Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Seattle Washington
930 PM PDT Friday Aug 29 2014

Synopsis...low pressure aloft will dominate the weather pattern
over the region through this weekend for cool and unsettled
conditions. A series of weather systems are expected to move across
the region through Tuesday. An upper level ridge will build over the
region Wednesday through Friday for a warming and drying trend.


Short term...water vapor imagery shows a weak upper level shortwave
trough just offshore near 130w at 02z/7 PM that is pushing a small
area of light showers toward the coast. Radar is picking up these
showers over the coastal waters which are associated with the higher
cloud tops showing up on infrared imagery. The showers are weak enough and
small enough in size for just some brief rain. This is covered by
the chance of showers tonight.

The large upper level trough over western Washington this evening will
move slowly east over Idaho by Saturday evening. While the large trough is
overhead the trailing small shortwave trough near 50n/145w at
01z/6 PM...and embedded in the fast west-northwest flow aloft...will cross the
area on Saturday. The air mass will remain somewhat moist which will
support a continuing chance of showers when the passing shortwave
moves overhead.

Fast zonal westerly flow aloft will continue over the region Sunday
and Monday. The embedded shortwave trough near 170w this evening
will eventually move across the area late Sunday afternoon and
evening. Models show this system to be a little stronger than the
previous two shortwaves so probability of precipitation increase from chance to likely
Sunday evening. Westerly flow aloft continues on Monday for about a
20 percent chance of showers.

Weak low level onshore flow will continue over the area through the
weekend. The NAM shows a weak convergence zone-like precipitation signature
over Puget Sound sound tonight through Saturday night. With just weak
onshore flow the surface convergence over the area is not this precipitation signature may be due to a combination of
confluent flow aloft downstream of the Olympics and periodic surface
convergence. Bottom line...chance of showers continues with slightly
higher probability of precipitation over Puget Sound sound. Kam

Long term...previous discussion from the 3 PM afd...the medium
range guidance - foreign and domestic - has generally come together
regarding days 4 through 7 of the forecast period. All dig a shortwave
into the Pacific northwest as the zonal flow pattern amplifies through the
week. Precipitation with the shortwave arrives Tuesday and exits by
late Wednesday - a bit quicker than previous guidance. Then rising
heights aloft and low level flow turns more offshore as thermally
induced low pressure noses north from northern cal and western
Oregon. There has been some speculation that perhaps we may not see
80 degrees again this year. This pattern change may produce some 80s
when it is all said and done.

Of interest...ksea has had 39 days of 80 degrees or better this year
making it the 6th most number of warm days on record. The most is 47
days of 80 or warmer set in 1958. In sharp contrast...1954 had only
2 days of 80 or better - whew! Buehner


Aviation...a broad upper trough axis will cross western Washington
tonight...then move to far eastern Washington by Sat evening. Large-scale lift
within a moist air mass will support showers at times for the next
24 hours. A few showers are currently focused within the Puget Sound sound
convergence zone between ksea and kpae...but a larger area of shower
activity is evident offshore. The offshore showers will gradually
spread across western Washington tonight and Sat. As a result...ceilings will become
lower through middle-day Sat...with heavier showers lowering ceilings below
020 and reducing visibilities into 3-5sm range.

Ksea...the terminal should remain south of the Puget Sound sound
convergence zone for the rest of light S winds are
expected. With the moist air mass...ceilings will lower overnight into
020-030 range. Shower activity will pick up in the 12z-20z time
frame...with heavier showers bringing ceilings into 010-015 range with
showers reducing visibilities to 3-5sm. Spottier shower activity may
continue well beyond 20z as the Puget Sound sound convergence zone again
gets within 5-10sm of the airfield...likely remaining north of the
airfield. Haner


Marine...weak onshore flow will continue tonight and Sat. A warm
front will cross the waters on Sunday...bringing an increase in S
and southeast winds. A cold front will move inland on Sun night or Monday
morning...with increasing onshore flow in its wake. Haner


Sew watches/warnings/advisories...




You can see an illustrated version of this forecast discussion at

National Weather Service Glossary of Abbreviations