Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Seattle Washington
935 am PST Thursday Dec 18 2014
Synopsis...an approaching frontal system will bring a chance of
rain today...followed by more continuous rain tonight as the cold
front moves inland. An upper level trough will bring scattered
showers on Friday. Strong moist west flow aloft over the northeast
Pacific will bring a windy and rainy pattern this weekend. A broad
upper level ridge will remain over the region early next week for
not quite as wet weather.
Short term...radar and observation indicate showers across much of western Washington
this morning. Although a weak upper ridge is traversing the area
today...the flow remains moist and southerly. Buffer sounds show the air
mass below 750 mb to be weakly unstable through midday so showers
will continue through the morning and probably up the point when
warm advection rain begins.
Infrared satellite imagery and the coastal radar show more steady rain
approaching the coast. This will spread inland late in the day.
Brighter cloud tops appear to indicate some splitting already
occuring with the front. A stronger portion is moving east-NE into
b.C./WA and another portion pushing due east into Oregon. Models
indicate the front holds together enough to produce more widespread
rainfall than past systems this week but it will be in a weakening
state as it arrives later today/early this evening.
Quantitative precipitation forecast amounts on various mesoscale models generally average a half to one
inch liquid in 12 hours from 00z-12z tonight over the mountains.
Areas that predominately remain as snow through the event could see
up to a foot of snow. Snow levels are the tricky part of the
forecast as current pass temperatures are in the upper 20s with Ely
gradient which may keep some passes cold for some time this evening.
The main snow level is currently near 3500 feet based on the kuil
freezing levels minus 1k feet. This will climb as warm advection
precipitation overspreads the area by late today or this evening.
Will go ahead and issue a Snow Advisory for 5 to 10 inch snow
amounts but stressing the lower end amounts will fall at higher
passes such as White Pass and Stevens Pass assuming some change over
to rain or freezing rain as the front pushes across later tonight.
Snoqualmie Pass will likely see snow change over to rain pretty
quickly with less than a couple inches there. The front will
actually help to mix the colder air out of the passes and raise snow
levels there. However...the main free air snow levels will fall back
to around 3000 feet Friday morning after lifting to 4500-5000 feet
tonight. Ski resorts exposed to S/SW flow such as Mount Baker and
Paradise should do best in this flow pattern if they remain as all
snow with up to 10 inches in these areas.
Lastly...windy conditions are expected to develop up north later this
afternoon through this evening. Southeast gradients tighten and a mesoscale low
will likely form around Port Angeles/Sequim. This should induce
locally stronger southeast winds west/ gusts to 45 miles per hour across Admiralty
Inlet...the San Juans...and exposed locations along the shores of
western Whatcom County. Both the hrrr/nam12 Show Low end advisory winds
and will plan to issue an advisory this morning for the late
afternoon and evening hours.
A moist onshore Post-frontal pattern develops Friday with weak
instability in the lower levels. This should translate to showers
for our area until the next system arrives. Latest 06z/12z models
bring warm advection rain to the coast late Friday night...then
overspreading the entire area by the middle morning hours on Saturday.
Models continue to show a lot of flow so rainfall rates will be
heavy along the S/SW facing slopes of the mountains...especially the
Olympics. A trailing moisture Plum will become directed at western Washington by
Saturday evening as the flow turns more westerly. This will generate
additional orographically enhanced rain along the west/SW slopes of the
Olympics/Cascades Saturday night into early Sunday. This could cause
some minor flooding on prone rivers...see the latest Hydro
discussion below for details. Southeast gradients will also be quite strong
with the warm front. Areas from Admiralty Inlet northward will likely need
a Wind Advisory and possibly the coast. Mercer
Long term...no update from previous discussion...European model (ecmwf) and GFS show
strong zonal flow continuing into early next week with a broad flat
upper level ridge over the region. Lots of flow through the ridge
will keep this a generally wet and breezy period through Sunday. The
main problem will remain heavy rain Saturday night. Fairly strong
westerly 850 mb flow will remain 30-45kt Saturday night into Sunday
morning...then gradually diminish over the north and central part of
West Washington Sunday afternoon. Stronger west flow will continue over northwest Oregon
and the southern Washington Cascades. There could be another 2-3 inches of
quantitative precipitation forecast over the central Cascades 12z Sunday through 12z Monday which
could bring some Cascade rivers close to flood stage.
The broad Flat Ridge maintains strong somewhat moist west flow aloft
over the area Monday and Tuesday...but the air mass is drier for
much less rain. GFS/European model (ecmwf) both bring an upper level shortwave trough
across the area on Wednesday...then develop cool dry north flow aloft
behind it for Thursday. We will see if this model agreement holds.
Hydrology...no update from previous discussion...river flooding is
not expected through Friday night. A moist westerly flow pattern
will develop Saturday and continue through Sunday night with heavy
rain at times in the mountains. A strong warm front will cross the
area Saturday with strong S flow over the Olympics becoming west by
late afternoon. The strong west flow will reach the Cascades Saturday
night. 2-3 inches of rain are expected over the Olympics and parts
of the Cascades during the 24 hour period from 12z Saturday through
12z Sunday. This could cause another round of flooding on the
Skokomish River and prime some of the Cascade rivers.
Low level west flow aloft over West Washington will diminish Sunday and Sunday
night as the core of strongest winds slides down over northwest Oregon and
the southern Washington Cascades. Still...another 2-3 inches of rain could
fall over the central Cascades between 12z Sunday and 12z Monday.
This added rainfall could push some of the more flood prone Cascade
rivers close to flood stage. Kam
Aviation...a strong Pacific frontal system over the offshore waters
will move across the area tonight. Southwesterly flow aloft will
increase and becoming strong this evening then becoming moderate on
Friday. The air mass is moist and stable...it will become somewhat
unstable late tonight and Friday.
At 1630z a strong front around 130w is moving east. It will push
across the area reaching the interior valleys just after 08z Friday
then east of the Cascades by 12z. Pressure gradients are
increasingly southeasterly in advance of the front...so despite rain
bands offshore moving inland...ceilings will become predominately
VFR. Expect ceilings and visibilities to deteriorate to MVFR along
the coast midday and in the interior shortly after 00z this evening
as precipitation develops and gradually increases with the approach
of the front.
There could be a short period of low level wind shear in the
interior from about kpae southward for a few hours ahead of the
front as winds at 2000 feet increase to south-southwest 40kt and surface winds
Post frontal onshore flow with scattered showers are expected behind
the front late tonight and Friday with MVFR conditions and some
clearing at times in the Lee of the Olympics. Isolated thunderstorms
are not out of the question along the coast on Friday as cold air
aloft arrives with an upper level trough. Albrecht
Ksea...expect broken-ovc040-050 today. Periods of -ra will develop this
afternoon. Ceilings will gradually lower to MVFR this evening with
ceilings bottoming out around 015 a couple hours ahead of frontal
passage 08z-10z Friday. Expect southeast wind 5 knots to increase to 9-11 knots
this evening then shift to S 10-14 knots behind the front 08z-10z.
There will be a couple hours of wind 18035-40kt at fl020 ahead of
the incoming front about 06z-08z time period. Albrecht
Marine...a front associated with a 968 mb low well west of haida
gwaii will move through the offshore waters today...then inland
tonight. A period of gale force winds can be expected over the
coastal waters beginning midday today through this evening with
Small Craft Advisory conditions for the Strait...northern inland
waters...and Admiralty Inlet. Gale force winds are now expected at
the East Entrance to the Strait of Juan Delaware fuca this afternoon and
tonight as a mesoscale low develops to the northeast of the
Expect moderate to strong onshore flow behind the front and with a
surface trough as it moves across the area on Friday. Small Craft
Advisory conditions will continue over the coastal waters and
possibly some of the interior zones. Isolated thunderstorms cannot
be ruled out over the coastal waters on Friday in a rather unstable
30-45 foot waves over a large area of the north central Pacific will
decay to a long period 22-23 feet swell that will reach the coast
Friday afternoon and continue to push onshore through Saturday.
Tidal anomalies along the coast are just over 1 foot above normal
and high tide occurs Saturday morning. If swell remains high into
Saturday morning as currently expected...some coastal flooding is
possible. A coastal Flood Watch may be required for portions of the
coast for the Saturday morning high tide.
Another strong front will arrive on Saturday with gale
force winds once again likely over the coastal waters...the northern
inland waters...and the Strait entrances. Albrecht
Washington...Wind Advisory through tonight for the San Juan Islands...
western Whatcom County...and the Admiralty Inlet area.
Winter Weather Advisory through tonight for the Olympics.
Winter Weather Advisory for the west slopes of the Cascades
4 PM PST this afternoon through tonight.
Pz...Gale Warning through tonight coastal waters and East Entrance.
Small Craft Advisory all remaining waters.
Small Craft Advisory for rough Grays Harbor bar tonight through
You can see an illustrated version of this forecast discussion at