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Area forecast discussion...updated 
National Weather Service Seattle Washington
1110 am PDT Thursday Aug 21 2014

Updated aviation ksea section.

Synopsis...a low pressure system aloft will move southeast over the
region today bringing a threat of showers mainly in the mountains
through Friday. Higher pressure aloft is then forecast to build over
the Pacific northwest over the weekend for more sunshine and warmer daytime
temperatures...likely continuing well into next week.


Short term...upper ridge is now inside 150w with the weak shortwave
visible in the water vapor image near 135w digging southeast. 12z forecasts in
thus continue the trend of digging this shortwave into the Pacific
northwest late today and developing a closed upper low in the interior Friday.
Enough instability late today and have included a threat of thunderstorms
for the north Cascades.

At the surface...low level onshore flow is weaker than at this time
on Wednesday. Ksea-keat gradient this hour is 5.2 mb vs 7.5 mb Wednesday. Marine
clouds blanket much of the western Washington lowlands but tops are between
2 and 3k feet. So should have an earlier breakout...around midday for
most locations. Will massage the short term forecast period.

Otherwise...the forecast looks on track. A threat of showers mainly
in the mountains Friday and then some ridging aloft over the weekend
for more sunshine and warmer daytime temperatures. Will keep an eye on what
the European model (ecmwf) showed with the next digging shortwave on sun vs what
other guidance showed with ridging in the upcoming 12z models.

Long term...from the previous discussion...confidence in this
period was not high. The medium range solutions were not in very
good agreement. For example...the latest European model (ecmwf) carved out an 570 dam
low right over the County Warning Area on Sunday whereas the GFS had the County Warning Area under
the influence of an upper trough centered to our east. Based on these
two solutions...the maximum temperature forecast for Sunday is too warm. For
now...thought it would be best to leave things unchanged. The
morning cycle might very well revert to a ridge solution for Sunday.


Aviation...a weak upper level trough sliding down the British
Columbia coast today will help the northwesterly flow aloft become
westerly. Low level flow will remain onshore and the air mass will
become slightly unstable this afternoon.

Pretty solid stratus coverage over The Lowlands this morning...with
the exception of western Whatcom and Skagit counties. Despite the
coverage the stratus layer not terribly thick...expect ceilings to
lift through the remainder of the morning and scatter out most
locales around 18z/11 am. The exception will be the coast...which
will become VFR early in the afternoon but not scatter out until
late in the day. The upper level trough approaching the area today
will bring increasing high clouds and a chance of thunderstorms to
the north Cascades.

Ksea...stratus and MVFR ceilings will continue to lift over the next
few hours and scatter out by 20z. Clearing has been a bit slower
than expected over the central sound...more solid breaks in the
stratus working their way up from the south. Conditions will remain
VFR tonight...lowering to MVFR levels early Friday
morning...improving through midday. Southwest winds 4 to 8 knots
becoming light and northeast after 06z.


Marine...the very static onshore flow pattern will continue for the
next several days. As is typical with this type of pattern...the
strength of the onshore flow will vary diurnally as well as with a
series of weak systems that are forecast to brush the region over
the next few days. Expect the strongest westerly winds over the
central and east Strait during the evening and early morning
hours...easing just before day break through the later morning.


Sew watches/warnings/advisories...
Pz...Small Craft Advisory central and east Strait of Juan Delaware fuca.




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