Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 171135
Area Forecast Discussion
National Weather Service Seattle WA
335 AM PST Fri Feb 17 2017
.SYNOPSIS...Weak high pressure aloft will bring dry weather
to the area today after early morning showers come to an end. A
weather system moving up for the south will clip the area
Saturday with a chance of rain. A series of weather systems will
move through the area Sunday through Tuesday or Wednesday for
another round of wet weather.
.SHORT TERM...Satellite imagery shows mostly cloudy skies over
most of western Washington early this morning with a break in the
cloud cover over the central Puget Sound. The convergence zone
over northern Snohomish and southern Skagit county has dissipated
but there is still scattered showers on the doppler radar west of
Puget Sound and in the Cascade Foothills from King county
northward. Temperatures at 3 am/11z were in the upper 30s to mid
Shower activity drying up this morning as a weak upper level ridge
builds east of the area. Low level flow becoming light northerly
by late in the day which will help dry out the lower layers of the
air mass a bit. Southwesterly flow aloft will spread some high
cloud cover over the area later today. Because of this will go
with just partly sunny wording for today. Highs will be within a
couple of degrees of normal...in the lower to mid 50s for the most
The break in the weather continues this evening then a chance of
rain returns to the forecast. Upper level ridge drifting to the
east. A weather system along the northern California coast will
move northward overnight with the leading edge of the rain will be
near Portland around 06z tonight and will get as far north as possibly
Seattle by 12z. The low level flow will remain dry northerly
which will help suppress the rain initially. Increasing clouds
overnight with lows a few degrees either side of 40.
Southwesterly flow aloft continuing on Saturday. Weather system
moving up from the south moving east of the area during the day but
not far enough east to keep a chance of rain out of the forecast.
The best chance for dry weather on Saturday will be the North
Coast. Highs will be near 50.
Another mini break in the weather Saturday night with the next
system to approach the area from the southwest moving into Oregon
overnight. Lows will again be around 40.
Some timing differences in the models with the GFS and ECMWF
faster than the NAM for the front on Sunday. Front negatively
tilted with the parent low well offshore. Because of this will go
with the slower timing of the NAM and have only chance pops in the
morning. Front moving through western Washington Sunday afternoon
into Sunday evening. Highs near 50 once again.
.LONG TERM...Extended models in good agreement with the jet to
the south of the area Monday and Tuesday and western Washington
getting the northern portion of the systems moving inland along
the west coast. Upper level trough moving through the region on
Wednesday keeping chance pops in the forecast. Another trough
moving down from the north on Thursday keeping a chance of
showers in the forecast. Highs will be a few degrees below normal
during the period...in the mid 40s to near 50. Felton
.AVIATION...Southwest flow aloft will continue through tonight.
Low level northerly flow will develop this afternoon as a trough
of low pressure builds northward from California into western
Oregon. The air mass will be weakly unstable this morning then
will become stable this afternoon. The air mass will gradually dry
Showers are lifting northeast across the region early this
morning. The showers will gradually end by 18Z as low level flow
weakens, then becomes northerly during the afternoon hours.
Conditions across the region are many VFR this morning, but there
are patchy MVFR ceilings in showers. Conditions will improve to
VFR across the area later this morning. Albrecht
KSEA...Scattered clouds at 018-020 early this morning will be
occasionally broken in light showers. There are some layers at 035
and 045. Expect the layers at 020 and 035 to dissipate by 18Z.
Light showers will also come to an end by about 18Z. South winds
7-10 kt will become light between 15Z and 20Z then will become
northerly 8-12 kt mid afternoon through tonight. Albrecht
.MARINE...Light southerly flow this morning will turn northerly
this afternoon and tonight as a trough of low pressure develops
northward from a deep low on the central California coast into
western oregon. Flow will return to onshore late Saturday
afternoon as a surface ridge develops to the south of the area.
A small craft advisory remains in effect for hazardous seas this
morning in the coastal waters out to 10 nm and through this
afternoon for the coastal waters beyond 10 nm. Seas will gradually
A front, associated with a 985-990 mb low moving through the
offshore waters will move into the coastal and inland waters on
Sunday. Another low may move through the waters late Monday night
or Tuesday. Albrecht
.HYDROLOGY...The Skokomish river is within a tenth of a foot of
flood stage and slowly receding. The flood warning for the river
will come to an end later this morning. Increasing flows upstream
of the White River at R st will keep the flood warning going at
least into the mid morning hours even though the river right now
is a little below flood stage.
Even with a break in the rain heavy rains earlier in the week have put
the USGS landslide guidance above the threshold at which
landslides typically occur. This means there will continue to be
an elevated threat of landslides for at least another day or two.
.CLIMATE...With another 0.50 inches of rain yesterday the monthly
total at Seattle-Tacoma airport is now 7.84 inches. Another 1.28
inches of rain is needed to break the record for precipitation for
the month of February which is 9.11 inches set in 1961. In this
water year, which began on October 1st, we have already had one
month with record breaking rainfall in Seattle, October with 10.05
PZ...Small Craft Advisory for hazardous seas until 3 PM PST this
afternoon for Coastal Waters From Cape Flattery To James
Island 10 To 60 Nm-Coastal Waters From James Island To
Point Grenville 10 To 60 Nm-Coastal Waters From Point
Grenville To Cape Shoalwater 10 To 60 Nm.
Small Craft Advisory for rough bar until 10 AM PST this morning
for Grays Harbor Bar.
Small Craft Advisory for hazardous seas until 10 AM PST this
morning for Coastal Waters From Cape Flattery To James
Island Out 10 Nm-Coastal Waters From James Island To Point
Grenville Out 10 Nm-Coastal Waters From Point Grenville To
Cape Shoalwater Out 10 Nm.
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