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501
FXUS66 KSEW 191651
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
851 AM PST Sun Jan 19 2025

.UPDATE...
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No changes made to the inherited forecast. Fog and low
clouds are still lingering over the Snoqualmie/Chehalis river
valleys and over parts adjacent to inland waters but should fade
into the afternoon. Temperatures are still forecast to top into
the lower 40s. Ample sunshine is expected. The previous discussion
is below along with an updated aviation section.
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&&


.SYNOPSIS...High pressure will dominate the weather into the
middle of this week, bringing dry conditions and cold overnight
temperatures. Overnight lows are expected to fall below freezing
over much of Western Washington tonight and into next week while
daytime highs will remain below normal.


&&

.SHORT TERM /TODAY THROUGH TUESDAY/...While clear skies are in place
for much of W WA this early morning, there are a couple of patches
of fog being picked up by satellite...the first being a narrow strip
over portions of southern Snohomish and northern King counties and
the other in the Chehalis river valley. This latter patch, given
that temps in the area are hovering around 30 at the time of this
writing /130 AM PST/, suggests this may be more along the lines of
freezing fog. Otherwise, the main story tonight is once again the
cold temperatures. Most obs indicating mid to upper 20s while the I-
5 corridor between Everett and Tacoma is in the lower to mid 30s.
Locations along the coast split the difference, hovering in that
narrow range between 30-32 degrees while the islands are benefiting
from their proximity to water and thus proving to be the warmest
with temps ranging in the mid to upper 30s.

The current ridge will slowly drift eastward today, although the
timing for the ridge axis to make its way to the coast has slowed a
little, now expected to arrive Monday morning. Models remain
consistent on the fact that once this happens, the ridge starts to
flatten in anticipation of a shortwave disturbance coming down from
the Canadian coast. This system passes through quickly on Tuesday
with any associated precip having fizzled out well before crossing
the border. The main impact here will be an increase in clouds
Monday night into Tuesday, helping overnight lows to nudge upward
somewhat.

Before that happens though, temps in the near term will certainly
retain their wintery nature. Highs today will largely be in the
lower 40s, although some water-adjacent locations may creep into the
mid 40s...and ditto that for Monday. With climo temps ranging in the
mid 40s to around 50, current expected highs place the area around 7
or 8 degrees below normal. The main focus, however, remains on
overnight lows. Much of the area will be cold again tonight with
lows ranging in the mid to upper 20s. Factoring wind in, we start to
see Apparent temperatures /temp plus wind chill or, more commonly,
feels like/ in the Seattle, Everett, Tacoma and Bremerton areas
ranging in a 20 to 25 degree window. This would trigger Cold Weather
Advisory conditions, especially in the overnight hours tonight and
early Monday morning. As such, will post this headline with the
morning forecast package. Overnight lows Monday night/early Tuesday
morning, do nudge upward in many locations, as per the
aforementioned fog, ranging in the mid 20s to around 30. Given this,
will allow further analysis before any decisions on further
headlines are made.

18

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...Another ridge pushes in
for Wednesday into Thursday, although deterministic models differ on
the amplitude of the feature. The GFS would keep any systems passing
over the ridge well into Canada while the ECMWF favors a lesser
amplitude, allowing for possible impacts to W WA. An upper level
trough pushes into the area for late Thursday and into Friday
although here again, deterministic models differ similarly. As has
been the case heretofore, the NBM has picked up on this and
introduced Pops for Thursday and Friday while ensembles again
downplay this solution, showing PoPs again up to around 30 pct,
mainly Friday, although the mean does not even break out of the
single digits. At this point, it is worth noting that 24 hours ago,
models, NBM and ensembles all pointed at nearly identical
output...but focused on Thursday instead. A pattern is starting to
emerge that the next potential round of precip, no matter how minor,
is simply being pushed out an additional 24 hours until there is
sufficient indications of the current ridge breaking down, and thus
allowing the forecast, including the long term, to unfold from there.

Daytime highs see a little bit of a warming trend Wednesday and
Thursday, with highs Wednesday generally in the lower to mid 40s and
in the mid to upper 40s Thursday. Should the expected trough enter
the area as discussed above, this would put highs back in the lower
to mid 40s for the remainder of the long term period. Overnight lows
also see a nudge upward at the start of this period, ranging in the
upper 20s to around 30 Wednesday night and in the lower to mid 30s
Thursday night. Cold conditions return after this though, with
overnight lows returning to the upper 20s to lower 30s.

18

&&

.AVIATION...
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Upper ridging remains offshore today with continued
northerly flow aloft. LIFR cigs and vsbys due to FZFG continue
primarily from OLM southwards this morning, with areas of localized
LIFR around BFI and river valleys with light flow this morning. MVFR
cigs also exist along the northern Olympic Peninsula. VFR elsewhere.
The LIFR areas will improve to VFR by late morning to midday.
Otherwise VFR this afternoon through this evening. Another round of
LIFR FZFG possible tonight into Monday morning, especially from OLM
southwards. Increasing N/NE flow by midday may be gusty at times
this afternoon and evening for BLI with gusts near 20-25 kt with
Fraser Outflow. Light E winds tonight into Monday.

KSEA...VFR this morning as of 1645z. A localized area of shallow
LIFR FZFG exists north of the terminal as of 1645z, particularly
around BFI. The expectation is that the LIFR vsbys remain N of SEA
through this morning, but there remains a slight chance of lower
vsbys/cigs through 18z due to light NE flow. Otherwise VFR
conditions are expected into tonight. VFR also expected into Monday
but will have to again monitor any localized areas of LIFR fog
development. Light NE winds this morning will increase to 7 to 12
kts after 19 to 20z. Winds will lighten tonight but remain NE. JD
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&&

.MARINE...High pressure offshore will build into the waters today
and into Monday. Offshore winds will continue through Monday,
with the gradient increasing again this afternoon and evening.
Outflow from the Fraser could bring some occasional gusts to
20-25kt across portions of the Northern Inland Waters. Gusty
offshore winds will also be possible for areas across the western
Strait of Juan de Fuca and portions of the northernmost coastal
water zones. Will continue to hold off on advisory issuance for
now with most guidance indicating winds will fall within the 15-20
kt range - however, this will bear watching throughout the day.

A frontal system looks to approach the region on Tuesday and
dissipate just north of Vancouver Island, bringing no significant
impact to the area waters. High pressure will then rebuild across
the area waters midweek for another round of offshore flow. Another
frontal system looks to move into the area waters from the north
on Friday, bringing gusty northwesterlies.

Seas will generally range between 6-8 ft through the first half
of the week as a long period wave group has moved into the region.
Seas will then subside towards 3-5 ft near midweek, before building
towards 9-11 ft late in the week as a stronger disturbance moves
through the region. 14

&&

.HYDROLOGY...


&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Cold Weather Advisory from midnight tonight to 8 AM PST Monday
     for Bellevue and Vicinity-Bremerton and Vicinity-Everett
     and Vicinity-Seattle and Vicinity-Tacoma Area.

PZ...None.
&&

$$