Scientific Forecaster Discussion
Area forecast discussion...corrected
National Weather Service Seattle Washington
345 am PDT Thursday may 28 2015
Corrected to fix a couple of typos.
Synopsis...high pressure aloft will prevail over the region today.
A low pressure system over northwest British Columbia will drift
into central British Columbia tonight where it will stall. This
will place the area under westerly flow aloft through Saturday.
Daytime temperatures are forecast to remain above normal.
stratus has made it as far east as the far southern Puget Sound sound
and Admiralty Inlet area at this time. The low clouds are forecast
to blanket much of the Puget Sound sound region after sunrise before
retreating back to the coast late this morning.
Meanwhile...a weakening upper level ridge will prevail over the
region today. The air mass will be a little warmer than
yesterday...so expect additional warming across much of the County Warning Area
this afternoon. There will also be a risk of showers over the
Cascades again late this afternoon through early this evening.
Decided to omit the chance of thunderstorms for today. The ingredients
/moisture...lift...and instability/ plus the steering flow did not
appear as favorable for thunderstorms today as yesterday. However...this
does not mean that the threat of thunderstorms is zero...just not high
enough to mention them in the forecast.
A cut-off low over British Columbia will begin to exert more
influence over the region tonight through Friday. Confidence in the
cloud and maximum temperature forecast was not very high. This will be a
tricky forecast. There is a possibility that the surface onshore
pressure gradient could be stronger than what the models
advertised...which would lead to cooler than predicted temperatures on
Friday. Otherwise...anticipate a threat of afternoon and early
evening showers over the Cascades on Friday.
It looks like another tricky forecast for Saturday...so confidence
in the forecast was not high. Chose to lean toward the cooler
solutions. Daytime temperatures will still be well above normal for this
time of the year...unless the onshore gradient ends up being
tighter than predicted.
an upper level ridge will build over the region on Sunday ahead of
a negative-tilted upper level trough approaching from the southwest.
Thus...temperatures should be a little warmer on Sunday compared to
Saturday due to weaker onshore flow.
The aforementioned upper low will move over Oregon on Monday.
Moisture coming up from the south with this low will bring an
increasing chance of showers to the area. Highs will cool into the
60s. The upper low will move inland to the south...close enough to
keep showers and clouds over the area through Tuesday with highs
remaining in the 60s.
Weak high pressure aloft will build over the region on Wednesday.
This should allow things to dry out and there will probably be some
sunshine by afternoon. Schneider/05
Aviation...an upper ridge over the offshore waters will weaken and
slide east today. North flow aloft will become westerly late this afternoon
and tonight. The air mass is dry and stable with a marine layer that
will burn back to the coastline midday.
Very low stratus with areas of fog has pushed east through the
Strait of Juan Delaware fuca onto Whidbey Island. This low level moisture
will slide southeast through Admiralty Inlet to around the kpae terminal by
12z. In the mean time low stratus and fog has pushed inland through
the Chehalis gap to the far southern portions of Puget Sound sound. Expect
this deck to reach the ksea terminal around 12z. Stratus bases are
003-005 mean sea level and tops appear to be 1000 feet or less based on estimates
from multispectral satellite images early this morning. Expect all
of the stratus to burn back to the Pacific coastline by 19z leaving
mainly clear skies in the interior. The air mass over the Cascades
will destabilize with daytime heating 21z-03z this afternoon giving
some cumulus buildups and a slight chance of showers. The risk for
thunderstorms is expected to remain east of the crest as the
steering flow aloft turns more northwesterly. Stratus along the coast will
push back inland overnight tonight and reach most of the inland
terminals Friday morning with coverage slightly less than this
Ksea...stratus with bases 002-003 will reach the terminal by 13z.
There is a 50 percent chance that visible will lower to 2sm or less as
the bases of the incoming stratus layer is quite low -fl003 at kshn.
Stratus will quickly lift and burn off 18z-19z then move back inland
13z on Friday. Light winds will become northwesterly 4-7 knots after 19z then
turn north and diminish this evening. Albrecht
Marine...a surface ridge along about 140w combined with lower pressure
east of the Cascades will give light onshore flow through the weekend.
The flow though the central and eastern Strait of Juan Delaware fuca will
likely increase to Small Craft Advisory criteria during the late
afternoon and evening hours each day in response to daytime heating
in the interior. Albrecht
Pz...Small Craft Advisory in effect until 6 am this morning for the
central and eastern Strait of Juan Delaware fuca.
You can see an illustrated version of this forecast discussion at