FXAK68 PAFC 191743
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
433 AM AKDT Thu Jul 19 2018
.ANALYSIS AND UPPER LEVELS...
The synoptic pattern features a broad ridge over Prince William
Sound that extends northward into the interior. Meanwhile
upstream, there is a surface low near Unalaska which is anchoring
an occlusion that is moving towards the AKPEN, and an inverted
trough that extends northward over the Pribilof Islands. The
latest ASCAT/advanced scatterometer pass has detected a secondary
area of high pressure over the western Bering which has trapped some
stratus underneath it. Just east of Kamchatka there is another low
with a frontal boundary that extends to mainland Japan.
The run to run continuity with the global model guidance has been
good. The models are in sync with the ridge axis over the Prince
William Sound/Gulf Of Alaska as well as low near Unalaska. This
phasing of the models is mirrored at both the surface and at 500
mb through 72 hrs. The models begin to diverge around 96 hrs
primarily over the Bering, but the models still look good with
the handling of the ridge axis into Southeast Alaska and the dome
of high pressure between Hawaii and the western Aleutians.
PANC...VFR conditions and light winds will persist.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
A drier and warmer pattern is expected to prevail across
Southcentral through the first part of the weekend as a ridge
resides across the gulf waters to the central interior. The stable
air mass along the ridge keeps moisture trapped near the surface
for marine areas, therefore, expect low clouds and patchy fog. A
front across the Alaska Peninsula slowly advances to Kodiak Island
by this afternoon and then stalls through Friday before diminishing.
Light rain along the front will not likely make it to the east
side of the island as the ridge axis extends toward the southern
tip of the Kenai. Another front moves up from the North Pacific to
Kodiak Island on Saturday which brings a higher chance for rain
as an upper level low begins to lift from south of the Alaska
Peninsula toward the mainland.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
Clear and dry weather will continue across much of the southwest
mainland and northern AKpen through the first half of Friday as
high pressure remains anchored over southern Alaska. With clear
skies and light winds, daytime highs will once again top out in
the upper 60s to lower 70s for the southwest interior. Along the
west coast, persistent areas of low stratus will keep temperatures
a few degrees cooler. The ridge axis begins to slowly slide east
for Friday. The resultant moist, southerly flow aloft will steer
a shortwave north from a stalled low south of Unalaska. This will
increase cloud cover from Bristol Bay to the YK Delta and bring a
chance of rain to the Aleutian and Kilbuck Mountains.
The main upper-level low tracks north on Saturday and continues
over the southwest mainland by Saturday night. This will result in
widespread clouds, areas of rain, gusty southeasterly winds, and
cooler temperatures for the weekend. A weakening front approaching
the west coast overnight Saturday into Sunday looks to keep the
unsettled weather in place as the weekend draws to the close.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
A stalled low south of Unalaska continues to keep clouds and
scattered rain across the central and eastern Aleutians today.
Gusty southeasterly winds with this system will diminish this
morning as the low continues to drift south and weaken. A second
system developing in the far western Bering will move northeast
today spreading rain and increasing winds across the western half
of the area. Its associated front will occlude as it moves east
across the Bering over the next day or two. Expect widespread
rain and cloud cover over the Bering and central and eastern
Aleutians for the weekend as the front slowly approaches the west
.LONG TERM FORECAST (Days 3 through 7: Saturday through Wednesday)...
This weekend's forecast continues to begin with strong upper level
ridging continuing over the eastern half of the state. Meanwhile
Southwest Alaska begins to get rain and cooler temperatures moving
in ahead of a large low over the western Bering and attendant
upper level wave moving into the Alaska Peninsula Saturday
morning. Under the ridging across Southcentral, Saturday will be
another beautiful day under sunny skies and very summer-like
temperatures. Saturday is likely to be the hottest day of this
stretch everywhere in Southcentral except the Copper River Basin
and coastal communities. Increasingly southerly flow will
gradually transport the fog and low stratus over the Gulf into the
north Gulf Coast. Thus, those along the coast may be dealing with
that instead and have a significantly cooler, cloudier day than
The negatively tilted trough moving into the Alaska Peninsula
Saturday slowly moves northward into Bristol Bay by late Sunday.
Southerly flow ahead of the trough will likely transport both the
associated cloud cover and cooler temperatures into Southcentral.
This unfortunately now looks to mean Sunday will not be as sunny
a day as previous forecasts have indicated. There is some
disagreement in the models regarding that at this point, with the
EC the fastest and bringing in overcast skies Sunday morning, the
GFS is in the middle waiting until early Sunday evening with mixed
sun and clouds for most of the day, while the GEM is the slowest
waiting until the predawn hours of Monday to bring in the thick
cloud cover. The GEM weakens the trough much more quickly than
the other models and was largely discounted, which unfortunately
means Sunday looks like it will feature more clouds than sun for
Southcentral. The clouds will likely hold off for much of the day
in the Copper River Basin, with Sunday likely being the hottest
day of this stretch of beautiful weather there. Rain begins Sunday
afternoon along the Kenai, moving north through the evening, rain
begins earlier across Southwest Alaska as the entire system moves
Monday now looks to be a wet day area-wide except the Copper
Basin and perhaps far eastern Prince William Sound as both the
weakening trough and more importantly its parent upper low move
eastward into the Bering coast of Southwest Alaska, causing
widespread rainfall. Tuesday should be a somewhat drier day from
Anchorage north and west. As the upper level low moves into the
southwestern Gulf, weak pulses of energy will continue to set off
scattered to widespread showers, even well north into the Copper
River Basin and Susitna Valley. Locations further south should see
the best chances for rain Tuesday being closer to the upper low
itself, but all areas will remain under threat through the day.
Wednesday will be much the same as the upper low largely stalls
somewhere over the southwestern Gulf, though exactly where is
still uncertain. This should result in more isolated rainfall
activity the further inland one goes. Most coastal areas will
continue to get nearly steady rainfall, especially along the Kenai
Peninsula and Kodiak as a stronger impulse of energy moves
northwestward into the coast.
SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
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