Avalanche in Washington, April 11, 2017

Area Forecast Discussion
National Weather Service Seattle WA
945 PM PDT Mon Apr 10 2017

.SYNOPSIS...Showers will decrease overnight as an upper level
trough over the area moves north. Short break in the weather on
Tuesday before another weather system brings rain to Western
Washington Tuesday night and Wednesday morning. Cool showery
weather will continue Thursday and Friday. Dry weather is expected
for the weekend as temporary high pressure builds over the
region.

&&

.SHORT TERM...Doppler radar shows plenty of shower activity over
western Washington this evening but the activity has decreased a
little in the last hour or two both over the interior and off the
coast. Water vapor imagery puts the upper level trough axis near
the Canadian border. Temperatures at 9 pm/04z are in the 40s.

Upper level trough will continue to lift north tonight. Air mass
stabilizing over western Washington overnight into Tuesday
morning. Shower activity will come to an end after midnight
tonight. The air mass over the area is extremely cold for April
with the 500 mb temperature on the KUIL 00z sounding -34C with
the 700 mb temperature -13C. Already some breaks in the cloud
cover and with the cold air aloft low temperatures Tuesday morning
will be chilly with 30s common and the colder locations getting
down to near freezing.

A break in the action on Tuesday as the next system, currently
near 38n/133w, moves northeast but remains south of the area.
Surface gradients going offshore on Tuesday but will be light
early on which brings up the possibility of some early morning fog
in the fog prone locations like the Southwest Interior. Model
cross sections show increasing moisture Tuesday afternoon above
500 mb. Will stay with the mostly sunny wording in the zones.
Highs even with the sunshine will only be near normal, in the 50s.

System to the south will continue to move north Tuesday night with
rain spreading over the entire area overnight into Wednesday
morning.

Steady rain coming to an end in the morning hours on wednesday as
the front moves north of the area. Model solutions remain
consistent keeping the central pressure of the surface low near
1000 mb with the low tracking just off the coast Wednesday
morning. Not looking for advisory level winds in the area but it
will be breezy along the coast. Upper level low off the Oregon
coast Wednesday afternoon with southwesterly flow aloft feeding
more showers into western Washington. Even with the rain highs
will be similar to Tuesday, near normal in the 50s.

Upper level low remaining off the coast Wednesday night and
Thursday with weak disturbances rotating around the low moving
through western Washington at times keeping showers in the
forecast. Current zone package has the trends covered. No update
this evening. Felton

.LONG TERM...Previous discussion follows. The 12z GFS shows a
band of rain around daybreak Thursday moving north into B.C. and
then winds aloft shift from SE to SW with a trough passage midday
Thursday. Thursday afternoon and night will continue to be showery
with the center of a cold upper low just offshore affecting the
Pacific Northwest. A lot of the cold air aloft should progress
inland Friday and by that time showers over Western Washington
should be decreasing. Saturday and Sunday both look dry in the 12z
global models before another cold upper low nears the area and
brings rain or showers into the area starting around next Monday.

&&

.AVIATION...The upper level trof over the region will continue to
weaken and lift northeast over British Columbia tonight. Flow aloft
will remain moderate and westerly. At low levels sly flow will
continue this evening, weakening after midnight before becoming
light northerly early Tuesday morning. Weakly unstable air mass over
the area still producing some showers this evening as the upper
level trough continues to exit the area. The will give way to drier
more stable conditions tonight. MVFR conditions still possible with
showers and patchy shallow fog remains possible late tonight into
early Tuesday morning with clear skies.

KSEA...VFR. Gusty s or sw winds still hanging on, but are expected
to fade over the next couple of hours becoming northeast 5 to 10 kt
around daybreak Tuesday morning.

&&

.MARINE...A the surface low over the northern offshore waters
this afternoon has moved inland over British Columbia this evening.
Rising pressure behind the low will help weaken the onshore flow
overnight. Westerly swell that has been producing hazardous seas for
much of the Coastal Caters and the West Entrance of the Strait is
expected to subside Tuesday morning. The Small Craft Advisory for
Hazardous seas remains in effect through 6 AM. Northerly flow on
Tuesday will give way to offshore flow as a 999 mb low near the
southwest OR coast moves north Tuesday night. This system will
weaken as it moves across the WA coastal waters on Wednesday but
will likely produce small craft advisory conditions for the Coastal
Waters and some of the interior Waters Wednesday afternoon and
evening.

&&

.HYDROLOGY...No river flooding is expected for the next seven days.

&&

.CLIMATE...It will be a cool morning around western Washington
Tuesday morning. The record low for Seattle is 35 degrees which
last occurred in 1991.( low temps of 35 degrees on the 11th in
Seattle were also recorded in 1981, 1966, 1947 and 1945 ). This is
the last time a record low was tied in the month of April in
Seattle. The last time a record low was set/broken in Seattle in
April is April 5th 1975 ( 29 degrees ). Since the last new record
low set on April 5th 1975 26 out of the 30 days in April have set
record highs. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory for hazardous seas until 6 AM PDT Tuesday
     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 for rough bar until 3 AM PDT Tuesday for
     Grays Harbor Bar.

     Small Craft Advisory for hazardous seas until 3 AM PDT Tuesday
     for West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until midnight PDT tonight for 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