Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 181054
Area Forecast Discussion
National Weather Service Seattle WA
354 AM PDT Tue Apr 18 2017
.SYNOPSIS...A frontal system will move through the area this
morning. Unstable air behind the front will bring a threat of
thunderstorms to Western Washington this afternoon. Another
system will arrive on Wednesday. An upper level trough will move
through on Thursday with a ridge of high pressure following on
Friday. Weak weather systems will arrive Saturday and Sunday, with
a stronger system possible Monday.
.SHORT TERM...Satellite imagery shows upper level low well
offshore with a weak front along the coast this morning. Doppler
radar has a consolidated area of precipitation echoes from near
Forks back down to near Portland but most of the precipitation is
not reaching the ground. With cloudy skies overnight temperatures
are mild, in the mid 40s to mid 50s at 3am/10z.
Front moving through the area this morning. Rainfall amounts will
be light. In general a tenth of an inch or less. Behind the front
the air mass becoming somewhat unstable this afternoon. Lifted
indexes range from 1 to -2, 500 mb temperatures are as cold as
-30C and the convective temperatures are in the mid 50s. Satellite
imagery shows some post frontal clearing which will push high
temperatures up to near 60 over much of the interior. In addition
to the shower activity, have expanded the chance of afternoon
thunderstorms to all of the land areas this afternoon.
Shower activity diminishing quickly tonight with the end of the
daytime heating and the approach of the next front. Weak upper
level ridge out ahead of the front moving over the area later
tonight into early Wednesday morning as the front offshore becomes
more north south oriented. Some clearing possible overnight but
mid and high clouds out ahead of the front will spread over the
area after midnight. This will keep lows in the mid 40s.
Front becoming more parallel to the flow aloft on Wednesday. This
will slow its eastward movement down. The 00z model runs don`t
have the front to the coast by 00z Thursday and have locations
east of the Puget Sound dry from 12z-18z Wednesday. Will cut back
on the pops for Wednesday morning for the eastern sections of the
area but will not eliminate them entirely yet. Rain over the
entire area in the afternoon as the front moves closer to the
coast. Highs will once again be near normal, 50s and lower 60s.
Front drags through the area Wednesday night with the
precipitation changing from rain to showers. Lows will be in the
Upper level trough moving through on Thursday with the
possibility of isolated thunderstorms in addition to the showers.
Highs will drop back into the 50s with the cooler air mass
.LONG TERM...Extended models in good agreement with an upper level
ridge moving over the area on Friday. Ridge axis over the interior
in the afternoon with the 500 mb heights up near 570 dms. Surface
gradients going offshore with surface high pressure over Eastern
Washington. Right now Friday looks similar to Easter Sunday
temperature wise. Have added a few degrees to the highs pushing
the warmer locations into the mid and upper 60s.
Western Washington weather returns to its regularly scheduled
programming since the first of October on Saturday as the upper
level ridge moves east and a negatively tilted front moves into
the area. Upper level trough moves through on Sunday followed by a
stronger front on Monday leaving Friday as the only dry day in the
extended period. Highs will lower back down into the 50s and
lower 60s on Saturday and remain in that range through Monday.
.AVIATION...A low pressure system offshore will send a series of
disturbances through Western Washington today through Thursday.
There will be showers at times and the air will be somewhat unstable
with southwest flow aloft. The mountains will be obscured at times.
KSEA...At 2am radars show some showers moving on to the coast which
should be into Puget Sound around daybreak. Another band of showers
should arrive later in the day.
.MARINE...A series of disturbances will move through the
waters over the next few days. This will bring off and on small
craft advisory winds to the normal breezier spots. High pressure
will build over the waters Thursday night with offshore flow
developing on Friday. Another frontal system will reach the area on
.HYDROLOGY...River flooding is unlikely over the next seven days.
.CLIMATE...With measurable rain on Monday in Seattle that makes
138 days with measurable precipitation since the water year
started October 1st. This is the highest number of days of
measurable precipitation on record in Seattle between October 1st
and April 30th. The old record number was 137 days in 2010-11 and
1998-99. The current water year total of 43.22 inches is the
third highest October through April precipitation amount surpassed
only by the record 44.52 inches in 2015-16 and 43.82 inches in
For the month of April in Seattle through the 17th the monthly
rain total is 2.44 inches. Normal for the entire month is 2.71
PZ...Small Craft Advisory until 5 AM PDT Wednesday 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.
You can see an illustrated version of this discussion at