Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 201347
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
447 AM AKST Tue Feb 20 2018

.ANALYSIS AND UPPER LEVELS...
The pattern has not changed much for the past week or so. It
continues to be dominated by transient shortwaves passing through
southern AK. In between the various shortwaves we see a resurgence
of a ridge from the North Pacific/Gulf of AK rebound back into the
area. The next in the series of shortwaves is currently pushing
from west to east across the AK Range. This system is quite
moisture starved, so not expecting much in the way of snow except
maybe an inch over the Northern Susitna Valley or along the
Talkeetna Mountains. There are a few returns on the Kenai radar
this morning, but very little reaching the ground yet per the
METAR observations. There is however plenty of low stratus and
fog lurking around Southcentral. This shortwave seems to be
helping to mix some of it out, but with more moist westerly flow
behind it, there is a chance it could linger through the day.
Meanwhile over the Bering, another gale-force front is approaching
the Central Aleutians. The upper-level jet is moving along at 125
kt helping to support that front. The jet then moves over
Southwest AK from south to north at 100 kt and is a big part of
the reason that most of the state is relatively warm in this
pattern.

&&

.MODEL DISCUSSION...
Models are in generally good agreement through the short term.
This is resulting in fairly high forecast confidence regarding the
overall pattern. However, the fog/low stratus details are still a
challenge. As much of the southern mainland keeps moist, westerly
flow, it will be tough to completely get rid of that low-level
moisture, so it could re-form at just about any point over the
next 48hrs. The bigger story though is how the models are handling
the next shortwave coming in late Wednesday into Thursday. Models
have been sorting out the nuances of this feature for several
days. The American models (GFS/NAM) show weaker energy passing
further north. This could lead to the potential for a rain/snow
mix and limited accumulations. Meanwhile, the EC/GEM are showing a
stronger developing low and keeping it further south. This could
bring several inches of snow across much of southern AK. At this
time, we have trended the forecast to the EC/GEM somewhat. Models
all agree that after the surface low exits the area, strong
offshore (west-northwest) flow will develop to end the work week.

&&

.AVIATION...
PANC...A very tricky forecast this morning. Plenty of fog/stratus
hung up along the Knik Arm. A weak-shortwave moving overhead
could help conditions improve to VFR later this morning, but it
is very unlikely that it will clear out all the low-level
moisture. Thus, expect at least some low stratus decks to linger
through the daylight hours with the potential for a ceiling and
more fog tonight.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
A weak shortwave will briefly flatten out the upper level ridge
as it crosses the northern half of the area this morning. There is
a slight chance of snow for Anchorage and the Matanuska Valley,
with a better chance further north through the Susitna Valley and
the Copper River basin. This system will then exit into Canada by
early afternoon. The ridge will then rebound over the area for dry
conditions tonight and Wednesday. Patchy fog and stratus are
expected for the Cook Inlet area as the flow inland remains weak
and the temperature inversion will stay in place.

A fairly strong and moist trough will approach Southcentral from
the west on Wednesday night and cross the area on Thursday. There
are some differences in how the numerical models handle this
system (see the "Model Discussion" above), but there is enough
confidence to put fairly high pops across much of the area. There
is also a possibility of mixed precipitation types, but for now
kept it as snow for most inland areas.

&&

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

The cold front has now crossed the Alaska Range into Southcentral
with onshore flow bringing fog and low stratus to much of Bristol
Bay and northward into the Kuskokwim Delta. Weak pressure
gradients today will likely keep some of this around until mid
day, but it is possible it lasts into the afternoon since the
surface winds will be weak.

Attention then turns to a trough moving into the western Bering
Sea today through Wednesday night. This will support a rather
robust warm front which will push another surge of warm air into
Southwest Alaska. Most of the precipitation will impact the
Kuskokwim Delta with less amounts farther eastward. Light snow is
possible before turning to all rain over the Delta. Eventually
the precipitation will spread east into Bristol Bay and the Lower
Kuskokwim Delta...largely as rain or a rain/snow mix, so
accumulations look to be paltry.

&&

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

The remnants of the once powerful storm force low over the western
Bering Sea have shifted into the Bering Strait, with attention now
switching to a developing gale force low which is already moving
into the southern and western Bering Sea/Aleutians. This low will
not be nearly as strong as the previous, but it will last longer
since it is slower moving, so expect gale force winds over parts
of the Bering Sea through Wednesday before the low weakens and
moves toward the Southwest coast Wednesday night.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The pattern in the long term looks to be an active one with
multiple systems moving through the area over the next week. The
main low centers look to track through the Bering with the fronts
still making it over the southern mainland. However, with the
associated low centers further away, the fronts do not look to be
very strong by the time they make it to the southern mainland.
There is decent confidence among the models on the overall
pattern but they are struggling with exact placement of the main
lows. Further out in time, ensemble means were preferred over the
deterministic solutions.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...GALE WARNING 173-179 185.

&&

$$

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



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