Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

428
FXUS66 KSEW 221009
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
309 AM PDT SUN MAY 22 2016

.SYNOPSIS...An upper low over the Pacific Northwest will result in
a mix of mainly light showers and sun breaks the next couple days.
Weak high pressure aloft will bring dry weather in the middle of
next week. Another upper low will move southeast over the area
late in the week for a return to cooler and showery weather.

&&

.SHORT TERM...A large upper trough is still over the region early
this morning, centered near the border of Montana, British
Columbia, and Alberta. Light spotty showers lingered over western
Washington during the night, mainly over the Cascades. Otherwise
weak onshore flow and lots of moisture in the lower 12000 ft of
the atmosphere are keeping skies cloudy. Temperatures at 3 am were
in the upper 40s to mid 50s.

The upper low will move slowly northeast across southern Alberta
today, and a weak upper shortwave extending west from the low will
move south from British Columbia into Washington. The shortwave
combined with low level onshore flow, moisture, and weak
instability will produce some showers again today -- mainly during
the afternoon over the mountains, in the south part of western
Washington, and probably in a weak Puget Sound Convergence zone
over the north sound. There will likely be few if any showers over
much of the greater Seattle metropolitan area, the coast, and the
north interior. It will be only a little warmer today with highs
around 60.

The upper low will move slowly east into southwest Saskatchewan
tonight and Monday, and the upper shortwave will move south of our
forecast area. There will be little change in the weather though,
with mostly cloudy skies and light spotty showers over the
mountains and from around the Seattle area southward. High
temperatures should be a couple degrees higher than today.

The upper trough will weaken Monday night and Tuesday, and drier
light northerly flow aloft will develop over western Washington.
The threat of showers should be limited to the mountains, and any
precipitation should be quite light. The lowlands should have a
bit more sunshine Tuesday with temperatures slightly higher yet.
McDonnal

.LONG TERM...Light northwest flow aloft will continue Wednesday,
ahead of an upper ridge well offshore. However, the leading edge
of another shortwave digging southeast could reach the area late
Wednesday. It should be similar to Tuesday, with partly sunny
skies and a chance of light showers over the mountains. The upper
trough will move southeast across the Pacific Northwest Thursday
and Friday for a better chance of more widespread showers over
western Washington. There is still a chance of showers Saturday,
but we will probably be getting into the somewhat drier northerly
flow on the west side of the upper trough. McDonnal

&&

.AVIATION...An upper level trough will remain over the Pacific
Northwest through the day today and into Monday. The low associated
with this trough is currently centered over western MT. As this low
continues its northeastward path into Alberta...flow aloft will turn
from easterly to become northerly as the morning progresses. The air
mass is expected to remain generally stable and moist...allowing for
MVFR cigs and a chance for showers area-wide today. Cigs may start
lifting during the afternoon somewhat...allowing for locations with
MVFR conditions to possibly creep back into VFR by evening. The very
slow movement of this trough along with a shortwave disturbance
within it will allow for the prospect for showers and low-end
VFR/high-end MVFR cigs to linger into Monday.  SMR

KSEA...VFR cigs over the terminal now will likely lower into MVFR by
sunrise this morning before rebounding to VFR by afternoon. Models
disagree on the prospects of precip today...so have opted to say
there may be showers in the vicinity this afternoon. Surface winds
will remain steadily S to SW. Morning and early afternoon speeds
generally around 10 kts before easing to around 5-8 kts by this
evening. SMR

&&

.MARINE...A weak surface low over Alberta will maintain
weak to moderate W-SW onshore flow through the day today and into
Monday. This flow will help SCA conditions in the Strait to persist
into the morning before starting to taper off. Only ob reading SCA
winds at the time of this writing is Race Rocks...so will leave
Strait in SCA at least through 5 AM PDT.  May need to extend by an
hour or two...but will wait and see.  Cancelled SCA for Puget Sound
and...given how wind speeds are tapering off in the east
Strait...considering dropping it for the N inland waters too. Winds
expected to remain below criteria today and at least into Monday.

Surface low pressure wobbles over portions of southern Canada for
the start of the week...but this will not really impact the onshore
flow pattern very much. Onshore flow should continue through mid
week with varying strength, probably weaker on Monday then stronger
on Tuesday and Wednesday. SCA winds will occur mainly in the
evenings. SMR

&&

.SEW Watches/Warnings/Advisories...
WA...None.
PZ...Small Craft Advisory until 5 AM PDT early this morning for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle

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



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.