Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 220118 CCB

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
438 PM AKDT Fri Jun 21 2019


A vertically stacked low is over the southern Gulf of Alaska with
a front pushing north from this low to the north Gulf Coast
region. This front has spread clouds and rain to Kodiak Island and
the Gulf of Alaska side of the Kenai Peninsula. Weak high
pressure is over the eastern interior extending down into south
central Alaska. A low is south of the western Aleutians with its
front spreading clouds across the western Aleutians. Between this
low and the low over the Gulf of Alaska is a weak ridge of high
pressure extending from the north Pacific across the central
Aleutians into the Bering Sea.

Under the high pressure over the interior is the surface thermal
trough. The 850 MB temperatures are up around 10-14 C across the
Copper River Basin into the Susitna Valley. The daytime heating
is contributing to increasing the lapse rates and thus
instability. This should be the region of focus for convection
this evening.



The models are in good synoptic agreement through the weekend. The
NAM and GFS do differ in resolving the convective threat for this
evening, Saturday and Sunday. The NAM is more aggressive and
indicates a higher threat for thunderstorms than the GFS through
the weekend. They both are in generally good agreement with the
areas of concern from the Copper River west to the Kuskokwim
Valley and the west side of the Alaska Range. The NAM is more
aggressive to bring an increased threat to the Kenai Peninsula
Sunday. How this plays out will be contingent on how the marine
layer evolves and whether it is pushed out from the Kenai Sunday.



PANC...VFR conditions and light winds will persist.



Initially moisture advecting into the southern mainland in
advance of the front will increase clouds and humidity that should
result in slightly cooler more humid conditions. As we progress
through this weekend, the low level flow becomes light northerly,
and will help to decrease cloud cover and decrease humidities.
However, the main concern and focus is with regard to the
increasing convective threat through the weekend. The influx of
moisture will feed into the more unstable regions of the Copper
River Basin to the Susitna Valley, thus increasing the potential
for thunderstorms. By Sunday this threat will spread south to the
Kenai Peninsula. Although, as mentioned above, there is some
model uncertainty with regard to this. This will bear watching.



The two main influences for weather across Southcentral this
weekend will be a ridge of high pressure currently over interior
Alaska moving southward, and a departing area of low pressure over
the Gulf drifting toward the southeast. As the ridge moves
southward, warmer and drier conditions are expected for the
weekend. Additionally, a series of upper level waves will move
over Southcentral from the east, enhancing instability and
generating afternoon thunderstorms. Storms are expected to be
concentrated over the Talkeetna Mountains and in the Copper River
Basin this afternoon and Saturday afternoon. By Sunday afternoon,
isolated thunderstorms are possible over the Kenai Peninsula,
however confidence remains low due to model uncertainty. This
possibility will be monitored over the next 24 hours.

Lastly, similar to yesterday, some smoke from the Swan Lake fire
may drift northward into the Anchorage area during the evening
hours tonight and into early tomorrow morning due to continued
weak southerly low level flow.


.MARINE (Days 3 through 5)...

Beginning the extended period marine forecast planning period,
there will be a weakening low over the western Bering Sea lifting
north with a swath of southerly winds/higher seas stretching
toward the Pribilof Islands. This low will continue to weaken
through Tuesday with a brief period of quiet weather until the
next front tracks into the eastern Bering Sea late Tue and Wed,
bringing a swath of stronger southerly winds from Unalaska to the
Pribilof Islands. Right now, there is good confidence these will
remain below gale force, but they will be stronger than the past
week or more.

Weak high pressure will continue to remain over Bristol Bay
eastward to the Gulf through the period as the storm track remains
to the west and south. Expect only light sea breezes with the
exception of the Barren Islands where gusty west winds will be
moving through with the offshore flow pattern Tue and Wed.


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

The global numerical models are finally beginning to settle down
with run-to-run inconsistency issues and are now showing a pretty
clear signal for the long range pattern. Beginning Tuesday evening,
there will be a low over the western Bering Sea and a shortwave
ridge extending into the Southern Mainland. Weak disturbances will
be riding over the top of the ridge, so expect both warm weather and
thunderstorms, with activity focused over Southcentral where the
much warmer and unstable air mass will be in place. All areas will
see potential thunderstorms, especially over the mountains and
across the interior (Copper Basin, Mat Valley, Susitna Valley).
Thereafter, Wed-Fri are now high confidence days for a return to
even warmer/hotter weather as a high amplitude ridge builds over the
Bering Sea, and strong offshore flow sets up over the Southern
Mainland. The ECMWF, GFS, and Canadian GDPS and statistical MOS all
depict this, and it also fits the conceptual pattern and pattern
recognition. This will inhibit cloud development and increase
adiabatic/compressional warming as that offshore flow increases. In
layman`s terms, it is going to get hot over the mainland, especially
Southcentral where interior locations will push safely into the 80s,
with 70s along the coast. This will be the warmest period of the
summer. The Bering will also be more active than normal with a
series of fronts moving through, keeping stronger winds and more
rain over the region than is normal at this time of the year.


MARINE...Gale 177 178.


MARINE/LONG TERM...JA is the U.S. government's official web portal to all federal, state and local government web resources and services.