Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 192247

Area Forecast Discussion
National Weather Service Seattle WA
347 PM PDT Sat May 19 2018

.SYNOPSIS...A weak upper low over Northwest Washington will move
north of the border this evening, taking the risk of thunder with
it. A weak upper level trough will move through the area on
Sunday keeping a chance of showers in the Cascades. An offshore
upper ridge will nose inland Monday and Tuesday for dry and
warmer weather. A turn to southeast flow aloft on Wednesday will
bring a chance of showers and thunderstorms over the Cascades.
Upper ridging will sharpen over the area on Thursday and Friday.


.SHORT TERM...Thunderstorms have developed along the west front of
the Cascades and near some of our foothill communities this
afternoon in a marginally unstable air mass. Storms have mainly
been in King and Snohomish Counties so far, with a northward trend
on lightning. Thunderstorms seem to have formed near the center
of a northward moving upper low. So based on current trends, the
remaining risk of lightning will mainly be in Skagit and Whatcom
Counties, though a stray strike further south is possible. Once
the upper low moves north of the Canadian border this evening, the
air mass will stabilize quickly. Convective showers and
thunderstorms will be fading quickly by 7 or 8 pm. In the
meantime, be ready to head for shelter if skies grow dark or you
start to hear thunder.

On Sunday, the weak splitty portion of an upper trough will move
in from the Pacific. Cooler and more stable low-level onshore flow
will have returned, and we will have lost the easterly component
of upper level flow. Stability indices will be considerably
weaker, so no thunder over the forecast area on Sunday. Any
showers will be more confined to the mountains.

A narrow upper ridge axis will nose in from the southwest on
Monday and remain in place on Tuesday. Surface gradients will turn
more northwesterly. This pattern supports clearer skies, rain-
free weather, and a warming trend.  Haner

.LONG TERM...On Wednesday, an upper low will lift northeast across
the Northern U.S. Rockies. At its closest approach on Wed, small
height falls and a brief turn to light southeast flow aloft will
occur over Western Washington. With some moister and more unstable
air coming in from our southeast, looking for convective
development over the Cascades Wed afternoon. 18z GFS shows CAPE
values of 500-800 J/kg near the Cascade crest, so included thunder
in the forecast for the Cascades on Wed PM. Steering flow will
shift from southeast to southwest at the most unstable time of the
PM, so convection should remain confined to the Cascades and not
stray into the Puget Sound lowlands.

As a cutoff upper low takes up residence off the central or
southern California coast, an upper ridge will pump up through the
Great Basin and then nose toward the Pacific Northwest. Therefore,
expect dry weather with above normal temperatures.

Next weekend, the upper low may try to lift northward, but precip
looks to hold off past next Saturday. Haner


.AVIATION...An upper trough will remain over the Pacific Northwest
through Sunday with southerly flow aloft. At the surface, onshore
flow will continue with high pressure offshore and lower pressure
east of the Cascades. The air mass is slightly unstable, especially
east of Puget Sound...with some afternoon and evening showers and
isolated thunderstorms developing over the Cascade foothills. These
storms may have enough energy to wander to the west some...impacting
some lowland locations...but threat does not look significant enough
to warrant mention in the TAFs.

Cigs generally VFR this afternoon and early evening...although
locations that find themselves under one of the aforementioned
showers could dip down into MVFR. As has been the pattern...MVFR low
clouds are expected to return after midnight as marine stratus
clouds set up over the area and once again lifting by late
morning/early afternoon Sunday.

KSEA...VFR conditions expected to persist into the evening before
cigs start to lower after midnight tonight. Seeing some showers
develop over the southern portions of Pierce Co. with a northward
trajectory. Hard to determine whether or not they will hold together
long enough to reach the terminal...much less impact it. Will likely
leave any mention out of the TAF due to a high degree of uncertainty
at this time. Cigs expected to lower once again with MVFR conditions
after midnight. Southwesterly winds generally 4-8 knots. SMR


.MARINE...High pressure offshore and lower pressure east of the
Cascades will result in onshore flow the next several days. Small
craft advisory westerlies are likely in the central and eastern
Strait of Juan de Fuca during the diurnal peak late afternoon
through early morning hours each day. Inherited headlines covering
this for this afternoon into early Sunday morning look good.


PZ...Small Craft Advisory until 3 AM PDT Sunday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.


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