Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 211550
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
850 AM PDT Fri Apr 21 2017

.SYNOPSIS...An upper level ridge will move across the area today.
This upper ridge combined with low-level offshore flow will give
a sunny and mild day to the area today. The dry spell will be
short lived. A series of weather systems will take aim at Western
Washington beginning Saturday and continuing into the middle of
next week.

&&

.SHORT TERM...A progressive 500 mb upper ridge over the coastal
waters this morning will shift eastward across the forecast area
today and east of the Cascades this evening. As the ridge aloft
moves east, surface pressures are increasing east of the Cascades
and over southern British Columbia. The combination of the surface
high developing off to the northeast of the area and an
approaching negatively tilted front offshore will result in nice
low level offshore flow today. Clouds are rapidly eroding early
this morning in response to the incoming upper ridge and
developing offshore flow. Temperatures are rather cool across the
area early this morning - in the mid 30s to 40s - but will
rapidly warm today with the developing offshore flow and strong
late April sun. Highs today near the water will likely stay in the
lower 60s, but they will rise into the mid 60s to lower 70s
inland and near the west slopes of the Cascades and Olympics by
late afternoon.

High clouds will spread into the area tonight and will gradually
thicken as the negatively tilted frontal system approaches. The
high clouds will help temperatures remain in the 40s overnight and
will keep fog from being a problem anywhere.

The next stretch of wet and slightly cooler than normal weather
will begin on Saturday as the incoming front moves northeast
through the area. Rain will begin on the coast early to mid
morning on the coast then develop rapidly inland as flow aloft
becomes increasingly diffluent. The front itself will push
through the area during the afternoon hours. Highs on Saturday
will be mainly in the 50s.

Post frontal showers will rapidly taper off Saturday evening with
the second half of the night being dry across much of the area.
Some clearing will allow lows to drop back down into the lower to
mid 40s.

The upper level trough axis will move through the area on Sunday.
The instability and low level onshore flow will contribute to
widespread shower activity by afternoon and temperatures will be
held mainly in the lower to mid 50s.

The short term forecast is in good shape this morning. No updates
are needed. Albrecht

.LONG TERM...From the previous long term discussion: Multiple
runs in a row of the extended models have shown the next weather
system moving into Western Washington on Monday. The jet is over
Southern Oregon/Northern California so we will not get a direct
hit from this system but there is plenty of moisture associated
with it so even though we are getting the northern portion will
still up the pops to categorical for Monday. A little wrinkle in
the extended runs this morning with the following system moving
inland a little further south on Tuesday. Not far enough south to
take the chance of rain out of the forecast but if the trends
continue precipitation on Tuesday could end up being fairly light.
Model solutions diverge on Wednesday with the GFS keeping a weak
trough over the area while the ECMWF has another organized front
approaching the area by the end of the day. The GFS is much slower
with this feature with the front not arriving until Thursday
night while the ECMWF is 12 to 18 hours faster and much stronger
with the surface low associated with the front. For now will stay
with chance pops on Thursday. Felton

&&

.AVIATION...An upper level ridge along the coast this morning will
move across W WA this afternoon then over E WA by 06Z. A cold front
about 400 NM offshore will slowly approach the region through
tonight. The ridge will keep the air mass dry and stable today,
except for some isolated valley fog over the SW interior through
18Z. Low level E-SE flow will develop this afternoon ahead of the
approaching front. However, surface winds through Puget Sound will
remain northerly through 06Z.

The cold front will bring light rain to the coast around 15Z
Saturday then spread NE across the interior 18Z-21Z Saturday.

KSEA...An upper level ridge will produce excellent VFR conditions
today with very little cloud cover. Surface winds will be northerly
4-8 KT through 20Z then rise to 6-12 KT after 21Z. Kam

&&

.MARINE...A surface ridge over W WA this morning will move E of the
Cascades this afternoon, while a cold front about 400 NM offshore
slowly approaches the region. E-SE offshore flow will gradually
increase this afternoon and tonight as the front approaches.
However, winds around Puget Sound will remain northerly today. The
front will become more negatively tilted with time and will curve NE
across the coastal waters and W WA Saturday morning and early
afternoon. This front should produce SCA winds over the coastal and
some of the inland waters.

A much weaker surface trough will move across the area Sunday
afternoon. The GFS and ECMWF are in good agreement that another
frontal system will affect the area on Monday. Both models have
about a 1003 MB eastward moving surface low approaching the coast
Sunday night. The low will weaken slightly before moving inland
across the N WA coast Monday afternoon. At this point the models
have the low generating SCA winds. Kam

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.
&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html


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