Scientific Forecaster Discussion
fxus66 ksew 171753
Area forecast discussion
National Weather Service Seattle Washington
953 am PST Sat Feb 17 2018
Synopsis...windy conditions and heavy mountain snow are expected
today as a deepening surface low tracks southeast past Vancouver
British Columbia and across northern Washington. A modified Arctic cold front
will move southward through western Washington early Sunday,
bringing the possibility of lowland snow and strong north winds.
Dry conditions are expected from late Sunday through Tuesday with
cold temperatures remaining in place. Another system could slide
in from the northwest around next Wednesday or Thursday.
Short term...active weather is underway, with the main impacts
today coming in the form of strong wind and heavy mountain snow.
An elongated 998 mb low is currently located squarely over central
or northern Vancouver Island, moving southeast-east. It is
expected to make its closest approach to western Washington as it
passes Vancouver b.C. Later today as a 995 mb low. Interesting to
see a southeast moving low with a baroclinic Leaf signature on
satellite imagery, which is a classic signature of
intensification. The pdx-bli gradient is expected to peak near +12
mb early this afternoon, hqm-sea around +4.5 mb, and uil-bli
around +4.5 mb. Southwest winds are already picking up and will
peak in most areas early this afternoon, though westerlies down
the Strait will peak later this afternoon. Current coverage of
wind advisories looks good, with those covered areas looking for
gusts of 45+ mph. Expect scattered power outages and minor tree
Mountain snow is the other big deal today. Heavy snow is already
underway, with snowfall rates of 1-2 inches per hour being
observed over the Cascades. Wsdot reports I-90 closed currently
eastbound across Snoqualmie Pass. The latest 24-hour snowfall
forecasts (ending at 4 am sunday) based on 12z model data are for
19 inches at Snoqualmie Pass and 25 inches at Stevens. With heavy
snow and strong wind in the mountains, nwac has issued an
avalanche warning for the Cascades today. Pack your patience and
an emergency supply kit if traveling through the mountains, and
stay in bounds if recreating.
The fun does not end today. In the wake of today's low center, a
a Continental polar air mass will quickly spread south through
interior b.C. And come pouring out of the Fraser later tonight.
The bli-cywl gradient is forecast to peak near -20 mb on Sunday
morning, making this a strong Fraser outflow event. Strong north-
northeast winds will ensue, most notably near Bellingham and the
San Juans, but other places will be breezy as well. The uncommonly
breezy wind for the normally less breezy north direction will
have greater tree impacts than a similar south wind. A modified
Arctic front will spread south and southwest through western
Washington on Sunday morning, with localized frontal convergence
and orographic lift leading to some lowland snow accumulations.
The average forecast snowfall up and down the I-5 corridor is
about 1 inch. In reality, some places, especially in the Seattle
Metro area will see little or none, while other places could see a
couple of inches.
The front will be followed by the coldest air mass of this winter.
A hard freeze is expected areawide on Sunday night, with Monday
night likely being the coldest night of the upcoming cold spell.
At least the cold will come with drier weather. The last showers
will taper off over the Cascades on Sunday night, with dry weather
expected Monday and Monday night. Haner
Long term...from previous discussion: no major changes to the
inherited forecast. There were some difference in the details
concerning the medium range solutions. The operational European model (ecmwf)
indicated an upper level low dropping into the area from the north
about the middle of next week while the GFS kept US more under
the influence of the offshore ridge. Beyond this period, the
models began to diverge even more. At any rate, it looks, for the
most part, temps and precip will be below normal during this
Aviation...a vigorous frontal system is moving through the
region this morning. The associated low pressure center is over
Vancouver Island this morning. Rain and areas of fairly low cigs
will give way to showers as the front moves through the I-5 corridor
by midday. It will be windy with strong onshore flow that eases
overnight quickly then picks back up as it turns to a north wind.
Ksea...rainy at times this morning then turning to scattered showers
by midday with some clearing periods in rather blustery weather.
Timing the modified Arctic front and the switch to northerly winds
Sunday morning was currently timed for 17z Sunday, but obviously we
will look at the mesoscale models closely and that could be off by a
few hours. Also, the snow level will be falling to sea-level Sunday
morning, so we will look to the mesoscale models for guidance on
that as well--it would not be unusual for an inch or two of snow to
fall quickly as the modified Arctic front moves through the Metro
area Sunday morning. Currently the taf shows no precip, but a tempo
shsn for a couple hours seems like a good idea for the 18z taf with
that switch to north winds.
Marine...a 995mb low will move to near the city of Vancouver this
afternoon. As the vigorous frontal system moves inland and the low
pressure falls with it, Washington state will be left with residual
slowly weakening low pressure on Sunday, with cold high pressure
moving south out of b.C. And forcing the residual low pressure
southward. The modified Arctic front will be moving south through
western Washington and the blustery southerly and westerly gales
today will be giving way to Fraser outflow northeasterly gales
before daybreak Sunday--and probably spreading down into the Port
Townsend to Port Angeles area pretty quickly. The switch to
northerly winds in Puget Sound will probably be around mid morning
and Sunday will be a raw brisk day.
Hydrology...river flooding is not expected the next 7 days.
Washington...Wind Advisory until 6 PM PST this evening for Bellevue and
vicinity-Bremerton and vicinity-central coast-Hood Canal
area-north coast-Seattle and vicinity-southwest interior-
Winter Storm Warning until 6 am PST Sunday for Cascades of
Pierce and Lewis counties-Cascades of Snohomish and King
counties-Cascades of Whatcom and Skagit counties.
Wind Advisory from midnight tonight to 6 PM PST Sunday for
western Skagit County-western Whatcom County.
Winter Weather Advisory until 10 am PST Sunday for Olympics.
Wind Advisory until 6 PM PST Sunday for San Juan County.
Winter Storm Watch from this evening through Sunday morning for
eastern Strait of Juan de Fuca.
Wind Advisory until 6 PM PST this evening for Admiralty Inlet
area-eastern Strait of Juan de Fuca.
Pz...Small Craft Advisory for rough bar until 4 PM PST Sunday for
Grays Harbor bar.
Gale Warning until 3 am PST Sunday 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.
Gale Warning until 4 PM PST Sunday 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.
Gale Warning until 4 PM PST Sunday for central U.S. Waters
Strait of Juan de Fuca-East Entrance U.S. Waters Strait of
Juan de Fuca-northern inland waters including the San Juan
Gale Warning until 3 am PST Sunday for Admiralty Inlet-Puget
Sound and Hood Canal-West Entrance U.S. Waters Strait of
Juan de Fuca.