Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
300 PM PDT Wednesday Oct 1 2014
Synopsis...a weak ridge will provide dry weather through Friday
with some afternoon sunshine each day. The south interior will
likely get some late night and morning fog. A weak front could bring
some rain Friday night through Monday but mostly confined to the
coast...north interior...and mountains. There will be a little
better chance of light rain Tuesday and Wednesday as a weak front
drifts through western Washington.
Short term...gradients are weakly onshore this afternoon and enough
mixing developed for partial clearing over some of The Lowlands. A
short wave over b.C. Will track southeast just brushing western Washington this evening
then moving off to the east. This may enhance the onshore flow just a
bit. There is one feature of interest to keep an eye on this
evening. The 12z/06z NAM-12/and high res-nmm are showing the flow
through the Strait to make the turn and surge S into Puget Sound sound
this evening. Forecast northerly gradients are quite weak but the NAM
tries to develop some light convergence induced precipitation during
the evening around Puget Sound sound. A quick look at NAM-12 time-heights
show good vertical motion up to about 800 mb but the 06z run has
delayed this period of lift. This solution looks overdone based on
the weak gradients forecast this evening and other counter model
solutions. The newly operational hrrr/WRF-GFS/high res-arw are all
dry for the evening and do not develop the amount of convergence
shown by the NAM. It appears residual moisture in place will settle
into the lower levels...resulting in some morning low clouds with
fog likely over southern interior areas.
High pressure builds stronger Thursday into Friday. There is good
agreement among the models that this will be a dry period.
However...night and morning fog will become somewhat more extensive.
Cold pocket locations will also notice lower nighttime temperatures
due to radiational cooling. Partial afternoon clearing each day will
help The Lowlands warm into the middle 60s to around 70.
The GFS/European model (ecmwf) are in good agreement on the strength of the ridge
Friday night into Saturday. However...the European model (ecmwf) brings moisture
associated with a stalled frontal boundary further S and faster than
the GFS. Probability of precipitation were trimmed back some as models have backed off again
but the far northern portion of the area could see some spotty light
rain. Southern interior areas will stay dry and could still see patchy
morning fog if middle/high clouds are not too thick. Areas S of this
frontal boundary will be relatively mild so much of the sound region
has a shot at reaching 70.
Long term...extended models are in somewhat better agreement today
not to say they won't flip flop again in subsequent runs. The 12z
European model (ecmwf)/18z GFS both show a stronger ridge now for Sunday into Monday
keeping precipitation to the north. Kept some low probability of precipitation over the coast
and mountains given the lack of continuity between model runs. If
the ridge maintains the dry weather then some fog should be expected
across some lowland areas...especially to the S.
Both the GFS/European model (ecmwf) drag the remnants of a front into the area on
Tuesday but show only light quantitative precipitation forecast. Since models agree...the Tuesday
through Wednesday period looks like the most best shot of bringing
some precipitation into the area. Chance probability of precipitation are indicated in the
Aviation...moderate northwest flow aloft will continue over West Washington tonight
and Thursday. An upper level shortwave trough near the central b.C.
Coast at 21z will brush by to the north of Washington as it moves southeast
tonight. A weak warm front moving toward Vancouver Island will brush
the far northwest part West Washington late tonight and Friday.
The air mass over West Washington is somewhat dry this afternoon with weak low
level instability allowing cumulus clouds to form over most areas.
The cumulus will dissipate this evening with only scattered clouds
expected 00z-06z. Overnight cooling will allow areas of low stratus
and fog to reform...especially over the SW interior. Middle level
clouds from the offshore warm front will reach coastal sections
Ksea...VFR conditions are expected through this evening with just
few-scattered clouds. IFR conditions from stratus and fog are expected to
develop 12z-18z. Kam
Marine...a 1024 mb surface ridge over the coastal and offshore
waters will shift over West Washington tonight as a warm front offshore heads
toward Vancouver Island. Weak onshore flow will continue through
this evening with marginal Small Craft Advisory winds expected in the central and east
Strait of Juan Delaware fuca.
The surface ridge will sit over West Washington Thursday and Friday for a
short period of weak offshore flow. During this period a frontal
zone will remain north of the area...over the central b.C. Coast. On
Friday night and Saturday the weakening front will finally move
southward over West Washington. The surface ridge is expected to survive the
passage of the weak front...then remain over the area Sunday and
Pz...Small Craft Advisory in effect from 3 PM this afternoon until
midnight tonight for the central and eastern Strait of Juan
You can see an illustrated version of the forecast discussion at