Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
935 am PST Friday Jan 30 2015
Synopsis...an upper ridge will provide dry and mild weather through
Saturday. While it will be dry...low level temperature inversions
will give fog to to mainly the interior lowlands during the night
and morning hours. A couple of systems will bring rain back to the
area Sunday and Monday.
Short term...an upper level ridge along 130w extends eastward into
western Washington this morning. A short wave trough moving over the
top of the ridge is squashing the top of the ridge...but clouds and
precipitation will remain well north of the area.
Areas of fog have formed in the interior from about Whidbey Island
southward with another patch of fog seen over the Strait of Georgia
and another just off the coast. Unlike the nam12 and GFS...the hrrr
maintains the fog over the central Puget Sound sound and the areas around
the East Entrance to the Strait of Juan Delaware fuca all day. This
appears overdone considering the shallowness of the fog and its
current narrow extend...but fog can persist below inversions well
into the afternoon this time of year. Will increase the fog and
decrease maximum temperatures a bit today...but will still go for
clearing in the afternoon.
Once the upper trough to the north passes the area by to the
east...the offshore ridge rebounds then shifts east across the area
Saturday afternoon and Saturday night. With a strengthening
inversion and light pressure gradients overnight...expect areas of fog
to reform...then persist into midday on Saturday. With the inversion
and fog...temperatures were lowered for Saturday afternoon as
well...about Half Way to an average of the model guidance.
A weak front will move across the area on Sunday bringing some rain
to the area...though the central Puget Sound sound lowlands may be
shadowed by the Olympics and only get a couple hundredths of an
inch. Another...bit more energetic front...will move across the area
on Monday bringing rain and breezy conditions.
Forecast updates were limited to the first 3 periods this morning.
Long term...from the previous long term discussion...some
differences in the models early on in the extended with the
different timing of the cold front Sunday into Sunday night. Both
the GFS and the European model (ecmwf) agree that another fast moving system will be
on the heels of the cold front and move into western Washington on
Monday keeping rain in the forecast. Beyond Monday forecast
confidence is low with the extended models showing little run to run
consistency. The European model (ecmwf) which on the 00z run last night was bringing
a warm front inland to the north of the area now has the precipitation
aimed at Oregon with a weak trough approaching Vancouver Island and
northwesterly flow aloft over western Washington on Tuesday. The GFS
also has the precipitation going into Oregon with a little bit more
ridge off the coast versus the European model (ecmwf). There is a general agreement
with a weak ridge over the area on Wednesday followed by a weakening
warm front moving up from the south on Thursday. The GFS ensemble
solutions are much more consolidated on the 00z run tonight versus
the previous runs. Even with this would like to see a couple more
runs with this solution before going with a drier forecast for
Tuesday and Wednesday. At this point will stay with the climatology
chance probability of precipitation. Chance probability of precipitation continuing on Thursday with the possible
warm front moving into the area. Felton
Hydrology...flooding is not expected for the next 7 days.
Aviation...an upper ridge will remain over the area. West flow aloft
will become northwest. The low level flow will be weak offshore
today...becmg nearly flat tonight. Areas of fog with visibilities below 1sm
and ceilings at or below 300 feet will dissipate by 2100 UTC. Expect fog to redevelop
after 0300 UTC Saturday.
Ksea...the surface visibility should improve to 2sm by 1900 UTC. Anticipate
VFR conditions by 2000 UTC. Fog will return after 0600 UTC
high pressure over the area will maintain a very weak or flat flow
through Saturday. This will also result in areas of fog with
visibilities of 1 nm or less. A weak cold or occluded front will
move across the area on Sunday for onshore or southerly flow.
You can see an illustrated version of this forecast discussion at