Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 281230
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
430 AM AKDT TUE JUN 28 2016
.ANALYSIS AND UPPER LEVELS...
The most prominent feature on the surface map this morning is an
occluded front currently draped across the Central Aleutians/Central
Bering Sea. This front has continued to make steady eastward
progress overnight and early this morning as an amplifying upper
level trough over the western Bering Sea continues to slowly
displace the upper level ridge toward the Alaska mainland. As this
front moves further and further away from it`s parent low near the
Kamchatka Peninsula the front will continue to weaken. The parent
low has also begun to weaken as much of the upper level support
has been cut off.
Fog continues to be quite the challenging issues across the
eastern half of the Bering Sea and parts of Southwest Alaska this
morning. As mentioned earlier, the ridge axis has continued to
shift eastward which has allowed a push of marine stratus and fog
to push inland across the Kuskokwim Delta and Bristol Bay zone.
12Z RAOBS from PASN/PACB/PAKN/PABE continue to depict a fairly
impressive subsidence inversion coupled with a sfc inversion.This
was also observed Monday morning which brings pretty high confidence
that the low clouds and fog could stick around through the morning.
The area of low pressure in the southern Gulf of Alaska which has
been spinning there for what seems like the last week is finally
on it`s way out as it continues to weaken before it appears to get
absorbed by an approaching sfc trough. The upper level low still
remains in place which will continue to push a few weak easterly
waves across Southcentral Alaska. Rain has been mainly confined to
coastal locations due in large part to the structure of the upper
level low with the gulf coast being along the northern periphery
of this upper low.
Still not a whole lot of change in model agreement through the
short term but a pattern shift toward the end of the weak begins
to expose some doubt. The front moving through the central Bering
Sea is expected to stall just to the west of the Pribilof Island
where it will shear apart into a weak sfc trough. This should help
finally bring an end to fog across much of the Bering Sea and at
the same time increase the wavelength of the upper level trough
which will likely expand from Kamchatka to the Alaska Panhandle.
Thunderstorms look possible today and Wednesday across Southwest
Alaska but discrepancies in a weak wave moving through as well as
trying to predict the amount of dry air aloft becomes a bit
challenging. Both the NAM and GFS depict decent instability
developing both days with a weak shortwave moving through during
peak heating. Regardless of model solution, it appears decent
thunderstorm chances will prevail.
It looks like much of the Alaska mainland will transition to a
wetter period by Thursday with that ever-persistent upper level
low in the Gulf of Alaska. This upper low is expected to make some
northern progress this time which will allow for more easterly
waves to traverse the mainland. It`s too early to nail down
location and timing of these waves so the forecast could change a
bit in the coming days based on how the models handle these features.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The large vertically stacked low which has been over the southern
Gulf has drifted southward over the past 24 hours and will begin
to swing eastward today. As a result, the most substantial short-
waves are now tracking westward across the Gulf toward Kodiak
Island and the south end of the Kenai Peninsula. This is where
the bulk of showers are expected today through Wednesday. Areas
farther north are still under cyclonic flow aloft, but with weak
impulses and overall weak instability expect mainly isolated to
scattered diurnal convection focused along the mountains. Although
cloud cover is extensive this morning expect breaks of sun to
develop over the valleys with day-time heating.
Beginning late tonight and continuing through Wednesday and beyond
the forecast becomes extremely challenging over Southcentral
Alaska. The upper level low will open into a trough and swing
northward across Southeast Alaska sending a steady series of
"easterly waves" across British Columbia and the southern Yukon
and into Southcentral. These usually produce 6 hours or more of
steady rain along with widespread clouds and cooler temperatures.
Models traditionally struggle with the track and intensity of
these waves, often not gaining a good handle on them until
6 to 12 hours out. The first of these upper level waves will
enter the Copper River Basin late tonight then track westward
across interior Southcentral during the day Wednesday. Used a
fairly broad-brush approach to precipitation forecast with this
do to varying representations in guidance and overall low
confidence in the details.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
It should be another cloudy and foggy start to the day across the
west coast of Alaska as the upper level ridge axis has shifted
east overnight. Current observations show low ceilings mixed with
some fog from a line extending from Toksook to Quinhagak to
Egegik. Light southerly winds will likely advect fog further
inland this morning but should mix out quickly by late morning
with conditions rapidly improving. By midday, showers will develop
near the Kilbuck Mountains and near the Alaska/Aleutian Ranges but
will likely remain near the mountains with storm relative motion
less than 10 kt. A weak wave should also pass through the area
near peak heating which should aid in some thunderstorm development
near the aforementioned mountain ranges. Thunderstorms will likely
increase in coverage Wednesday afternoon as a more potent upper
level wave pushes through Southwest Alaska. Drier air aloft will
also likely increase lapse rates which will also aid in thunderstorm
Fog and low stratus should should be less of a problem tonight
and Wednesday morning as the high pressure begins to fall apart.
However, it wouldn`t be surprising to see a few places fog in
during the early morning hours if some locations see some decent
rainfall today from showers and/or thunderstorms and winds become
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A weakening occluded front will continue to push eastward today
but should stall just to the west of the Pribilof Islands by late
tonight. A narrow band of decent rainfall will be associated with
the frontal boundary with just scattered showers observed behind
the front with decent low level moisture and broad cyclonic flow
aloft. Fog and stratus continue to be a problem across the eastern
third of the Bering Sea this morning as it looks like some fog has
banked up along the Alaska Peninsula. Going with persistence from
yesterday, it appears that fog should burn off along the Alaska
Peninsula late this morning or early this afternoon with a partial
clearing of skies.
.LONG TERM FORECAST (Days 3 through 7)...
The extended forecast beginning late week will feature a a large
upper low over the western Bering Sea and another stretching
across the Gulf of Alaska. Narrow ridging (and the associated
thermal low) will be centered over the interior of Alaska north of
the Alaska Range. This will mean easterly flow aloft will be
present across southern Alaska. These patterns can be conducive
to inland heavy rain events as thermal lows forming across the
Yukon interact with shortwave troughs embedded within the trough
before retrograding west. There is currently low certainty with
timing and exact placement of these lows as they move west, but
there is increasing confidence that large parts of southern Alaska
will potentially see moderate to heavy rain and shower events
Thursday through Saturday. The best chances for heavier rains will
be along and near the Alaska Range, with lesser chances farther
south. After Saturday, the Gulf trough will weaken and move east
with the Bering trough moving in and replacing it. This will keep
a wetter and more moist pattern in place with periods of rain
along the coast and shower threats inland.
SYNOPSIS AND MODEL DISCUSSION...MMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MMC