Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 190500
Area Forecast Discussion
National Weather Service Seattle WA
900 PM PST Sat Feb 18 2017
.SYNOPSIS...Scattered showers will persist tonight. Two frontal
systems in quick succession will keep rain at times over the area
Sunday and Monday. An upper trough will move over the region next
week for cooler weather with a chance of showers each day.
.SHORT TERM...Radar shows numerous showers across the area but rain
amounts overall today were generally near or under a quarter inch.
Showers will probably continue with little break in activity
overnight as the next system seen on IR/WV imagery near 135w
approaches from the west.
The shield of steady rain can be seen on the Portland radar offshore
lifting northeast toward WA. Steady rain will likely pick up later
tonight and through the day Sunday. Rain amounts look to be a little
higher than today but not hydrologically significant. Some areas
could get a half inch of rain or locally more which does not help
the potential landslide situation. This amount rain would not
normally heighten the threat of slides but the antecedent conditions
already support an elevated risk. Snow levels will be lower with
this system, currently around 2500 feet and only rising slightly. A
few inches of snow could fall in the mountains, but amounts at pass
level will probably be rather light. Will hold off on any advisory.
Rain will taper Sunday night, then yet another system will arrive
Monday afternoon or evening. The new 00z gfs just in is much much
weaker than past runs. The surface low fills and almost dissipates
as it reaches the area with the secondary low heading inland south
of our area into Oregon or even Nrn Ca. Similarly, the then Nam-12
is further south with the low and also lower on overall QPF. If the
weaker and further south trends shown by the models are correct,
rain amounts will be even lighter than expected but still upwards of
an inch of rain in spots, especially higher terrian. See the
hydrology section below for details.
Several inches of snow could fall in the mountains Monday and Monday
night, possibly advisory amounts at higher locations like Paradise
and Mount Baker. The passes could also see a few inches of snow
Monday. May need to consider an advisory, with 6 inches or more
possible at higher passes like Stevens and especially above 4500
feet with ski resorts seeing the most snow. Mercer
.LONG TERM...From previous discussion...An upper trough will move
over the area Wednesday, followed by drier northwest flow aloft on
Thursday. Another upper trough will dig offshore on Friday and
Saturday. The upshot is cooler weather and a chance of showers.
Highs will be in the lower 40s, with lows near freezing. Some
showers could occasionally have some snow, with the snow level
bouncing around the 500-1000 foot level. Burke
.HYDROLOGY...All rivers are below flood stage and continue falling.
Rain during the next three days should not present a threat of
flooding, with total amounts of roughly 1 to 3 inches in the
mountains and the snow level in the 2000 to 3000 ft range. Trends on
the most recent 00z (4pm) model runs are lighter yet on
precipitation which supports the lighter amounts.
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. Even
if rain amounts are not as heavy as expected through Tuesday, the
risk remains elevated. Continue to monitor the latest forecasts and
statements closely. Mercer
.AVIATION...Southwest flow aloft through Sunday evening. A 988 mb
low over the Oregon offshore waters will lift northeast to south-
central Vancouver Island Sunday evening. The air mass is moist and
stable, and will become somewhat unstable Sunday afternoon as the
low offshore moves by to the west of the area.
Showers continue to move northeast across the area this evening.
Precipitation will decrease overnight, then will increase from the
southwest early Sunday morning as a front extending east and
southeast of the low over the Oregon offshore waters approaches the
area. The front will move through the area midday Sunday with low
level flow turning southerly and the air mass destabilizing
Conditions are variable across the area with conditions ranging from
VFR to LIFR. Expect conditions to become mostly MVFR with pockets if
IFR late tonight and Sunday morning as the front to the southwest
approaches the area. Conditions will improve to VFR with some MVFR
in showers Sunday afternoon behind the front. Forecast confidence is
lower than usual tonight. Albrecht
KSEA...Conditions are VFR at the terminal this evening, but low
ceilings around BKN009 are not far off in KTIW. Expect conditions to
settle into a broken ceiling around FL025 by midnight as pressure
gradients relax. 4SM -RA BR BKN-OVC010 will develop by 13Z as the
offshore front approaches. Improvement to low VFR conditions are
expected about 19Z Sunday behind the front. Winds becoming SE 4-6 kt
tonight and Sunday morning then southerly 15G25KT behind the front
Sunday afternoon Albrecht
.MARINE...A low at around 990 mb over the Oregon offshore waters
will lift northeast to south-central Vancouver Island early Sunday
evening. The front associated with this low will lift northeast
through the waters Sunday morning. Small craft advisories are in
effect for the coastal waters and for the eastern 2/3 of the Strait
of Juan de Fuca for Sunday for the front and the low.
A 10-12 ft west to southwest swell will follow the low Sunday night
into early Monday. Small craft advisories for hazardous seas will
likely be needed for the coastal waters Sunday night and for rough
Grays Harbor bar conditions. Southerly flow behind the front will
also likely bring winds to small craft advisory levels in Puget
Sound and Hood Canal Sunday afternoon and Sunday night.
Another low will approach the area from the SW on Monday. It now
appears that the low will move inland just to the south of the
waters. With the track of the low to the south, it appears that
winds will generally remain below small craft advisory criteria
across most of the waters.
After the Monday low, a large surface ridge will develop offshore,
putting W WA under NW flow Tuesday and Wednesday. Albrecht
PZ...Small Craft Advisory from 2 AM to 4 PM PST Sunday for Coastal
Waters From James Island To Point Grenville 10 To 60 Nm-
Coastal Waters From James Island To Point Grenville Out 10
Nm-Coastal Waters From Point Grenville To Cape Shoalwater
10 To 60 Nm-Coastal Waters From Point Grenville To Cape
Shoalwater Out 10 Nm.
Small Craft Advisory from 6 AM to 4 PM PST Sunday for Coastal
Waters From Cape Flattery To James Island 10 To 60 Nm-
Coastal Waters From Cape Flattery To James Island Out 10 Nm.
Small Craft Advisory from 4 AM to 7 PM PST Sunday for Central
U.S. Waters Strait Of Juan De Fuca-East Entrance U.S.
Waters Strait Of Juan De Fuca.
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