Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 231107
Area Forecast Discussion
National Weather Service Seattle WA
407 AM PDT FRI SEP 23 2016
.SYNOPSIS...A weak front will move inland today for some rain. A
strong ridge will bring dry and warmer weather Saturday through
Monday with the warmest weather in September possible on Monday. A
marine push will cool things down on Tuesday. Upper level trough
moving out of the Gulf of Alaska in the middle of the week will
bring a threat of more rain to the area by late in the week.
.SHORT TERM...Satellite imagery shows cloudy skies over most of
Western Washington early this morning. Doppler radar has precipitation
echoes as far east as Mason county but most of the precipitation
is staying aloft. Rain just began at KUIL in the last hour. There
is a big hole in the cloud cover over the metro area right now
with the Moon and Orion shining brightly in the early morning sky.
Temperatures at 3 am/10z were in the upper 40s and 50s.
Weak cold front offshore near 129w at 10z will slowly move east
today. The front will continue to weaken with its parent low well
north of the area moving north throughout the day. Satellite
imagery already shows cloud top temperatures warming offshore. A
good sign the front is already weakening. Front reaching the
Washington Coast midday with what is left of the front moving
through the interior in the afternoon. Rain will spread inland
later this morning and continue into the afternoon hours.
Precipitation amounts will be light. The flow aloft will continue
to be westerly so expect a little bit of a rain shadow over the
central sound. With the front weakening will only go for chance
pops from about Seattle to Everett because of the rain shadow.
With plenty of cloud cover today will be the coolest day in over
three months with highs a couple of degrees either side of 60
( Forecast high of 61 for Seattle coolest day since 60 on June
14th. The last time Seattle had a high only in the 50s was May
21st with a high of 58.)
No follow through behind the front this evening. With the front
splitting, weakening and slowing down will leave some chance pops
in the evening hours for the eastern portion of the area. Weak low
level onshore flow will keep the lower levels moist so plenty of
cloud cover around even without any precipitation. Lows will be in
the upper 40s to mid 50s.
Upper level ridge offshore building quickly on Saturday with 500
mb heights rising into the lower to mid 580 dms by late Saturday
afternoon. Surface high pressure building in as well. The
combination of these two factors will push the next frontal system
to the north of the area late Saturday. Even with the warming
temperatures aloft there is little flow in the lower levels so
expect some areas of fog and low clouds in the morning hours
before the sun breaks out in the afternoon. With a slow start to
the sunshine highs will be near normal on Saturday, in the 60s and
Upper level ridge continuing to build over the area Saturday night
and Sunday. By late Sunday afternoon model 500 mb heights are in
the lower 590s dms over the southern portion of the area. Flow in
the lower levels turns northerly but does not really go offshore.
This will keep high temperatures from warming up too much even
with the warm air aloft. Highs will be above normal on Sunday but
not near record levels, mostly in the 70s to lower 80s.
.LONG TERM...Extended models in good agreement early on in the
extended period with the low level flow turning offshore briefly
on Monday before the flow turns back to onshore during the day.
Upper level ridge shifting east of the area on Monday but 850 mb
temperatures peak on Monday, in the plus 13 to 16c range. The
little bit of offshore flow in the morning will help temperature
get off to a warm start. Expect Monday to be the warmest day in
the forecast period...except on the coast where the transition to
onshore flow will take place by Monday afternoon capping high
temperatures near 70. For the remainder of the area highs in the
70s and lower 80s.
Onshore flow continuing Monday night into Tuesday. Model
consistency starts to break down on Tuesday with the GFS bringing
a very weak shortwave into the area while the ECMWF does not have
this feature. Either way with the marine push Monday night Tuesday
will be much cooler with highs as much as 15 degrees lower than on
Monday, mostly in the 60s. Both models show an upper level trough
digging well offshore on Wednesday. The GFS keeps the trough
offshore on Thursday while the ECMWF moves the trough into Western
Washington. Did not add any pops to the forecast for the interior
on Thursday but did increase the cloud cover. Highs in the 60s
will be common both for Wednesday and Thursday. Felton
.AVIATION...An approaching front will bring increasing clouds,
lower ceilings and rain to the area today. Light westerly flow aloft
will strengthen this morning, with strengthening southerly flow at
low level flow ahead of the front. The generally dry and stable air
mass this morning will give way to lower ceilings and rain along the
coast this morning, spreading inland via the strait and Chehalis Gap
and through the remainder of the interior by around midday. In
addition to MVFR ceilings with rain, there will also be gusty
southerly winds ahead of the front.
KSEA...VFR conditions this morning will give way to increasing
clouds at all levels. Light rain is possible this morning, but
expect rain shadowing to mitigate the impact of the front through
the metro area today. Ceilings may dip to MVFR levels early this
afternoon with the front. Southerly winds 6-10 KT will become
southwesterly 10-12 KT gusting to 15 KT during the afternoon.
.MARINE...A front working into the region this morning
has already boosted southerly winds over the coastal waters to small
craft advisory levels this morning. Expect the strengthening
southerly winds to spread to all waters but the Central Strait
through the morning and afternoon as the front continues to work
into the area. In addition to the strengthening winds, westerly
swell at 10 ft and 11 seconds will also move into the coastal waters
this afternoon and evening. Winds will decrease considerably behind
the front, with a brief shot of small craft advisory westerlies via
the central and east strait this evening. Despite the easing winds,
a small craft advisory for hazardous seas and rough bar conditions
will remain in place for the coastal waters and Grays Harbor Bar
this evening for the 10 ft swell, which should subside after
midnight. Winds and seas will remain generally light through the
weekend as high pressure builds over the area.
PZ...Small craft advisory all waters.
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