Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 261114
Area Forecast Discussion
National Weather Service Seattle WA
314 AM PST Sun Feb 26 2017
.SYNOPSIS...An upper trough centered off the northern Washington
coast will slide south into northwestern Oregon this evening.
Another upper trough will develop off the north Washington coast
on Monday and move across the Monday afternoon through Tuesday
morning. Both of these systems will give showers to the area with
snow levels around 300 feet during the late night and morning
hours and 1000 feet during the afternoon and evening hours. A
series of systems will move through the area Wednesday through the
rest of the week, but snow levels will rise to over 2000 feet
from Wednesday onward.
.SHORT TERM...A cool upper level low sitting off Cape Flattery
this morning will slide south and southeast into southwestern
Washington and northwestern Oregon this evening. Models agree
that a second weak upper low will form off Vancouver Island on
Monday. This second feature will track east and southeast across
the area Monday afternoon through Tuesday morning. Snow levels
will be low in the interior, on the order of 300-500 feet during
the late night and morning hours and 1000 feet or so during the
afternoon and evening hours through Tuesday morning.
Precipitation across the area early this morning has been mainly
in the form of rain showers. However observations and traffic
cameras show the snow level now down to about 400 feet north of
Seattle. The snow levels will fall another couple hundred feet
early this morning as cold air aloft associated with the offshore
low filters in and low level flow turns more easterly. Most places
below 300 feet elevation will see only wet snow with little or no
accumulation. The zone from 300 to 500 feet will average around
an inch. Some higher hills in western Snohomish County and east of
Interstate 405 in the central Puget Sound area will see up to 3
inches. A winter weather advisory is out for all of the lowland
zones until 10 AM this morning to cover this event. Midday today
temperatures will rise to the upper 30s to mid 40s resulting in
snow melting faster than it falls. Then precipitation this
afternoon will turn to all rain showers at lower elevations. A few
places will see snow showers again later tonight and Monday
morning as precipitation lingers around todays passing low.
The mountains have see 3 to 7 inches of snow overnight and will
see another 4 to 8 inches through this evening. A winter weather
advisory is in effect for all of the mountains except for the
Cascades of Whatcom and Skagit counties -where snowfall amounts
Both the GFS and the ECMWF show another round of showers Monday
afternoon into Tuesday morning. Some snow accumulations of 1 to 3
inches in a convergence zone from about Seatac airport eastward
into the Cascades cannot be ruled out with this system late Monday
night into Tuesday morning, but uncertainty is high enough to
keep POPS in the chance category and snow accumulations below 1
inch at this time.
Temperatures through Tuesday will be cold for late February with
lows in the upper 20s to around freezing and highs in the upper
30s to mid 40s. Albrecht
.LONG TERM...A third system will form off Vancouver Island on
Tuesday and will track across northern portions of the area
Tuesday night and Wednesday morning. This third system will have
more onshore flow, and as a result will have higher snow levels,
around 2000 feet.
A series of systems will move through the area Wednesday through
early next weekend. Snow levels will be rather low, 2000-3000
feet. The GFS shows stronger zonal flow than the ECMWF -which
results in the potential of large snowfall amounts in the Cascades
late in the week. Breezy to windy conditions will rule the
lowlands with temperatures mainly in the 40s. Albrecht
.AVIATION...Upper trough digging SE off Vancouver Island early
this morning is expected to continue into NW Oregon late today.
Westerly flow aloft becoming SW later this morning, then NW this
evening. Associated air mass cool moist and unstable for showers.
A clearing trend this evening as the trough heads into Oregon.
Mainly MVFR ceilings and vsbys with the showers, locally IFR. Air
mass cool enough for precipitation to change to snow showers many
locations this morning. Terminals on higher terrain such as KPAE
have the better chance of accumulating snow - 1-3 inches. Showers
should change back to rain by midday today as temperatures warm.
KSEA...MVFR ceilings and vsbys this morning - may briefly go IFR.
Showers mainly as snow til about 19z. The winter weather advisory
remains in effect for 1-3 inches of slushy snow at the terminal as
temps to remain just above freezing. Decreasing showers this
afternoon with improving ceilings and vsbys. SE winds 7-11 kt
turning northerly around 21z today for your traffic pattern
.MARINE...A 1009 mb low off central Vancouver Island early this
morning is forecast to deepen a bit to 1004 mb as it tracks
southeast near the mouth of the Columbia River by late this
afternoon. Low level flow turning offshore as the low tracks
southeast. Lower pressure should remain over the region into
A Pacific frontal system is forecast to move ashore Tuesday night
with stronger low level onshore flow. The next stronger frontal
system is due to arrive Thursday. Buehner
.HYDROLOGY...No flooding is expected during the next week.
WA...Winter Weather Advisory until 10 AM PST this morning for
Admiralty Inlet Area-Bellevue and Vicinity-Bremerton and
Vicinity-East Puget Sound Lowlands-Everett and Vicinity-
Hood Canal Area-San Juan County-Seattle and Vicinity-
Southwest Interior-Tacoma Area-Western Skagit County-
Western Whatcom County.
Winter Weather Advisory until 10 PM PST this evening for
Cascades of Pierce and Lewis Counties-Cascades of Snohomish
and King Counties-Olympics.
PZ...Small Craft Advisory until 7 PM PST this evening for Coastal
Waters From Cape Flattery To James Island 10 To 60 Nm-
Coastal Waters From Cape Flattery To James Island Out 10 Nm-
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.
You can see an illustrated version of this discussion at