Scientific Forecaster Discussion
fxus66 ksew 211138
Area forecast discussion
National Weather Service Seattle Washington
338 am PST Wed Feb 21 2018
Synopsis...another weak system moving down the British Columbia
coast will reach western Washington tonight. Light snow out ahead
of the system will begin along the north coast this afternoon
spreading to some locations in the interior this evening. A break
in the weather is expected between systems on Thursday. A stronger
system will give wet weather to the area Friday into Saturday
with low snow levels.
Short term...satellite imagery shows the next system to reach
western Washington up near haida gwaii early this morning. Closer
to home skies are mostly clear over western Washington at 3
am/11z. With the clear skies temperatures have dropped into the
20s for the most part with the colder places in the upper teens.
Forecast problem for the day is the track of the low moving down
the British Columbia coast today. The GFS solution is amount 100
miles further west with the low center versus the NAM solutions.
The GFS solution keeps most of the precipitation confined to the
coast this afternoon into the evening hours. The 00z NAM with the
further east track spreads light precipitation, which will be in
the form of snow, over the entire area beginning late this
afternoon in the north with the precipitation spreading over the
remainder of the interior in the evening hours. With the further
east track on the NAM the moisture will less than the more over
water GFS solution. So...for the morning package will go ahead and
issue a Snow Advisory for the coast because with either solution
there is the potential for a couple of inches of snow this
afternoon into the evening hours. Will start and end the advisory
3 hours sooner along the north coast ( start 21z/1pm end 03z/7 PM
) compared to the central coast ( 00z/4 PM start end 06z/10 PM ).
Now what model to go with this morning for the remainder of the
forecast area. Current satellite trends at 11z favor the NAM
solution. Because of this have likely pops over the northern
portion of the interior tonight with chance pops over most of the
remainder of the interior, higher than the GFS pops but not as
high as the NAM leaving some wiggle room for the day shift.
Precipitation amounts in the NAM are limited, a tenth of an inch
or less, so have not issued an advisory for the northern interior
with snow accumulations forecast to be an inch or less. For the
Metro area current NAM timing brings the light snow into the area
at the tail end of the evening commute ( start time around 7 PM )
with just flurries after midnight. Skies will begin clearing from
the north early Thursday morning and by 12z the chance for light
snow will be confined to the southwest interior. Before all the
snow action clear skies this morning will give way to cloudy skies
this afternoon. With the increasing cloud cover highs will only
be in the 30s with overnight lows in the mid 20s to lower 30s.
Sunny skies return on Thursday in the morning hours with the
upper level trough continuing to dig south and dry northeasterly
flow aloft over the area. Still a chance for some light snow over
the southwest interior but that threat will be gone by 18z as the
system moves south of the area. Even with the sunshine highs will
have a hard time getting out of the 30s.
Clouds on the increase Thursday night as yet another system
starts to make its way down the British Columbia coast but not in
time to prevent temperatures from dropping below freezing once
again with lows in the 20s and lower 30s.
Tough call on the precipitation type for the Friday system.
Surface gradients go southerly ahead of the system but model
1000-850 mb thickness values stay below 1300 meters over all but
the coast during the day. For now will go with low snow
levels, less than 500 feet, but not on the surface. Northwesterly
flow aloft will help rain shadow the central Puget Sound area a
little but precipitation amounts north and south in the interior
are enough that another round of winter weather advisories is
possible. Highs on Friday will be near 40 except on the coast
where afternoon temperatures will be a couple of degrees warmer.
Long term...honestly did not spend a lot of time on the extended
forecast this morning with two possible snow producers coming down
the Pike and 00z European model (ecmwf) model not getting ingested into the
computer this morning. GFS brings another system down from the
north on Saturday with stronger low level onshore flow ahead of
the system. This should lift the snow levels off the surface but
the air mass aloft is still relatively cool keeping snow levels
low, around 1000 feet. Another possible system on Sunday with
another cool upper level trough settling in over the area for the
first part of next week keeping the below normal temperature trend
of the last half of February intact. Felton
Aviation...flow aloft is northwesterly across western Washington
today. The air mass is stable with increasing high and mid level
moisture across the region ahead of an approaching weak upper level
trough moving southward along the British Columbia coast. MVFR
ceilings will develop along the coast this afternoon along with
areas of light snow. A period of MVFR ceilings with light snow is
possible over portions of interior western Washington late this
evening. The northwest flow aloft could lead to some shadowing over
parts of the Puget Sound area. VFR conditions will return by early
Thursday along with northerly flow aloft as the trough moves
southward to the northern Oregon coast.
Ksea...VFR conditions through this afternoon with increasing high
and mid level moisture ahead of a weak upper level trough sliding
down the British Columbia coast. A period of MVFR ceilings in light
snow is possible in the 04z-09z time frame. Recent model runs have
shifted ever so slightly in favor of a very light snow accumulation
tonight. There's a 30 to 40 percent probability of up to an inch of
accumulation in the late evening. The approaching system does not
have a lot of moisture with it and the northwest flow aloft favors
some shadowing of the Puget Sound. VFR conditions should return by
10z-12z Thu as the system departs to our south. Light east wind this
morning...becoming light and variable midday...then becoming NE 5-10
knots late tonight. 27
Marine...northeast pressure gradients will weaken this morning as
a low near haida gwaii drops southeastward. Small Craft Advisory
conditions have ended except in the northern inland waters where
they will persist until about 7 am.
The low to the north of the area will move south-southeast through the
Washington coastal waters this afternoon and onto the northern
Oregon coast this evening. Northeasterly flow will increase
somewhat behind the low later tonight into early Thursday. At this
time, winds are expected to remain below Small Craft Advisory
criteria as this rather weak low moves through the waters.
A ridge of high pressure will move southeast across the waters Thursday
afternoon. Another low, this one somewhat stronger, will move southeast
through the waters on Friday. Widespread Small Craft Advisory
conditions are expected on Friday.
Stronger systems will affect the region Saturday and Sunday and
may give gales to some of the waters over the weekend. Albrecht
Hydrology...river flooding is not expected the next 7 days.
Washington... Winter Weather Advisory from 1 PM this afternoon to 7 PM PST
this evening for north coast.
Winter Weather Advisory from 4 PM this afternoon to 10 PM PST
this evening for central coast.
Pz...Small Craft Advisory until 7 am PST this morning for northern
inland waters including the San Juan Islands.