Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 231028

Area Forecast Discussion
National Weather Service Seattle WA
228 AM PST Thu Feb 23 2017

.SYNOPSIS...Cooler than normal temperatures are expected to
persist over Western Washington for the next several days. Systems
embedded in northwesterly flow aloft, along with an unstable air
mass, will result in showers at times. Temperatures aloft will be
cold enough that any showers in the lowlands that fall during the
late night and morning hours may fall as snow, or mixed rain or
snow. Brief accumulations in the lowlands, if they occur, will be
light over the next few days. Temperatures will begin to moderate
to near normal toward the middle of next week.


.SHORT TERM...Area radars show showers over the central and south
Washington coastal waters and over eastern portions of the Strait
of Juan de Fuca. An isolated rain shower is also seen around
Seatac airport. Temperatures across the area are near freezing, so
showers will likely be mixed with snow at times this morning.
Expect showers to remain rather isolated this morning, then
increase a bit during the afternoon and early evening hours as
daytime heating provides instability. Lapse rates will remain
rather steep this afternoon with rather low freezing levels, so
some small hail is a possibility with the showers. Showers will
likely be less widespread than what we saw on Wednesday.

Patchy fog or freezing fog is most likely this morning along the
coast where there has been more persistent clearing overnight.

Late February insolation and a maritime air mass means that
temperatures this afternoon will climb into the lower to mid 40s.
Temperatures tonight will quickly fall after sunset, especially if
there is any clearing. So another cold night is expected with
lows near or a few degrees below freezing.

Showers will increase Friday as a cold upper level trough drops
southeastward into the region. The ECMWF is farther offshore with
the track of the upper trough than the GFS/NAM solutions. In any
case, any precipitation that falls will be light. Precipitation
type will more likely be snow or a mix of snow or rain in the
morning hours, then rain or small hail during the afternoon and
evening hours as temperatures rise into the lower to mid 40s.

Conditions will dry out later Friday night and Saturday as a weak
upper ridge drops southeast into the area. Temperatures will fall
to near or a few degrees below freezing at night and rise into the
lower to mid 40s during the day. Albrecht

.LONG TERM...Another cold upper level trough will drop southeast
into the area late Saturday night and Sunday. Both the ECMWF and
the GFS bring the trough closer to the area and give more
widespread shower activity to Western Washington than the Friday
morning system. Temperatures will again be cold enough that
precipitation will likely be in the form of snow or mixed rain and
snow during the late night and morning hours and rain with small
hail during the afternoon hours. The operational ECMWF and GFS
show the possibility of snow accumulations across the area, but
they are generally more robust with the amounts than the various
ensemble solutions. Uncertainty with the location or amount of
snow with this type of system is usually very high due to the
convective nature of the precipitation.

Yet a third upper level trough is likely to swing southeast across
the area later Monday and Monday night. Then toward the middle of
next week, temperatures will return closer to normal for late
February or early March. Albrecht


.AVIATION...An upper trough will gradually move through Western
Washington today with the trough axis exiting the area by late this
afternoon. Upper level flow will remain northerly for much of the
day before turning northwesterly late tonight. Air aloft associated
with this trough remains very cold and as such may yet see scattered
showers arise again this afternoon. Cigs remain a mixed bag over the
area although the majority of sites seem to be either VFR or MVFR.
Plenty of low level MVFR conditions will likely hold
on past mid-morning but lifting by late morning or early afternoon
thanks to generally unstable conditions...allowing for more
widespread SCT skies in the afternoon and carrying over into the
evening. By late tonight/very early Friday morning...persistent low
level moisture will make for yet another round of low clouds and
potentially fog.

KSEA...Earlier brief shower has moved to the south of the terminal
and current radar does not show anything in vicinity. Still
expecting cigs to fall to MVFR levels this morning before lifting in
the afternoon and becoming more SCT in the evening. With low level
moisture remaining in place...would expect conditions similar to
this morning for late tonight and into Friday morning. Light and
variable winds less than 5 kts expected for much of the
day...becoming north to northwesterly this evening. SMR


.MARINE...Weak onshore flow is expected at times through this
evening with the exception of low end small craft advisory level
winds continuing in the outer coastal waters into late this morning.
A slowly strengthening surface low will sink southward about 80 to
120 nm off the coast on Friday. Weak flow is then expected on
Saturday. Another surface low will sink southward closer to the
coast on Sunday. Haner/SMR


.HYDROLOGY...No flooding is expected during the next week. USGS
landslide guidance remains near or above the threshold at which
landslides typically occur. The guidance depends on rainfall
amounts during the past 3 days and the past 15 days; the three-
day amounts are fairly low, but the total for the last 15 days is
still high.


PZ...Small Craft Advisory until 10 AM PST this morning 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.



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