Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
939 am PST sun Dec 8 2013
Synopsis...dry and cold weather will continue today. Another weak
system will move over the area on Monday with a chance of flurries.
The air mass will start to warm Monday night into Tuesday with
another weak system moving down from the northwest. Temperatures
will return to near normal Wednesday. A cold front will bring more
typical rain and winds Thursday...followed by another front Saturday.
Short term...the forecast challenge today will be determining phase
and type of precipitation on Monday and Tuesday as western Washington transitions out of the
cold and dry Continental polar air mass that is in place right now.
Before diving into specifics of the forecast...here is a quick
weather lesson. It is possible to get liquid falling from the
sky...even when the entire sounding is below freezing. The key is
that precipitation has to be generated in a layer that is -5c or
warmer...in which case supercooled water droplets are formed...
rather than snowflakes. When these contact a surface that is below
freezing...it will freeze upon contact and be observed as freezing
drizzle or freezing rain. This becomes a concern in a setup like Monday
and Tuesday...where precipitation is being generated at fairly low and
relatively warm layers of the atmosphere...even if those layers are
a little below freezing.
Okay...on Monday the upper ridge out around 140w will start to
flatten...backing the flow aloft from due north to northwest. This will
initiate weak onshore flow above the low-level cold air. Moisture
will rapidly increase from 850 mb down to the surface on Monday...and weak
warm advection lift will lead to light precipitation out of this low-level
moist layer. Light precipitation will first develop on the coast during the
day on Monday...then spread inland on Monday evening. The immediate coast out
near Ocean Shores and Westport will likely pop above freezing pretty
quick on Monday...making mere drizzle or light rain predominant there.
By going inland as far as Aberdeen...below freezing temperatures
will likely support a mix of light snow and freezing drizzle until
the surface air temperature rises above freezing early Tuesday morning.
Further inland along the I-5 corridor...the stretch from Olympia
south through Lewis County is looking favorable for freezing drizzle
on Monday night into Tuesday morning. Precipitation from Tacoma to Everett is being
generated in a layer that will be cold enough for a mix of freezing
drizzle and light snow. North of Everett...light snow will be the
dominant precipitation type through Tuesday morning. For the entire I-5
corridor...surface air temperatures should rise above freezing around
middle-day on Tuesday...ending concerns for anything but the wet and
liquidy type of precipitation.
Want to emphasize that precipitation on Monday and Tuesday is expected to be
light...on the order of one-tenth of an inch or less. However...it
only takes a little freezing precipitation to make untreated roadways and
sidewalks slippery and hazardous. For the afternoon forecast
package...will be figuring out more specifically where and when to
include freezing and frozen precipitation in the forecast. Based on morning
model data...freezing precipitation will likely be an aspect of the
forecast...especially from Everett on south. Haner
Long term...from previous discussion...extended models showing the
same trends as the 00z runs from Saturday morning. European model (ecmwf) digs the
trough off the coast on Wednesday deeper than the GFS resulting in a
weak ridge popping up over the area and a slower eastward advance of
the next incoming system. The GFS is weaker with the ridging which
allows a warm front to hang over the northern portion of the area on
Wednesday and then has the onset of the precipitation from the
trailing cold front over most of the area by 12z Thursday. The European model (ecmwf)
is a good 12 hours slower with the rain out ahead of the cold front
not starting until 00z-06z Friday. Better agreement on day 7 with
both the GFS and European model (ecmwf) indicating another frontal system moving into
the area although the GFS is once again a little faster with the
timing. Current extended forecast follows the GFS solution and will
stay with this forecast this morning. Felton
Hydrology...river flooding is not expected for the next 7 days.
Aviation...strong northerly flow aloft will continue over the region through
tonight. Very weak low level onshore flow. Anticipate VFR conditions
today...with flurries possible mainly on the coast. Areas of MVFR
ceilings /1-2k feet/ will develop tonight...mainly after 0600 UTC.
Ksea...VFR conditions will prevail over the terminal. Winds will be
a 1028 mb low over British Columbia will result in light southerly
flow over the area through tonight. Expect a weak surface trough to
move across the coastal waters on Monday for continued weak onshore
flow. A weak frontal system will move across the area on Tuesday.
For the graphical afd...visit