Scientific Forecaster Discussion
fxus66 ksew 212128
Area forecast discussion
National Weather Service Seattle Washington
228 PM PDT Wed Mar 21 2018
Synopsis...moist southwest flow aloft will continue tonight, and
a cold front will move inland early Thursday. A deep upper level
low will make the air mass cool and unstable Thursday through
Saturday. The snow level will fall to around 1000 feet Friday and
Saturday, and isolated thunderstorms are possible Friday. A ridge
will build inland early next week for warmer and drier weather.
Short term...a pattern change will bring wetter and cooler
weather to the region tonight through the weekend. A deep upper
level low off the b.C. Coast will continue to dig offshore
tonight, setting up moist S/SW flow over western Washington. Rain will
increase late tonight into Thursday morning as a cold front moves
inland. Strong onshore flow will develop behind the front. Showers
will continue through Thursday afternoon as an upper level trough
axis swings through. The mountains will see a few inches of snow
with snow levels lowering to around 1500 feet Thursday night.
The broad upper level low will remain over the region on Friday
for more cool and showery weather. We are still under moist,
diffluent flow and there's a slight chance of thunderstorms across
much of western Washington. By this point in time, 500 mb temps will drop
to around -37 c, setting up steep lapse rates and a lifted index
near 0. Small hail is possible too.
The deep upper level low will finally weaken and shift inland as
an open trough on Saturday. Snow levels will remain low during the
morning, only a few hundred feet, but showers in The Lowlands are
light and spotty. Significant snowfall accumulations are not
expected. Temperatures will continue to trend below normal with
highs mainly in the 40s. 33
Long term...an upper level ridge will develop over the NE Pacific
on Sunday for northwest flow over western Washington. A weak passing upper level
disturbance may trigger scattered showers. The upper level ridge
will start to drift east and inland on Monday then stall over the
region through early next week. This sets up a drying trend with
temperatures rising a few degrees above normal. The ridge may
flatten toward the end of next week from a passing trough over
western Canada. 33
Aviation...high and mid level moisture continues to increase this
afternoon as an upper level trough approaches the region from the
northwest. Rain is still expected to spread into the area from the
southwest later this evening...with current radar putting the
leading edge of this activity along the Washington/or border at the time of
this writing. Once started...rain is expected to continue well into
Thursday as a front starts to move through the area. VFR conditions
in place over the entire County Warning Area and will remain there into this
evening. Although cigs will gradually lower due to increasing cloud
cover...MVFR conditions may hold off until early Thursday morning.
As the aforementioned front starts to move through the area
Thursday...expect winds to increase with some gusts...especially
Thursday afternoon and evening...and generally MVFR conditions.
Ksea...above discussion applies. Cigs likely to remain VFR into
early to mid morning Thursday...falling to MVFR between 12-15z.
Surface winds light and variable for the remainder of the afternoon
with speeds generally below 6 kt. Developing offshore gradients
ahead of approaching system will likely bring some east winds 5 to
10 knots after 03z tonight. Winds expected to turn southerly 10-15
kt by noon Thursday with gusts possible. 27/smr
Marine...exiting high pressure will give way to a low pressure
system that will drop down over the region through Thursday.
Stronger southerly winds are likely to develop Thursday and Small
Craft Advisory winds expected over the coastal waters and west
Strait starting Thursday morning and will have headline in place for
afternoon package. Small craft winds are also possible over portions
of the area waters...such as the central and eastern Strait as well
as the northern inland waters Thursday evening...but any potential
headlines can wait for additional analysis of new model data.
Hydrology...river flooding is not expected in the next 7 days.
Pz...Small Craft Advisory from 5 am Thursday to 11 am PDT Friday 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-coastal waters from Point Grenville to Cape
Shoalwater 10 to 60 nm.
Small Craft Advisory from 11 am Thursday to 11 am PDT Friday for
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.
Small Craft Advisory from 11 am to 11 PM PDT Thursday for West
Entrance U.S. Waters Strait of Juan de Fuca.