Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 131325
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
425 AM AKST Fri Dec 13 2019

.ANALYSIS AND UPPER LEVELS...

For the third night in a row, the overall upper air pattern
remains somewhat unchanged as a weakening fully-stacked low in the
Gulf of Alaska dominates the overall flow regime. There is a
retrograding mid-level PV anomaly riding over the Alaska Range,
with the leftover cloud cover lifting into the interior. A
weakening cold upper trough over the Bering Sea is the remnant of
a once stronger upper trough, and it sits over a remnant Arctic
airmass over most of the Bering Sea. A dynamic mobile trough is
now moving into the far Southwest Bering/Aleutians, which is
bringing mixed rain/snow to Shemya, Adak, and Atka. Over the
Mainland, weather remains mostly quiet as the pressure gradient is
weak and mostly offshore (weak offshore flow), with most of the
precipitation impacting the northern Gulf Coast. Some light
freezing rain is still occurring over the Kuskokwim Delta in
association with weak easterly waves moving overtop a shallow near
surface warm layer. Over Southcentral, light rain/freezing rain
is occurring across parts of Anchorage, and this is also observed
on the 12z sounding as a shallow warm layer above the surface.
Outside of the Copper Basin, temperatures continue to run well
above normal for both daytime highs and lows.

&&

.MODEL DISCUSSION...
The numerical weather models are in excellent agreement with the
general synoptic pattern through Sunday. They differ, however,
with the placement and orientation of a storm force low which will
move over the Aleutian Chain as they seem to continue trending
eastward, which currently has the storm force surface low
directly over Dutch Harbor by Saturday. This forecast will trend
this feature eastward favoring the NAM hi-res, with strong storm
force winds on both sides of the low.

Thereafter, while the synoptic pattern is being handled well, the
models differ with the track of a secondary NPAC low moving up
toward the AKPEN Saturday night and Sunday, which would impact gap
flows through Bristol Bay area and the front as it lifts toward
the Gulf Coast. This will also make precipitation forecast
challenging for Southwest Alaska as bands of warm air form into
deformation zones as they move north. Forecast confidence is
moderate.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist with spotty
sprinkles today and this evening. Much uncertainty develops
tonight as a weak disturbance passes through, bringing the
possibility of a rain and snow mix and MVFR CIGS. For now will
favor a more conservative approach introducing light rain/snow and
VFR conditions for late tonight and Sat morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Fri and
Sat)...
The pattern will start to become slightly more active as we head
toward the weekend. The vertically stacked low in the Gulf
remains the dominant feature. It continues its slow drift to the
north and as of 4am, is nearly due east of Kodiak City. Steady
rain continues to fall along the Gulf Coast from Cordova over to
Western Prince William Sound courtesy of this feature. There is
enough upper level energy and mid level instability wrapping
around the center to allow some of the showers to drift over the
mountains and into areas around Cook Inlet. These showers are
extremely light, however, with temperatures below freezing across
most of the Anchorage Bowl and parts of the Matanuska Valley,
anything that does fall could make for slippery road conditions.

The low will push right up against the southern Kenai Peninsula
coast late this evening as it makes "landfall." This will
continue to produce gale-force level winds into Prince William
Sound as well as bring some gusty winds to the Turnagain Arm and
Anchorage Hillside. The system will then drift right over
Anchorage Saturday morning as it weakens. This should bring a
brief period of gusty southeasterly winds into town. It will also
provide our best chance of some light precipitation. At this
point, this will likely fall as a wintry mix of snow and rain,
and possibly some freezing rain. There will initially be some
warm air at mountain top levels which could allow for some super
cooled water droplets. However, cold air will filter in on the
backside of the low and cause any lingering precipitation to fall
as snow. Little to no accumulation is expected near sea level. For
the higher elevations of Turnagain and Thompson Pass, a few
inches of snow are likely, so weekend travelers should be prepared
for that.

Late Saturday into Sunday, the pattern will continue to become
more active as a warm front approaches the western Gulf of AK
moving towards the Kenai Peninsula. This will bring southeast
winds and steady rain back to the Gulf Coast with down-inlet winds
for the Matanuska Valley and Cook Inlet area.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

Several weak upper level waves have helped to allow light
precipitation to continue over the inland areas of Bristol Bay
this morning. Farther north, light precipitation has also been
falling over areas along the Kuskokwim Delta coast. As midlevel
instability persists, showers will likely continue over the area
with little to no snow accumulation expected by the afternoon.

The next system arrives by Saturday as a front lifts inland,
bringing gusty easterly flow. While downsloping is likely to
occur, areas that do see precipitation will likely see snow to
start with a change to rain rather quickly (or a mix of rain and
snow in places) as southerly flow raises temperatures, especially
along the Alaska Peninsula.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

A weak ridge over the central Aleutians will move eastward today,
bringing generally calmer conditions to the eastern Bering Friday
evening. The next system, estimated at 957 mb and currently
centered southwest of Shemya, is a strengthening low moving south
along the Aleutian Chain. This low quickly regenerates south of
Unalaska Island by late Friday night. As the low drifts slightly
northward on Saturday, storm force marine winds are expected
around the core of the low. The most recent change models have
shown is a slightly shifted position to the east, which was
updated in this morning`s forecast.

&&

.MARINE (Days 3 through 5/Sunday through Tuesday)...
Overall, confidence with positions of lows and fronts is above
average. The primary forecast challenge is whether windspeeds top
out at gale force or storm force. For now, will largely max winds
out at gale force until have more confidence in the depth of
these storm systems. The areas which will experience the strongest
winds include the northern Gulf and eastern Aleutians Sunday
through Sunday night, then out along the Aleutian Chain Sunday
night through Tuesday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The global models are in good agreement with the northern Pacific
low through Wednesday December 18th. There are some minor
difference with the timing of two different cyclones that develop
in the north Pacific and move into the Aleutians but the forecast
intensity is only a few millibars off, and the forecast track is
very similar. With above normal confidence, expect back to back
lows to impact the Aleutians. The respective precipitation shields
and pressure gradient force will move from west to east...in
typical mid-latitude fashion. There is a dome of high pressure in
the central Pacific with a ridge axis that moves into the Chain
and serves as a lull between systems.

For Southcentral/AKPEN and southwest Alaska, the forecast period
commences with a mature cyclone entering the region. Even through
the Canadian/GFS and ECMWF are not in sync with the placement of
the surface low, there is widespread consensus for onshore flow
from the Kenai Peninsula to Kodiak and the Alaska Peninsula. This
low decays and fills rapidly, ultimately becoming absorbed into
the longwave pattern. The first aforementioned Pacific low is
moving straight into the Gulf of Alaska and will undergo the
process of frontolysis there...which means minimal impacts for
Southcentral.


&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gales: 119, 120, 125, 130-132, 150, 155, 160, 165, 170,
171, 175-180, 352, 413.
         Storm: 172-174, 414.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...JA
SOUTHCENTRAL ALASKA...MSO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...PJS


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