Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 120117
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
517 PM AKDT Mon Jun 11 2018

.ANALYSIS AND UPPER LEVELS...
Southcentral has finally started feeling the impacts associated
with the highly amplified Arctic trough that has been anticipated
for the past few days. Showers that started last night are still
continuing for the region with the heaviest precipitation
occurring along the north Gulf coast and Prince William Sound, as
well as the Copper River basin. Northerly flow and cooler
temperatures are being felt as the trough moves south and has even
brought new snowfall to higher elevations along the passes in the
Copper River Basin.

Southwest Alaska is experiencing quiet and mostly dry weather. Low
stratus continues over the western and central Bering, associated
with high pressure. Offshore flow near Bristol Bay is causing
clearing for the eastern Bering. The front associated with the low
south of Shemya is staying just south of the western Aleutians.

&&

.MODEL DISCUSSION...
Models are in good agreement as the Arctic trough continues to dip
down into the northern Gulf through the next few days.

&&

.AVIATION...
PANC...Following the potential for a few more showers and possible
isolated thunderstorm to move through this evening briefly
dropping conditions to MVFR, VFR conditions are expected to
prevail tonight and tomorrow.

&&

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

There are several challenges in the forecast through Tuesday as
an Arctic low sends several shortwaves around the base of its
trough which extends to the southern tier of the state. The main
impacts from this storm are thunderstorms across Anchorage, the
Matanuska Valley and eastern Alaska Range, heavy rain for the
northeast Gulf coast and snow along Thompson Pass.

The surface low center which developed along the base of the
trough today remains nearly stationary overnight near the
Talkeetna Mountains. This positioning brings a convergent zone
along the Matanuska Valley to Anchorage area. A significant upper
wave moves across this boundary during the evening which creates
additional lift. This combination will be a focus for thunderstorm
and small hail development through early tonight. For the
northeast gulf, a low level jet helps to pump abundant moisture
towards Cordova through early Tuesday which could potentially
bring up to an additional three inches of rainfall. Lastly for
tonight is snowfall for Thompson Pass...temperatures cooled to
freezing at the surface earlier today and is expected to drop just
below the freezing mark overnight as the cold core system sags
southward. An over running pattern will bring several inches of
snow as moist gulf flow glides over the cold pool at the surface
through Tuesday morning. There is still uncertainty with the snow
amounts as models have a difficult time capturing this situation
during this time of year. Snow and heavy rain is expected to diminish
with the low level jet rapidly weakening Tuesday morning.

Scattered rain showers are expected Tuesday and Tuesday evening
with isolated thunderstorms possible across the eastern Alaska
Range where instability increases along a thermal ridge nudging
across the Alcan boundary. Drier and warmer conditions are
expected on Wednesday as mid level ridging brings a short reprieve
before the next front moves up from the North Pacific.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 to 3)...

Southwest Alaska remains in a dry, northerly flow, situated
between high pressure anchored over eastern Russia and an upper-
level trough slowing over eastern Alaska. Gusty winds across the
YK Delta today will diminish overnight as the pressure gradient
between the surface high to the west the low over Southcentral
Alaska relaxes. A weak shortwave will drop south early Tuesday
morning, reinforcing the northerly flow. This shortwave will stall
over Bristol Bay allowing a weak surface trough to develop and
extend northeast over the Nushagak and Lime Hills. Although the
airmass overall will remain dry, an onshore flow ahead of the
trough could push enough moisture into the interior to produce an
isolated shower over the Alaska Range.

A second arctic trough will drop south toward the region late
Wednesday. This time, however, the shortwave should only graze the
YK Delta before sliding east as high pressure building in ahead
of a developing north Pacific low halts its southern progression.
The occluded front associated with this low will move north and slide
over the eastern Aleutians and AKPen by Thursday afternoon. Gusty
southeasterly gap winds are possible in advance of this system in
favored locations along the AKPen Thursday afternoon and evening.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 to 3)...

High pressure remains the dominant weather feature over much of
the Bering and Aleutian chain through the next two days. With
that said, a strong northerly flow and plenty of low-level
moisture in place has meant lingering low stratus and occasional
fog for the Pribilofs and along the Bering side of the eastern
Aleutians and northern AKPen coast. This will remain the case
overnight tonight. Some clearing, or at least a lifting of any
fog, is expected by tomorrow afternoon. But a developing trough
over Bristol Bay tomorrow will produce a weak onshore flow that
may allow another round of fog to develop from Dillingham to King
Salmon overnight Tuesday into Wednesday morning.

By Thursday, all eyes turn to a developing 962 mb low barreling
toward the central Aleutian from the north Pacific. This system
will move north, pushing an occluded front across the eastern
Aleutians Thursday afternoon. Easterly gale-force winds will
develop over the Pacific side of the eastern Aleutians ahead of
the low Thursday morning and continue through the evening hours.

&&

.LONG TERM FORECAST (Days 3 through 7...Thu through Mon)...

The long term will start with much of southern AK caught between
two systems. To the north, the cold Arctic trough continues to
hang on, even in mid-June. Meanwhile, to the south, another
feature that is somewhat rare this time of year will move toward
the southern mainland. It is a fairly potent storm system made
stronger by incorporating the remnants of former Tropical Storm
Maliksi. So for sensible weather, Thu will see a fair amount of
sunshine across the area with another round of afternoon shower
and some isolated thunderstorms. But by early Fri, the extra-
tropical system will bring widespread moderate rain to Southwest
AK. This rain will reach Cook Inlet and the eastern half of
southern AK by Fri evening. We will be watching the associated QPF
amounts for any potential impacts to rivers across the region.

This system will be strong enough to reorient the entire synoptic
pattern allowing a broad trough to establish itself over the
Eastern Bering into Western AK. The result of this longwave
pattern is a much cloudier and wetter pattern than what we saw to
start June. Temperatures will remain rather seasonal, but
afternoon highs will certainly stay (and feel) a bit colder
without the sunshine and morning lows will likely be a bit warmer
with some clouds for isolation. Models due start to diverge (as
usual) later in the forecast with the ECMWF being the most bullish
on the Eastern Bering trough. But none of the models are painting
any sign of a strong ridge bringing hot and dry conditions to the
southern mainland through early next week. Thus, expect fire
weather concerns to remain low.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...None.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...BL/AH
SOUTHCENTRAL ALASKA...KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
LONG TERM...MSO



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