Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 261157
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
357 AM AKDT THU MAY 26 2016
.ANALYSIS AND UPPER LEVELS...
A stacked mid- and upper-level cut-off high currently dominates
the eastern and central Bering and southern mainland. It is
currently centered over the southern Kuskokwim Delta zone but is
progressing slowly north. Water vapor and infrared satellite
imagery depict high-level clouds rotating around the cut-off high
in all directions, while in the lower levels fog and low stratus
cover much of the North Pacific and Bering regions and continues
to try to push into parts of coastal southern Alaska due to
onshore flow. Meanwhile a well-organized low pressure system is
centered in the western Aleutians just northeast of Shemya.
At the surface, with the north to northwest flow over the southern
mainland in the mid-levels, a lee-side trough resides along the
North Gulf Coast, while surface high pressure resides over
Southwest Alaska and the central Interior. This combination is
resulting in a fairly weak offshore pressure gradient over
Models remain in relatively good agreement as the upper level
high continues to push to northwestern Alaska through this
weekend. Models show some disagreement with closed mid-level lows
that will attempt to merge and form a weak west-east trough this
weekend into early next week from the southern Bering through the
Gulf to the south of the high. Overall forecast confidence through
the holiday weekend is high.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)..
A building upper ridge over the Eastern Bering Sea amplifies
(intensifies) as it builds east over the Alaska mainland today and
friday. Moisture overshooting the upper ridge will keep mid to
high level cloud cover across Southcentral today. This moisture
heads east on Friday allowing for clearing skies over much of the
area. Warm and dry conditions are expected to carry into the
weekend with generally northerly flow allowing temperatures to
climb into the mid 60s to 70s inland with cooler temperatures
along coastal locations that experience afternoon/evening sea
breezes. Downslope drying/warming with northerly flow across the
Alaska Range will bring widespread 70s and very dry conditions
with potentially gusty northerly winds to the Susitna Valley and
Copper basin Friday. These conditions will most likely carry over
into Saturday. Periods of gusty outflow winds are expected over
the Eastern Kenai and North Gulf during the overnight and morning
hours into Saturday.
A building ridge spreading East will bring very warm and dry
conditions across inland areas of Southcentral Friday and
Saturday. This warming and drying will be enhanced by the
northerly flow across the Alaska Range creating downslope
conditions across the Susitna Valley and Copper River Basin. As a
result, widespread temperatures in the 70s with RH values in the
20s are expected in these areas Friday and Saturday. How strong
the afternoon pressure gradients and resultant northerly winds
will be across the Susitna Valley and Copper River basin is still
in question. However, confidence is increasing that critical fire
weather conditions may be realized during the afternoon hours
Friday and Saturday.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
Cloud cover will continue to diminish over the area today as high
pressure continues to build over the Southwest Mainland through
Sunday. This will make way for sunny skies, warming temperatures,
and light offshore flow persisting over much of the Southwest
Mainland through Saturday morning. As the high pressure moves
further into the interior and weak surface low pressure builds
toward Bristol Bay Saturday into Sunday, winds will turn to more
of an onshore flow. The onshore flow could lead to fog and marine
stratus over the Eastern Bering to advect over the coastal areas
along the Southwest coast.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
Most of the activity over the region will remain confined to the
Western and Central Bering/Aleutians as a weakening front will
continue to remain nearly stationary over the area through this
afternoon. This front will continue to bring gusty winds and rain
to the Western and Central Aleutians/Bering through this evening.
Winds and rain will then diminish throughout the region as the
associated low moves north into the northwest Bering and
dissipates. As this happens, winds throughout the region will fall
below small craft advisory levels by Friday, and remain there
into Saturday with clearing skies. One exception to this would be
the Eastern and Central Aleutians Friday night through Saturday
night, where a weak North Pacific low will bring some rain and
a slight increase in winds. The other exception would be a series
of fronts moving into the Western Aleutians overnight Friday
through Sunday morning, bringing rain and gusty winds to the area.
.LONG TERM FORECAST (Days 3 through 7)...
On Day 3, models remain in fair agreement with a pair of weak lows
over the Aleutians and a northwest to southeast orientated ridge
over the Western Gulf of Alaska. Starting late on Day 3, models
begin to diverge with a fairly weak-flow pattern developing over
the region. Models are unanimous in developing a surface low on
Sunday southwest of Shemya, but the model agreement ends there
with the GFS developing a low that meanders just south of the
Western Aleutians through the end of the period before washing out
and the EC aggressively developing a 980 mb low that migrates into
the Southcentral Bering Sea. With this wide spread, the extended
forecast settled heavy on WPC Guidance, which was mainly based on
the 00Z EC Ensemble with some 00Z EC deterministic data.
SYNOPSIS AND MODEL DISCUSSION...CC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP