Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 170158 AAA
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion...UPDATED
National Weather Service Anchorage AK
558 PM AKDT Sun Jul 16 2017

UPDATED LONG TERM DISCUSSION

.ANALYSIS AND UPPER LEVELS...
In the upper levels a ridge is centered over the northern to
eastern portions of mainland Alaska with a trough (and 2 large
closed lows) over the northern to western Bering Sea. A short-wave
trough which has been nearly stationary from interior Southwest
Alaska southeastward into the western Gulf is finally starting to
drift northeastward in response to energy exiting the Bering
trough. The atmosphere across most of the southern mainland and
Kodiak Island is stable with abundant low level moisture. The sun
will create some breaks in the clouds as it did yesterday, but
overall conditions will remain quite cloudy. The Copper River
Basin is the one location with some more significant breaks of sun
along with some low level instability, so afternoon and evening
convection is expected, primarily along the mountains.

The focus of the forecast for the next few days is the deep upper
low over the far southwest Bering Sea which will march eastward
toward the mainland. An old warm front extending from a surface
low in the same location along with some upper level short-waves
are producing widespread rain over much of the Bering Sea today,
including the Pribilof Islands. Back behind this front, there has
been quite the lightning show this morning from just north of the
central Aleutians (Adak to Atka) and into the central Bering Sea.
The thunderstorms are forming right where mid-level dry air and
jet streak intersects the low level theta-e ridge (focus for warm
moist and unstable air). The GLD lightning network indicated
thousands of lightning strikes with these storms. The jet streak
is re-orienting as the upper low over the southwest Bering
progresses eastward, so conditions will become less favorable for
thunderstorms as we head through the day.

&&

.MODEL DISCUSSION...
Models are in lock step with key features through Tuesday, but
then diverge Monday night through Tuesday night with track of new
surface low forming near Kodiak Island as well as with the parent
low. The differences with the low in the Gulf (and associated
differences with frontal position) are the most significant,
affecting how far north and east rain will spread into Southcentral
and determining role which downslope flow may play in rainfall
totals. This system looks very dynamic, so continued trend of
forecasting high PoPs. Due to the continued large differences in
surface low track, will go with an average position for now.

&&

.AVIATION...
PANC...With little change in low level stability and moisture the
threat for marginal ceilings moving back into the terminal
overnight tonight looks good. At a minimum, expect low VFR
ceilings. A weak upper level low moving up the Inlet could also
produce a few sprinkles or even drizzle. Ceilings should improve
Monday afternoon as the upper low passes by. Wind will remain
light through the period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The fairly benign weather pattern over much of Southcentral
Alaska will persist through late Monday night. Within this weather
regime we will continue to see some showers developing along the
mountains, patchy fog along the Northern Gulf Coast and Cook Inlet
region during the overnight/early morning hours, and partly to
mostly cloudy conditions. There could also be some drizzle and
sprinkles over parts of the Cook Inlet region, as a small upper
level disturbance traverses over the area this evening through
tomorrow afternoon.

The overall weather pattern then changes with a front moving into
the region from the west, as its parent low over the northeastern
Bering Sea dips southeastward through the Alaska Peninsula by
Wednesday afternoon. There is some uncertainty with regards to
the positioning/timing of the front as it tracks into and through
the Gulf, which has lead to lower confidence in the timing and
amount of rain that will fall, as well as gusty gap winds
developing. This is especially true for the areas susceptible to
downsloping, like the western Kenai Peninsula, Anchorage bowl, and
Matanuska Valley, where the positioning of the front and specific
flow pattern has a large affect on the amount of precipitation
that actually falls and how strong the gap winds will become. As
of now, we are going with more of a middle ground solution
bringing rain into the area and ramping up the Turnagain Arm wind
Starting Tuesday Morning through Wednesday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
There is a frontal boundary approaching the Kuskokwim Delta
coastline this evening. This will bring rain with marine layer
stratus to the aforementioned coastline before spreading inland
through the overnight hours. By Monday morning a low pressure
system in entering the northern Bering with an associated front
tracking into the Kuskokwim Delta region. Therefore, widespread
stratus with rain will spread from west to east through the
Kuskokwim Delta into the Bristol Bay, and the Lower Kuskokwim
Valley by late Monday afternoon time-frame. This weather front
exits the region by Tuesday morning with continued onshore flow
for the Southwest Alaska region.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A 994 mb low is dropping down off the Kamchatka Peninsula with an
associated weather front sweeping west to east across the northern
Bering extending southward through the Eastern Aleutians this
evening. The remnants of lightening will exit across the Eastern
Aleutians into the North Pacific Ocean this evening as it looses
its support as the jet streak tracks farther south of the Aleutian
Chain this evening tonight. Meanwhile, the Bering Sea/Aleutians
look to remain in southwesterly flow as an upper level closed low
tracks from west to east across the Bering before parking itself
over the Bering Strait by Tuesday morning. Therefore, look for
continued widespread stratus with areas of fog accompanied by a
showery weather regime through Tuesday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Forecast thinking is largely unchanged in the long term as an
upper low moving from the Y-K Delta region to the Gulf of Alaska
during the middle of the week looks to remain the dominant weather
feature into the latter half of the week. The southern mainland
will be in the midst of a relatively short lived cool and rainy
pattern Tuesday night into Wednesday as the low tracks along the
southwest coast and an associated front moves through Southcentral
AK. Precipitation chances will remain higher along the North Gulf
coast through the end of the week as the low remains over the
southern Gulf. For inland areas and Southwest AK, Thursday should
mark the beginning of an improving trend as temperatures rebound
to near or slightly above normal seasonal values and a diurnally
driven shower activity once again becomes the primary mode of
precipitation. Meanwhile, the Bering will settle into a more
benign cloudy pattern in the wake of the low as high pressure
builds over the region through the end of the week. The exception
will be along portions of the Aleutian chain, where another North
Pacific disturbance will bring an increased chance of rain on
Thursday.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...TP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PLD
LONG TERM...CB



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