Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
817 PM PDT Sat Aug 1 2015
Synopsis...an upper ridge will gradually weaken over the next
couple of days. Onshore flow will return early next week for cooler
weather. An upper trough will bring a chance of showers to the
mountains later in the week
Short term...an upper ridge over the area will weaken a little
Sunday with slightly stronger low level onshore flow developing.
In general...the weather will be about the same as today...sunny
and very warm. Temperatures will probably cool slightly but
inland areas will be flirting with record highs again. A heat
advisory remains in effect through Sunday from Seattle south.
An upper trough will move over the area on Monday with increasing
low level onshore flow. Low clouds will probably be limited to
near the coast Monday morning but highs will cool over the
interior by several degrees. There will also be a slight chance of
thunder near the Cascade crest as moisture and instability
increase in the southerly flow aloft.
The upper trough will remain over the area on Tuesday. There
should be morning clouds followed by afternoon sunshine for the
interior and highs will be near normal. Schneider
Long term...previous discussion...the upper trough in British
Columbia continues to influence the weather over western
Washington Wednesday and Thursday. Mainly it is a matter of
depressing The Heights and giving a chance of showers to the
mountains. Have kept The Lowlands dry with high temperatures
mainly in the 70s.
The upper trough is past by Friday and an upper ridge actually
builds over the area. This should keep the whole area including the
mountains dry...with sunny skies and high temperatures rebounding
into the low to middle 80s inland. Another trough could bring a chance
of showers to the mountains by next weekend. Burke
Aviation...a broad upper level ridge will remain over the Pacific
northwest through Sunday with light west flow aloft tonight will
become SW on Sunday. The air mass will be stable and dry...except
for shallow marine moisture mainly along the coast.
Marine stratus will increase along the coast tonight and move over
the lower Chehalis valley late tonight...bringing local IFR
visibility and ceilings through Sunday morning. Otherwise skies will
remain mostly clear across West Washington.
Ksea...mostly clear skies tonight and Sunday for good VFR
conditions. Surface winds will be north 5-10 knots this evening
becoming NE 3-7 knots after midnight. Kam
Marine...high pressure offshore and low pressure east of the
Cascades will result in stronger onshore flow through the middle
of next week. Small Craft Advisory northwest winds are likely over
the outer coastal waters through at least Sunday morning...with
Small Craft Advisory northwest winds this evening over the
southern two inner coastal waters zones. High end Small Craft
Advisory westerlies are likely in the central Strait of Juan Delaware
fuca and East Entrance Strait tonight and Sunday night...and gales
are not out of the question. Gale force westerlies are a good bet
there Monday night. Mcdonnal/kam
Fire weather...previous discussion...the upper level ridge which
has been responsible for the very dry air mass in recent days will
start to loosen its grip on the area on Sunday. In fact...an upper
ridge axis will pass western Washington tonight and exit east to
the northern rockies by Sunday evening. This will allow the air
mass to moisten just enough to let the red flag warnings expire
this evening. Once the red flag warning does expire late this
evening...it just means that weather will be less of an
aggravating factor in the fire environment. Background fire danger
will still be elevated due to dry fuels.
The new wrinkle in the fire weather forecast is the introduction of
isolated dry lightning to the Cascades from late Sunday night until
early Monday evening. As the upper ridge axis moves east...southerly
flow aloft will develop and bring north the needed moisture and
instability. Today's models came around to showing the Cascades at
the northwest edge of a larger-scale lightning risk area. The first
threat over the Cascades will be from high-based nocturnal
convection that could bring isolated dry lightning to the Cascades
from Mount Rainier on south in the hours around sunrise on Monday.
The lightning threat will reach the north Cascades on Monday
afternoon...then end on Monday evening. A dry sub-cloud layer and
the steady forward movement of the storms is a recipe for sub-
wetting rains with any storms. Haner
Washington...heat advisory until 8 PM PDT Sunday for Bellevue and vicinity-
east Puget Sound sound lowlands-Seattle and vicinity-southwest
Red flag warning until 10 PM PDT this evening for central
Cascades-east portion of the Olympic Mountains-north
Cascades-west portion of the Olympic Mountains.
Pz...Small Craft Advisory until noon PDT Sunday 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 midnight PDT tonight for coastal
waters from James Island to Point Grenville out 10 nm-
coastal waters from Point Grenville to Cape Shoalwater out
Small Craft Advisory until 3 am PDT Sunday for central U.S.
Waters Strait of Juan Delaware fuca-East Entrance U.S. Waters
Strait of Juan Delaware fuca.
You can see an illustrated version of this discussion at