Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 270007
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
407 PM AKDT TUE JUL 26 2016
.ANALYSIS AND UPPER LEVELS...
A gradually weakening vertically stacked low south of the Alaska
Peninsula continues to control the weather pattern over southern
Alaska. Weak upper-level disturbances rotating around the low
through the southern mainland are bringing more precipitation to
the Gulf of Alaska coast and the Bristol Bay region. Morning
soundings show a continuation of deep saturation in these areas
with onshore flow, which has supported persistent rainfall over
the last couple of days. As the low pressure system has shifted
southward over the last 24 hours, flow over Southcentral Alaska
has gained more of a southeasterly component. This is allowing for
enough downsloping in the lee of the Chugach and Kenai Mountains
for a precipitation shadow to form. As a result, Anchorage and the
western Kenai Peninsula are experiencing a much drier day than
yesterday, with only some isolated showers moving through the
Further west, offshore flow is keeping the Yukon-Kuskokwim Delta
drier than the Bristol Bay region. Weak instability and a thermal
trough over the area is supporting some isolated convective
showers. Northerly flow over the Bering Sea continues to bring low
stratus and fog to the area.
Models are in fairly good agreement synoptically through the
short term. Recent model runs have come into better agreement with
the evolution of the broad low in the Gulf through the middle of
the week. With low pressure remaining over the western Gulf, the
showery pattern will continue through the end of the week. As is
typical with this pattern however, models struggle with timing and
exact placement of the individual easterly waves past the near
term. This results in a high confidence of the general showery
pattern, with lower confidence in the exact placement and timing
of showers through the end of the week.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Drier and cloudy conditions will continue through Wednesday
afternoon, as broad and deep southeasterly flow continues to keep
the inland portions of Southcentral in a downsloped regime. The
atmosphere remains quite moist with very weak instability, and the
downslope component will remain on the weak side, so still expect
isolated to scattered showers to pass through the area. An upper
level low developing over the Gulf on Wednesday will swing a
series of waves onshore over the Northern Gulf coast through
Thursday. These approaching short-waves combined with upslope
flow will maintain wet conditions across the Prince William Sound
and the southern Kenai Peninsula through Thursday. Expect
continuous rain along the coast through Thursday, with rain likely
spreading inland across Southcentral Wednesday night. This looks
like another potent upper level low with impressive instability
and a decent jet streak on the east side of the low. Thus, expect
a transition to heavy showers, or maybe even a thunderstorm, as
the low reaches the North Gulf coast Wednesday Night.
High pressure at the surface will continue to build out ahead of
an approaching warm front this evening, producing some gusty winds
through the usual gaps (Turnagain Arm/Knik Arm/Copper River). The
forecast has trended toward more widespread Small Craft
Advisories, though guidance is hinting at potential for gales
along the North Gulf coast on Wednesday.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Scattered to numerous rain showers will remain in the forecast
for the next couple days, with a slight chance of thunderstorms
each afternoon/evening...mainly over the Kuskokwim Delta and along
the Alaska Range. Wednesday night into Thursday the pattern begins
to change, bringing flat ridging aloft and westerly flow. At that
time most of the shower activity will be confined to the Kuskokwim
Valley and Greater Bristol Bay area. The westerly flow will bring
marine layer stratus to the Delta and along the Alaska Peninsula.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A disturbance dropping down from the north will bring a chance of
rain to the central Bering Sea overnight. Another northerly
disturbance will bring the same to the eastern Bering Sea
tomorrow. Generally westerly flow will become established over
most of the Bering. Areas of dense fog will spread over the
Western Bering sea tonight dropping ceilings and visibility in
the area. These conditions will remain for the next couple days.
.LONG TERM FORECAST (Days 3 through 7...Fri through Tue)...
A fairly active pattern will remain in place for the mid-long term
portion of the forecast. We will end the week across the southern
half of the state with the low currently in the northern Gulf of
AK finally exiting the area. Zonal west to east flow will develop
which should lead to drier conditions. However, do not expect to
see much in the way of sunshine as plenty of moisture will still
be streaming into the area and any ridging that does build in will
For the weekend...another fairly deep trough will start to slide
down from Northwest AK towards Bristol Bay. This set-up looks similar
to the one we saw around the 16th of July as the Arctic low sends
energy down the western flanks of the AK and Aleutian Ranges.
There are some key differences from that event. The North Pacific
ridge in advance of this system will not be nearly as strong. And
cooler, cloudier conditions will be the rule this time around vs.
sunny, dry, and hot. There is a rather large amount of model
spread at this time from the EC, the GFS, and the GEM. Many of the
differences stem from the orientation of the jet at 300mb. The GEM
keeps most of the jet energy over the northern half of the state,
while the GFS and EC try to carry it down in a much more
meridional N to S oriented trough. The EC and GFS open up much of
Southcentral to SW flow and would make for a very wet end of the
weekend/start of the week. However, the GEM seems much more
reasonable developing a closed low further west near Bristol Bay
as the remnant "front" falls apart along the terrain (This was
what happened with the set-up in mid July). And even as we write
this, the newest run of the GFS has trended toward the GEM model.
So in this forecast package, we have left at least a chance of
rain across the area due to model uncertainty. However, we will be
watching to see if we need to trend drier if indeed the GEM
For the middle of next week, all models are hinting at (at least) a
brief drying trend with some ridging building over the Bering.
SYNOPSIS AND MODEL DISCUSSION...rd
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ml