas late this afternoon or early evening and
spread inland overnight. Initially, snow levels will be in the
3000 to 4000 foot range as milder air associated with the system
infiltrates the region. The front is expected to slow and stall
somewhat offshore as some fairly healthy QPF (the most we`ve seen
in weeks) spreads over the area. This will translate to some
significant snowfall in the passes. The heavier snowfall will be
initially over the higher passes, but snowfall throughout the
event will warrant converting the existing Winter Storm Watch for
the Cascades above 3000 feet to a warning. Storm total snowfall
between Thursday night and Saturday will range from around a foot
at Snoqualmie Pass to 2 feet or more at Stevens Pass and Mount
Baker.

For the lowlands, we`re talking rain Thursday night through much
of Friday evening. But then significant changes occur. Colder air
follows a front Friday night with 1000-850 millibar thicknesses
falling below the crucial 1300 meter threshold and 850 millibar
temperatures dropping to -7 or -8 C. Models are advertising a
fairly potent convergence zone Friday night into Saturday morning.
The numbers outlined above combined with the convergence zone`s
ability to draw snow levels locally downward points toward
increasing probabilities for accumulating lowland
snowfall...especially Snohomish County northward. NBM
probabilities show a 50-70% chance of a fraction of an inch from
north King county northward by mid-morning Saturday. Those
percentages fall to 20-30% for an inch of accumulation. But keep
in mind that convergence banding can play a significant role in
localized higher amounts and this feeds uncertainty. A secondary
upper disturbance is expected to swing onshore Saturday night.
Portions of Puget Sound may see some shadowing by the Olympics,
but the air mass by this time is expected to be sufficiently cold
for accumulating snowfall down to sea level for most of the area.


.LONG TERM /SUNDAY THROUGH WEDNESDAY/...A vertically stacked
trough is expected to remain just offshore by the time we get to
Sunday. At the same time, a modified arctic boundary will be
sinking southward over southern British Columbia. Models have the
Williams Lake to Bellingham gradient approaching -20 millibars by
mid-morning Sunday which will initiate Fraser River outflow. The
modified arctic front is then expected to slowly sink southward
over Western Washington Sunday into Sunday night. While
uncertainty with regard to snowfall amounts is still high, the
probabilities of widespread snowfall accumulation are highest
during this period. Additionally, the Fraser outflow may act to
enhance upslope snowfall along the Strait of Juan de Fuca as
we`ve seen a number of times before.

Broad upper troughing remains just offshore Monday through
Wednesday with the the potential for additional precipitation to
move onshore for additional snowfall though uncertainty is quite
high in terms of amounts. 27

&&

.AVIATION...Generally VFR to MVFR with areas of localized IFR/LIFR
in patchy fog and low stratus. Cloud cover will continue to increase
throughout the morning and bring terminals down towards MVFR
conditions, except for terminals along the Kitsap Peninsula which
will trend towards IFR. Precipitation will expand inland Thursday
evening as a cold front approaches the region. Southerly to
southwesterly winds will continue through the period generally 3 to
6 kts.

.KSEA...VFR with calm/light SW winds early this morning. Ceilings
will trend towards MVFR this morning with potential for brief
improvement to VFR this evening. Rain will expand over the terminal
late Thursday evening through Friday. Southwesterly flow with winds
4 to 6 knots.

15

&&

.MARINE...A frontal system will move across area waters throughout
the day today, bringing in Small Craft winds across the outer waters
and through the East Entrance of the Strait of Juan de Fuca and
Northern Salish Sea. Another front later on Friday will maintain SCA
winds for most areas through Saturday. Winds will ease into Monday,
with another round of potential SCA winds on Tuesday.

Seas will gradually build to 6 to 8 feet throughout the day today,
then continue to build on Friday to 10 to 13 ft. Seas will remain
elevated through Sunday before gradually easing to 4 to 7 feet by
the beginning of next week.

15

&&

.HYDROLOGY...No river flooding is expected over the next 7 days
as abnormally dry conditions continue. However, with the
precipitation coming towards the end of the week, the Skokomish
River is forecast to rise above Action Stage. There will be much
uncertainty with the river forecast all the way into the event
given the current below normal flows and the snow and ice build up
in the basin over this recent dry but cold period. As far as
other rivers, only low elevation basins like the Chehalis, Samish,
and Stillaguamish Rivers will see much response at all.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Watch from Thursday evening through Saturday
     afternoon for West Slopes North Cascades and Passes-West
     Slopes North Central Cascades and Passes-West Slopes South
     Central Cascades and Passes.

PZ...Small Craft Advisory from 1 PM to 10 PM PST Thursday 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.

&&

$$