Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 011004
Area Forecast Discussion
National Weather Service Seattle WA
304 AM PDT FRI JUL 1 2016
.SYNOPSIS...Weak low pressure systems will keep the weather
pattern over the area somewhat unsettled through the Fourth of
July weekend. Temperatures will be near normal.
No major changes in the overall pattern are expected during this
period. An upper level trof and associated cold front will bring a
threat of light rain to mainly the northern and western parts of
the forecast area today. The front is anticipated to dissipate
as it moves onshore later today.
The satellite derived total precipitable water (TPW) product
showed a large area of precipitable water values of over 100
percent of normal just off our coast. Although the low pressure
system has tapped into this moisture, the forcing is too weak to
efficiently squeeze out the available moisture. Instead of
significant rainfall amounts, expect amounts to be less than a
quarter of an inch.
A weak disturbance embedded in the strong (for this time of the
year) zonal flow aloft will bring a chance of light precipitation
to much of the area on Saturday. Another low pressure system will
impact the region on Sunday for possibly a better chance of
The Pacific Northwest will remain under the influence of an upper
level trof through at least Wednesday. However, the medium range
solutions varied quite a bit in the details. For example, the GFS
solution developed a closed Low over the area on Tuesday while the
other models didn`t. In other words, expect generally more of the
same type of weather we have been having recently.
On Thursday, there may or may not be a weak ridge over the area.
The ECMWF says, "No," while the Canadian and GFS say, "Yes." The
inherited forecast reflected the ridge solution.
.AVIATION...Upper flow will be westerly today and tonight. A weak
upper trough and weak front will brush the area today, and onshore
flow will increase at the surface. The air mass is stable and moist
in the lower levels due to onshore flow. Generally dry air in the
mid and upper levels will become somewhat moist aloft as the
decaying front pushes across the area. Sprinkles or spotty light
rain is possible today as the weak front moves into Western
Still expecting stratus to form early this morning, but it has been
slow to develop. Stratus that does form will probably be more patchy
than previous mornings, and it will likely break up sooner. Some fog
is also possible. Any ceilings that form will be around 2k ft.
KSEA...Discussion above applies. Light and variable wind this
morning will become northwest 6-10 kt in the afternoon. CHB
.MARINE...Higher pressure offshore with low pressure inland will
maintain moderate to strong onshore flow through Tuesday. Small
craft advisory strength winds are likely in the strait each
afternoon and evening. A weak front today could enhance the onshore
flow tonight but for now have kept the wind below gale at 20-30
knots. Another system on Saturday night and Sunday could also bring
stronger onshore flow. CHB
PZ...Small Craft Advisory in effect until 6 AM this morning for
the Central and Eastern Strait Of Juan De Fuca.
An illustrated version of this forecast discussion can be seen at