Scientific Forecaster Discussion
fxus66 ksew 162220
Area forecast discussion
National Weather Service Seattle Washington
320 PM PDT Mon Sep 16 2019
Synopsis...scattered showers and isolated thunderstorms will
precede the next organized cold frontal system. The cold front
will bring widespread rainfall to the area late tonight thru
Tuesday afternoon. Scattered showers and isolated thunderstorms
will be possible again Tuesday night and Wednesday. A brief break
in the rainy weather is expected Thursday and again part of of
Saturday before another cold front approaches.
Short term /tonight through Thursday/...an upper trough axis is
pushing across western Washington this afternoon, evidenced by
looping the water vapor imagery. Associated with this upper trough
are cool mid level temps in -20c to -22c and hence steep mid level
lapse rates in the 6.5-7c/km range. This environment is supportive
of some weak instability that may allow for the development of a
few thunderstorms through the afternoon and evening hours. Current
mrms radar imagery shows shower activity increasing in coverage
over the last several hours, with a few isolated lightning strikes
across southern Washington and east of the Cascade crest. Any
heavier convective cell has the potential to produce moderate
downpours along with brief gusty winds and potentially some very
Activity should decrease in coverage overnight, but only to give
way a deep trough digging quickly southeast down the British Columbia coast.
This trough will carry with it an organized and fairly strong cold
frontal system with it. This cold front will bring another round
of widespread rain to the area through much of the first half of
Tuesday, with breezy winds too as winds in the 925-850 mb layer
average between 25-40kts. Do not have any zones meeting Wind
Advisory criteria at this time so winds will likely be most
impactful across the waters (see marine section below). Rainfall
total at the onset of this event and thru Tuesday night will be
fairly similar to sunday's rain event.
Another round of scattered showers will follow the front late
Tuesday into Wednesday and the upper trough axis progresses
through the local area. As has been similar to today, cool mid
level temps will likely yield and unstable environment and will
likely support isolated thunderstorms. The area may then catch a
brief break in active weather systems as a weak ridge moves in.
Long term /Friday through Monday/...despite the weak ridge in
place going into the long term, there appears to be a subtle
shortwave traverses thru its flow. The energy associated with this
shortwave appears to be sufficient for the development of a few
showers thru the day. This event will be nowhere near as
widespread as the previous several systems, but could be enough to
be a nuisance for outdoor plans. Guidance then suggests another
semi-organized frontal system late Saturday into Sunday which
could yield another widespread stratiform rain event. A ridge may
then build into the region in the wake of this system for Monday,
which may allow for a dry day.
Aviation...westerly flow aloft tonight will become southwesterly
Tuesday as a front moves through the region. Scattered showers
(and a possible thunderstorm or two) will taper this evening, then
rain will develop on the coast by 12z and spread inland by 18z.
At the surface, strong southeast flow will develop on the coast
tonight and over the interior Tuesday. The air mass is moist and
slightly unstable this afternoon. It will stabilize later this
evening, then become slightly unstable Tuesday afternoon.
Ceilings are VFR this afternoon, with occasional dips to MVFR in
showers. After the showers end this evening, conditions should be
VFR for several hours. Then after midnight or so the approaching
front will lower ceilings to MVFR with IFR pockets in rain on
Tuesday morning. Improvement to VFR is likely Tuesday afternoon.
Ksea...mainly VFR but perhaps brief MVFR in showers this evening and
overnight. Becoming MVFR in rain Tuesday. Southerly wind 6-10 kt
tonight, increasing to 10g20kt Tuesday afternoon. Chb
Marine...a strong weather system will bring gale force southerlies
to the coastal waters beginning late tonight. These gale winds will
spread to the East Entrance and northern inland waters Tuesday
morning, with Small Craft Advisory strength winds elsewhere. Winds
will ease Tuesday night, and remain light to moderate through the
end of the week. Chb
Hydrology...the daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed
Pz...Gale Warning from midnight tonight to 5 PM PDT Tuesday 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.
Gale Warning from 5 am to 5 PM PDT Tuesday for East Entrance
U.S. Waters Strait of Juan de Fuca-northern inland waters
including the San Juan Islands.
Small Craft Advisory from 5 am to 5 PM PDT Tuesday for Admiralty
Inlet-central U.S. Waters Strait of Juan de Fuca-Puget
Sound and Hood Canal-West Entrance U.S. Waters Strait of
Juan de Fuca.