Areal Flood Warning 2, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
452 AM AKDT Mon Sep 25 2017

.ANALYSIS AND UPPER LEVELS...
An elongated upper level low is currently centered just to the
north of Bristol Bay. A secondary low center has dissipated into
an embedded shortwave trough over Southcentral. The upper level
low complex is reflected down at the surface and is producing
areas of rain and showers over Southwest and Southcentral Alaska.
Areas of heavier rain showers are embedded within the main
precipitation field over the Gulf Coast and the Northern Cook
Inlet due to low and mid level instability underneath the
dissipating upper level low. Out west, an upper level ridge is
located between the Kamchatka Peninsula and the Western Aleutians
leading to northerly flow across most of the Bering with plenty of
cloud cover and a few rain showers.

&&

.MODEL DISCUSSION...
Numerical models are doing a good job at handling the evolution of
the low pressure system over Southwest Alaska as it continues to
rotate and drift south through Tuesday. Models have continued to
struggle with a low moving up into the Southern Gulf on Wednesday,
with varying run-to-run solutions and inconsistencies between
models. The GFS has been the most consistent over the past couple
of days and is keeping the low center south of Kodiak Island. The
other models have begun to trend towards this solution so the GFS
was the model of choice for the morning forecast package.

&&

.AVIATION...
PANC...Low stratus will be an issue again this morning in the
Anchorage Bowl. The lower clouds look to stay north over the Knik
Arm and the JBER area but may creep down towards the terminal,
though this is unlikely with periodic gusty up inlet flow. For
now, just included a FEW stratus layer for the morning period.
Looking further down the Cook Inlet, satellite imagery and
observations show no sign of low stratus that could advect north
towards the terminal, so Anchorage is expected to remain VFR
throughout the day.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Rain showers will continue to diminish this morning as the upper
level trough axis slowly moves northward and dissipates. As
the clouds slowly clear from south-north, there remains
uncertainty as to how much fog/stratus will develop over the Cook
Inlet region this morning with light southwesterly flow. So far
this morning any fog development has been very spotty and short
lived, so at this point it doesn't appear that anything more than
patchy fog will occur this morning. By this afternoon the focus on
the forecast will begin to shift towards the gulf as the next gale
force low begins to push northward. While there remains some
uncertainty in the exact track of this low, there is good
confidence that impacts from this low will be confined to the
Central Gulf and Kodiak Island until late Tuesday when rain and
gusty winds will begin to spread into the North Gulf Coast. The
low center will however remain near or south of Kodiak Island,
leaving much of Southcentral dry but generally cloudy with
persistent offshore flow through Tuesday. Rain then looks to
spread into Southcentral for Wednesday as a stronger shortwave and
deep/moist southerly flow spreads throughout the Gulf.

&&

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

A weakening low in Bristol Bay continues to support areas of
showers along the immediate coastline this morning. A separate
weak deformation zone aloft north of the upper trough is
supporting a narrow strip of rain across the Kuskokwim Delta, but
this feature should weaken today. Eventually, the Bristol Bay low
will move southwest with another North Pacific low moving toward
Kodiak Island Tuesday. This will allow for the development of an
offshore pressure gradient, allowing for a gradual drying trend
along with decreasing fog potential heading into Wednesday.

&&

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

A low in Bristol Bay today will weaken and then drift southwest
into the North Pacific. Broad northerly flow will encompass all
of the Bering Sea into Wednesday with periodic light rain and
showers. The northerly winds will remain at or below 25 knots as
the pressure gradient is not particularly strong.

&&

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

Models have come into better agreement with an area of low
pressure moving across Kodiak Island on Wednesday. The recent
forecast reflects the increase in model agreement with gale force
winds likely across the northwestern Gulf of Alaska and Shelikof
Strait Tuesday night and Wednesday. In addition, it looks like a
piece of the subtropical jet will bring an influx of moisture
allowing for some decent rainfall over Kodiak Island and the gulf
coast. Interior locations will remain showery as the low doesn't
take a favorable track for any type of downsloping conditions.

Otherwise, the forecast hasn't deviated much from yesterday in
thinking that upper level trough digging out across Alaska late
this week will quickly be shunted eastward by a transitory
shortwave ridge. Behind this ridge will be yet another upper level
trough but this time it's the arctic jet stream that is
strengthening rather than any subtropical influence. With colder
air expected to move over the Bering Sea and much of Alaska, a
cooler and showery period of weather is expected through the
upcoming weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Warning 121 125.
MARINE...None.

&&

SYNOPSIS AND MODEL DISCUSSION...KVP
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...MO