Washington State Forecast Discussion

Western Washington


211
FXUS66 KSEW 040429
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
829 PM PST Tue Dec 3 2024

.UPDATE...Fog and stratus continues to spread across Puget Sound
this evening, with lowering visibilities. In addition,
temperatures are generally in the low to mid 30s, with lows
dropping into the upper 20s in some areas tonight. This may result
in areas of freezing fog, along with dense fog, which will
continue to be monitored. If you're traveling tonight, give
yourself extra time through the morning commute. No major forecast
updates this evening. JD

&&

.SYNOPSIS...High pressure aloft will maintain dry conditions with cool
overnight temperatures, morning fog and low stratus through
Wednesday evening. The upper level pattern will begin to shift
Thursday into Friday as the next storm system approaches, bringing
widespread precipitation across the region through the weekend.

&&

.SHORT TERM /TONIGHT THROUGH SATURDAY/...The persistent upper level
ridge over the region will remain in place through Wednesday,
maintaining the status quo of dry and stable conditions with
widespread morning fog and low stratus with pockets of greatly
reduced visibility possible again overnight. Cold overnight lows
will also give way to the development of freezing fog in areas
that fall below freezing overnight, primarily south of the Puget
Sound. Lows will again dip to near freezing across the lowlands
including the Cascade valleys and highs Wednesday will rise into the
upper 40s to lower 50s.

The large scale pattern is expected to shift Thursday as an upper
level trough moves into the Pacific Northwest, displacing the
persistent upper level ridge. This weak system will allow light
rain to move into western Washington late Wednesday night into
Thursday morning, perhaps more importantly it will also boost
mixing in the boundary layer and limiting fog into Thursday
morning. On Friday a warm front will lift across the region,
bringing even more widespread rainfall and a warmer air mass -
boosting snow levels to near 7000-9000 ft. Temperatures Friday
will peak well into the 50s across the lowlands.

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...Warm frontal precipitation
will continue into Saturday morning. A trailing cold front will
follow closely behind on Saturday as an upper level trough settles
over the region, bringing in more precipitation and lowering snow
levels to 3000-3500 ft by Saturday night. The deterministic
precipitation forecast from 06z Saturday through 06Z Sunday -
capturing both the heavier warm frontal precipitation and expected
accumulations from the cold front is highlighting over 3 inches in
portions of the North Cascades. This has trended towards the
wetter portion of the ensemble distribution somewhere near the
80th percentile mark. Given that the area has been relatively dry
for the last week or so, this will be a noticeable shift to more
typical Pacific Northwest Fall weather. Showers are expected to
continue into Sunday under the trailing upper level low, with
orographic enhancement over the Cascades and snow levels at or
below pass level. Ensembles and medium range models continue to
highlight a shift back to a drier, more benign patter early next
week with high pressure aloft and northerly flow aloft.

&&

.AVIATION...Another round of widespread fog is expected to fill
into the majority of the terminals this evening bringing IFR/LIFR
conditions through the night and into Wednesday morning. Some
guidance suggests that KCLM and KBLI may remain MVFR/VFR as
guidance hints at lower probabilities for LIFR/IFR conditions.
Conditions should begin to improve to VFR after 21Z, the
exception being KOLM, as they may improve to MVFR.

KSEA...Fog has started to fill in around the terminal this
evening, with LIFR conditions taking over through majority of
tonight into Wednesday morning. Winds will be from the
north/northeast, however guidance at this time indicates that
winds will be light. Similarly to today, model guidance has LIFR
fog beginning to lift around 21Z (20%) into VFR conditions on
Wednesday afternoon.

MGF/Mazurkiewicz

&&

.MARINE...Another round of fog tonight over the interior waters
into Wednesday morning. Visibilities may decrease below 1 NM.
Light winds will keep waves between 5-8 ft. A series of storm
systems are expected to move through the area beginning on
Thursday, followed by a stronger system on Friday. Southerly flow
will allow for steep seas to develop before waves build with the
Friday system. Stronger winds and westerly/northwesterly winds
will help waves build between 14-17 ft on the coastal waters
during the weekend. Waves should gradually begin to subside on
Monday.

Currently there are no advisories out for the coastal waters,
however ensemble guidance does hint at winds possibly reaching Small
Craft criteria on Friday.

&&

.HYDROLOGY...An upper ridge will remain in place across Western
Washington through Wednesday. A frontal system will move through on
Thursday for lighter QPF amounts. Another weather system Friday into
Saturday is forecast to bring heavier QPF amounts. In addition, snow
levels are expected to rise later Friday into early Saturday to 7000
to 9000 feet. This, combined with rainfall, will result in rises on
rivers across Western Washington. At this time, the Skokomish River
is the main river of concern for potential river flooding during
this period. There is a fair amount of uncertainty as to the highest
amounts of rainfall that are possible with Saturday's storm, with
some models producing over 4 inches over the Olympics and Central
Cascade north. This could drive some rivers to close to or over
minor flood. Due to this and the higher snow levels Friday into
Saturday, other area rivers will need to be monitored for flood
potential as well.

JBB/JD

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.
&&