Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 161732
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
932 AM PST Thu Nov 16 2017

.SYNOPSIS...An upper level trough offshore will move inland
through tonight giving showery weather to the area. There will be
a threat of thunderstorms along the coast. A flat upper ridge will
move across the area Friday through Saturday, bringing a
diminishing trend on showers. A wet frontal system will bring
rain to the area Saturday night through Sunday night. The shower
threat will decrease Monday as another ridge moves across the
area. Rain will return to the area Tuesday or Wednesday.

&&

.SHORT TERM...A broad upper trough axis is located offshore near
130W this morning. There is very cold air aloft, with 500 mb
temps of -32C to -34C. Until the trough axis has completed its
trek across Western Washington late tonight, the resulting
instability and lift in advance of the trough will lead to loosely
organized bands of showers. So the weather through late this
evening will be characterized by short-lived rounds of heavy
showers, separated by an hour or more of dry weather and perhaps
a sun break or two.

The Winter Weather Advisory for the Mt. Rainier area still looks
valid, and will be monitoring radar trends for any persistent
areas of showers that possibly dump more snow than forecast on
other mountain zones. Snow level will be 2000-2500 feet today.

Once the trough axis passes east overnight, large-scale lift will
lessen, and the depth of the unstable layer will become more and
more shallow through Friday. So there will be an overall
decreasing trend in showers on Friday. The remaining showers will
be weaker. The combination of onshore flow but weaker large-scale
lift means the remaining showers will be more anchored to terrain
features, namely the windward side of our mountain ranges. Parts
of the lowlands, especially west-flow rain shadow country from
Edmonds to Port Townsend, could remain dry on Friday.

By Saturday, a flat upper ridge axis will develop over the Pac NW.
Especially on Sat afternoon, dry weather appears likely over the
entire forecast area, except perhaps the northwest Olympic
Peninsula.

Looking further north, a wet frontal zone will develop Saturday
further up the B.C. coast on the periphery of the upper ridge. As
the flat ridge axis moves east to the Rockies and beyond, the
front will start to steadily sink south down the B.C. coast. This
front has a juicy moisture tap from the central North Pacific. As
the front spreads in from the north, rain will overspread the
northern half of the area late Saturday night, then spread across
the rest of Western Washington on Sunday. Hourly rainfall rates
could become intense. However thankfully, the front will be
progressive and moving steadily without any hang-ups, so the
somewhat short duration of intense rainfall will limit the risk
of river flooding for all but the Skokomish. Haner

.LONG TERM...The 12z GFS has moved toward the ECMWF`s
consistently shown solution of mostly dry weather on Monday, owing
to a shortwave ridge in the wake of Sunday`s wet front. If the
12z ECMWF remains consistent, we may be able to remove rain from
the forecast from parts of the area late Monday or Monday night.

Models all show quite wet and possibly windy weather across the
area toward the middle of next week. The 12z GFS brings rain in
early Tuesday, and the 00z ECMWF holds off til Tue evening, so
there are still timing differences to resolve. Haner

&&

.AVIATION...Moist westerly flow aloft today will turn northwesterly
tonight as an upper trough passes. The air mass will remain slightly
unstable. At the surface, a moderate southeasterly pressure gradient
will keep breezy conditions over most areas today. Winds will ease
tonight.

Cigs running a mix of VFR to MVFR conditions generally. Isolated
locations under showers may dip down into IFR and locations prone to
fog may also see some reduction in visibility until conditions
improve by 18Z this morning. The air mass remains unstable enough
for a chance of thunderstorms along the coast.

KSEA...The moist and showery air mass will keep cigs around or just
around 3k ft until 19Z, then improving to more solidly VFR
conditions afterward. Surface winds will remain southerly 8-12 kt
today, then ease tonight. CHB/SMR

&&

.MARINE...A surface low will remain near northern Vancouver Island
today. The low will move toward northwest Washington and dissipate
tonight. Small craft advisory southerly winds will continue over
most waters through late this afternoon before easing over the
entire area tonight. A weak ridge will cross the region Friday.

A strong front will slowly approach the area late Saturday then move
inland Sunday morning. Inherited forecast suggests gales are likely
over the coastal waters, East entrance to the Strait, and the north
inland waters. Will evaluate the need for a gale watch for the
afternoon forecast package. CHB/SMR

&&

.HYDROLOGY...The Skokomish River at Potlatch is receding but still
remains above flood stage for now. It will almost certainly fall
below flood stage this afternoon. Elsewhere, no flooding is
expected across the area through Saturday.

The rain expected Sunday through Sunday evening will be heavy at
times with snow levels 4000-5000 feet. Rivers will rise, but the
progressive nature of the front will limit duration. As a result,
the only river that appears that it may flood is the normally
flood-prone Skokomish River in Mason County. With weather models
now in line with both amount and timing of rain, should be able
to better assess QPF today and determine by this afternoon if a
Flood Potential Outlook is warranted.

Around next Tuesday or Wednesday, Western Washington may see a
prolonged period of heavy --- but not necessarily intense ---
rainfall. If this occurs, all rivers will see rises with some
approaching bankfull. However without a particularly intense
period of rainfall, flooding concern for now is once again focused
on the Skokomish.  Haner

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until midnight PST tonight for Cascades
     of Pierce and Lewis Counties.

PZ...Small Craft Advisory until 4 PM PST this afternoon for Admiralty
     Inlet-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-Puget Sound and Hood
     Canal.

     Small Craft Advisory until noon PST today for West Entrance U.S.
     Waters Strait Of Juan De Fuca.

&&

$$

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



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