Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
900 am PDT Tuesday Mar 11 2014
Synopsis...high pressure aloft will prevail over the region today.
The ridge will weaken on Wednesday with a system moving by to the
north of the area Wednesday night. The ridge will rebuild slightly
on Thursday then shift east Thursday night. This will allow a pair
of weather systems to move through western Washington Friday into
Short term...visible imagery and observation show areas of fog mainly from
Tacoma southward through the SW interior and along the coast. The moisture
layer is quite shallow and most of this will burn off by midday.
Highs today will rise into the middle 50s given light surface gradients
and sunshine...but probably not yet reaching 60 with 850 mb temperatures
still struggling at +1c this afternoon.
The upper ridge axis holds over the Pacific northwest through Wednesday
morning...then starts to shift eastward during the afternoon. Light
offshore gradients in the morning will become light around midday
then weak northwest onshore flow late in the day. Expect another repeat of
morning fog around the S sound/SW interior and Chehalis gap/and
coast. This Burns off midday similar to today with mostly sunny
skies. A few middle/high clouds come in during the afternoon but
probably not enough to affect temperatures. 850 mb temperatures are prognosticated
to reach +3 to +4 with the NAM met MOS likely the better guidance
giving a 60 for a high at sea-tac. Will likely tweak high
temperatures up slightly with the afternoon forecast package.
GFS/ECMWF/NAM all show a dry trough passage late Wednesday/Wednesday
night. Will maintain the dry forecast.
Onshore flow is surprisingly weak behind the trough Thursday
morning. Therefore...don't expect much of a status surge inland if
any. Weak upper level high pressure and light gradients dominate
during the morning hours...so expect not much more than areas of fog
over the same areas S of Seattle. It will be another dry day with
partly cloudy conditions as a weak trough brushes north of the area
Thursday afternoon. Highs probably a few degrees cooler than
Wednesday with increased onshore flow and lower heights below 5600m.
Long term...from 330 am discussion...extended models in good
agreement once again this morning for Friday with a front moving
through the area. Have raised the probability of precipitation into the high end of
likely...70 percent. Model consensus falls apart after Friday with
the European model (ecmwf) hanging on to the precipitation longer into Saturday
morning versus the drier GFS. The GFS builds a ridge over western
Washington on Saturday with the ridge remaining intact on Sunday.
The European model (ecmwf) is weaker with the ridge allowing the next system to move
into the area later Saturday into Sunday. The European model (ecmwf) is indicating
there could be a sub tropical tap with this system resulting in
another shot of significant rain for western Washington on Sunday.
The GFS drags a weakening system over the area Sunday night with
some ridging for Monday. The European model (ecmwf) doesn't kick the slow moving
system out of the area until Sunday night then also has a bit of
ridging for Monday. With the lack of any model consensus beyond
Friday have left the current forecast alone which has chance probability of precipitation
through the period except for likely probability of precipitation for the coast and
northwest interior on Sunday. Felton
Hydrology...minor flooding has come to an end on area rivers.
The Chehalis river at Porter is the one remaining river running just
below flood stage. However...the gauge shows the flow is turning
over from the peak so flood stage will not be reached at Porter. The
Flood Watch runs through noon and will plan to let this expire on
Precipitation through Saturday will not be hydrologically
significant...and no further flooding is expected. The European model (ecmwf) models
is the outlier but does show a much wetter front on Sunday than
other models. This leaves some uncertainty in the forecast for
Sunday with a small chance of flooding concerns Sunday and Monday.
Continue to monitor forecasts. Mercer
Aviation...high pressure at the surface and aloft will continue
over western Washington today. The high will shift east of the
Cascades tonight and early Wednesday giving light offshore flow. The
air mass is stable. Patchy low level moisture over western
Washington will dry by midday.
Observations...web cameras...and satellite imagery shows fog over
portions of the south Puget Sound sound area...valleys of the south
interior...the lower Chehalis valley...and along the immediate
coastline. Fog is shallow and will burn off by shortly after 18z. An
area of scattered-bkn030 is sitting around the Everett area and the east
end of the Strait of Juan Delaware fuca. These clouds will thin and
scatter midday. By 20z expect just a few cumulus and scattered middle level
clouds at times.
A stable air mass and light offshore flow will give just a few middle
clouds tonight. Patchy fog tonight into Wednesday morning from about
12z-18z should be limited to fog prone river valleys of the south
interior...more sheltered river valleys around Arlington...and the
immediate coastal stretch not impacted by outflow from the Chehalis
gap and the Strait of Juan Delaware fuca. Albrecht
Ksea...expect only few-sct030 few-sct120 today. Light north to northwest wind
5-7 knots will develop about 19z and continue until 03z before turning to
the NE/east 5-7 knots this evening. Albrecht
Marine...a 1036 mb surface high centered over the inland waters
will give light flow to the waters today. The high will shift east
of the Cascades tonight into early Wednesday giving light offshore
A weak front will move by to the north of the area late Wednesday
afternoon and evening. This will induce some light to moderate
onshore flow late in the day Wednesday...but at this time winds in the
Strait of Juan Delaware fuca are expected to remain just below Small Craft
The global wave model is showing a 12 to 14 feet swell in the central
Pacific moving east and impacting the coastal waters beginning
sometime late Wednesday morning or Wednesday afternoon. A Small Craft Advisory
for hazardous seas and rough bar conditions will likely need to be
issued with the afternoon forecast package for this swell train.
A weakening frontal system will move through the waters on Friday
and may bring Small Craft Advisory conditions to the coastal zones.
Another front may move east across the waters on Saturday. Albrecht
Washington...flood watches and/or warnings are in effect for parts of the
County Warning Area.
For the graphical afd...visit