Scientific Forecaster Discussion
fxus66 ksew 221556
Area forecast discussion
National Weather Service Seattle Washington
900 am PDT sun Sep 22 2019
Synopsis...a cold front will move through western Washington today
with rain at times through the day. Showers will follow tonight. A
weak ridge will build over the area Monday, with the next system
moving through the ridge Monday night into Tuesday. Another system
arriving Wednesday night into Thursday will bring considerably
Short term /today through Tuesday/...a front is moving into western
Washington this morning, with radar and observations showing rain
everywhere. Expect steady light rain this morning, then more showery
conditions in the afternoon. Highs today will be in the 60s.
Showers will taper tonight. A convergence zone seems probable,
setting up in Snohomish County late this afternoon, then moving
south into King County this evening. Monday will start dry, with
only scattered showers in the morning, but a weak impulse in moist
northwest flow aloft will enhance showers on the coast in the
afternoon, and elsewhere overnight into Tuesday. Some clearing is
likely Tuesday afternoon. Highs will be in the low to mid 60s
Monday, and in the mid to upper 60s Tuesday. Burke
Long term /Wednesday through Saturday/...previous discussion...the
ridge weakens Wednesday but still manages to keep conditions dry in
the morning and maybe even through the early afternoon. An upper
level trough moving toward the area will initiate a generally
troughy pattern over West Washington that will linger for the remainder of the
forecast period. Models agree this initial system will move through
pretty fast...being out of the area by Thursday morning and allowing
for a brief 6-hour window for dry conditions. Models diverge from
here though. Both agree on bringing the next system in Thursday
evening but the GFS still plunges it further south by Friday morning
which would allow for activity to taper off and provide for a pretty
much dry day. The European model (ecmwf) is a little slower...although its trajectory
does line a bit better with the GFS than it did 24 hours ago. This
slow down though would keep rain in the forecast throughout much of
Friday before dry conditions set up overnight Friday and into
Saturday. The GFS keeps this low pretty stationary for
Saturday...which would again hint at a continuation of dry
conditions while the European model (ecmwf) lifts the system back up to the north
which could return rain to the eastern half of the County Warning Area Saturday.
Given this continued discrepancy in the extended...opted not to get
too fancy with the forecast and generally kept the slight chance to
chance pops inherited from previous shift but did initiate a little
bit of drying from west to east to at least hint at where a very
broad strokes consensus might be forming with the model solutions.
Aviation...southwesterly flow aloft will persist today and
gradually become northwesterly late this evening as an upper
level trough moves through the region today. The surface front
will move through western Washington this morning into early this
afternoon, brining widespread MVFR/IFR conditions to the terminals.
Current radar imagery has indicated more widespread rain moving
into the region this morning ahead of the frontal passage.
Visibilities will be reduced in heavier bands of precipitation.
Some spots could improve back to VFR conditions late this
afternoon in the wake of the frontal passage, however it's more
likely that conditions will remain MVFR for most terminals throughout
the day today. A convergence zone may develop in the vicinity of
King/Snohomish counties this afternoon into this evening, which
would keep cigs MVFR if it does develop. Winds will generally be
southerly with westerly winds along the coast by the afternoon.
Winds 5 to 10 knots with a few gusts possible with the front.
Ksea...VFR cigs will lower to MVFR as the front approaches. Expect
widespread rain through the morning hours with IFR conditions
possible in heavier bands of precipitation (16z-19z). A convergence
zone may develop during the afternoon and evening. Winds generally
southerly 5 to 10 knots. Jd/sb
Marine...a front will move through western Washington today,
bringing widespread rain to the region. Small Craft Advisory
southerly winds will remain possible for the coastal waters,
the eastern Strait, and northern inland waters through the
Winds will transition more northwesterly following the front this
afternoon with Small Craft Advisory level northwesterlies for the coastal waters
through Sunday night. A Small Craft Advisory is also in effect
for westerlies in the central and eastern portions of the Strait
of Juan de Fuca in the wake of the frontal passage. Small Craft Advisory winds
could bleed into adjacent Admiralty Inlet and northern inland
waters this evening. Winds will then ease Monday morning with
another frontal system moving through the area Monday afternoon.
Hydrology...the daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed
Pz...Small Craft Advisory until 2 am PDT Monday 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.
Small Craft Advisory until 5 am PDT Monday for East Entrance
U.S. Waters Strait of Juan de Fuca.
Small Craft Advisory until 11 PM PDT this evening for northern
inland waters including the San Juan Islands-West Entrance
U.S. Waters Strait of Juan de Fuca.
Small Craft Advisory until 5 am PDT Monday for central U.S.
Waters Strait of Juan de Fuca.
Small Craft Advisory from 6 PM this evening to midnight PDT
tonight for Admiralty Inlet.