Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
252 PM PDT Sat Jul 30 2016
Synopsis...an upper level low pressure trough will track across
British Columbia tonight through Sunday morning, brushing western
Washington and increasing our cooling onshore flow. A closed upper
low will move across northwest Washington on Monday night and
Tuesday morning, bringing scattered showers and lower
temperatures. Sunny skies and higher temperatures will return by
the middle of next week.
Short term...it's noticeably cooler across the region today.
Temperatures are mainly in the 60s to mid 70s across western
Washington this afternoon. This is 5-15 degrees cooler compared
to yesterday. The cooler weather is due to onshore flow and a weak
upper level trough over the region. Onshore flow will increase
overnight and stratus clouds will surge inland...expect a cloudy
start to Sunday. An upper level low will continue to spin over
b.C./Alberta with a weak trough over western Washington. Temperatures on
Sunday will remain mild, close to what we are seeing today.
On Monday, an upper level low will spin off the coast then move
inland during the evening. The air mass will be moist and unstable
with a chance of showers across the region, mainly toward the late
afternoon and evening hours. Isolated thunderstorms may develop
over the north Cascades where instability is the greatest.
The upper level low will continue to spin overhead on Tuesday for
additional showers. Showers will taper off Tuesday night as this
system exits east. 33
Long term...weak upper level ridging will bring dry and slightly
warmer weather on Wednesday. Otherwise, the rest of the long term
period is dominated by a long wave trough over the west. The
forecast is for mainly dry conditions although isolated showers
may develop over the north Cascades toward the end of next week.
The surface pattern will remain onshore for late night and morning
clouds, with some clearing in the afternoon. Temperatures are
close to normal for early August. 33
Aviation...an upper trough over the Pacific northwest will bring
westerly flow aloft today and tonight. At the surface, onshore
flow will prevail with high pressure offshore and lower pressure
inland. The air mass is stable.
Most of the clouds from this morning have burnt off...with VFR
conditions over much of the area. A few locations along the south
sound and khqm however remain MVFR. Expect to see conditions at said
locations to continue to improve as the afternoon progresses. Still
look on track for clouds to begin to re-emerge late tonight to give
rise to another cloudy start to tomorrow morning.
Ksea...sky clear to sct250 this afternoon with a few lingering mid to low
level clouds this afternoon and evening with low clouds expected to
return tonight after midnight. Winds remaining southerly 5-10 knots.
Marine...onshore flow will prevail for the next several days with
high pressure offshore and lower pressure. Small Craft Advisory
westerly winds are likely most evenings in the central and
eastern Strait of Juan de Fuca.
For the outer coastal waters, still looks to be the Prospect for
some borderline gale force northwest winds this afternoon before
winding down in the early evening...transitioning to Small Craft
Advisory speeds. As such...will Post a Small Craft Advisory for that
area once the current Gale Warning expires. Winds look to remain
fairly consistent over the coastal waters...so have opted to extend
inherited small craft advisories out into Sunday afternoon.
For remaining waters under small craft advisories...winds not really
quite up to criteria just yet...but models remain consistent that
winds should start picking up by early this evening...so will leave
inherited headlines in place.
There will also be some 10-13 foot short period northwest swell over
the outer coastal waters through Sunday morning. This will subside
to more normal summertime levels by the start of the new week.
Pz...Small Craft Advisory until 11 am PDT Sunday for coastal waters
from Point Grenville to Cape Shoalwater 10 to 60 nm.
Small Craft Advisory from 6 PM this evening to 11 am PDT 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.
Gale Warning until 6 PM PDT this evening 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.
Small Craft Advisory until 5 am PDT Sunday for central U.S.
Waters Strait of Juan de Fuca-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-East
Entrance U.S. Waters Strait of Juan de Fuca.
Small Craft Advisory until 3 am PDT Sunday for Admiralty Inlet-
northern inland waters including the San Juan Islands.
Small Craft Advisory until 3 am PDT Sunday for West Entrance
U.S. Waters Strait of Juan de Fuca.
You can see an illustrated version of this discussion at