Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 180454
Area Forecast Discussion
National Weather Service Seattle WA
900 PM PST Fri Feb 17 2017
.SYNOPSIS...Dry weather will continue most of tonight. Moisture
arriving from the south early Saturday morning will bring some light
rain to most the area. Another weak system from the west will bring
more light rain on Sunday. A stronger storm system will bring steady
rain and breezy conditions Monday into Tuesday. An upper level
trough will bring cooler weather later next week with a chance of
showers each day.
.SHORT TERM...A weak ridge will continue shifting east of the region
overnight as a weak northern stream trough approaches from the west.
This system will entrain some moisture northward into Western
Washington by early Saturday morning. Expect cloudy skies and some
light rain at times with highs only in the upper 40s. Rain amounts
appear light, on the order of one to two tenths with locally a
quarter inch over the mountains. Light rain tapers to showers
Another weak system will arrive from the west on Sunday with more
light rain at times. Rain amounts again appear light, around a
quarter inch or less. These amounts are not normally associated with
an elevated risk of land slides. However, the antecedent soil
conditions remain very moist from recent heavy rainfall which means
the landslide risk will likely stay elevated this weekend, and
especially by Monday as a stronger system arrives. The system
affecting the area Monday into Tuesday could produce more
significant rainfall and perhaps breezy or locally windy weather.
See the hydrology section below for details. Mercer
.LONG TERM...Rain and breezy conditions will start to ease off on
Tuesday. An upper trough will move over the area around Wednesday,
brining cooler conditions and a chance of showers. Another upper
trough will swing by on Friday. The snow level will fall to 1000
feet or so. Highs will be mostly in the 40s Wednesday through
.AVIATION...SW flow aloft will continue through Saturday. A
surface trough of low pressure extending northward from a deep low
moving into the central California coast will give dry northerly low
level flow to western Washington tonight. Moisture will increase
from the south early Saturday morning as a low forms at the northern
edge of the trough then moves north across western Washington during
Good VFR conditions will continue tonight with thickening mid and
high level clouds. The exception will be on the north slopes of the
Olympic Peninsula where upslope northerly flow gives stratus with
bases FL025-035 there. As a weak low moves north through the area on
Saturday expect light precipitation across mainly the interior
valleys to spread northward by about 15Z. Despite the precipitation,
it appears that ceilings will remain above FL030 Saturday morning.
Ceilings will likely fall to FL010-030 in the afternoon as low level
flow turns onshore behind the passing low.
KSEA...High clouds this evening will thicken overnight, but
visibility will remain good. Expect 5SM -RA BR OVC035 to overspread
the terminal about 15Z. Cigs will likely fall to 020-025 during the
afternoon hours with showers occasionally dropping vis to 4SM -SHRA.
North wind 10-12 kt this evening will turn NE 5-7 kt after 08Z then
will switch to SW 5-9 kt after 20Z.
.MARINE...A surface trough extending northward into Oregon and
Western Washington from a deep low moving inland over central
California, will give northerly flow to the waters tonight. A weak
low will form at the north end of the trough over western Oregon
early Saturday morning, and that low will move northward through the
waters during the day on Saturday. Winds will switch to south to
southwest, but will remain below small craft advisory criteria.
A series of systems will move through the waters starting later in
the weekend. The first will be a weakening 990-995 mb low that will
be moving through the offshore waters Sunday with its accompanying
front moving east through the coastal and inland waters. Another
low, this one a little stronger, will move northeast through the
waters late Monday night or Tuesday morning.
Models differ with the exact track of the low with some taking it
inland over the northern Olympic Peninsula and others taking the low
inland over the central or southern Washington coast into the inland
waters. The strength of the wind behind the low will depend on the
exact strength and track of the low center.
A ridge of high pressure will develop well offshore on Tuesday
giving increasing northwesterly flow to the coastal waters mid week.
.HYDROLOGY...All rivers are below flood stage and levels continue to
fall. Rain will not be hydrologically significant over the weekend
with generally less than a quarter inch of rain each day. A stronger
system Monday into early Tuesday may produce more significant
rainfall. Rain amounts in the mountains may reach one to locally two
inches which is not likely enough to cause river flooding. Forecasts
should be monitored closely as there remains some uncertainty as to
how much rain will fall early next week.
USGS landslide guidance remains above the threshold at which
landslides typically occur. This means there will continue to be an
elevated threat of landslides through at least early next week.
You can see an illustrated version of this discussion at