Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 232326
Area Forecast Discussion
National Weather Service Seattle WA
326 PM PST Thu Feb 23 2017
.SYNOPSIS...Cooler than normal temperatures will persist over
Western Washington through Tuesday, and low pressure systems
moving south through the coastal waters will bring a chance of
snow showers to the lowlands Friday morning and again Sunday
morning before temperatures warm in the afternoon hours. A chance
of showers will continue through much of next week.
.SHORT TERM...Scattered showers are moving southeast through the
area this afternoon. Some of these showers may contain graupel or
a rain/snow mix. Expect these showers to dwindle around sunset as
diurnal heating ends.
Late tonight and Friday, an upper low and its associated surface
low will move southward through the offshore waters, bringing a
good chance of showers over much of the area. The 18Z GFS and NAM
models show the system staying slightly further offshore than
previous runs, so precipitation amounts may be lower than earlier
anticipated. Since snow levels will be at or near sea level during
the morning hours, any precipitation that falls will likely start
off in the form of snow or hail and change to rain later in the
day as temperatures warm from the lower to mid 30s into the 40s.
Expect showers to move onto the coast late tonight and inland
during the early morning hours. Highest chances for accumulations
are along Highway 101 near the coast and in the Hood Canal Area.
Lowest chances for accumulations are south of Renton and Tukwila
in southern King and Pierce counties away from the Cascade
foothills. Since the precipitation will be convective, some areas
will see an inch or two of accumulation and others will not see
anything at all.
Saturday will be a lull day as an upper ridge builds overhead,
with mostly sunny skies and a lack of precipitation. Temperatures
will remain cool and crisp.
Saturday night into Sunday, another upper low and its associated
surface low pressure system will drop south through the coastal
waters, likely bringing more precipitation than the Friday system.
However, the most recent model runs have shown slightly warmer
surface temperatures with this system, meaning that snow levels
are unlikely to reach down all the way to the surface. This system
is also convective in nature, so amounts and locations are highly
uncertain at this time. JSmith
.LONG TERM...Northwesterly or westerly flow aloft will keep a
chance of showers in the forecast from Sunday night through
Thursday of next week. The GFS shows a wetter solution with higher
precipitation amounts and the ECMWF shows a drier solution with
precipitation more limited to the mountains. However, both models
have been consistent that snow levels will rise and the threat of
any lowland snow will diminish beginning late Tuesday. JSmith
.AVIATION...A brief period of weak upper ridging is expected this
evening, then an upper trough will dig southward on Friday between
the coast and 130W. Light northerly flow aloft this evening will
become southerly on Friday. Cold air aloft will continue to
support an unstable air mass. Ongoing air mass shower activity
will decrease in coverage after sunset but may not completely
dissipate tonight. Early Friday morning, the upper trough digging
off the coast will spread numerous showers onto the coast, with a
few making it into the interior. With the cold morning air mass,
snow showers are possible down to sea level, especially away from
the immediate coast. Prevailing lowland precip type by aftn will
be rain, given daytime warming of the lower atmosphere.
KSEA...Plenty of small showers in the vicinity of the terminal
right now (3 pm). Will cover it with VCSH in the TAF, but a few
minutes of rain/snow mix are possible with any showers that stray
onto the airfield. Shower activity should subside around sunset
with the loss of daytime heating. For Friday morning, the risk of
snow showers at the terminal appears a bit less than earlier
indicated. There is still some risk, but will go easier on the
cig/vsby impacts in the upcoming TAF package. Risk of snow showers
should end by 18z Fri, with Fri aftn cloud bases in the 035-050
.MARINE...Weak onshore flow is expected late this afternoon and
this evening. A 1015 mb low will sink southward about 130-170 NM
off the coast on Friday. Weak flow is expected on Saturday. Then a
1007 mb low will sink southward close to the coast on Sunday.
Light northerly onshore flow is expected Monday, followed by
stronger southerly onshore flow on Tuesday. Haner
.HYDROLOGY...No flooding is expected during the next week.
PZ...Small Craft Advisory from 6 AM to 3 PM PST Friday for Admiralty
Inlet-East Entrance U.S. Waters Strait Of Juan De Fuca-
Northern Inland Waters Including The San Juan Islands.
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