Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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153
FXUS66 KSEW 230501
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
901 PM PST Wed Feb 22 2017

.SYNOPSIS...
Cooler than normal temperatures are expected to persist over
Western Washington for several days. A broad area of low pressure
in the upper levels of the atmosphere will remain over much of the
Northwest. This will allow for temperatures to stay unseasonably
cool along with continued chances of rain and snow the next
several days. The cool and unsettled weather pattern will persist
until the middle portion of next week as temperatures will
moderate some but remain near normal.

&&

.SHORT TERM...
Daytime heating driven showers continue to fizzle in the absence
of the sun this evening. Coverage has steadily decreased and as
the upper low weakens and drift south there will be little in the
way of forcing left for further activity. Readings have already
dipped well down into the low and mid 30s even in urban areas.
With cloud cover and relatively high surface dewpoints, not
expecting too much more cooling, but a lot of low temperatures
right around freezing will occur for Thursday morning. Any areas
that do see better clearing have a shot at fog development but at
present it does not look too widespread unless more clearing
develops. Expect some more diurnally driven shower activity on
Thursday. Forecast soundings by early afternoon show very steep
low level lapse rates and surface based CAPE. Freezing levels down
below 2000 ft again so small hail a good possibility in any
heavier showers. Temperatures will warm into the mid 40s with some
sun but should remain below normal again. A little more
interesting on Friday morning in terms of rain and possible
lowland snow with a cold core upper low dropping south along the
coast and marginally favorable temperatures. Will bear watching,
given time of year it is unlikely to get sea level accumulation
but this system is a cold one. On Saturday, a weak ridge between
upper lows will bring mostly sunny skies and relatively fair
weather to the area. Johnson

.LONG TERM...Recent model runs have been more in agreement that
the upper trough and associated surface low forecasted to move
southeastward through the area on Sunday will likely bring some
clouds and a chance for precipitation to Western Washington,
closer to the previous ECMWF solution. Thus I have kept a chance
of precipitation in the forecast for Sunday. The rest of the week
has shown inconsistency from model run to model run in terms of
precipitation, however confidence is decently high that snow
levels will rise after Monday so the threat of any lowland snow
will diminish.

&&

.AVIATION...An upper trough will pass across western Washington
tonight and early Thu morning before exiting east to the northern
U.S. Rockies on Thu afternoon resulting in upper level flow being
northerly. Shower activity associated with this trough has tapered
off over much of the area...although a few widely scattered showers
remain on radar. Air aloft associated with this trough remains very
cold and as such may yet see scattered showers arise again Thursday.
Models remain consistent on abundant low level moisture which may
bring cigs down to MVFR levels for some locations overnight. This
low level moisture will likely also result in fog in all the usual
favored places. By Thursday morning...cigs are expected to lift once
more under unstable conditions...which will allow for more
widespread SCT skies in the afternoon.


KSEA...SCT clouds this evening and early tonight with skies filling
in and cigs lowering potentially to low-end MVFR or high-end IFR
overnight tonight thanks to plenty of low level moisture over the
area. By mid-morning should see cigs start to lift as unstable
conditions return with clouds scattering out and lifting in the
afternoon. Northerly winds 3-5 kts this evening and tonight becoming
variable in the early morning before turning southwesterly by the
afternoon and speeds remaining generally light.  SMR

&&

.MARINE...Weak onshore flow is expected at times through Thursday
evening. Models remain consistent on wind speeds in outer coastal
waters still reaching low end SCA criteria...so advisory will remain
in place through Thursday 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...

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 10 PM this evening to 10 AM PST
     Thursday 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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