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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 171009

Area Forecast Discussion
National Weather Service Seattle WA
309 AM PDT Wed Jul 17 2019

.SYNOPSIS...A well organized for July frontal system will move
through Western Washington later this afternoon into the early
evening hours. A Puget Sound convergence zone is likely behind the
front. An upper level trough over British Columbia will keep a
mention of showers in the forecast through Thursday night. Drying
trend Friday. Warming trend over the weekend as an upper level
ridge builds to the east with dry weather continuing into the
first part of next week. 


.SHORT TERM /TODAY THROUGH FRIDAY/...Satellite imagery shows
cloudy skies over Western Washington early this morning. Rain out
ahead the approaching front has already reached the North Coast
and portions of the Strait of Juan de Fuca. Temperatures at 3
am/10z were in the mid 50s to mid 60s. 

Frontal system moving down the British Columbia coastline reaching
Western Washington late in the day. The system is well organized
with the support of a healthy 140 knot jet over the Eastern
Pacific ( it is going to be howling on top of Mount Rainier
beginning later this morning ). Rain will spread over the area 
from northwest to southeast this morning. With the flow aloft 
westerly there will be a rain shadow in the Central Puget Sound.
Eventually this afternoon the moist air mass will overcome the 
rain shadow with even the Seattle metro area getting a couple of 
hundredths of an inch of rain. In addition to the rain, breezy 
conditions developing this morning and continuing into the 
afternoon hours. The Olympia to Bellingham surface gradient is 
forecast to be around 3 mb for most of the day. Clouds and rain 
will keep the temperatures from warming up very much from the 
current readings with high only in the 60s.

Front moving into the Cascades early this evening with a Puget
Sound convergence zone forming quickly behind the front. Flow
aloft west northwesterly which will push the convergence south
into the northern portion of Seattle. Outside of the convergence
zone little in the way of cool air behind the front so just a
chance of showers for the remainder of the area. Lows tonight in
the 50s.

Convergence zone dissipating early Thursday morning. Upper level
trough settling over the area during the day with most of the
energy associated with the trough to the north over British 
Columbia. Will dry out the southern portion of Western Washington
in the afternoon keeping a chance of showers from about Olympia
northward. Highs will still be below normal Thursday, in the mid
60s to lower 70s.

Weak upper level trough continuing to hang around Western
Washington and Southern British Columbia through Friday. As the
trough continues to weaken shower chances will eventually go away
and cloud cover will slowly decrease to the point of partly to
mostly sunny skies Friday afternoon. Highs will start to warm up
a little with mid 60s to mid 70s Friday. Lows Thursday night in 
the 50s. 

.LONG TERM /SATURDAY THROUGH TUESDAY/...Extended models in good
agreement with the upper level trough kicking out to the east and
a weak ridge moving through the area Saturday. Four corners high
beginning to build back to the northwest with another upper level
trough digging off the coast Sunday through Tuesday. The trough is
far enough to the west to keep any mention of showers out of the
forecast. Southwesterly flow aloft with 500 mb heights rising
into the lower 580 dms. Varying degrees of low level onshore flow
will keep temperatures from getting too warm Sunday through
Tuesday. An increase in onshore flow Tuesday will cool highs back
down into the upper 60s to upper 70s from the 70s to lower to mid
80s readings Sunday and Monday. Felton


.AVIATION...Increasing westerly flow aloft today as an upper level 
trough and associated frontal system approach the region. 
A warm front draped across the coastal waters will shift 
onshore this morning with ceilings falling to low MVFR or 
occasional IFR as light precipitation spreads across the area. 
The mountains will be obscured. The trailing cold front will sweep 
across the area by early this evening. Low level onshore flow will 
increase behind the cold front for gusty surface winds for many area 
terminals this evening. Some modest improvement in ceilings possible 
late tonight as post-frontal showers dissipate.

KSEA...Ceilings lowering to MVFR this morning as some light 
precip spreads into the area. Ceilings could lower to occasional IFR 
at times between 18Z and 00Z as a warm front pushes into the area. 
Surface winds S-SW 8 to 14 knots rising to 12 gusting 20 knots at 
times with the passage of a cold front this evening.  27


.MARINE...A front will reach Western Washington today and 
south winds will pick up with that front with small craft advisory
strength winds likely most waters. Westerly gales are
expected behind the front tonight behind the front in the central 
and eastern Strait of Juan de Fuca. Onshore flow Thursday 
will ease Friday and then the winds over the weekend should more 
typical summertime diurnal sea-breezes with advisory level 
winds in the central and eastern strait each late afternoon and 
evening. 27 


.HYDROLOGY...The daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed
until then.


.CLIMATE...With the forecast high of only 67 degrees in Seattle 
today will be the 4th day this month with a high in the 60s.
The total number of days with highs in the 60s for the last four
Julys, 3. The last time Seattle had more than 3 days with highs 
in the 60s in July, 2012 with 10 days. Felton 


PZ...Small Craft Advisory until 6 AM 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-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 until 7 PM PDT this evening for East 
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Warning from 7 PM this evening to 3 AM PDT Thursday for 
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance 
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 11 PM PDT this evening for Northern 
     Inland Waters Including The San Juan Islands.

     Small Craft Advisory until 2 AM PDT Thursday for Admiralty Inlet.

     Small Craft Advisory until 7 PM PDT this evening for Puget Sound 
     and Hood Canal.