Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
315 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...infrared satellite imagery shows the moist baroclinic band
still pinned to Vancouver Island and stretching SW across the
NE Pacific. Models have been consistent the past few days with the
evolution of this system...and have also reached better consensus
with regard to quantitative precipitation forecast. An upper level trough along 143w at 09z will
slowly push the baroclinic band southeast as a cold front today. This will
put the band southeast squarely over West Washington today allowing the rain to
continue. The advancing upper level trough will finally push the
cold front southeast of the area later tonight with rain changing to
showers after midnight.
Models agree on the recent quantitative precipitation forecast trends...showing lower total
accumulations over the mountains today...but somewhat higher amounts
over The Lowlands. Precipitation intensity will drop off rapidly this
evening and even more after midnight tonight. This easing of the
precipitation intensity today and tonight will help keep most rivers below
flood stage. See the Hydro section below for more detail.
Colder air aloft will filter in over the area today as the upper
level trough approaches. This will cause a problem over the north
Cascades as snow levels are expected to lower down to 3500-4000 feet
this morning over The Mount Baker area...then spread S to around
Stevens Pass this afternoon. Enough precipitation is still expected to fall
over the far north Cascades to day for 6 to 12 inches of snow to
accumulate. Precipitation rates will be declining this afternoon...so by
the time the snow level drops down to Stevens Pass this
afternoon...accumulations will be below warning criteria. The Winter
Storm Watch for the north Cascades has been upgraded to a warning.
Over the central Cascades Snoqualmie Pass should only receive a few
inches. Paradise could receive advisory amounts of snow by this
evening but I will let the day shift take another look at that.
Drier northwest flow aloft will develop over West Washington late tonight...between
the departing upper level trough and an approaching low amplitude
upper level ridge over the NE Pacific. Low level onshore flow will
increase...leading to breezy winds on Monday and a Puget Sound sound
convergence zone. Onshore flow will ease and shower activity will
diminish Monday afternoon as the upper level ridge nears the area.
The bad news is that over the last few model runs the ridge has
become dirtier...with weak warm advection developing ahead of the
ridge. Clouds and a chance of light rain have been added to the
forecast Monday night through Tuesday to account for the warm
advection. An upper level trough and its associated cold front will
approach the area Tuesday night for increasing rain. This change to
a wetter forecast follows the GFS solution more closely than the
European model (ecmwf). The European model (ecmwf) has been trending toward the GFS solution the last
couple of runs...but still does have quite as much precipitation as the
Long term...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
Hydrology...2 to 2.5 inches of rain has fallen over the Olympics
and north Cascades in the past 24 hours ending at 09z. Each area also
had one station with a bullseye of 3 inches or more. Rainfall
intensity has diminished in the past few hours and is not expected
to be as intense through this evening. The 00z WRF-GFS and 06z NAM
both show about another inch of rain in the next 12 hours...ending
around 21z...or about 1.5 inches through this evening.
With lower precipitation intensities expected today and this evening the
threat of flooding on area rivers will be lower. For the north
Cascades...some rivers are already experiencing sharp
rises...particularly parts of the Nooksack...Skagit...and
stillaguamish river basins...as well as the tolt. So far...although
some river forecast points may crest just below flood stage...only
the stillaguamish at Arlington is forecast to go above flood.
For the Olympics...only the bogachiel and the skokomish look like
they have a chance at reaching flood stage. And for the
bogachiel...the trace is starting to drift below the current
hydrograph so I will watch it for a few more hours before deciding
on a warning. Kam
Aviation...west to southwest flow aloft will prevail today...then
become northwest tonight as an upper trough moves through. A
baroclinic zone across the far northwest part of western Washington
will sag slowly southeast across the area today...pushing east of
the region tonight. The air mass is moist and stable.
Generally MVFR ceilings and visibilities today with some IFR at
times. Conditions should improve tonight as the frontal band pushes
east of the area.
Ksea...ceilings mostly 1000-2000 feet today with visibilities 3-5
miles but some lower stuff is possible early this morning and again
in the afternoon as heavier rain redevelops. Ceilings and
visibilities should improve tonight. Southerly wind 8-12 knots.
Marine...a cold front will move slowly inland today and tonight.
Most waters will have Small Craft Advisory winds.
Strong onshore flow will follow tonight and Monday with gale
westerlies expected down the Strait of Juan Delaware fuca. The coastal
waters will also rise to near gale later tonight but are expected to
stay just below. West swell in the 15-20 foot range will affect the
coastal waters and western Strait of Juan Delaware fuca later today
A brief ridge of high pressure will build over the area on
Tuesday...then another frontal system will move through the area
Tuesday night and Wednesday. Schneider
Washington...flood watches are in effect for much of the County Warning Area.
Winter Storm Warning for the north Cascades today and tonight.
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