Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 282246
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
300 PM PDT Tue Mar 28 2017

.SYNOPSIS...Another warm front will bring more rain and higher
mountain snow to the area tonight. Low pressure will move into
Vancouver Island with the associated cold front pushing through
Western Washington Wednesday morning. An upper trough will follow on
Thursday resulting in showers and sun breaks. High pressure will
build Friday and Friday night providing mostly dry weather. Weak
fronts may affect the region Sunday and early next week.

&&

.SHORT TERM...Radar shows a slight let up in rainfall intensity as
the first warm frontal system begins to move east of the area. IR
satellite imagery shows the next batch of warm advection rainfall
already moving inside 130 w and is just coming into range of the
coastal radar. This next system will be at least as strong, and will
likely bring another 2 to 4 inches to the Olympics and perhaps 1 to
2 inches to the Cascades with locally higher amounts. The lowlands
could still get another inch of rain on top of the .25 to 75 inch
amounts that fell through this morning. Areas northeast of the
Olympics remain mostly rain shadowed with a pronounced portion of
the eastern Strait and part of Admiralty Inlet getting very little
or no measurable rain so far. This additional rainfall will likely
cause flooding on the Skokomish river in Mason county. Other rivers
will run near bankfull and the Bogachiel running off the Olympics
could come close to flood stage, although flooding is not currently
forecast. See the hydrology section below for more detail and refer
to the latest flood statement for current information. Additional
rain will also cause the landslide risk to be even higher through
Wednesday as rain falls on saturated soils. The special weather
statement will remain in effect to address this concern.

Snow levels are on the rise this afternoon and it appears
precipitation may be close to changing over to rain at Stevens Pass
after about 5 inches fell today. Less fell at lower passes like
Snoqualmie which should stay mostly rain until the cold front brings
colder air back in on Wednesday. Snow levels remain near 4500 feet
across the north Cascades and Mount Baker is up to around 9 inches
of snow with little change in temperatures near freezing.
Eventually, even some warmer air should cause a period of mix or
rain up to at least 5000 feet later tonight. But 6 or more inches
will probably fall with total amount over a foot likely by this
evening. A winter weather advisory is in effect for west slopes of
the north Cascades of Whatcom and Skagit counties through late
tonight.

Rain will become showers on Wednesday as low pressure moves into
Vancouver Island and cooler, somewhat drier air filters into Western
Washington. Snow levels will drop back down to most pass levels and
the strong flow give some orographic help in snowfall. But the lack
of moisture should still keep amounts limited to just a few inches
or less, including the passes. A meso low is still expected to
develop on the north Kitsap peninsula. Latest models are a little
stronger that yesterday but the kbli-kpdx gradient never reaches +10
mb on most models and the wrf-gfs keeps winds generally sub-advisory
from late morning to the afternoon. Strongest southerly gusts will
occur from central/north Puget Sound to around Admiralty Inlet,
reaching 35 to 40 mph at times. A westerly push down the Strait may
also bring some gusts to the land portions of the Strait and west
side of Whidbey Island but still sub-advisory. With recent rainfall
and saturated soil, it will not be surprising to see some trees
topple in gusts of 35 or 40 mph.

The weather begins to quiet down Wednesday night and Thursday. An
upper trough will move across the region bringing showers. A Puget
Sound convergence zone is also possible, with the best chance of
this bringing enhanced showers to Snohomish and King counties.
Showers will taper off Sunday afternoon and most areas will see sun
breaks.

High pressure aloft builds Thursday night and Friday giving a period
of dry weather. The low level flow will be light and 500 mb heights
over 5700m will support warming temperatures, possibly reaching near
60s from around the central Sound to the southwest interior,
especially inland from the water. If Sea-Tac reaches 60, it will be
the first time this year.

.LONG TERM...The gfs/ecmwf are in fair agreement agreement on
showing the ridge flattening Friday night and Saturday as a system
rides over the top of the ridge. Increased cloud cover should be
expected, and a chance of light rain, especially toward the coast
and further north. Western Washington will still be in the warm
sector so high temperatures should again be mild, possibly close to
60. If less clouds prevail, some areas like the southwest interior
could reach the low 60s. But any stronger onshore flow would
certainly keep the area stuck in the 50s but still nice compared to
recent temperatures.

Models show mainly weak systems to affect the region early next
week. Some solutions keep cut off lows more offshore with less
threat of precipitation while other models are a bit wetter. Neither
the gfs or ecmwf show any significant rainfall so a lull in the
active weather is expected. All signs point toward more typical
spring like weather with some clouds, some sun, and a chance of some
showers with near average temperatures. Mercer

&&

AVIATION...A front will continue to bring rain to Western Washington
tonight through Wednesday morning. There is strong westerly flow
aloft becoming more southwest tonight. There is low pressure moving
into British Columbia and breezy southerly pressure gradients over
the area--although the gradients will relax for awhile this evening
and then pick up again overnight through Wednesday as a second
frontal wave rides up along the frontal zone. A cold frontal passage
will occur Wed afternoon with the rain turning to showers and the
air becoming unstable.

KSEA...Rainy middling TAF looks good with a decent southerly breeze
through Wednesday morning. Periods of rain will give way to showers
later Wednesday afternoon as a cold fropa occurs.

&&

.MARINE...Pretty decent southerly flow today, although that
will ease ahead of the next wave riding up the frontal zone. The
winds might increase again later tonight and Wed morning more than
the forecast calls for, so we will have to watch the developing
frontal wave and see if it spins up a deeper low. As that passes
through, around midday Wed, there will be a cold frontal passage
followed by westerlies coast and Strait of Juan de Fuca.  High
pressure will build into the region for Thursday and Friday and then
a front may reach the coast Friday night.

&&

.HYDROLOGY...Rain amounts over the past 18 hours reached roughly 3
inches along the south and west facing slopes of the Olympics. The
Cascades have seen a somewhat less, generally a half to 2 inches
with localized amounts near 3 inches in central portions. At this
time, flooding is not expected on rivers in the Cascade basins. Some
rivers will run near bankfull and bare a close watch the next 24
hours.

The only river forecast to flood is the Skokomish river in Mason
county. Minor flooding is expected tonight into Wednesday. See the
latest flood statement for current information. The Bogachiel river
is also forecast to rise tonight, but is forecast to remain just
below. However, forecasts should be closely closely the next couple
days for any changes or updated watches and/or warnings. Refer to
the latest flood watch statement on our website at
www.weather.gov/seattle

Rainfall over the lowlands will reach around 1 to locally 1.5 inches
through Wednesday morning from around Everett southward. Lowland
areas north of Everett will get less rainfall due to rain shadowing
off the Olympics. The risk of landslides is already elevated across
all of Western Washington due to saturated soil moisture due to
excessive rain the past couple months. However, the additional
rainfall will cause an even higher risk of landslides through
Wednesday over greater Puget Sound southward and toward the coast.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch for Mason county.
PZ...Small craft advisories for all waters except the central Strait
     of Juan de Fuca. Small craft advisory Grays Harbor bar for
     rough bar conditions.



&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html


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