Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 201616
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
916 AM PDT Mon Mar 20 2017

.SYNOPSIS...The upper level ridge that brought a beautiful day to
the area on Sunday has moved off to the east. Clouds will increase
today and rain will become likely over the Olympic Peninsula as a
trough of low pressure moves approaches the area from the south. A
series disturbances embedded in southwesterly flow aloft will move
through the area starting late tonight and continuing through
Friday; each will bring clouds and some rain along with locally
breezy conditions. The best chance for a break in the wet weather
looks to be on Thursday. The upper level flow will become
westerly over the weekend with the precipitation type changing to
showers.

&&

.SHORT TERM...Upper level ridging has moved into Eastern
Washington early this morning. The surface high responsible for
the north to northeast surface flow is now located over the
Canadian prairies and moving away. Today will be a transition into
a pattern we know well - cloudy and cooler than normal with a
little rain at times to make things a bit dreary. Rainfall now
seen on the NWS radar out of Portland that extends from around
Salem westward will lift northward into the Olympic Peninsula this
afternoon. With the low level dry air in place due to moderate
offshore flow, expect precipitation amounts to be light. The
precipitation will have a difficult time moving north of Olympia
in the interior. Temperatures today will rise into the 50s.

The first frontal feature over western portions of Oregon is
disorganized and will likely not survive in tact all the way to
the Canadian border. A second front wrapping around a 999 mb low
centered near 40N 138W will lift northward into the area late
tonight and Tuesday morning increasing rain chances a bit. Again,
low level flow will be offshore and the front will be becoming
increasingly ragged as it moves to the north, so expect
precipitation amounts to be light. With the clouds and moisture
around, lows tonight will remain in the 40s and highs will get
back into the upper 40s to mid 50s.

Another band of moisture with a frontal system will move into the
area Tuesday night and Wednesday. Lows Tuesday night will be in
the 40s and highs on Wednesday again in the upper 40s to mid 50s.

Current forecasts are in good shape. No morning update is needed.
Albrecht

.LONG TERM...From the previous long term discussion: Extended
models hinting a brief dry period Thursday with the next system
getting pushed to the south by a digging upper level trough out
near 145w. Confidence not real high but after all of the
precipitation in March will try and single out a dry period on
Thursday. System moves into western Washington Thursday night into
Friday for another round of rain. Upper level trough moving
through on Saturday keeping showers in the forecast with a
splitting system approaching on Sunday keeping a chance of
precipitation in the forecast. High temperatures will be a little
below normal through the period, in the upper 40s and lower 50s.
Lows will mostly be in the 40s except for Wednesday night where
lows will be in the 30s to lower 40s. Felton

&&

.AVIATION...An upper ridge over the area will shift inland today.
Southwest flow aloft today will become southerly tonight. At the
surface, moderate to strong northeasterly gradients will become more
easterly tonight. A frontal system will spread moisture into the
area from the south later today and tonight. The air mass is stable
with increasing mid and high clouds. Conditions are forecast to
remain VFR today and tonight.

KSEA...High clouds this morning. Mid clouds will arrive from the
south this afternoon and evening. Northerly wind 10-15 kt. CHB

&&

.MARINE...Northerly and offshore flow will produce small craft
advisory north and east winds at times over all waters today. Winds
over the inland waters should ease tonight as as weak frontal system
arrives from the south. Additional frontal systems - around Tuesday
night and Thursday night - will affect the waters later in the week
for off and on small craft advisory winds this week. CHB

&&

.HYDROLOGY...The White River at R Street in near Auburn is just
below its flood stage of 5500 cubic feet per second. The river is
forecast to recede slowly over the next few days. Elsewhere, river
flooding is expected for the next 7 days.

The ground is saturated due to the heavy rainfall of the past 2
months. Some shallow landslides continue to be observed this
morning. Rain at times over the next few days will keep the threat
for shallow landslides elevated across the area for the next few
days, and the special weather statement that is out for the
landslide threat will be continued. Albrecht

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 6 PM PDT 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 Point Grenville To Cape Shoalwater 10
     To 60 Nm-West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until noon PDT today for Central U.S.
     Waters Strait Of Juan De Fuca-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 Out 10 Nm-Puget
     Sound and Hood Canal.

&&

$$

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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