Preview of NWS' New Version of Forecast
This preview is not operational and should not be used for support decisions.

Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 170922

Area Forecast Discussion
National Weather Service Seattle WA
222 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...Northwesterly flow aloft will continue with a flat
upper level ridge over the area. A weak disturbance moving through
tonight will keep spotty showers around through 08z. Otherwise, 
mainly dry conditions following through Wednesday. The air mass 
will be moist and generally stable. Cigs will transition from VFR 
to MVFR tonight as the system moves through. Some IFR is possible
during the morning hours, mainly around the South Sound and along
the coast. Cigs will become VFR during the afternoon as the upper
level ridge shifts east over the area.

KSEA...Spotty showers will be around through 07z. VFR cigs will
become MVFR tonight and continue through the morning. Improvement
to VFR cigs expected during the late morning on Wednesday. Winds
will be mainly southerly ranging from 8-14 knots. JD


.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.


PZ...Small Craft Advisory from 11 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.