Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 180432

Area Forecast Discussion
National Weather Service Seattle WA
930 PM PDT Sat Jun 17 2017

.SYNOPSIS...A warm front will bring a few areas of light rain at
times to the area tonight. The front will shift north out of the
area on Sunday and the air mass will warm and dry through Monday.
Cooler onshore flow will develop Tuesday and Wednesday.


.SHORT TERM...A little light rain fell at times over parts of the
area this evening. There will probably be a little more light rain
overnight as a warm front brushes the area. The threat of rain
should diminish from south to north during the day Sunday and a weak
ridge and warmer drier air mass will shift over the area later
Sunday through Monday. Temps are forecast to be above average
Monday, with some lowland sites warming into the lower 80s.
The ridge will shift east Monday night and Tuesday, allowing strong
zonal flow aloft to become established across the region. This will
also cause the surface onshore pressure gradient to tighten in
response, leading to cooler temps on Tuesday. Additionally, a weak
disturbance moving across British Columbia will cause the marine
layer to deepen Monday night for a threat of light rain or drizzle
late over parts of the CWA late Monday night and Tuesday morning.

.LONG TERM...Zonal flow aloft will prevail Wednesday through
Thursday. This combined with continued low level onshore flow will
result in typical late June weather.

Beyond Thursday, confidence in the forecast was low due to
disagreement between and lack of continuity in the medium range
models. On Friday, there may be a deep upper level low
(operational GFS solution) over the area or an upper level ridge
(ECMWF). On Saturday, there may an upper level trough over the
CWA (ECMWF) or a ridge (GFS). For now, decided to keep the
forecast dry and stick with above normal temps.


.AVIATION...Westerly flow aloft will continue into Sunday. A
weak warm front over the area tonight will dissipate in place on
Sunday. Light flow at the surface. The air mass is stable. Clouds
will thicken and lower tonight as the warm front arrives. MVFR
ceilings from about Seattle southward tonight into Sunday morning
with improvement Sunday afternoon. Pockets of IFR ceilings are
possible at times through 15z Sunday. North of Seattle ceilings VFR
through Sunday.

KSEA...Ceilings 1500 to 2500 feet into Sunday morning with
visibilities 2-4sm in light rain at times. Ceilings improving Sunday
afternoon. Variable winds to 6 knots through Sunday morning. Felton


.MARINE...A weak warm front will affect the area later tonight and
Sunday with winds expected to remain below 20 knots all waters.
Moderate to strong onshore flow will return on Monday and continue
Tuesday through Thursday. Felton





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