Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 121723
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 AM PST Sun Nov 12 2017

.SYNOPSIS...A frontal system will bring rain and mountain snow to
Western Washington today and tonight. A vigorous front will add
gusty winds to the rain and mountain snow on Monday. The weather
will become somewhat cooler Tuesday through Thursday, with lots
more rain and the snow level around 3000 feet. Wet and slightly
warmer weather is likely Friday and into the weekend.

&&

.SHORT TERM...Radar shows bands of moderate rain lifting north
across the area this morning. Heaviest rainfall overnight was over
the Olympics with roughly 2 to 3 inches. This is driving the
Skokomish river higher and a flood watch is now in effect for Mason
county. Refer to the hydrology section below and flood statement for
more details. Snow levels are around 4500 feet and somewhat higher
over the Olympics. Precipitation amounts will not be very
significant over the Cascades today except for a couple spots that
do well in southerly flow such as Mount Baker and Paradise. While
several inches of snow are likely in those spots today, heavier snow
seems like a better bet on Monday. The next system will bring wind
and colder air aloft which should produce some heavy orographic
snowfall. Will hold off an an advisory as more significant snow is
likely later Monday and Monday night, at least solid advisory
amounts above 4500 feet.

A quick look at the new 12z models this morning show little change
the past 24 hours. The GFS/NAM/WRF still indicate rapid low
development will begin later today and tonight arising from a short
wave feature out near 135W west of the Oregon coast. This is not the
typical wind storm pattern with jet support lacking, but models do
eventually develop a 100+ kt jet aimed at Oregon. Several other
weaker short waves are embedded in the trough but all models, even
the ECMWF/Canadian, show these system merging into a single low
center. The 00z ECWMF last night is a little stronger and also
develops a meso low northeast of the Olympics which would create
some windy conditions around Puget Sound. The current watch for high
wind looks good for the coast and north interior. There is a good
chance Puget Sound will get at least advisory levels winds. Will
make the final decision on this with the afternoon forecast
issuance. Mercer

.LONG TERM...Previous discussion...The models are in fairly close
agreement that the upper trough following the Tuesday front will
bring cool showery weather Wednesday through Thursday. The snow
level will probably be 2500 to 3000 feet, with more snow piling up
in the mountains.

It looks like the long wave trough position will shift inland at
the end of the week for a transition in the pattern Friday and
Saturday, but models differ considerably on how that will play out.
The GFS shows a warm southwest fetch of moisture taking aim at
the area Friday and Saturday. However, the model trend over the
last few runs is for this pattern to develop more slowly and for
the moisture stream to be further north. The ECMWF shows a higher
amplitude ridge, which keeps the moisture stream even further
north over British Columbia and delays its setup until Saturday.
Confidence at this point is insufficient to change the forecast,
but the GFS could be trending toward the ECMWF solution. McDonnal

&&

.HYDROLOGY...Rain will continue at times through mid week. There are
breaks between fronts and snow levels are not that high. This will
help minimize the flood threat. The only river currently at risk of
flooding the next couple days is the Skokomish river in Mason
county. A watch is in effect and it could reach flood stage as early
as tonight. The current lull in rain may cause it to come up just
short, but additional rainfall later today could drive it higher.

Besides the Skokomish river, flooding is not expected through at
least Thursday. On Friday, a transition to a warm and wet pattern
could develop, which would present a more widespread threat of river
flooding. Confidence in this is rather low, though, with poor
agreement between the models in the longer range. We`re still at a
point where we`ll have to see how this develops in the next few
days. Mercer/McDonnal

&&

.AVIATION...An upper trough remains just offshore with SSW flow
aloft over Western Washington that will strengthen through Monday.
There is a lot of shower activity or rain bands on the radar and the
mountains will be obscured much of the time. We might see a little
break as the next system gets stronger--and the low pressure area
deepens: That will turn the low level flow more easterly tonight and
could help dry the low levels a bit and break up the showers and
clouds for awhile. Rain will increase Monday however and it will
become windy Monday as this next system arrives.

KSEA...Showery rainy and a few breaks at times. Monday will be windy
with periods of heavier rain, so I will be adding some lousy weather
toward the end of the TAF for Monday.

&&

.MARINE...There are a lot of disorganized showers over the area and
a few spots still have a decent breeze--but today is a lull between
weather systems. A low pressure system offshore is forecast to
strengthen and affect the area Monday and that will be a good day to
stay off the water. The satellite image does not make it look like
this next system is going to be all that strong--but the models have
been showing a windy day on Monday pretty consistently. The 12z GFS
takes the 1000mb low that is out there now down to a 979mb center at
daybreak on Monday--and then this low center is around 985mb as it
tracks just off the north coast of WA in the afternoon. So Monday
will be windy with full 40kt gales over the waters--and the coastal
waters have a good chance of getting up to storm force. Smith Island
in the east entrance to the Strait of Juan de Fuca should also get
up around 50kt on Monday. The pressure gradient on the 12z GFS for
AST-UIL is forecast to peak at +15mb early Monday afternoon and the
gradient for OLM-BLI is forecast to peak at +9mb later Monday
afternoon, with PDX-BLI at +15mb so that is pretty impressive--
especially when you look at the satellite image and all that is out
there now is a trough and showery comma cloud at 135w. We will see
what the ECMWF does and if it matches the GFS this morning.

The forecast remains active beyond Monday as a series strong weather
systems is expected to affect the region through next week.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...High Wind Watch from Monday morning through Monday afternoon for
     Admiralty Inlet Area-San Juan County-Western Whatcom County.

     High Wind Watch from late tonight through Monday afternoon for
     Central Coast-North Coast.

     Flood watch for Mason county.

PZ...Gale Watch from this evening through Monday 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.

     Small Craft Advisory from noon today to 10 PM PST this evening
     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.

     Gale Watch from Monday morning through Monday afternoon for
     Admiralty Inlet-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 6 AM PST early this morning for
     Admiralty Inlet-East Entrance U.S. Waters Strait Of Juan De
     Fuca-Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory from midnight tonight to 6 PM PST Monday
     for Central U.S. Waters Strait Of Juan De Fuca-West
     Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

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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