Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 170110
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
410 PM AKST Sat Dec 16 2017

.ANALYSIS AND UPPER LEVELS...

A broad upper low stretching from the Bering Sea to the eastern
border of Alaska continues to dominate the regional upper air
pattern. The main upper low center remains over the eastern Bering
Sea with a series of weak embedded shortwave troughs in southwest
flow tracking toward the North Gulf Coast. One of these is clearly
visible on satellite imagery with an associated deformation band
moving into Prince William Sound. Behind this shortwave, there is
an extensive region of post-frontal unstable air, clearly evident
in satellite imagery with a large region of convective cumulus.
Some lightning has been observed on the GOES lightning detector in
this unstable air mass, including over Kodiak Island. Finally, a
powerful low is beginning to develop just south of the Kamchatka
Peninsula. This low will become a major weather "player" over the
next few days as the storm force front moves across the Bering
Sea.

&&

.MODEL DISCUSSION...

The numerical weather models are in excellent agreement with the
general synoptic pattern for the next 4 days with little to no
discrepancy. All the models are progging a powerful storm force
front expected to cross the Bering Sea then move inland across
Southwest Alaska Monday and into Southcentral Tuesday. Agreement
is very good and all the models are depicting a similar scenario
with the exception of the ECMWF which shows more overrunning snow
across parts of Southcentral Tuesday. For now, a more "moderate"
approach will be favored since the GFS/NAM are depicting a
stronger cross-barrier gradient which would act to limit some
inland snow amounts for select.

&&

.AVIATION...
PANC...VFR conditions are expected to prevail through most of the
TAF period in a showery southwest flow regime, however occasional
MVFR conditions will remain possible, although unlikely, through
early Sunday morning before shower threats end.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Snow and rain showers will persist over the coastal areas tonight
as a shortwave embedded within the base of an upper level low to
the west sweeps through the Northern Gulf/Prince William Sound
area. Precipitation should stay mostly to the east of the Chugach
Front this evening, though a few stray snow showers may make it
into Anchorage and the Mat-Valley. Showers will taper off some
overnight before a surface low moves into the gulf tomorrow
bringing another round of precipitation to the coastal areas
during the day. Cold air will wrap around the backside of the low
which should help keep temperatures cool enough for the
precipitation to stay as mostly snow. The surface low will weaken
and move off to the east allowing a ridge of high pressure to
build in over the area by Monday. Fog could develop over the Cook
Inlet area, including Anchorage and the Mat Valley, on Sunday
night/Monday morning as the ridge axis moves overhead, allowing
strong subsidence to combine with clearing skies and cold surface
temperatures. Areas without fog on Monday will see mostly clear
skies though temperatures will be noticeably cooler than the
previous weeks, though around average for this time of year.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The eastern Bering low will track south along the southwest Alaska
coastline tonight before being absorbed into the low forming in
the western Gulf Sunday morning. As it passes, it will bring
light snow showers to southwest Alaska. Ridging will then build in
over the area on Sunday clearing out the showers and also helping
with fog formation over the area Sunday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Ridging is building in over the western Bering and tracking east
which will help to diminish the remaining showers over the
eastern Aleutians and Alaska Peninsula. Behind the ridging a new
system will deepen in the far western Bering to sub 960 mb.

The front with this system will move over the central Aleutians
Sunday afternoon bringing storm force winds as it passes. It will
then make it to the eastern Bering by Monday morning as the winds
begin to slowly diminish. This system will pull up enough warm air
from the south for the majority of the precip with the front to
fall as rain. Behind the front, cold air will move back in
bringing snow showers in its wake.

&&

.LONG TERM FORECAST (Days Tuesday through Saturday)...

On Tuesday, all of southern Alaska will be under the influence of a
weakening weather front moving from west to east. In general, a
front oriented southwest to northeast should not cause too much
downsloping, so the usually drier areas have a pretty good chance
of seeing some precipitation from this storm. Most coastal areas
will probably warm up enough to see rainfall, but from Anchorage
northward the airmass might just be cold enough for a few inches
of snow at lower elevations. Details are uncertain, so stay tuned.
We are more confident that the northern Susitna Valley and
Hatcher Pass areas should see some significant accumulations of
snow from this storm.

For the 2nd half of next week, models continue to indicate the
resurgence of a strong ridge over the eastern Gulf, that produced
deep-layer (and warm) southerly flow across most of southern
Alaska. This is the same type of pattern that we`ve been stuck
under for much of December so far. The orientation and strength of
the ridge will dictate what coastal locations are most likely to
get slammed by heavy precipitation (somewhere from Kodiak to
Cordova), as well as how warm and windy interior locations like
Anchorage will get. Stay tuned.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Storms 173-178 411-414 119 120 130-132 138 150 155 165
170-172 179-181 185.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...JA
SOUTHCENTRAL ALASKA...KVP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
LONG TERM...AD



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