Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
940 am PDT sun Mar 16 2014
Synopsis...a moist frontal system will remain over western
Washington today for periods of rain. An upper level trough will
push the cold front southeast of the area tonight with rain changing
to showers after midnight. Northwest flow behind the trough will
maintain a chance of showers with partly sunny skies on Monday. A
weak warm front will bring a chance of rain Monday night and
Tuesday...followed by an upper level trough and cold front Tuesday
night and early Wednesday. Showery and breezy weather will follow
the cold front Wednesday and Thursday.
Short term...moist southwest flow will continue to train over
western Washington today as a cold front or baroclinic bands slowly shifts
S/southeast through the region. So far the forecast appears on track with
precipitation amounts of 1-3 inches across the olympic peninsula and north
Cascades. Meanwhile...heavy snow is falling at Hurricane Ridge and
Mount Baker. Heavy precipitation in the lower basins is forcing sharp rises
on the rivers and a Flood Watch remains in effect. Minor flooding is
forecast for a few rivers - see the hydrology discussion below. We
should see another 0.25-0.75 inches in the interior today putting
rainfall totals close to 1 inch. Precipitation will start to taper down
this evening as an upper level trough shifts inland - forcing the
front to push out of western Washington. The trough will also drop the snow
levels down to around 2500 feet tonight.
Northwest flow will prevail on Monday with scattered showers mainly
over the Cascades. We may see a convergence zone form over Snohomish
County. Models are now showing some light rain on Tuesday with weak warm air advection.
Long term...previous discussion...GFS and European model (ecmwf) have been
consistent in bringing the upper level trough and its associated
cold front through the area Wednesday morning. Onshore flow will
increase behind the cold front for breezy conditions...and a Puget Sound
sound convergence zone will mostly likely form. Northwest flow aloft and
onshore flow will continue Thursday and Thursday night. Another
trough could reach the area on Friday. Kam
Hydrology...rainfall over the last 6 hours has been heaviest in the
central Cascades centered on the Skykomish river basin. Both the
tolt and the Snoqualmie near Carnation are forecast to crest
slightly above flood stage. The main stillaguamish near Arlington
will also probably crest right around flood stage. These crests will
likely occur late this afternoon or this evening.
By contrast...rainfall over the Skagit and especially Nooksack
drainages has tapered considerably. The Nooksack and its Forks
appear to have crested already and flooding is looking unlikely
there. The Skagit was never forecast to flood and that looks
accurate. Over on the Skokomish River rainfall was forecast to be
about half what is required for flooding and the river seems to be
cresting about half a foot below flood stage. The bogachiel river is
not quite done rising but at this point the heaviest rain has moved
south of the northern Olympics and the bogachiel should also not
Later today the baroclinic zone is forecast to move south...ending
any heavy rain threat over northern basins and giving southern
basins some rain but not any hydrologically significant rainfall.
Over the next week some fast moving fronts will bring rain at times
but any flooding is unlikely over the next week. Burke
Aviation...a stalled cold front will persist over western Washington this
afternoon...with strong west-SW flow aloft running parallel to the front.
This pattern with a moist and stable air mass will cause widespread
rain to continue into early evening. This evening...an upper trough axis
will pass across western Washington. The resulting turn to northwest flow aloft will
finally push the cold front off to the southeast. Rain will taper off to
just a few showers overnight...aided by middle-upper level drying late
tonight and Monday morning. However...lingering low-level moisture will
cause ceilings of 015-030 to continue over much of The Lowlands.
Ksea...rain will continue today...perhaps picking up in intensity
from 22z-04z. Rain will taper off nicely after 04z. Though there
will be some variability...predominant ceilings of 010-018 will prevail
through at least 09z. Some drying of the air mass on Monday morning
will allow for more breaks in the clouds...as well as a lifting of
ceilings into the 020-030 range around 12z Monday. Haner
Marine...a cold front was stalled over western Washington this morning. The
front will start to move and accelerate off to the southeast this afternoon and
evening...bringing an increases in northwest onshore gradients and flow. Uil-bli
gradient expected to peak near +3.5 mb early Monday morning...which is
typically strong enough for a low-end gale in the central and
Otherwise...a long-period swell is arriving along the coast right
now...with the Grays Harbor buoy showing 18-foot seas at 18 seconds.
This should lead to severe bar conditions during the Ebb current
late this afternoon. Buoy 89 peaked at 21 feet at 8 am. Will soon issue
a high surf advisory for the beaches.
A brief ridge of high pressure will build over the area on
Tuesday...then another cold front will move southeast through the area Tuesday
night and Wednesday morning. Haner
Washington...high surf advisory along the coast.
Winter Storm Warning north Cascades and Olympics.
Winter Weather Advisory central Cascades.
Pz...Gale Warning central and east Strait tonight and Monday morning.
Small Craft Advisory all waters except Puget Sound sound and Hood
Small Craft Advisory for rough Grays Harbor bar.
You can see an illustrated version of the forecast discussion at