Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 271248 CCA
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
448 AM AKDT WED JUL 27 2016

corrected Southwest Ak forecast

.ANALYSIS AND UPPER LEVELS...

The vertically stacked low south of the Alaska Peninsula is filling
this morning as a frontal wave entering the southern Gulf develops
into a closed low. Further north, a surface ridge along the
northern Gulf Coast and moist southeasterly flow will continue to
stream precipitation over much of southcentral Alaska this morning
with isolated to scattered showers over the Copper River Basin and
Susitna Valley. West of the Alaska Range, showers continue as
well, as troughs rotate both around the north side of the low
south of the Alaska Peninsula and drop into the area from the northwest.
The phasing of two of these troughs led to a more organized band
of precipitation stretching from the Kuskokwim Delta, across the
western capes and into Bristol Bay which earlier brought some
embedded thunderstorms to coastal portions of the Kuskokwim
Delta. A strong upper level ridge persists over the western
Aleutians and Bering bringing continued northwesterly flow to the
Bering.

&&

.MODEL DISCUSSION...

Models are in fair agreement through the mid term with the larger
scale features. Both the NAM and GFS however depict a stronger and
better defined handling of the developing low moving into the Gulf
of Alaska. Given the jet support and upper level forcing, this
stronger solution was preferred. The NAM was preferred for the
western domain as well for its better handling of details.

&&

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

An upper level short-wave crossing Southcentral during the
overnight hours will exit to the north and west during the morning
hours. This will provide a break in steady precip for inland
areas, though a few showers are still possible in this very moist
and weakly unstable atmosphere.

All eyes will turn to a potent upper level low and numerous
short-waves making there way into the Gulf today, along with a
deepening surface low out ahead of the upper center. This is a
very strong and dynamic system which will produce a lot of rain
and wind. Expect rapidly deteriorating conditions along the Gulf
coast and Kodiak Island this afternoon while inland areas will
see increasing showers as the area of upper diffluence on the
leading edge of the system moves overhead. The low will become
vertically stacked tonight causing it to stall over the northwest
Gulf. A jet streak on the east side of the low will drive frontal
precip and upper short-waves inland, so expect some rain to fall
just about everywhere tonight.

Models have been keying in on an area of instability on the east
side of the low behind the surface occlusion, with sfc LIs below
zero degrees Celsius and CAPE values of 200 J/KG or more. Have
gone ahead and added isolated thunderstorms tonight over the
Northern Gulf and into southeastern Prince William Sound. Even if
thunderstorms do not develop, expect very heavy downpours and
gusty winds in showers.

An Arctic trough dropping in to western Alaska on Thursday will
help kick the low over the Gulf eastward. After a rainy/showery
start to the morning this will lead to improving conditions
across much of Southcentral and the Gulf during the day Thursday.
The trough will reach Southcentral Thursday Night bringing another
shot of rain to the region. With west to southwest flow ahead of
it rain will be focused along the west side of mountain ranges.

&&

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

As the pattern changes, instability showers along the AK range
can be expected through Thu evening, with some rain over the rest
of the forecast area. An upper level trough moving across the area
will interact with the thermal trough as it returns to the AK
range. This will enhance convection so in addition to showers,
expect a slight chance of thunderstorms near the AK range this
evening and Thu afternoon and evening.

&&

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

Persistent west to northwest flow will keep fog and drizzle over
portions of the area through Thu. Some showers are also possible
over the eastern Aleutians. Rain will begin to increase Thu night
across the central Aleutians and Bering as a trough moves east.

&&

.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
be weak.

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
verifies.

For the middle of next week, all models are hinting at (at least) a
brief drying trend with some ridging building over the Bering.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...FLOOD ADVISORY 141.
MARINE...GALES 119.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...JER
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DS
LONG TERM...MSO


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