Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 222359
Area Forecast Discussion
National Weather Service Seattle WA
359 PM PST Wed Feb 22 2017
.SYNOPSIS...A series of upper lows will move south across the
offshore waters tonight through Friday, producing cool weather
for Western Washington with scattered rain and snow showers. An
upper ridge will bring cool, mostly sunny weather Saturday, with a
few showers in the mountains. Another upper trough will arrive
Sunday, and the chance of showers will continue early next week.
.SHORT TERM...Heavy convective showers are bringing a
graupel/snow/rain mix to areas of northern King County. Any
accumulations with these showers should melt away quickly. A weak
upper low currently over the coastal waters will weaken and move
southeastward onto the Oregon Coast this evening, cutting off
support for the showers after midnight.
The western edge of a broad upper trough covering much of the
northwestern US is keeping cold air aloft over Western Washington.
Expect this cold air aloft to stay in place through Monday, with
850 MB temperatures in the -6 to -8 degree Celsius range.
Thursday will see another upper low embedded in the broad trough
pass southward well offshore for a hint at showers through the
day. I have kept the POPs low for this time period. Any showers
that occur Thursday morning will likely be in the form of snow or
a rain/snow mix, however as showers are light, no significant
accumulations are expected. Diurnal warming will cause any showers
later in the day to mainly fall as rain.
On Friday, a moderate upper low and its associated strengthening
surface low will move southward off central or northern Vancouver
Island and through the coastal waters for a better chance of
precipitation. I have increased the POPs for Friday as recent
model runs have showed this low as being more impactful. Again,
temperatures are cold enough for morning showers to fall as snow
or a rain/snow mix. The most recent University of Washington high
resolution model runs show accumulations for the coast and some
locations in the interior in the morning. I have held off on
adding snow accumulations into the forecast for now, but will be
checking to see if future model runs show the same. In addition, I
have added a slight chance of thunderstorms into the coastal
waters Friday afternoon and evening, based on instability and
forcing over the coastal waters from the upper low.
On Saturday, a weak ridge between upper lows will bring mostly
sunny skies and relatively fair weather to the area.
.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
.AVIATION...An upper trough was spreading southeast down the B.C.
coast this afternoon, and it will pass across western Washington
tonight and early Thu morning. It will exit east to the northern
U.S. Rockies on Thu afternoon. The trough is coming with very cold
air aloft, which is leading to an unstable air mass and vigorous
cold-topped shower activity. Showers will be enhanced into this
evening along a southward-moving Puget Sound Convergence Zone.
Fewer showers are expected on Thursday as the upper trough exits
to the east. Otherwise, cloud bases of 025-045 will be common this
evening. Areas of fog and low clouds expected around sunrise on
Thu, but the unstable air mass will allow the clouds to easily
lift and scatter out around mid-day Thu.
KSEA...A broadly defined Puget Sound Convergence Zone (PSCZ) will
move from north to south across the terminal around 4-5 pm. A
shower will likely accompany this feature. A north wind shift is
likely with the passage of the PSCZ. Wind speeds could exceed 6
knots for a couple hours at first. Speeds will ease during the
evening rush, though a northerly component is still expected to
the direction. Otherwise, SCT-BKN clouds this evening with bases
around 030-050. Some low clouds and the outside possibility of
some fog expected in the 2-3 hours either side of sunrise on Thu,
but clouds will readily lift and scatter out around mid-day Thu.
.MARINE...Weak onshore flow is expected at times until Thursday
evening. 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
.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
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.
You can see an illustrated version of this discussion at