Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
345 am PST Sat Dec 7 2013
Synopsis...cold and dry weather will continue through tonight with
the coldest low temperatures in a couple of years Sunday morning. A
weak system will brush the coast Sunday with more weak systems
moving into the area Monday and Tuesday. Temperatures will begin to
moderate across the region Tuesday and Wednesday as the low level
flow becomes more onshore. Light snow is possible on Monday with
snow or mixed rain and snow Tuesday. The air mass will warm up
enough that by Wednesday the precipitation becomes all rain.
Short term...satellite imagery shows clear skies over most of
western Washington early this morning. The only area with cloud
cover is the north slopes of the Olympics with northeasterly flow
aloft causing some upslope conditions. Temperatures at 3 am were in
the middle teens to middle 20s.
Sunny skies expected over most of the area today with dry
northeasterly flow aloft. The exception will continue to be the
north slopes of the Olympics with the upslope flow. Surface high
pressure over southern British Columbia will begin to slide south
today. This will weaken the kbli-cywl gradient with the northeast
outflow winds slowly easing as the day wears on. Temperatures aloft
remain cold with the 850 mb temperatures -10 to -14c. High
temperatures today even with the sunshine will be below freezing.
Surface high pressure continuing to slide south tonight. This will
weaken the surface gradients across western Washington resulting in
lighter winds. With the lighter winds low temperatures Sunday
morning will be the coldest so far with the colder locations getting
into the single digits and teens common over the rest of the area.
The NAM has backed off on the light precipitation along the coast
and over the southwest interior for Sunday into Sunday night with
the 00z and 06z runs weaker with the already weak system versus the
previous 3 runs. GFS solution has been dry the whole way and
continues to be dry on the 00z run. Will confine the precipitation
to the coast on Sunday and scale back some of the cloud cover over
the interior. The air mass over the area remains cold on Sunday but
the 850 mb temperatures do warm up a couple or three degrees c. On
the flip side there is potential for more cloud cover on Sunday. The
end result will be highs just a little warmer on Sunday versus today
with plenty of locations still not getting above freezing.
All of the models have a weak system embedded in the northwesterly
flow aloft moving into the area Monday or Monday night. This feature
is very weak and there is little consensus on the timing. Model
cross sections have a good amount of moisture in the lower levels
developing with some weak lift. Will go with a broad brush chance of
flurries forecast for Monday with accumulations...if we get
any...under an inch. With the increase in cloud cover morning lows
on Monday will be a little warmer versus Sunday morning.
Temperatures aloft continue to warm with the 850 mb temperatures in
the -5 to -10c range. Still with the cloud cover expect highs around
freezing to be common with the warmer locations getting up into the
Long term...extended models are in pretty good agreement on the 00z
runs for Tuesday and most of Wednesday. Tuesday has a continuation
of the warming aloft and another weak system moving into the area
from the northwest. The GFS is a little more aggressive in getting
rid of the cold air versus the European model (ecmwf) but both models still have
1000-850 mb thickness values below 1300 meters over some portion of
the area still at 00z Thursday. Will stay with the chance of rain or snow
forecast for Tuesday. A short break on Wednesday before the next
system arrives. Some timing differences start to appear in the
models with this feature. The GFS starts the precipitation Wednesday
night while the European model (ecmwf) holds the precipitation off until Thursday.
Both models warm the air mass up enough so the precipitation is in
the form of rain. With the faster timing the GFS is quicker to bring
the precipitation to an end on Friday. Current forecast has likely
probability of precipitation Wednesday night through Friday and with no consensus in the
models will stay with this idea in the morning package. Felton
Hydrology...river flooding is not expected for the next 7 days.
Aviation...NE flow surface and aloft with Fraser outflow from a
1037 mb high center over the interior of British Columbia. Clear
skies except a patch of clouds in the far south of Washington and over the
Olympics early this morning. Air is dry and stable.
Ksea...NE breeze and mostly clear skies. No frost to speak
of...single digit dewpoints...the air is Bone dry.
Marine...bli-ywl gradient peaked Friday evening and is starting to
ease. Gale Warning and freezing spray advisory up in the cold windy
Fraser outflow over the water. There is a 1037 mb high centered over
b.C. With low pressure over Nevada giving strong dry offshore flow.
Pz...Small Craft Advisory coastal waters...Strait of Juan Delaware
fuca...Puget Sound sound and Hood Canal.
Gale Warning and freezing spray advisory in effect until 10 am
northern inland waters.
For the graphical afd...visit