Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 221308

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
508 AM AKDT FRI JUL 22 2016


The upper level ridge that was in place across the eastern Bering
continues to flatten and retrograde to the west in response to
the next upper level low that is moving across the Bering Sea. The
jet traversing between the low and ridge continues to stream
subtropical moisture into southwest mainland. This is causing the
region to experience widespread clouds and rain. Meanwhile, across the
Susitna Valley, remnants of a shortwave that passed through the
area earlier in the afternoon yesterday, continues to produce
scattered showers this morning.


Models continue to be in good agreement with the current wave of
energy moving into southwest mainland this morning. There is some
uncertainty as to how much of the subtropical moisture will be
pulled into the region from the associated front which would
ultimately determine QPF amounts. Nonetheless, given the westerly
winds, enhancement from upslope seems likely, and have gone with a
blend of models for the next 24 hours. Models remain in good
agreement with another piece of energy coming from the northwest
Saturday with some finer details still in question, mainly with

Looking further east the models are handling well the shortwave
ahead of the main low that is moving across Southcentral this
morning. It becomes a little more murky as we head into this
afternoon and Saturday. Models are in good agreement as the upper
level low in the west traverses east, a surface low will take
shape near the vicinity of Cook Inlet. Meanwhile, high pressure
will develop in response across Prince William Sound. Where the
differences lie is with the pressure gradient strength that will
develop between these two features. Lo-res models show a weaker
gradient and thus downslope winds are not as strong and the QPF
amounts are higher. The hi-res models on the other hand show a
tighter gradient and a stronger downslope component to the wind,
thus cutting QPF amounts. By early Saturday, however, models are
in good agreement that the low will move into the northern gulf
and winds across Southcentral will switch to the northwest with a
wider coverage of rain.


The pattern change across southern Alaska is in full swing with
wetting rains falling across the Copper River Basin along with increased
cloud cover and cooler temperatures. The trend of wetter and
cooler weather is expected to continue into the weekend with a
potent upper level system poised to move across the area. Upper
air soundings sampling this feature show a very moisture rich air
mass moving across the Alaska Peninsula but it looks like the
majority of the moisture will pass south of the mainland and into
the Gulf of Alaska. However, this does not mean that rain will not
be seen across southern Alaska. Upper level forcing looks quite
impressive and should be able to squeeze out plenty of moisture
from the atmosphere and help produce widespread wetting rains
across Southwest Alaska this morning and afternoon. Rain showers
will encompass Southcentral Alaska throughout the day today with
the steadiest and heaviest moving in by late afternoon. Rainfall
amounts across Southcentral Alaska will likely range from half of
an inch to an inch with higher amounts along the coastal range.

Less rainfall will be seen across Southwest Alaska as rainfall
comes to an end much quicker as the upper level low quickly swings
through tonight. Looking further ahead, it looks like another
significant chance of rainfall will impact much of southern Alaska
on Sunday. Confidence in timing if quite low as it depends on the
evolutions of a weak shortwave currently near the Bering Strait.
However, confidence is very high that many locations will see
wetting rains again on Sunday


The greatest challenge remains the amount of rainfall that will
occur across Southcentral today into Saturday morning. There is a
trend for models to indicate more dynamical forcing resulting in a
stronger surface low developing and moving into the gulf of
alaska. Therefore pressure gradients will be stronger across
Turnagain arm, resulting in a more downsloping effect than
previous model runs. While rainfall will be moderate at times by
this afternoon, rainfall totals will be a little less across the
Anchorage Bowl than previously anticipated. Meanwhile, more robust
rainfall amounts are anticipated across Prince William Sound,the
Kenai Peninsula and the Copper River Basin as the low moves into
the Gulf of Alaska.

On Saturday, a few lingering showers will occur on the backside of
the low, but generally be limited due to northwesterly offshore
flow across eastern portions of southcentral. In contrast, the
low in the gulf will continue to support generally wet conditions
across the Copper River Basin.


A very moist air mass with tropical origins continues to stream up
the Aleutians and Alaska Peninsula this morning with the fringes
of this moisture plume feeding the frontal system pushing into
Southwest Alaska. Additionally an upper level trough over the
Bering will swing inland over Southwest Alaska today further
enhancing the precipitation potential.

Behind the upper level trough precipitation will taper off briefly
tonight with northwesterly flow increasing. A shortwave trough
traveling through this northwesterly flow will reach the Kuskokwim
Delta Saturday morning then rotate across the remainder of Southwest
Alaska Saturday afternoon through Saturday night bringing another
round of rainfall to the area. The associated upper level low will
settle south to become centered over Southwest Alaska Sunday and


Following the upper level trough swinging inland today a flat
upper level ridge will build over the Bering. Northwesterly flow
aloft will bring a couple of troughs south along the western
Alaska Coast while generally west to east running frontal
boundaries linger south of the Aleutians.


.LONG TERM FORECAST (Days 3 through 7)...
The upper levels have a northerly 90 knot jet streak over the
Kamchatka Peninsula before strengthening into a 120 knot
northerly jet streak over the Western ALeutians by Monday. This
synoptic feature continues to dig into the base of the longwave
trough located south of the Central Aleutians. By Tuesday this
jet streak transitions to a 110 knot southwesterly jet streak
south of the Eastern ALeutians extending into the Gulf of Alaska.
This pattern will result in an upper level closed low over the
Seward Peninsula that will drop down into the Southwest region
while, a secondary deeper closed low develops just south of
Unalaska by Monday evening. These two features phase into one
closed low by wednesday morning located just south of the Eastern
Aleutians. This will result in a deep southwesterly flow in the
North Pacific ocean as it streams towards the Gulf. Therefore,
look for a wetter pattern for the Southwest/Southcentral Alaska
regions as shortwaves track through the Southwest before swinging
around the base of the longwave trough just south of the
Aleutians. These troughs will then follow the southwesterly jet
stream into the Southcentral region bringing abundant moisture,
and precipitation accompanied by cooler temperatures heading into
the midweek time-frame.





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