Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
303 PM PDT Sat Apr 18 2015
Synopsis...high pressure aloft will remain over the region
through Monday for continued warm weather. The high will move
east on Tuesday...allowing a low pressure system to approach. The
main impacts will be cooler temperatures and increased cloud
cover. Unsettled conditions will prevail Wednesday through the end
of the upcoming week.
smoke from large wildfires in Siberia and thin cirrus were
drifting across the County Warning Area this afternoon. There was also a
stubborn area of stratus lurking just off the coast. Temperatures this
afternoon were running above normal across much of the County Warning Area.
However...northerly sea breezes were keeping a number of locations
immediately adjacent to the Puget Sound sound...northern inland waters...
and Strait of Juan Delaware fuca from reaching their true potential.
An upper ridge will remain over the Pacific northwest through Monday.
The ridge axis will move over the interior west on Tuesday...allowing
an upper trough to approach the coast from the west. The main
forecast problem during this period will be the onshore pressure
gradient...in particular the onset of the marine push...which will
have an impact on daytime temperatures.
It looks like the northwest to southeast pressure gradient will
remain tight enough through Sunday for a risk of late night and
morning stratus and/or fog on the coast. Also...this will likely
keep maximum temperatures from realizing their true potential across those
areas immediately adjacent to major water bodies.
The forecast for Monday was a bit tricky. The upper ridge axis will
be right over the County Warning Area in the afternoon...which should result in
warmer temperatures...especially over the interior. The question was how
much. Earlier model solutions had the ridge axis to our east Monday
afternoon. There is a chance that they will revert back to this
solution...thus decided to play it conservatively for now. Temperatures
on the coast Monday should be similar to Sunday or perhaps a few
For now...it looks like the marine push will occur Monday night or
during the day Tuesday. If it occurs Monday night...the forecast temperatures
for Tuesday will be too warm...especially for the interior. If it
occurs during the day Tuesday than the forecast temperatures may hold. At any
rate...anticipate increased cloud cover on Tuesday. There will also be
a risk of thunderstorms over the Cascades...mainly near the crest...Tuesday
no major changes to the overall synoptic scale picture. It still
appeared that the Pacific northwest will enter a cooler and
unsettled pattern during this period. However...confidence in the
details were low due to lack of agreement and consistency in the
medium range solutions. The models were also out of phase at
times. For example...on Wednesday...the European model (ecmwf) had a weak upper ridge
over the area whereas the GFS had a relatively strong...cold
upper low over the County Warning Area. Decided not to make significant changes to
the inherited forecast for the aforementioned reasons.
Aviation...an upper level ridge over the Pacific northwest will give clear
skies and a dry stable air mass over western Washington this weekend. The flow
aloft is northwest. At 2 PM there was a patch of marine stratus
along the coast.
Ksea...clear skies with a northerly breeze.
Marine...at 2 PM the wind at Destruction Island was still less than
10 kts so I dropped the Small Craft Advisory for the coastal waters.
The 2 PM sea level pressure analysis shows not much pressure gradient
over the coastal waters. The wind should still increase into the
late afternoon and evening with a 10-20kt forecast for most areas.
There is high pressure just offshore that extends through Queen
Charlotte Sound and into b.C. With thermally low pressure centered
over calif. A cold front will arrive Tuesday.
Pz...Small Craft Advisory in effect until 11 PM tonight for the
Small Craft Advisory for rough bar in effect until 9 PM
tonight for the Grays Harbor bar.
You can see an illustrated version of this forecast discussion at