Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 222132

Area Forecast Discussion
National Weather Service Seattle WA
232 PM PDT Thu Mar 22 2018

.SYNOPSIS...A deep upper level low will keep the air mass cool
and unstable through Friday with isolated thunderstorms possible.
The upper low will weaken as it moves inland on Saturday. A ridge
will build over the Pacific Northwest next week for warmer and
drier weather.


.SHORT TERM...The cold front pushed through Western WA early this
afternoon but we`ll continue to see showers across the area
tonight as an upper level trough axis lifts through. The upper
level low is spinning offshore with lots of cellular clouds and
showers noted on the satellite. All of this activity will be
streaming inland. Isolated thunderstorms with small hail are
possible over the coast. Lows will drop into the 30s overnight.

The broad upper level low will remain over the region on Friday
for more cool and showery weather. We are still under moist,
diffluent flow and there`s a slight chance of thunderstorms across
much of Western WA. By this point in time, 500 mb temps will drop
to around -37 C, setting up steep lapse rates and a Lifted Index
near 0. Small hail is possible too.

The deep upper level low will finally weaken and shift inland as
an open trough on Saturday. This system is diving farther south
in the models with the bulk of moisture over Oregon. Across
Western WA, snow levels will remain low during the morning, only a
few hundred feet, but showers in the lowlands are light and
spotty. Significant snowfall accumulations are not expected.
Temperatures will continue to trend below normal with highs mainly
in the mid to upper 40s.

The upper level trough will shift toward the Intermountain West on
Sunday with northwest flow aloft over Western WA. A weak upper
level disturbance passing in the flow may generate scattered
showers across the area. Snow levels will remain low in the
mountains. 33

.LONG TERM...A strong upper level ridge will develop over the NE
Pacific early next week, then gradually shift inland by midweek.
We still have a chance of showers in the forecast Monday and
Tuesday, mainly coast and mountains, as moisture from a system in
B.C. clips area. Wednesday and Thursday are looking warmer and
drier with the ridge moving in. Temperatures will trend a few
degrees above normal with highs in the 50s to lower 60s. The ridge
may flatten toward the end of next week from a passing trough over
western Canada. 33


.AVIATION...Moderate southerly flow aloft continues over western
Washington with a moist air mass in place. The cold front has moved
through the western two-thirds of the CWA and is expected to exit
the area this evening. As such...rain has already tapered to showers
behind the front and cigs have responded in kind...with most
locations reporting either MVFR or VFR conditions. Once the front
fully exits...given current satellite trends...would expect more
widespread VFR conditions this evening and overnight...with more
cloud/fog prone locations likely to linger right on the MVFR
threshold. Gusty winds expected to remain into this evening before
subsiding...but another round is expected by late morning Friday and
continue into the afternoon. Showers are expected to linger over the
area over the next 24 hours as low pressure continues to linger off
the coast.  SMR

KSEA...Above discussion applies. While rains associated with the
front have moved away from the terminal...radar indicates plenty of
echoes from showers. Future TAFs may be justified in changing -SHRA
to VCSH...but will opt for wetter solution for now. Cigs already VFR
and while they may drop a bit overnight...generally expected to
remain VFR. Fairly strong post-frontal onshore flow will lead to
some gusty S-SW winds of 20-25 knots at times for the remainder of
the afternoon before tapering off after 03Z...but look to pick
up again around 15Z Friday morning.  SMR


.MARINE...A cold front continues to exit the area with gusty winds
in its wake. Have opted to extend inherited Small Craft Advisories
for most waters into at least Friday morning. An area of low
pressure area will linger offshore Friday and then weaken and move
ashore over northern California over the weekend. High pressure will
build into the area next week, but weather systems will still brush
the area.


.HYDROLOGY...River flooding is not expected in the next 7 days.


PZ...Small Craft Advisory until 11 AM PDT Friday 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 until 5 PM PDT Friday for Central U.S.
     Waters Strait Of Juan De Fuca-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-East
     Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland
     Waters Including The San Juan Islands.

     Small Craft Advisory until 5 AM PDT Friday for Admiralty Inlet-
     Puget Sound and Hood Canal-West Entrance U.S. Waters Strait
     Of Juan De Fuca.


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