Small Craft Advisory 322, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
456 PM AKDT Thu Apr 9 2020

.ANALYSIS AND UPPER LEVELS...

In the upper levels, a ridge of high pressure stretches from the
Eastern Pacific through the Alaska Peninsula (AKPEN) and into
Bristol Bay/Southwest Alaska in a negatively tilted fashion.
Further west, a trough extends from Eastern Siberia into the
Western Aleutians. Several disturbances in the flow are rounding
the base of the trough and then ejecting northeast as they try to
top the downstream ridge.

Closer to the surface, clear skies dominate much of Southcentral,
thanks to an upper level ridge building in. A surface reflection
of this ridge can be seen as an area of high pressure extending
from the AKPEN into central Alaska. Light snow and rain are
affecting southwest AK today as an afore-mentioned disturbance
grazes the region.

&&

.MODEL DISCUSSION...

The models remain in very good agreement through the weekend, with
the main change seen today being an eastward shift of the ridge
axis that extends from the eastern Pacific into mainland Alaska.
Today's runs indicate this axis will extend from the eastern
Pacific to the Yukon border by Saturday evening, which is an
eastward shift from yesterday. Otherwise, the remaining changes
are minimal.

&&

.AVIATION...

PANC...VFR conditions and light winds will prevail through the
forecast period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Tonight
through Sunday)...

High pressure transiting from the western Gulf to the eastern gulf
this afternoon through Saturday will lead to dry conditions with
mostly clear skies. Lingering gap winds across the bays and passes
of Prince William Sound will wind down this evening as the ridge
builds in from the west. With clear skies and diminished winds
tonight, overnight lows will once again fall well below normal.
However, as the ridge works from west to east, the flow aloft will
shift more southerly, yielding a warming trend that should last
through the next several days. By the weekend, daytime highs are
expected to top out in the mid to upper 40s across most inland
locations. A weak shortwave riding over top the ridge Friday will
bring a slight increase in high clouds for the northern half of
Southcentral.

As the ridge slides east, the resultant southerly flow will also
begin to push warm, moist air from the Pacific north in advance
of a digging trough to the west. A north Pacific low will lift
toward Kodiak on Friday and swing a front over the western Gulf
by early Saturday. Gusty easterly winds and widespread
precipitation will accompany the front, with precipitation
beginning as a mix of rain and snow quickly changing to rain for
Kodiak City as temperatures warm well into the 40s. This front
will continue to lift slowly into the northern Gulf by late
Saturday night, with precipitation spreading along the eastern
Kenai Peninsula coast. A second system then looks to follow on the
heels of the first as the initial frontal system stalls and
weakens along the coast. This second system will enhance winds and
precipitation from the western Gulf north across the western half
of Southcentral by the end of the weekend. Timing and track of
this system is still very much uncertain; however, once it does
move over the region, the added southerly fetch of warm, moist
air should drive up temperatures warm enough to produce a mix of
rain/snow changing to rain for most locations with the exception
of the higher elevations and northern Susitna Valley.

&&

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

The front along the Kuskokwim Delta and Valley lifts north tonight
with snow diminishing in its wake. Southerly flow across the
region will keep temperatures from plummeting tonight as a
maritime air mass advects inland. Patchy fog may develop tonight
along the Bristol Bay coast where winds stay relatively light. A
warming trend is expected Friday through the weekend as a long
wave pattern brings several subtropical storm systems to the
Bering. The next surface low to monitor approaches the Alaska
Peninsula Friday evening, slowly tracking across to the central
Bering on Sunday. In the meanwhile, several significant waves act
to pull the associated front northward. Gusty east to
southeasterly winds develop across the Aleutian Range on Saturday
with increasing winds for the remaining areas of the southwest on
Sunday. Expect rapidly melting conditions across the region during
the weekend with the combination of gusty winds and rising
temperatures into the mid to upper 40s.

&&

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

An occluded North Pacific front spreads rain from the southwestern
Bering to the northwestern waters tonight. The low lifts north
across the western Bering tonight with a general low center
remaining over the western waters through Saturday. Rain
concentrates over the central Bering Friday along a weak
deformation zone between the western Bering low and the ridge
across the Gulf of Alaska. The next North Pacific low moves a
front up to the Alaska Peninsula Friday morning, spreading rain
and gusty easterly winds along the boundary. This front pushes
across to the central and eastern Bering late Friday afternoon.
The associated low begins to lift north on Sunday with the next
front pushing toward the Central Aleutians Sunday afternoon.

&&

.MARINE (Days 3 through 5/Saturday through Monday)...

Bering Sea/Aleutians:

The theme for this extended forecast will be warmer and wetter air
masses as a period of southerly flow begins thanks to a broad
area of high pressure over the Gulf of Alaska/Mainland Alaska. As
a frontal boundary lifts north out of the north Bering on
Saturday, high pressure over the eastern half of the Bering begins
to break down as a 1007 mb low in the north Pacific pushes a warm
front north into the eastern two thirds of the Aleutian Chain on
Saturday morning. This wave will push north of the Pribilofs
Saturday afternoon and keep western Alaska, Bristol Bay, and the
Alaska Peninsula in a wetter pattern through Monday morning.
Another warm front lifts into the Western Aleutians Sunday
afternoon, with its 995 mb parent low in the north Pacific set to
cross the chain late Monday night.

Gulf of Alaska:

The forecast period will start with dominant high pressure pushing
off to the northeast Saturday and a warm front lifting across
Kodiak Island Saturday afternoon as a north Pacific low pushes
into the Aleutians. This frontal boundary will maintain a presence
across the north Gulf Coast through Sunday night and then fade
Monday morning as high pressure builds back in from the east.

&&

.LONG TERM FORECAST (Days 4 through 7/Sunday through Wednesday)...

Model guidance has come into much better agreement for the Sunday
through Tuesday time period showing an upper level ridge in place
across much of Alaska with confidence very high. With this upper
level ridge in place, the warming trend will continue every day
into the middle of next week. For the mainland, expect
temperatures warming to near normal on Sunday and then to above
normal early next week. Unsettled conditions are also possible on
Sunday as a weak system from the North Pacific south of Dutch
Harbor moves northward toward Bristol Bay with high confidence for
this system. A different closed off system west of the Aleutians
Sunday will move east towards the Bering Monday due to the upper
level trough in place over eastern Russia and the North Pacific.
This may bring another period of unsettled weather to Southwest
and Southcentral Alaska Tuesday to Wednesday time frame next week
as it continues east but gets pushed north due to the blocking
ridge to the east. Confidence is average for this system.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE: 173 413.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...PD
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
MARINE/LONG TERM...LB/MV