Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
926 am PDT Fri Oct 21 2016
Synopsis...a front will bring a little rain today and tonight.
There will be a break in between fronts Saturday, then another
front will affect the area Saturday night and Sunday. A stronger
front associated with a low pressure system offshore will bring
wetter weather on Monday.
Short term...a front will spread rain onto the coast this morning
and then inland this afternoon and evening. The front is fairly weak
and quantitative precipitation forecast will be pretty light. Weak high pressure aloft will dry
things out Saturday. Morning clouds will likely give way to a
mostly sunny afternoon. Another relatively weak front will bring a
little rain Saturday night. Models dry things out again across the
area on Sunday behind the weak front. The current forecast might
be a bit pessimistic in that regard. Schneider
Long term...previous discussion...on Monday a stronger front
associated with a deep low pressure system well offshore will
impact the area. Wetter conditions will likely remain in place
through at least Tuesday night. Beyond Tuesday night, there has
not been much consistency between models and model runs with
regard to what will happen. It looks like a weak ridge could build
in, but models have been inconsistent as to how much moisture the
ridge will bring up into the region from the south. Both wet and
dry options are on The Table at this point. Jsmith
Aviation...a weak cold front in moderate southwest flow aloft
will cross western Washington this afternoon and evening,
accompanied by a few showers. Along the north coast and over the
coastal waters, a thunderstorm is possible from now through 21z.
Behind the front tonight, low-level onshore flow will support the
development of widespread IFR cigs below 2000 feet over the course
of this evening, then lasting into Saturday morning. Later Saturday
morning, drying easterly low- level flow will develop in advance
of an offshore front. This will readily help low clouds to scatter
out around mid-day Saturday, except perhaps where they get banked
up against the east side of the olympic mtns. For most of western
Washington, Saturday afternoon will be a time window featuring VFR
Ksea...little change through this afternoon in the prevailing
scattered-broken cloud layers around 040-060. A few showers will accompany
the passage of a weak cold around 00z-03z (5-8 pm). Following the
weak front, low-level flow will become more southwesterly (onshore)
this evening, leading to slowly lowering cigs through the rest of
this evening and tonight. Sat morning will start with IFR cigs most
likely around 010-015. Definitive scattering will take place mid-day
Saturday as a drying easterly component to the surface wind
Marine...a series of fronts will move through the area over the
next several days.
The first front --- a weak one --- is currently bringing small
craft southeast-S winds to the coastal waters and to the West Entrance.
This will continue until about mid afternoon. The front will be
weakening as it moves inland, so winds in the inland waters are
expected to remain below 20 knots. While winds behind the front
over the coastal waters and in the western Strait will weaken, a
Small Craft Advisory for hazardous seas is in effect for late
this afternoon through Saturday as a westerly swell of 10 to 13
feet moves into the coastal waters.
A stronger front will bring Small Craft Advisory winds to most
waters later Saturday or Sunday. While gales are possible for
portions of the outer coastal waters with this system, winds this
strong are not currently indicated in the models or forecast.
A vigorous frontal system associated with a low that will deepen
from around 990 mb to possibly below 970 mb over the offshore
waters, will push east across the waters on Monday. Gales are
likely with this front over the coastal waters and possibly at the
entrances to the Strait of Juan de Fuca. Seas behind the front
over the coastal waters may build to 15 to 20 ft or higher
depending on the strength and track of the offshore low.
Hydrology...rivers are receding and are all below flood level
now. The Snohomish river at Snohomish turned over a few hours ago
below flood stage and the flood threat should be over now. The
Flood Watch that was in effect for the Snohomish river today has
Otherwise, river flooding is unlikley for the next seven days.
Washington...Flood Watch until 1 PM PDT this afternoon for Bellevue and
vicinity-Cascades of Snohomish and King counties-east Puget
Sound lowlands-Everett and vicinity.
Pz...Small Craft Advisory until 3 PM PDT this afternoon for coastal
waters from Cape Flattery to James Island 10 to 60 nm-
coastal waters from Cape Flattery to James Island out 10 nm-
coastal waters from James Island to Point Grenville 10 to
60 nm-coastal waters from James Island to Point Grenville
out 10 nm-coastal waters from Point Grenville to Cape
Shoalwater 10 to 60 nm-coastal waters from Point Grenville
to Cape Shoalwater out 10 nm-West Entrance U.S. Waters
Strait of Juan de Fuca.
Small Craft Advisory for hazardous seas from 3 PM this afternoon
to 6 PM PDT Saturday for coastal waters from Cape Flattery
to James Island 10 to 60 nm-coastal waters from Cape
Flattery to James Island out 10 nm-coastal waters from
James Island to Point Grenville 10 to 60 nm-coastal waters
from James Island to Point Grenville out 10 nm-coastal
waters from Point Grenville to Cape Shoalwater 10 to 60 nm-
coastal waters from Point Grenville to Cape Shoalwater out
10 nm-West Entrance U.S. Waters Strait of Juan de Fuca.
Small Craft Advisory for rough bar from 3 PM this afternoon to 6
PM PDT Saturday for Grays Harbor bar.
You can see an illustrated version of this discussion at