Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
342 PM PDT Wednesday Jul 9 2014
Synopsis...high pressure aloft will remain over the Pacific
northwest through at least early next week. Meanwhile a surface high
offshore will maintain varying degrees of low level onshore flow
over western Washington. The result will be a warm and dry weather
pattern with mostly sunny skies punctuated by areas of morning low
Short term...high pressure remains over West Washington this afternoon
displacing the main storm track northward over British
Columbia...where an upper level trough is crossing the area.
The upper level trough over b.C. Is heading east and an upper level
ridge is moving in behind it toward the b.C. Coast. These changes
have shifted the low level pressure field so that there will be a
stronger southwest component to the onshore flow over West Washington tonight.
Pressure gradients are already showing this difference with the
hqm-sea and AST-sea gradients around 1.5 mb at 21z and the uil-bli
gradient through the Strait at 0.6 mb. The stronger SW component
will help stratus spread farther north through Puget Sound sound early
Thursday morning. The low clouds will probably come in with slightly
higher ceilings Thursday morning but will still be shallow...so the
clouds should burn off back to the coast by midday. The air mass
will still be a little warmer than normal with highs expected to be
in the 70s to lower 80s.
The upper level ridge just west of the b.C. Coast Thursday will shift a
little farther east on Friday. This will weaken the onshore flow
Thursday night and also warm the air mass. It looks like there will
be 6 to 9 degrees of warming on Friday with inland highs in the 80s
to lower 90s. The stratus will not make it very far inland Thursday
night and will burn off quickly. Most of West Washington will be sunny. Weak
onshore flow will keep coastal maximum temperatures in the 70s.
Saturday will be another very warm day with the upper level ridge
over the region. Maximum temperatures will be about the same as Friday. Onshore
flow will remain weak for minimal stratus formation...and mainly on
Middle level flow will shift to S-SW Friday night as the ridge axis
moves inland. A weak upper level shortwave trough could generate
some convection over the Oregon Cascades...with the debris clouds
moving up over West Washington late Friday night and on Saturday. Kam
Long term...the upper level ridge axis will shift over East Washington
Saturday night with S-SW middle level flow continuing over West Washington.
Another weak upper level shortwave trough over west Oregon may generate
additional convection with clouds moving north over the Washington Cascades. In
addition...lifted indices over the Washington Cascades Sunday afternoon are
forecast to be around -2...so some elevated convection is possible.
Confidence remains low since the pattern is weak and the models have
had trouble deciding which days will be convective. If the signals
become stronger and the models more stable then a chance of
thunderstorms may be added to the forecast.
Otherwise the air mass over West Washington on Sunday will remain very warm
with highs in the 80s to lower 90s. Weak onshore flow will continue.
The upper level ridge will shift far enough east for onshore flow to
increase on Monday or Tuesday. However the ridge will remain close
enough to the region so that a strong marine push will probably not
happen. The increased onshore flow will provide a little cooling to
afternoon highs. Kam
Aviation...moderate westerly flow aloft will continue through
Thursday. At the surface moderate onshore flow will also continue.
The air mass is stable and dry except for stratus right along the
coast. Stratus will again spread inland over most marine areas
tonight with adjacent land areas and the southwest interior also
developing stratus ceilings.
Ceilings under the stratus will be around 1000 feet with conditions
elsewhere VFR clear. The stratus will evaporate Thursday morning
with clear skies nearly everywhere by afternoon.
Ksea...VFR clear through tonight. There is a chance of a few hours
of stratus with 1k feet ceilings Thursday morning. Northwest wind 4-8
knots this evening will become southerly late. Chb
Marine...high pressure offshore combined with lower pressure east of the
Cascades will give moderate onshore flow through Thursday. Pressure
gradients will weaken Thursday and Friday and remain weak through
the weekend. Small craft advisories remain in effect for the coast
and the Strait tonight. One more afternoon and evening of these
small craft advisories is possible before winds generally fall below
20 knots. Chb
Pz...Small Craft Advisory coast except north inner coast.
Small Craft Advisory central and east Strait.
You can see an illustrated version of the forecast discussion at