Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 161216
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
416 AM AKDT Thu May 16 2019

.ANALYSIS AND UPPER LEVELS...
The longwave pattern features a broad area of troughing over the
Bering and into the north Pacific encompassing the southern tier
of Alaska. There are a number of weak disturbances but there are
three decent lows. There is an upper level low between the
Pribilof Islands and Goodnews Bay. There is another low over the
southern Gulf which is wrapping moisture and cloud coverage into
southcentral Alaska. There is a third closed low south of Adak
which still has a well defined frontal boundary attached to it.
The jet core is south of 50N latitude.

&&

.MODEL DISCUSSION...
The models are in good synoptic agreement for today. But as the
low in the Gulf Of Alaska moves northwest towards the Kenai
Peninsula the models vary with the handling of this feature Friday
and Saturday. There seem to be some run to run consistency issues
which will add an element of uncertainty over the weekend.
Sensible weather-wise, there are some minor differences in the
precipitation, as well as convective parameters. The NAM continues
to be more aggressive, ginning up thunderstorms compared to the
global models.

&&

.AVIATION...
PANC...VFR conditions will persist. There is a decaying low in the
Gulf of Alaska which is wrapping moisture and cloud coverage into
the region. At this point, expecting ceilings at PANC to remain
above 5,000 feet. But the ceilometers at PAMR and PANC have
detected FEW to SCT low clouds over night. Expect the cigs to
bounce around this TAF package.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Friday night)...
A low over the southern Gulf will lift northwestward toward
Southcentral today, leading to generally cloudy and cool
conditions with isolated to scattered showers. A short-wave out
ahead of the low will track from the southern Copper River Basin
to the Mat-Su today. This will produce areas of steady rain
through about mid afternoon before exiting to the west.

On Friday, the upper level center will continue northwestward
across the Kenai Peninsula, Cook Inlet, and the Susitna Valley.
With cold upper level temperatures (-30 degrees C or colder at
500mb) precipitation will be showery. There could even be a few
thunderstorms, though forecast soundings show a "skinny CAPE"
type of profile. Thus, would need some extra help from upper
level wave or upslope flow along Alaska Range (western Susitna
Valley). For now, have included in the forecast as "slight chance
of thunderstorms". Also, will need to keep an eye out for
additional upper waves tracking up from Southeast Alaska on
Friday. Models seem to be keying in on a wave train along the
Yukon/Copper River Basin border on up along the Alaska Range.
Thus, there is potential for some steady rain to affect the
eastern to northern basin.

With cooler temperatures today and presence of low over the Gulf,
generally expect a light wind regime. On Friday, the coastal ridge
will build and combine with unstable air and favorable synoptic
flow to produce gusty winds through the usual gaps (Turnagain Arm,
Knik River Valley, Copper River). These winds will spread inland
and affect Anchorage, the Matanuska Valley, and the heart of the
Copper River Basin.

Conditions will dry out Friday night as the upper low exits
across western Alaska. Expect partial clearing which will allow
temperatures to drop into the 30s for most inland valleys.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
The upper level low over the eastern Bering will track south
through today making it into the North Pacific on Friday. A
separate upper level low will then track over southwest Alaska
from the Gulf and push to the northwest through Saturday. This
setup will bring continued showers and light winds to the area.
There will also be a slight chance for thunderstorms to develop
this afternoon and Friday afternoon for areas from the Alaska
Range into the Middle Kuskokwim Valley as shortwaves move through
the area. However, if cloud cover remains over the area as
forecasted, it would inhibit afternoon heating which would
decrease the likelihood of thunderstorm development.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
Broad, weak northerly flow will continue for the Bering and
Aleutians into Saturday with high pressure to the west and low
pressure to the east. This setup would also bring rain to more
eastern locations that are closer to the low pressure. The
heaviest of the rain would be on the Bering side of the islands.
Finally on Saturday a low will track into the central Bering from
the north that will start to shift the pattern to bringing rain
farther to the west.

&&

.MARINE (Days 3 through 5)...
North to northwesterly winds will continue over the Bering and
Aleutians with an area of 25 knot winds in central and eastern
Bering diminishing gradually Friday night. Winds will generally be
20 knots or less through the weekend and turn progressively more
northerly Sunday and Monday. For the Gulf, winds will also
generally remain below small craft advisory levels, although more
uncertainty exists regarding the strength and timing of the front
swinging from east to west across the northern Gulf on Saturday.
Current model runs are now keeping the Sunday night and Monday low
well south of the Gulf but this is another time period with high
model uncertainty.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Easterly flow will continue to bring periodic waves of showers
across southern Alaska and the possibility of isolated
thunderstorms at times this weekend through early next week. By
Friday night, the multi-center upper level low will have
progressed to have one center south of the Alaska Peninsula with a
trough extending north over western Alaska and a second low
center over the southeastern Gulf. The more western center will
track slowly east to the southcentral Gulf through Saturday night
while the eastern low weakens and is pulled around the north side
of it. This pinwheeling motion will pull a frontal system across
the northern Gulf Saturday, over the Kenai Peninsula and Kodiak
Island Saturday night and into Southwest Alaska Sunday. A second
upper level short wave trough further to the northeast, but in
phase with this rotation will track through the Yukon territory
Sunday morning and into interior Alaska Sunday afternoon through
Sunday night.

A rapidly developing North Pacific low tracking east well to the
south of the Gulf will cause the upper level low in the
southcentral Gulf to open up to a trough Sunday and lift to the
northwest over the Alaska Peninsula and into Southwest Alaska
Sunday night and Monday. In response to this deeper and much
further south (in comparison to yesterday`s models runs) track of
the North Pacific low, the upper level ridging over northwestern
Canada may build further south into Mainland Alaska Monday night
and Tuesday. If this solution holds true, the strengthening
ridging will bring a warmer air mass into Southcentral Alaska for
Tuesday. Depending on the timing and how far south the ridge
builds, convection could be suppressed briefly. The potential for
the thermal trough building more strongly into the Copper River
Basin on Tuesday, however, will need to be closely watched as it
could support the formation of more abundant thunderstorms
Tuesday evening if a weak easterly or southeasterly wave rotates
around the south side of the ridge.

For Southwest Alaska, troughing and showery weather will be more
persistent with lows continuing to rotate up the Southwest Alaska
coast and through the eastern Bering for the first half of next
week. By midweek, models are hinting at the possibility of a
pattern shift with a more active westerly storm track into the
Bering and a southerly storm track up over the Alaska Peninsula
and into Southwest Alaska. A great deal of uncertainty remains
however, regarding where the ridge to the east will set up and
how much of the storm energy will push north into the Gulf and
Southcentral Alaska.

&&


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

&&

$$

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK
MARINE/LONG TERM...JR


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