Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 161608

Area Forecast Discussion
National Weather Service Seattle WA
908 AM PDT Mon Sep 16 2019

.UPDATE...The forecast is in good shape this morning with only a
few minor tweaks to the sky forecast based on current satellite.

Water vapor loop this morning shows a narrow, but fairly high
amplitude trough extending roughly from BC southward through
California. There appears to be a weak sfc reflection across the
offshore waters associated with this trough. Cold front from
yesterday is well east of the area and given rather cool mid level
temps within the upper trough, western Washington lies in an
unstable post-frontal environment. This can also easily be
verified by the "bubbly" appearance of the clouds currently
offshore in IR, water vapor, and vis imagery.

A few lightning strikes are seen off the coast of Oregon this
morning, with a few spotty showers across southern Washington and
up near the Canadian border. SPC mesoanalysis shows some
instability offshore this morning (100-250 J/kg MUCAPE), so any
isolated lightning strike thru the remainder of the morning is
most likely to happen over the Ocean or near/along the coast. As
the trough axis moves east across western Washington this
afternoon, mid level lapse rates will steepen and likely provide
for enough instability for an isolated thunderstorm just about
anywhere across the area. Localized moderate rainfall and perhaps
small graupel will accompany any thunderstorm. Forecast aligns
well with SPC`s Day 1 outlook for general thunder across western

Next organized, seasonably strong front is still on tap for
Tuesday with another round of widespread rain. Forecast for this
event has been left as is for now. Previous discussion follows
with updated marine and aviation sections.



.PREV DISCUSSION... /issued 332 AM PDT Mon Sep 16 2019/

.SYNOPSIS...Cool upper level trough moving through Western
Washington today. Fairly strong for September front arriving on
Tuesday bringing rain and windy conditions. Another cool upper
level trough following the front Wednesday. Weak upper level
ridge over the area Thursday. Much weaker system moving through
Friday. Another frontal system will arrive Sunday.


.SHORT TERM /TODAY THROUGH WEDNESDAY/...Satellite imagery shows
cool upper level trough offshore with some showers embedded in the
trough moving on to the coast at 10z/3 am. these showers have been
producing a lightning strike or two every hour since 10 pm. Over
the interior a few breaks in the cloud cover has allowed
temperatures to drop into the upper 40s. The remainder of the
area has temperatures in the 50s.

Upper level trough moving inland today with the trough axis right
over Western Washington at 00z Tuesday. As the trough moves
slowly east today shower activity will spread east as well.
Convective variables are not very impressive. The lifted index is
the best of the bunch between 0 and -2 today. With the showers
already producing lightning offshore will stay with the idea of a
chance of thunderstorms over the interior this afternoon and
isolated thunderstorms along the coast all day. It will be another
cool day with highs only in the mid 60s.

Upper level trough moving east of the area early this evening.
Next organized system to reach Western Washington, currently near
the Alaska panhandle, will approach the coast tonight. Rain out
ahead of the front will reach the coast by 12z Tuesday with a
chance of rain inland after midnight. Lows will be in the 50s.

Well organized and fairly strong front for September moving
through the area on Tuesday. Rain associated with the front will
start early in the day for the interior. While the surface
gradients are not too impressive model cross sections show 30 to
40 knot winds right off the surface ahead, with and just behind the
front. It will not take too much mixing to get winds close to
this range down to the ground. For now will keep winds under
advisory criteria ( sustained winds at least 30 mph and/or gusts
at least 45 mph ) but if the front intensifies a little more than
the models indicate at this time could see wind advisories,
especially for the coast, on Tuesday. Steady rain will give way to
showers behind the front in the afternoon. Highs once again will
only be in the lower to mid 60s.

Another cool upper level trough moving down over Western
Washington Tuesday night with the trough remaining in the area
Wednesday. Shower activity will taper off Tuesday night then
increase again on Wednesday with a little daytime heating. Lows
Tuesday night in the upper 40s and 50s. Highs Wednesday in the
lower to mid 60s. Felton

.LONG TERM /THURSDAY THROUGH SUNDAY/...Extended models in good
agreement with a weak upper level ridge nosing into British
Columbia Thursday with dry northerly flow aloft over the area.
Weak system embedded in the northerly flow aloft Thursday night
into Friday for a chance of showers. Ridge briefly rebuilding over
the area Saturday for another dry day. Models showing good
consistency with another organized front Sunday with the rain out
ahead of the system reaching some of the area Saturday night.
Will go ahead and start increasing the rain chances on Sunday with
this feature. Highs will remain below normal through the period,
in the 60s with a few lower 70s possible Thursday and Saturday.


.AVIATION...An upper level trough will slowly move east today into
this evening. A front offshore will approach the coast late tonight.
Flow aloft will be light and variable today, becoming moderate
westerly tonight. At the surface, light to moderate onshore flow
today will become strongly southeast tonight as the front
approaches. The air mass is moist and stable this morning with local
fog. Showers and isolated thunderstorms are possible this afternoon.
Rain will develop on the coast late tonight.

Ceilings are VFR to LIFR at 8 AM. The trend will be for improving
conditions, with VFR likely all areas this afternoon. Despite the
general improvement, MVFR ceilings and visibilities are possible in
the afternoon during showers and even isolated thunderstorms as the
upper level trough moves through the area. Some stabilization will
occur this evening, and mid and high clouds will increase.

KSEA...Mostly MVFR this morning with 2-3k ft ceilings. Visibility
restrictions due to fog seem to be finished. VFR this afternoon with
showers and a slight chance of thunderstorms. Southerly winds 4-6 kt
becoming 6-8 kt in the afternoon. CHB


.MARINE...Winds and seas are generally light this morning ahead of
an approaching system that will arrive early Tuesday morning. This
system will bring gale force southerlies over the coastal waters
beginning late tonight, and could bringing gales to parts of the
interior waters Tuesday morning. A gale warning is in effect for the
coastal waters and a gale watch remains in effect for the Northern
Inland Waters, the entrance to the Strait of Juan de Fuca, and
Admiralty Inlet. Remaining waters will likely see small craft
advisory level winds and seas Tuesday. Winds will ease again
Wednesday and Thursday. CHB


.HYDROLOGY...The daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed
until then.


.CLIMATE...With the rain on Sunday most locations in Western
Washington are already above the normal rain for the entire month.
This is only the 4th time in the last 40 years Seattle has gotten
over the normal monthly rain for September in the first half of
the month. Some numbers ( Seattle rain total through the 15th
1.65 inches normal for the entire month 1.50 inches, Bellingham
3.24 inches monthly normal 1.78 inches, Quillayute 5.02 inches,
monthly normal 3.82 inches, Hoquiam 2.88 inches monthly normal
2.28 inches, Olympia 1.77 inches monthly normal 1.71 inches ).

The record 1.75 inches at Bellingham yesterday was only 0.03
inches less than the normal for the entire month. Felton


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 Watch from late tonight through Tuesday afternoon for
     Admiralty Inlet-East Entrance U.S. Waters Strait Of Juan De
     Fuca-Northern Inland Waters Including The San Juan Islands.


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