FXAK68 PAFC 211325
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
425 AM AKST Sun Jan 21 2018
.ANALYSIS AND UPPER LEVELS...
A highly amplified pattern remains over the region this morning,
with a broad longwave trough encompassing much of the Gulf and
mainland Alaska while a stout ridge persists over the central
Bering Sea. This is supporting fairly benign clear to partly
cloudy conditions across much of the southern mainland as
temperatures drop near or below zero inland and into the teens or
single digits along the coast. A north-south oriented jet streak
between the two primary synoptic features is also supporting brisk
northerly winds over the southwest mainland, allowing wind chill
values to dip a little lower. Light snow has started to spread
into eastern portions of Prince William Sound as a weak surface
low pushes westward across the Gulf. A developing upper low
embedded within the longwave trough is also bringing a few snow
showers to portions of the western Gulf and the Alaska Peninsula.
Marine stratus dominates most of the rest of the Bering beneath
the ridge, although a quasi-stationary front is bringing rain and
gusty southeast winds to parts of the Western Aleutians.
Models remain in good synoptic agreement through the short term as
the longwave trough remains over the mainland and the disturbance
near Kodiak Island evolves into a broad upper low over the
southern Gulf. This will send several smaller waves towards the
Gulf coast through Tuesday, keeping a higher chance for
precipitation along the coast as a modest offshore gradient
remains over Southcentral. Some differences begin to emerge with
respect to these individual waves moving through the northern
Gulf early in the week, with the NAM becoming the biggest outlier
PANC...VFR conditions and light north winds are expected to
persist through Monday morning. It is possible that some light
snow will spread into the terminal this afternoon after 00z. This
may lower ceilings below 5000 ft, but conditions are expected to
remain at VFR for the duration of any precipitation.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Sunday &
Outflow conditions will continue across The northern and western
Gulf through today with localized gap flows inland. A weak low
and associated trough move west across the Gulf today spreading
snow to the Gulf coast this morning and to the eastern Kenai
Peninsula by early afternoon. Isentropic lift/overrunning light
snow/flurries will likely make it into the Anchorage
Bowl/Matanuska Valley late this afternoon and should linger into
the early evening hours with little accumulation expected.
Tonight and Monday
Localized gusty outflow conditions will continue across the
Southcentral, though weaker then previous days. However Gales/Gale
gusts will continues across the Barren Island marine areas and
Gaps along the Aleutian Range. The next in a series of surface
lows and associated frontal boundary heads west across the Gulf
Monday/Monday night continuing elevated chances of snow across the
Gulf coast/eastern Kenai Peninsula. This system will even bring
increased chances of snow across the Anchorage Bowl and Matanuska
Valley late Monday night into Tuesday.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Over the next couple of days the story remains the same for the
southwest Mainland, as arctic air pours into the region and causes
persistent offshore flow and declining temperatures. This will
continue to push both daytime highs and lows downward with
increasing threats for bitter wind chills approaching -40F. This
will be most likely where the winds are the strongest, so it is
likely the Kuskokwim Delta will realize the coldest wind chills.
As the light snow showers along the Kuskokwim Delta coast taper
off early this morning, cold and windy conditions with clearing
skies will resume today and carry into early next week.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
High pressure settling over the central part of the Bering Sea
will induce more of an easterly flow component as cold air pours
over the eastern Bering Sea, and a gale force front moves into
the western Aleutians then stalls. This front looks like it will
not make further progress than the western Aleutians and begin to
dissipate this afternoon. With the colder air and winds pouring
over the Bering Sea, expect freezing spray to be a threat from the
Pribilof Islands eastward, with heavy freezing spray over the far
eastern Bering and along the ice edge.
.LONG TERM FORECAST (Days 3 through 7)...
Beginning the extended period forecast Monday night, the main
feature of interest will be a very cold upper vortex situated over
the North Slope of Alaska. Another low will be in the vicinity of
the Gulf of Alaska. A deep arctic air mass will be in place over
all of Alaska, although the bitterly cold air will still be north
of the Alaska Range. The first item of interest will be the
potential for snowfall across Southcentral. As the upper trough
digs south and interacts with the Gulf low, this will set up a
potential favorable setup for light snow. As usual, however, the
degree of phasing between the two upper troughs is rather low
confidence at this point with some runs of the GFS, NAM and ECMWF
showing the possibility of weak deformation banding from the Kenai
Peninsula northward. However, run-to-run consistency is low, so
stuck with a continuation of broad chance snow POPS until there is
more clarity with regards to the interaction of the two troughs.
Eventually, the cold upper vortex will move south into Bristol Bay
and dig into the southwest Gulf of Alaska, bringing down all of
the deep arctic air with it. This will bring the coldest
temperatures to Southwest Alaska, but Southcentral will also drop
well below average region wide by Wednesday and likely lasting
through Thursday. As the low forms over the Gulf, this will
introduce more of an easterly flow component aloft, and with
offshore flow, will likely keep precipitation predominantly along
the coast with much lesser chances inland. Temperatures will
moderate some by the weekend, but will generally run below average
across all of southern Alaska.
MARINE...Gale Warning 127 130 131 132 137 138 150 155 165 172 177
178 180 411
Heavy Freezing Spray 130 131 138 139 160 180 181
SYNOPSIS AND MODEL DISCUSSION...CB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP