Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
315 PM PDT sun Aug 2 2015
Synopsis...an upper ridge will move east tonight and Monday as a
deep upper level trough moves into central British Columbia. Onshore
flow will gradually return by Tuesday...bringing areas of morning
clouds and lower temperatures through middle week. The trough will give
a chance of showers to the mountains Wednesday and Thursday. After a
dry day Friday showers could become more widespread next weekend.
Short term...middle and high level clouds have persisted over the
southeast but most areas have received plenty of sun today.
Temperatures are notably lower today from about Seattle north and it
appears the streak of 90+ days at ksea is over. There probably will
be some 90+ degree highs over the southwest.
Later tonight thunderstorms are possible over the south Cascades and
these will spread north to the rest of the Cascades Monday.
Meanwhile moderate onshore flow tonight should bring some stratus to
the coast and some distance inland...then it will evaporate back to
the coast by noon or so. Heights Monday will be considerably lower
than today as the upper trough in British Columbia moves closer. The
upshot will be a mostly sunny and dry day for most areas with high
temperatures in the low 80s inland and near 70 on the coast.
Onshore flow is forecast to be stronger Monday night and marine
clouds should penetrate farther inland and persist longer. Heights
are lower yet on Tuesday. High temperatures will be mainly in the 70s
though the coast may not reach 70. The pattern will be similar
Tuesday night and Wednesday with similar temperatures Wednesday. There will
be a chance of showers in the mountains as the upper level trough
moves a bit closer. Burke
Long term...heights actually start to rise on Thursday as the upper
trough in British Columbia moves well to the east. Onshore surface
gradients also weaken. A warming trend should begin with highs back
in the low to middle 80s Thursday and Friday and generally dry weather
except for a chance of showers in the mountains Thursday.
The transient upper ridge responsible for warm and dry weather will
move out of the area by Saturday. Both the GFS and Euro show light
precipitation over most of the area on both weekend days. Have
introduced a chance of showers to the whole County Warning Area on both Saturday and
Sunday. High temperatures will be mainly in the 70s. Burke
Aviation...a broad upper ridge will remain over the region through
tonight with light southwest flow aloft. The air mass will be
somewhat unstable but generally dry...except for shallow marine
moisture mainly along the coast.
There is little or no stratus over the coastal waters this
afternoon...with only high convective debris clouds over the
forecast area. Marine stratus will probably form over the coastal
waters tonight and move into the coastal zones...lower Chehalis
valley...and Strait of Juan Delaware fuca for local IFR conditions later
tonight and Monday morning. It should burn off in most areas midday
Ksea...northwest wind 4-10 knots...becoming variable 6 knots or less
tonight...then northwest 4-10 knots Monday afternoon. Mcdonnal
Marine...high pressure will remain offshore with low pressure east
of the Cascades through this week. Onshore flow will increase Monday
and Monday night and remain fairly strong through midweek. Small
Craft Advisory northwest winds will continue over most of the
coastal waters zones tonight...and then again at times early next
week. Brief westerly gales occurred in the central Strait of Juan Delaware
fuca and East Entrance Strait Saturday evening... and conditions
look similar this evening. In fact the Quillayute to Bellingham
pressure difference -- a key indicator -- is slightly higher this
afternoon. Confidence is not especially high...but I think there is
a good enough chance for brief westerly gales this evening to issue
a Gale Warning with the afternoon forecast. Pspc Vancouver also has
a Gale Warning in effect. Gale force westerlies are a good bet
Monday night...and the forecast is for 30 to 40 knots. They could occur
Tuesday night and Wednesday night as well. Mcdonnal
Pz...Small Craft Advisory tonight coastal waters except zone 150.
Small Craft Advisory central and east Strait until 6 PM today.
Gale Warning central and east Strait from 6 PM to midnight.
Gale watch central and east Strait Monday night.
An illustrated version of this forecast discussion can be seen at