Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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135
FXUS66 KSEW 242332
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
332 PM PST Fri Nov 24 2017

.SYNOPSIS...A frontal system over Vancouver Island should bring a
little light rain to mainly the north part of Western Washington
tonight. After short lull with mostly dry weather Saturday
morning, another frontal system will arrive Saturday afternoon and
night, with wet and locally windy weather through Sunday. An
upper level ridge will cross the area Monday but will be too weak
to end the chance of showers. Another frontal system will move
through the area Monday night and Tuesday. An upper level ridge
could bring some dry weather and a little sunshine Wednesday and
possibly Thursday.

&&

.SHORT TERM...An east-west oriented frontal system will remain
quasi-stationary over Vancouver Island tonight. W WA is in the
warm sector and some light rain has been reaching the WA coast and
the north interior. The front will continue to bring a little
light rain to mainly the far N part of W WA through tonight.

Snow levels will be between 4000 and 4500 feet tonight, which could
bring a little snow to the northern passes. However, amounts will
be light.

A large longwave trough deepening offshore along roughly 145W
tonight will amplify the flow offshore allowing stronger SW flow
to develop downstream over W WA on Saturday. A shortwave trough
embedded in the SW flow will spread rain northward over W WA
Saturday afternoon through Saturday evening. At the same time an
associated deep surface low will move N offshore along 130W. This
low will generate windy conditions along the coast and possibly over
the Cascade foothills late Saturday and Saturday night. There is
a chance the winds could reach advisory levels along the coast and
possibly near the Cascades Foothills.

There is also potential for another period of heavy rain over the
mountains Saturday night. The warm and wet system embedded in the
SW flow will push snow levels up to 6000 to 7000 feet Saturday
night, before lowering back down to 3500 feet Sunday afternoon. A
number of the 12Z mesoscale models show up to 2 inches of rain,
but if the models are low, then potentially up to 3.5 inches in
12 hours over the Olympics, with a little bit less over the north
Cascades Saturday night, which could push river levels back up a
little. However, widespread flooding is not expected.

The pattern offshore becomes a little more complicated and tricky
Saturday night and Sunday morning. The models have a small cold
front moving inland Sunday morning. N-S pressure gradients could
become strong enough for at least windy conditions over the
interior lowlands. Advisory level S winds are also a consideration.
However, this is a small scale feature a few days out, so it
wouldn`t take much for the models to change it.

The remnant of the large longwave upper level trough offshore
will move E across the area Sunday night into Monday morning for
showery weather. Kam

.LONG TERM...The GFS and ECMWF have regained some consensus for
Monday night through Wednesday. The latest solution is for a warm
front to roll through the area Monday night, followed closely by a
cold front Tuesday morning. The trailing upper level trough will
be right behind, crossing the area Tuesday afternoon and early
evening.

A moderate amplitude upper level ridge will follow with
potentially dry weather and some sunshine on Wednesday and
possibly Thursday. Bear in mind that this fog season, so morning
low clouds and fog could mess up any sunshine Wednesday and/or
Thursday mornings. Looks like a weak front may follow either
Thursday afternoon (ECMWF) or Thursday night (GFS). Kam

&&

.AVIATION...A weak upper level ridge over Eastern Washington
combined with an expansive trough over the Eastern Pacific will
give strong west to southwest flow aloft to the region through
Saturday. High pressure at the surface centered over west- central
Oregon combined with a frontal system that extends from central
British Columbia southwestward into the Pacific will give low
level southerly flow to the area. Showers associated with a weak
upper level trough is seen over and west of the Olympic Peninsula
with less extensive shower activity over the interior. Showers
will rapidly diminish from the south early Saturday morning. Rain
will increase from the south Saturday afternoon into Saturday
night as a warm front approaches the area from the south.

Conditions are mainly VFR across the area this afternoon with
pockets of MVFR on the Kitsap Peninsula and in the north interior
from the San Juan Islands to around KBLI. Expect conditions to
gradually improve to VFR from the south overnight as moisture
associated with a weak trough over the area lifts northward into
British Columbia.

VFR conditions will be the rule on Saturday, but conditions will
deteriorate to MVFR in CIGS and precipitation from the south late in
the afternoon as the next warm front approaches the area. Albrecht

KSEA...Southerly winds 9-13 knots will persist through tonight and
will become more southeasterly 5-7 knots Saturday morning then
northeasterly Saturday afternoon. Expect broken cloud layers
3500-4000 feet and 9000 feet through much of tonight with only
scattered to broken high clouds Saturday. Albrecht

&&

.MARINE...A frontal system extending from the Queen Charlotte
Sound southwestward into the Pacific in combination with surface
high pressure over west-central Oregon will give southerly flow to
the waters through tonight. Flow will become more easterly on
Saturday as a deepening low pressure system moves northward along
130W longitude and a warm front lifts northward through the
waters. Small craft advisory conditions will diminish in Puget
Sound and Hood Canal late this evening, but elsewhere they will
continue into Saturday.

Behind Saturday`s warm front, expect flow over the coastal waters,
at the entrances to the Strait of Juan de Fuca, and in the
Northern Inland Waters to become more southerly and to possibly
strengthen to gale force. The gale watch has been expanded to
cover the entrances to the strait and the northern inland waters.
Other waters will likely see small craft advisory conditions that
will continue into Sunday as a frontal system moves through the
area.

Another strong frontal system will arrive Monday Night or Tuesday.
There are some forecast models that suggest that seas near 30
feet could accompany the system on Tuesday, but a majority of
ensemble members and wave guidance from the ECMWF hold seas to 20
feet. Per discussion with the Ocean Prediction Center and WFO
Portland, will keep the seas to 20-22 ft per the model consensus.
Albrecht

&&

.HYDROLOGY...Rivers have crested and are receding at this time.
For those that are still above flood stage, most will fall below
by tonight. The White River will remain above flood likely into
Sunday as they drain the pond - Mud Mountain Dam.

For a list of current flood watches and warnings, please
check: www.wrh.noaa.gov/sew/get.php?pil=RVA&sid=SEW

More fronts will move through Western Washington this weekend and
early next week, but rainfall will not be as heavy and snow levels
will be mainly in the 3000 to 5000 foot range, except 6000 to 7000
feet Saturday night. 2 to 3.5 inches are expected over the
Olympics and most will be rain over the Skokomish Basin, so that
presents a threat of further flooding there.

With the heavy amounts of rain we have had and continued storms for
several more days, the landslide risk remains elevated and any of
the rainstorms have the potential to trigger one or two. JBB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch in effect for Mason County from Saturday
     afternoon through Sunday afternoon.

PZ...Gale Watch from Saturday evening through Sunday afternoon 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-East Entrance U.S. Waters Strait Of Juan De Fuca-Northern
     Inland Waters Including The San Juan Islands-West Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 6 PM PST Saturday 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.

     Small Craft Advisory until 3 PM PST Saturday for Admiralty Inlet-
     East Entrance U.S. Waters Strait Of Juan De Fuca-Northern
     Inland Waters Including The San Juan Islands.

     Small Craft Advisory until 4 AM PST Saturday for West Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 10 PM PST this evening for Puget
     Sound and Hood Canal.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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