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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

                            
000
FXUS66 KSEW 171543
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
843 AM PDT Wed Apr 17 2019

.UPDATE /TODAY THROUGH SATURDAY/...Visible satellite imagery
shows abundant low level moisture across the region today. Taller
mountains are popping out above the clouds meaning tops are about
7000 feet or so. Some clearing is seen NE of the Olympics and 
appears to be popping up in portions of the Puget Sound region. 
Forecasts were adjusted to reduce shower chances today and refine 
cloud cover toward observations and satellite imagery.

On Thursday, the frontal system wrapping around a low in the
central Gulf of Alaska will move toward the area bringing an
increase in clouds and rain from the northwest. The front will
move across the area Thursday night into early Friday morning,
then showers will follow the front on Friday. Saturday will see
gradual clearing.

Temperatures, sky cover, POPS and QPF were updated toward latest
model solution tonight through Saturday. Albrecht

&&

.SYNOPSIS...Scattered showers will gradually decrease today as a
weak frontal system dissipates over the region. Another frontal 
system will reach Western Washington Thursday and Friday. The 
weekend will be dry before fronts arrive next Monday and Tuesday.

&&

.AVIATION...An upper ridge will move over the area today and then 
shift inland tonight as a frontal system approaches from the west.
West to northwest flow aloft today will become west to southwest 
tonight. At the surface, a ridge along the coast today will weaken
tonight. The air mass is moist and stable.

There will be a mix of low and mid clouds today. In general, 
ceilings should improve a bit this afternoon and evening as the
low levels dry ahead of the next approaching frontal system. Sun 
breaks are likely in the afternoon.

KSEA...MVFR ceilings this morning will rise to VFR in the 
afternoon and evening. Southerly wind 10-15 kt. Schneider/CHB

&&

.MARINE...A ridge along the coast will sharpen today, giving onshore 
flow. Small craft advisory strength south winds are forecast in 
Puget Sound today.

A frontal system will reach the area Thursday and move inland 
Thursday night and Friday. Small craft advisory strength southerly 
winds are expected for most of the coast Thursday ahead of the 
front. Westerly advisory level winds will develop on the coast 
and in the strait Friday behind the front, as onshore flow 
increases. Gale force west winds are possible in the strait Friday 
evening. 

Onshore flow will prevail Saturday and Sunday with high pressure
offshore and lower pressure inland. Schneider/CHB

&&

.PREV DISCUSSION... /issued 227 AM PDT Wed Apr 17 2019/ 

SYNOPSIS...There will be a break between weather systems today. A 
frontal system will reach Western Washington Thursday and Friday. 
The weekend will be dry before fronts arrive next Monday and Tuesday.

SHORT TERM /TONIGHT THROUGH FRIDAY/...A weak temporary upper ridge 
will build over the area today through this evening. The ridge axis 
shifts inland overnight and by daybreak Thursday it will be raining 
on the north coast--and by late afternoon it should be raining over 
most of Western Washington. Thursday night will see the front move 
slowly through the area. On Friday the areas of rain will turn to 
showers with a good chance of a convergence zone. The snow level 
will be rather high Thursday and Thursday night before dropping 
back down around pass level Friday.

LONG TERM /SATURDAY THROUGH TUESDAY/...The weekend should be dry 
with some sunshine. A quick moving front will arrive Monday and yet 
another quick moving front should arrive later on Tuesday. Both 
systems look like they ought to produce blustery weather with a 
pretty decent zonal jet across the eastern Pacific, but the sea 
level pressure forecast in the GFS is pretty unremarkable. The ECMWF 
has a deeper low with the first system--but it tracks even further 
away than the GFS--into Queen Charlotte Sound on Monday. The second 
system in the Euro is but a 1010mb low into Tofino Tuesday evening, 
but we might get some blustery weather out of that one.

AVIATION...An upper ridge will move over the area today and then
shift inland tonight as a frontal system approaches from the west.
West to northwest flow aloft today will become west to southwest
tonight. At the surface, a ridge along the coast today will weaken
tonight. The air mass is moist and stable.

There will be a mix of low and mid clouds today. In general,
ceilings should improve a bit this afternoon and evening as the
low levels dry ahead of the next approaching frontal system. 

KSEA...Ceiling will probably bounce up and down this morning then
settle into a mid level layer this afternoon and evening.
Southerly wind 10-15 knots. Schneider

MARINE...A ridge will be over the coast today with onshore flow
and small craft advisory strength south winds for Puget Sound. 

A frontal system will reach the area Thursday and move inland 
Thursday night and Friday. Southerly small craft advisory strength
winds are expected for most of the Coastal Waters Thursday ahead 
of the front. Westerly small craft advisory strength winds will 
develop for the Coastal Waters and the Strait Friday behind the 
front as onshore flow increase. Gale strength west winds are 
possible in the Strait Friday evening. 

Onshore flow will prevail Saturday and Sunday with high pressure
offshore and lower pressure inland. Schneider

HYDROLOGY...Rivers like the Nooksack and Stillaguamish are forecast
to rise--but crest well below flood stage on Friday. Flooding is not
expected in the next seven days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 8 PM this evening to 5 PM PDT Thursday 
     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.

     Small Craft Advisory until 8 PM PDT this evening for Puget Sound 
     and Hood Canal.

&&

$$

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