Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion...correction
National Weather Service Anchorage Alaska
420 am akst Tuesday Dec 10 2013
***dramatic pattern change expected for southcentral Alaska through
Analysis and upper levels...
Weak flow continues to persist over the region this morning...with
the 12z Anchorage sounding depicting a very deep saturated low level
layer. Areas of fog and freezing drizzle will linger over much of
Cook Inlet...the northern Gulf...and the Matanuska Valley through the
day before drier air clears the region out.
Beginning tonight...the region will begin a dramatic shift in the
large scale pattern...as the persistent high pressure aloft over
southern Alaska will give way to a deepening upper level
vortex/trough late today into the weekend. A high latitude blocking
high over eastern Siberia will shift east to the Beaufort Sea on
Wednesday...allowing for a cold offshore flow pattern to develop over
the eastern Bering and Gulf Coast of Alaska as a low currently south
of the Alaska Peninsula slides into the Gulf. This pattern will usher
in much colder and drier polar air...which will finally bring an end
to the persistent fog and drizzle over much of southern Alaska.
The cold upper vortex over southern Alaska and a strong North Pacific
low moving into the Gulf by late week will eventually set the region
up for a potential significant precipitation event...including
widespread snow across southcentral Alaska (see extended discussion).
The numerical guidance is in excellent agreement with the large scale
pattern heading into the late week across the entire region...with
only small differences resulting in negligible sensible weather
differences. By late Thursday into Friday...more substantial
differences begin to emerge with the pressure pattern in the Gulf of
Alaska as a strong North Pacific low moves into the Gulf...as well as
the onset of light precipitation along the coast. The GFS and European model (ecmwf)
were favored for the entire forecast period.
Short term forecast...
areas of freezing drizzle continue across much of the Anchorage bowl
this morning...with patchy drizzle and isolated flurries extending
north and south into the mat valley and Kenai
Peninsula...respectively. These conditions will persist through the
day today before increasing dry and cold air filters into the region
overnight...finally bringing an end to the low clouds...drizzle...and
Wednesday through Thursday...all southcentral will see temperatures
plunge as polar air seeps into the region...with clearing skies and
otherwise fair and dry conditions dominating until Friday.
Across the Copper River basin and Susitna Valley...light snow showers
will expand across the region today...bringing light accumulations
mainly over the Copper Basin. Flurries may persist near the mountains
into Thursday...but the general weather regime will become much
colder and drier with clearing skies into Friday.
a very cool and moist airmass is stagnant across the southwest with
a warm inversion just above the surface. The areas of dense fog that
developed overnight across the southwest Mainland will persist
through early afternoon with relatively weak north to east flow
across the surface. Periods of rain will continue to develop and
move north across a stationary front over the Alaska Peninsula as
an upper low advects moisture northward through tonight. A cold
airmass will move west from the interior spreading a cold air mass
over the region Wednesday. Weak low pressure in the Bering will keep
scattered precipitation across the peninsula through midweek.
a weak surface low over the western Bering will slowly move east and
south through middle week. Precipitation will mainly be rain along the
front with light mixed precipitation in its wake. Winds will change
direction out of the north to east behind the low returning cooler
temperatures across the Aleutians and Bering.
Long term forecast...
Beginning Friday...while small but important details remain somewhat
uncertain...the large scale synoptic pattern will feature a cold
upper level vortex centered along and west of the Alaska/Aleutian
ranges...with a strong North Pacific low moving into the Gulf of
Alaska. With cold air across all of southern Alaska...there is no
doubt that precipitation will begin as snow across portions of the
Gulf Coast as early as Thursday night into Friday. Differences arise
with the position of the upper level vortex...and the wave
configuration of the deep low in the Gulf. Pattern recognition and
climatology favor a vortex that sets up well west of the Alaska
Range...which would favor more warm air advection and cross barrier
flow to form along the coastal mountains...limiting snow across
interior locations as the front moves inland. However...the European model (ecmwf) and
GFS ensemble support a much more impressive snow event for portions
of southcentral including Anchorage...with a deep vortex right along
the Alaska/Aleutian ranges...and a weaker warm front eventually
sliding farther east as it transits the northern Gulf. These
solutions would support a prolonged and impressive snow event as a
long lasting deformation zone/front sets up somewhere along and/or
near the Aleutian Range...with snow and snow showers lasting through
much of the weekend. For now...the ongoing forecast will take a
somewhat conservative middle ground approach...which calls for
widespread precipitation across most of southern Alaska Friday into
Saturday. Localized snowfall potential still remains somewhat
uncertain...especially for locations near mountains where the small
details can result in dramatically different sensible weather
public...Freezing Rain Advisory 101 111.
Marine...gale 126 127 128 130 132 138 139 150 155 177 178.
Heavy freezing spray 126 127 180 181.
Ahsenmacher/kh Dec 13