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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 251600

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

.UPDATE...Northwesterly flow aloft this morning extends from the
northern Pacific into the Pacific Northwest, with 100kt+ jet
streak nosing into the region. At the surface, a large area of high
pressure remains centered across the northern Pacific with weak 
trough noted across far eastern Washington, allowing for onshore
low level flow.

Within the NW flow aloft this morning, a shortwave trough is seen
skirting the northern counties of Washington. Most of the deeper
layer moisture and bulk of the larger scale lift exists to the 
north and east of the area across southern BC into the ID 
panhandle and western Montana, where best shower activity is seen
in MRMS radar imagery. Nevertheless, local radar imagery from KATX
is picking up a few light echoes across Skagit and Whatcom
Counties, which is where best shortwave influence would exist.
Have added slight chance showers to this area this morning to
account for current activity. These showers will not amount to
much and are likely to fizzle out in the next few hours. Elsewhere
conditions are mostly cloudy, tho a few spots across the Olympic
Peninsula are already clear. 

Mid level heights will rise thru the day today and skies will
clear as a ridge of high pressure across the Pacific /well
offshore the southern Oregon and California coasts/ begins to
slowly build north into the northern Pacific and Pacific
Northwest. Low level flow will continue to be onshore as sfc high
pressure remains locked across the Pacific. Should be a nice day
with highs in the 60s along the coast and mountains and low to 
mid 70s interior.

Forecast is largely on track this morning so aside from slight
adjustment to morning POPs, the previous forecast has been left as
is. Previous discussion can be referenced below with updated
marine and aviation sections.



.PREV DISCUSSION... /issued 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. 
Generally a mix of VFR to MVFR this morning with marine stratus. 
Expect VFR for all by late this morning into early this afternoon as 
upper level ridging builds over the area. Winds generally light 
becoming northerly later this afternoon.

KSEA...VFR cigs this morning with MVFR possible briefly until 18z-
19z. VFR this afternoon. NE winds becoming SW this afternoon 
becoming northerly this evening. Winds 4-8 knots. JD


.MARINE...Onshore flow continues into Monday. 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 monitor latest data this afternoon. 
Offshore flow returns on Tuesday and will persist through much of 
the week. JD


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