Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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733
FXUS66 KSEW 202224
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
325 PM PDT Wed Mar 20 2019

.SYNOPSIS...A transition to onshore flow starting tonight will bring
marine air inland resulting in clouds, moisture and cooler
temperatures into Thursday. Light rain will spread northeast across
the area Friday marking the start of an active weather pattern
through the weekend. Temperatures for the remainder of the week will
be near normal.

&&

.SHORT TERM /TODAY THROUGH FRIDAY/...Looking at current
observations...temperatures at any given location appear to be
dependent on wind direction. Locations that retain low level
offshore flow...AKA easterly winds...are seeing temps in the mid to
upper 70s while locations that are seeing northerly or westerly
winds are reporting temps more in the mid to upper 60s range. The
most notable stand out regarding temperature would be Sea-
Tac...reaching 79 at 120 PM PDT this afternoon...that happening
after the temp there jumped 8 degrees in the span of 20 minutes all
thanks to the change in wind direction.

This looks like it will be the last hurrah of the little summer
preview that has been in place over W WA the past couple of days. A
transition to onshore flow will start to usher in some cooler air
and additional clouds overnight tonight and into tomorrow. An upper
level low passing well south of the area...down in central CA...may
wrap some moisture into the area by Thursday afternoon/evening...but
models continue to be rather meh about it. Would not rule out a
stray shower...mainly confined to higher terrain...but overall
tomorrow should remain dry...albeit cooler and cloudier.
Temperatures tomorrow will end up being more in line with seasonal
averages...although still may end up a few degrees warmer.

The next chance for widespread precip comes in the form of an
approaching front expected to reach the WA coast early Friday
afternoon...spreading inland late in the afternoon and into the
evening. The GFS has the front breaking apart as it moves eastward
over the area although the ECMWF has it remaining far more cohesive.
Opted to not deviate too much from inherited forecast keeping POPs
in the high-end chance to low-end likely range. Temps Friday look to
not stray too far from what is expected on Thursday...near to
slightly above normal.  SMR

.LONG TERM /SATURDAY THROUGH TUESDAY/...The upper level trough
associated with the aforementioned Friday front will keep showers in
the forecast for Saturday with the ECMWF proving to be the wetter
solution with more moisture remaining over the area. This is
probably a bit more true to what can be expected...as the GFS
solution might be a touch on the optimistic side. Both models show
the trough exiting Saturday night with a shortwave ridge expected
behind it for Sunday. Neither model is terribly impressive with this
feature but there is the general agreement that dry conditions will
persist through the day and into Monday morning. A storm system out
over the Pacific will be the next system down the conga
line...bringing moisture up from the south and thus a return of
precip by late morning/early afternoon Monday. This system looks to
churn over the Pacific for the remainder of the forecast period...so
although models agree that there might be a bit of a dry window for
late morning/early afternoon Tuesday...certainly not anything worth
putting any stock in at this time.  SMR

&&

.AVIATION...Southeasterly flow will continue aloft with low level
offshore flow continuing into tonight. The air mass will remain dry
and stable. VFR conditions. Easterly winds will remain strong across
portions of the southern Sound through the evening. Winds generally
east-northeasterly from 5 to 15 knots. Gusts up to 25 knots possible
for KSEA and KHQM. Winds will turn more onshore later tonight with
increasing high to mid level cloud cover tomorrow morning ahead of a
weak frontal system.

KSEA...VFR conditions with easterly winds continuing through the
evening. Gusts up to 27 knots possible. Increasing mid to high level
cloud cover tonight with onshore winds generally following 09z. JD

&&

.MARINE...Offshore flow will persist through this evening. Breezy
winds near the gaps of the coastal terrain - strongest easterly
winds will be along the Central Strait of Juan de Fuca. SCA issued
for tonight into tomorrow morning for coastal waters as a small
scale surface low will push north along the coast.

Light onshore flow will develop on Thursday ahead of a weakening
front spreading into the region on Friday. Westerly swell 10 to 12
feet will begin to affect the coastal waters Friday night rising to
14 feet at 16 seconds Saturday.

&&

.HYDROLOGY...River flooding is unlikely over the next seven days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 11 PM this evening to 5 AM PDT
     Thursday for Coastal Waters From Point Grenville To Cape
     Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville
     To Cape Shoalwater Out 10 Nm-Northern Inland Waters
     Including The San Juan Islands.

     Small Craft Advisory for rough bar until 9 PM PDT this evening
     for Grays Harbor Bar.

     Small Craft Advisory from 2 AM to 11 AM PDT Thursday 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.

     Small Craft Advisory until 5 PM PDT this afternoon for Central
     U.S. Waters Strait Of Juan De Fuca-Coastal Waters From Cape
     Flattery To James Island Out 10 Nm-West Entrance U.S.
     Waters Strait Of Juan De Fuca.

&&

$$

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