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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 251027

Area Forecast Discussion
National Weather Service Seattle WA
327 AM PDT Sun Aug 25 2019

.SYNOPSIS...Upper level ridge offshore slowly building today as 
an upper level trough moves east of the area. The ridge will
continue to build, moving inland on Tuesday and remaining in place
into next weekend. The ridge will weaken somewhat on Thursday. Low
level flow turning offshore later Monday with the strongest
offshore flow Tuesday into early Wednesday. Low level flow 
turning back onshore Thursday.


.SHORT TERM /TODAY THROUGH TUESDAY/...Satellite imagery shows
mostly cloudy skies over most of the area at 3 am/10z with a break
in the cloud cover from the central coast southeast into Cowlitz 
county. Temperatures at 3 am were in the mid 50s to lower 60s.

Pretty "normal" late August day on tap for Western Washington
today. Upper level trough well east of the area this afternoon
with weak upper level ridging developing over the area. In the
lower levels light onshore flow. Marine layer is shallow and with
the low level flow trending toward neutral there will be no 
reinforcement of the marine layer. End result plenty of afternoon 
sunshine after the morning cloud cover. Temperatures near normal, 
mid 60s to mid 70s. 

Upper level ridge offshore continuing to build and move east
tonight into Monday. Surface gradients going northwesterly
overnight which does not bode well for morning stratus on Monday
for the interior. Temperatures aloft warming and by 00z Tuesday 
500 mb heights in the mid 580 dms. Afternoon seabreezes will put 
a cap on the high temperatures near the water but even with the 
seabreezes most places a few degrees warmer Monday with highs in 
the 70s. Lows tonight in the 50s.

Upper level ridge moving over Western Washington Monday night into
Tuesday with the low level flow turning offshore. Winds aloft in
the lower levels also turning offshore but not very strong, 850 mb
winds easterly less than 10 knots. Temperatures aloft continuing 
to warm with 850 mb temps around plus 20C by 00z Wednesday. All 
this adds up to much warmer temperatures on Tuesday with highs in 
the 80s even along the coast. Warmest locations will be near 90. 

agreement with upper level ridge over Western Washington and low
level offshore flow weakening on Wednesday. Right now it looks 
like the high temperatures will peak Wednesday for the interior 
with a little cooling along the coast with the lack of offshore 
flow allowing afternoon seabreezes to kick in. Highs in the 
interior from the upper 70 to lower 90s with highs cooling back 
down into the 70s along the coast. Upper level ridge weakening on 
Thursday with temperatures aloft cooling and the low level flow 
going light onshore. Weak surface low off the coast preventing a
stronger onshore flow scenario from developing. The weak marine 
push Thursday will drop high temperatures up to 5 degrees with mid
70s to mid 80s for the interior and lower to mid 70s for the 
coast. Weak surface low off the coast will continue to put a 
damper on the low level onshore flow Friday. 00z model runs are 
much slower with the approach of the next upper level trough 
next weekend. The slower approach actually pumps the upper level 
ridge back up a bit over the area. Low level flow remaining weakly
onshore Saturday as well. With the lack of any significant low 
level onshore flow will stay on the warm side of guidance for the 
high temperatures Friday and Saturday, in the 70s to lower 80s. 
Will also take the chance of showers out of the forecast for 
Saturday with the slower approach of the upper level trough. 


.AVIATION...Northwesterly flow aloft as an upper level trough 
departs the Pacific Northwest. Onshore flow at the surface. VFR 
conditions as of 8Z. Brief period of MVFR ceilings later this 
morning as marine stratus makes another push inland. Clearing 
expected by the late morning/early afternoon as upper level ridging 
begins to build into the area.  

KSEA...VFR conditions early this morning with mid level clouds. 
Brief period of MVFR ceilings possible later this morning as marine 
stratus moves inland before improvement again by this afternoon. NE 
winds 5-10 kts become southwesterly after 12z. CEO 


.MARINE...Onshore flow continues into Monday. A westerly push down 
the Strait tonight behind a weak front has resulted in small craft 
advisory winds. Winds will gradually weaken this morning. Another 
round of small craft advisory winds through the Strait possible this 
evening but for now have forecast just under criteria and will let 
day shift monitor trends. Offshore flow returns on Tuesday and will 
persist through much of the week. CEO


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


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.