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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 211619

Area Forecast Discussion
National Weather Service Seattle WA
919 AM PDT Thu Mar 21 2019

.SYNOPSIS...Low level onshore flow will bring some marine air
inland today resulting in high temperatures nearly 15 degrees
cooler than Wednesday. There will also be an increase in cloud 
cover, especially along the coast. A front will bring light rain 
and breezy to windy conditions to the area later Friday into 
Friday night. Showers and cool conditions are expected on 
Saturday. After a dry day on Sunday, another front will move 
northeast across the area on Monday.


.SHORT TERM /TODAY THROUGH SATURDAY/...Areas from about Seattle
southward in the interior and along the coast are seeing the
effects of a shallow marine layer this morning. Winds are more
west to southwest, temperatures are mainly in the 40s to lower
50s, and some stratus is being seen along the coast and in the
Chehalis Gap west of Shelton. Conditions from about Everett
northward in the interior is seeing the last gasps of dry
northeasterly flow with exposed locations in the mid 50s and
dewpoint temperatures mainly in the 20s to mid 30s. The marine air
will continue to move northward today, but it will not be deep
enough to provide any cloud to the interior north of Seattle. Mid
and high level clouds will spread into the area from the southeast
as an upper level low moves inland to the south of the area. Highs
today will be quite a bit cooler than what we saw on Wednesday
with highs in the lower to mid 60s in most of the interior and
only in the upper 50s on the coast.

Clouds will develop and spread into the area from the south and
southwest tonight as the marine layer deepens. Lows tonight will
be more uniform than what we have seen recently with mainly 40s
across the region. 

Clouds will thicken from the south on Friday with rain spreading
NE across the area during mainly the afternoon and overnight
hours. Light offshore flow may result in high temperatures
reaching around 60 on Friday before the rain starts.

The combination of a weak upper level trough and onshore flow will
give a cool and showery day to the area on Saturday with
temperatures near normal for late March. Albrecht

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...Forecasts for Sunday will
continue to favor the more consistent and drier ECMWF model today
that shows a weak upper trough moving NE of the area Sunday
afternoon and low level flow becoming offshore. Current thinking
is that forecast highs on Sunday will be in the 50s to around 60.
Another NW/SE oriented front will lift northeast across the area
later Monday or Monday night as high pressure remains over the
Canadian Prairies and a strong jet stream pushes systems mainly
into California. The remainder of the period during the early to
middle part of next week will see clouds and a chance of showers
as systems rotate around a trough to the south of the area and get
blocked from moving off to the north by high pressure over western
Canada. The extended forecast represents a model blend today.


.AVIATION...Satellite imagery reveals increasing clouds skies over
western Washington early this morning. Expect passing cirrus
through the morning hours with areas of brief patchy lower MVFR
clouds. Conistent MVFR at KHQM through the morning. Clouds will
continue to increase into the afternoon, remaining mainly VFR. A
stray shower would be not impossible this afternoon, especially
across southern TAF sites. The next frontal system will be moving
towards the area tonight and into Friday, providing a much better
chance for showers. Winds will remain generally southerly through
the period at under 10 kts.

KSEA...Generally VFR conditions with clouds increasing through the
day. Patchy, brief MVFR cigs possible this morning with low
clouds, otherwise mainly VFR. Conditions should remain mostly dry.
Winds generally between SE and SW through much of the period.
Speeds under 10kts. JD


.MARINE...Small Craft Advisory conditions this morning along the
Strait with onshore flow. Winds and seas will ease this afternoon
into tonight ahead of the next frontal system. Increasing
southerlies are expected on Friday along with a signficant
westerly swell - 10 to 12 feet - over the coastal waters and west
entrance Friday night rising to 14 feet at 16 seconds on Saturday.
Gale Watch issued for outer coastal waters with SCA issued for
inner coastal on Friday.


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


PZ...Gale Watch from Friday morning through Friday evening for 
     Coastal Waters From Cape Flattery To James Island 10 To 60 
     Nm-Coastal Waters From James Island To Point Grenville 10 
     To 60 Nm-Coastal Waters From Point Grenville To Cape 
     Shoalwater 10 To 60 Nm.

     Small Craft Advisory from 1 PM to 11 PM PDT Friday for Coastal 
     Waters From Cape Flattery To James Island Out 10 Nm-Coastal 
     Waters From James Island To Point Grenville Out 10 Nm-
     Coastal Waters From Point Grenville To Cape Shoalwater Out 
     10 Nm.

     Small Craft Advisory until 11 AM PDT this morning for Central 
     U.S. Waters Strait Of Juan De Fuca.