Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
409 am PDT Friday Aug 28 2015
Synopsis...a series of Pacific storms will impact the region
through this weekend for below normal temperatures and above
normal rainfall. The first system will impact the area today...
followed by a much stronger system on Saturday. The Saturday
system promises to bring strong winds and locally heavy rain.
rain was falling across much of the County Warning Area early this morning. The
areas where it was not currently raining will receive rain by
sunrise or shortly thereafter. The satellite derived blended total
precipitable water product /tpw/ indicated that the system
has tpw values of up to near 1.5 inches associated with it.
Upward forcing did not appear impressive with this system...so
not anticipating significant rainfall today. Amounts should
generally be near a quarter of an inch or less across The
Lowlands with amounts of up to 0.75 inch in the amounts. Of
course...these are generalized amounts. Amounts could be locally
higher...especially in the mountains and on the coast.
Look for the steady rain to taper off to scatter showers this
afternoon. There will be about a twelve hour /relative/ lull
before the next system impacts the area.
The next system will begin impacting the region later tonight.
This system will be stronger and promises to bring strong...
gusty winds for this time of the year along with locally heavy
rain. There were differences between the models concerning the
details...with the GFS being much stronger with the wind field
than the NAM...for example. The GFS MOS indicated high winds for
knuw Saturday afternoon whereas the NAM MOS indicated breezy for
the same site and time period. This will be the forecast
problem...how strong the winds will be on Saturday.
At present...the thinking was that wind speeds will be in the
advisory criterion range or 25 to 35 miles per hour with gusts of 45 to 50
miles per hour...especially over the coast and parts of the northwest
interior. However...there is also a chance that southerly winds
could blow this strong across the I-5 corridor.
Unlike the typical Autumn Wind events...this potential wind event
will likely have more of an impact due to the fact that the
deciduous trees are still in Leaf. This will make the branches
more prone to snapping in winds that are below high wind
criterion. Thus...the day shift /depending on what the morning
model solutions advertise/ should consider issuing a high wind
watch for parts of the County Warning Area for Saturday.
In addition to the wind...rain will be locally heavy on Saturday
ahead of and with the cold or occluded front. At this time...
anticipating a storm total of 1-3 inches of rain in the mountains
and on the coast with a half to one inch elsewhere from the
Saturday system. Look for the rain to taper off to showers late
Saturday afternoon or early Saturday evening behind the front.
An upper trough will move across the region on Sunday. Dynamics or
upward forcing will be decent enough for a good chance of showers.
Daytime temperatures will be about 5-10 degrees below normal.
cool and unsettled conditions will persist over the area during
this period. An upper level trough will dominate the weather pattern
over the Pacific northwest. Therefore...anticipate below normal
temperatures and above normal precipitation. Snow levels are forecast to fall
into the 5000-5500 foot range on Wednesday before rebounding slightly on
Thursday. At this time...not anticipating significant snowfall in the
Aviation...a front will move through the area today with
southwest flow aloft. The air mass is moist and stable. There
will be mostly low and middle clouds today as the front moves
through...with some improvement this afternoon and evening behind
Ksea...ceilings will gradually lower this morning as rain
develops with low and middle clouds for much of the day. Ceilings
should lift a bit this afternoon and evening behind the front.
Southerly wind 4-8 knots...becoming light northerly tonight.
Marine...southerly Small Craft Advisory winds are likely this
morning for the coastal waters and West Entrance Strait of Juan Delaware
fuca as a front moves through.
A stronger front will move through the waters Saturday. Gale
force winds are likely for most waters. A couple of additional
frontal waves will move through the waters Sunday and Monday for
Small Craft Advisory winds at times...but not as strong as the
Saturday system. Schneider
Pz...Small Craft Advisory in effect until 1 PM this afternoon for
the coastal waters and West Entrance to the Strait of
Juan Delaware fuca.
Gale watches are in effect for all waters.
An illustrated version of this forecast discussion can be seen at