Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
325 am PST Thursday Jan 29 2015
Synopsis...a weak upper disturbance will move through the area this
morning keeping skies mostly cloudy. An upper ridge will provide dry
and mild weather later today through Saturday. A front will bring
rain back to the area Sunday and early next week.
Short term...satellite imagery shows a very weak shortwave over the
southwest interior this morning. Nothing on the radars or the
surface observations indicating any precipitation with this feature
just cloudy skies over most of the area. Even with the cloud cover
some locations...especially in the southwest interior...have already
fogged in with visibilities a mile or less. Temperatures at 3 am/11z
ranged from the middle 30s to the middle 40s.
Shortwave will dissipate over the area this morning. With the
satellite imagery already showing the shortwave starting to break up
will keep precipitation out of the forecast this morning. Flat upper
level ridge...already over the area this morning...will begin
building this afternoon. This combined with the dissipating
shortwave will result in some sunshine this afternoon across the
area. High temperatures will remain above normal...in the upper 40s
to middle 50s.
Upper level ridge continuing to slowly build over the area tonight.
Surface gradients are flat/light offshore and with the lack of cloud
cover expect more fog especially in the fog prone areas like the
southwest interior. Temperatures will once again drop into the middle
30s in the colder locations with upper 30s to lower 40s common.
Upper level ridge remaining over the area Friday with the surface
gradients remaining flat/light offshore. Model cross sections show
the low level moisture is shallow so expect what fog does form
Friday morning to burn off leaving sunny skies in the afternoon.
Surface gradients are still light so will go with a persistence
forecast for the high temperatures...upper 40s to middle 50s.
Upper level ridge remaining over the area Friday night and Saturday.
With the lack of cloud cover Friday night into early Saturday
morning and the light surface gradients around round of patchy
morning fog with areas of fog over the southwest interior. Tail end
of a warm front moving into Vancouver Island Saturday afternoon.
With the surface pressures falling over the coastal waters the low
level offshore flow increasing. Temperatures aloft don't warm
any versus Friday with the model 850 mb temperatures remaining in
the plus 3 to plus 8c range. With the increasing low level offshore
flow and western Washington potentially being in the warm sector
with the warm front to the north will add a couple of degrees to the
highs on Saturday versus Friday and go for 50s across the area. Lows
Saturday morning will once again be in the middle 30s to middle 40s.
Long term...next frontal system still timed into the area late
Sunday although the 00z model runs were a touch slower with the
onset of the precipitation Sunday versus the previous runs. Took the
probability of precipitation out of 00z-06z Sunday time frame and lowered the probability of precipitation for
06z-12z Sunday. This system is weak and not expected to be much of a
rain producer with just a few hundredths. Model solutions start to
diverge a bit for Monday with the European model (ecmwf) bringing a stronger system
into the area while the GFS brings a weaker system into western
Washington. With both models indicating at least some light rain
will stay with the rain likely forecast on Monday. Model solutions
continue to diverge Tuesday into Wednesday with the GFS building a
ridge over western Washington on Tuesday keeping the weather dry
while the European model (ecmwf) has a warm front approaching...close enough to keep
a chance of rain in the forecast. The European model (ecmwf) brings the warm front
into western Washington Tuesday night and stalls the front over the
area on Wednesday with a bit of atmospheric river scenario setting
up with the bulk of the precipitation aimed at Vancouver Island. The
GFS is more progressive with the pattern with the warm front
brushing western Washington Tuesday night into Wednesday and a cold
front moving through Wednesday night. GFS ensembles area all over
the place Tuesday into Wednesday not lending much confidence in the
model solution. With the lack of consistency in the models and the
scrambled ensembles will stay with the chance probability of precipitation forecast for
Monday night through Wednesday. High temperatures will remain near
or slightly above normal through the period. Felton
Hydrology...flooding is not expected for the next 7 days.
Aviation...a weak upper level ridge will remain over western
Washington today and tonight. A surface ridge over East Washington will
maintain weak low level offshore flow. A weak upper level shortwave
trough along the coast at 10z will move east across the area this
morning. So far it has brought areas of middle level
clouds...bkn050-100...to the west part of West Washington...interfering somewhat
with the fog trying to form in the moist surface air mass ahead of
the clouds. Temperature-dew point depressions remain mainly 5f or
less in most areas...so some patchy fog will probably
continue...particularly over S Puget Sound sound and SW interior...despite
the middle level cloud cover.
Whatever fog or low stratus that forms will dissipate by late
morning...and the clouds from the shortwave trough will diminish by
21z...for good VFR conditions.
Ksea...VFR conditions are expected this morning. Middle level clouds
scattered-bkn050-100 from a weak upper level shortwave trough will be
present through midday which should be enough to prevent fog or low
stratus from forming. Good VFR conditions expected after 18z after
the trough and the middle level clouds move east. Surface winds will be
north-NE 3-7 knots. Kam
Marine...a surface ridge centered over eastern Washington will
maintain weak offshore flow through Saturday. Pressure gradients
across western Washington will remain weak...producing northerly
winds along the coast and through Puget Sound sound...and easterly winds
through the Strait. Other than mentioned below...winds are expected
to remain less than 20 knots.
A weak frontal system will approach the region from the west on
Saturday. Falling pressure offshore will strengthen the offshore
flow through the Strait...with Small Craft Advisory winds expected Saturday afternoon
through Sunday morning at the West Entrance and the adjoining coastal
Models agree on bringing the approaching front across the area
Sunday. Recent models have trended weaker with the front and are
keeping winds below 20 knots. A stronger system will follow on Monday
but models are in poor agreement regarding the strength. The 00z
European model (ecmwf) came in much stronger with a 1006 mb surface low crossing
Grays Harbor Monday morning. GFS is the weakest while the Canadian
holds The Middle Ground. The current forecast is based on the weaker
You can see an illustrated version of this forecast discussion at