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Area forecast discussion 
National Weather Service Seattle Washington
920 am PST Monday Dec 22 2014

Synopsis...a weak upper level ridge will move east across western
Washington today giving dry conditions with occasional middle and high
clouds. A frontal system will spread light rain into the area
tonight and rain will increase Tuesday and Tuesday night as the
front moves slowly eastward. An upper level trough will keep the
rain going on Wednesday along with cooler conditions. Drier
northwest flow aloft will keep the area cool on Christmas day with
only a chance of showers early in the day. Additional systems will
move southeast into the area Thursday night through Saturday.

&&

Short term...a broad long wave ridge is along 135w this morning
with a trough over the Great Plains. While models seem to have a
feel for the approximate timing of short wave features moving over
the ridge now through the upcoming weekend...they appear to be
having problems with the details concerning possible block
development offshore and its strength and placement. Problems with
the details of The Block development are a function of model
differences...both model-to-model and run-to-run...in the strength
of development of low pressure north of Hawaii and handling of deep
low development in the western Bering Sea. While these problems
effect mainly the long range portion of the forecast...some of these
problems creep into the forecast as early as Wednesday morning.
Expect the forecast over the next couple of days to be
challenging...low skill...and dependent on details that cannot be
resolved well at this point.

For today...flat high pressure aloft will move across the area. The
ridge is rather dirty...so while it will be dry...there will be
plenty of middle and high clouds moving across the area. Temperatures
are already in the middle 40s at many locations...so will update the
forecasts for high temperatures a couple degrees warmer than current
values.

Precipitation associated with a warm front that sits from off the S
coast of Oregon to a low near 48n 145w will reach the coast tonight.
Then precipitation will increase Tuesday and Tuesday night as the
cold front portion of the system moves through. This front has some
tapping to convection south along the inter-tropical convergence zone south of Hawaii.
Precipitation on Tuesday will briefly become moderate to somewhat
heavy as freezing levels rise to around 8000 feet...but the system
is expected to be progressive enough to keep precipitation from
becoming heavy enough to cause flooding concerns at this time. Will
be closely evaluating incoming guidance to see if precipitation may
be a bit heavier and thinking may need to be changed.

The nam12 and European model (ecmwf) both cool the air mass significantly behind the
front late Tuesday night into Wednesday morning as precipitation
continues to fall behind the front. Snow levels fall to around 1800
feet by Wednesday morning. That combined with moderate precipitation
rates appear to cause the higher terrain southeast of the
Seattle-Tacoma-Olympia metropolitan area...places like out toward
Enumclaw...Orting...and Yelm to get close to the snow level. At this
time both mesoscale models and the European model (ecmwf) keep snow levels just high
enough to limit any accumulations that occur to the Cascade zones.
This will need to be monitored. Behind that front...an upper trough
crosses the area later Christmas even and gives showers in northwesterly flow
-along with possible convergence zone activity on Christmas morning.
Temperatures are marginal...but there is a chance that snow levels
in heavier showers and convergence zone activity...if it
occurs...could result in some surprise snowflakes for Christmas
morning in portions of the east Puget Sound sound lowlands and around the
Everett area. This will not be something to hold your breath for
though. Albrecht

Long term...the 12z GFS brings a weak short wave trough down from
the northwest Christmas night and this is followed by a stronger
short wave trough on Friday night. The first will only bring a
chance of light precipitation...but the second may give the
mountains some much needed snowfall. Behind Friday nights
trough...the GFS builds a ridge along 140w that promises cool and
dry weather for later in the weekend through next week. The
operational GFS appears to bring cool air in quickly from the north
while the parallel run is slower and stronger with a potential
modified Arctic front. This is due to much stronger low development
north of Hawaii...a less progressive northern stream short wave
coming from the western Bering Sea...and better resolved terrain
over south central British Columbia. Will likely need to make some
pop adjustments for the Thursday night to Friday night period for
the lead short wave features...then dry things out for later in the
weekend. The challenge will be temperatures. Will not make changes
to the extended before looking at the 12z Gem and European model (ecmwf). Albrecht

&&

Hydrology...a period of heavier rain and high snow levels on
Tuesday is expected to be brief enough to not result in flooding
across western Washington. Flooding is unlikely later Wednesday
through early next week. Albrecht

&&

Aviation...weak high pressure aloft over the Pacific northwest through Tuesday
morning...flattening during the afternoon and evening. A warm front
will move into the area tonight...followed by a cold front on
Tuesday. Air mass somewhat moist and mostly stable. Patchy morning
MVFR ceilings and patchy fog with visible below 3sm...improving by midday.
Drier in the lower levels this afternoon.

Ksea...lower stratus now appears unlikely to affect the terminal
this morning. VFR through tonight with middle/high clouds more
dominate. MVFR ceilings may develop late tonight/Tuesday morning as rain
increases along a cold front.

&&

Marine...west swell 10-13 feet will continue over the coastal
waters. Winds generally less than 20 knots with higher pressure at the
surface over the waters.

A warm front will lift north across the area tonight...bringing Small
Craft Advisory winds to most areas except Puget Sound sound. A trailing
trough or surface low will affect the waters Tuesday into Wednesday.
Onshore flow strengthens behind the front Tuesday night and
Wednesday with stronger westerlies in the Strait and southwesterly winds over
Puget Sound sound possibly reaching small craft.

Some of the higher astronomical high tides of the year will be
occurring this week along with slight tidal anomalies due to passing
weather systems. It looks like minor tidal overflow is possible for
some locations around Puget Sound sound early Tuesday morning with a
better chance early Wednesday morning. Dtm

&&

Sew watches/warnings/advisories...
Washington...none.
Pz...Small Craft Advisory for hazardous seas coastal waters and
West Entrance Strait of Juan Delaware fuca.
Small craft for winds coastal waters and West Entrance to the
Strait this evening through Tuesday.
Small Craft Advisory for Admiralty Inlet...the East Entrance to
the Strait...and north inland waters from late this evening
through Tuesday afternoon.
Small Craft Advisory for rough bar Grays Harbor bar.

&&

$$

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