Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 220114
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
514 PM AKDT Mon May 21 2018

.ANALYSIS AND UPPER LEVELS...

The pattern that has been observed over the last week has no signs
of letting up as a long wave troughs persists over the Bering Sea
and southern mainland Alaska. The upper level low that once lived
in the eastern Bering has entered the southwestern Gulf. This
system is setting the stage for several more fronts to advance
toward the northern Gulf. These fronts are the cause of the dense
cloud cover and showers along the North Gulf Coast which are now
migrating into the western Kenai Peninsula. Trailing the trough is
a minimal ridge over the Bering. West of the ridge, a second
trough is barreling toward the western Aleutians.

A second surface low, stemming from Kamchatka, is advancing
toward the western Aleutians preceded by a strong front. The jet
stream skirting just south of the western Aleutians is strong
along the frontal boundary and is reaching speeds of 150 kts
within the jet streak. The placement of the jet stream is helping
to intensify this surface low. The result is most locations in
the Aleutians are experiencing surface winds exceeding 25 kts with
clouds and rain coating the chain behind the front.

&&

.MODEL DISCUSSION...
Models initialized well and move forward in lock step through 48
hours. We begin to see some deviation in the GFS and NAM,
especially in the Western Gulf, as the upper-level trough becomes
more negatively- tilted and deepens into the weekend.

Southcentral forecasters preferred the NAM. The Aleutian and
Southwestern Alaska forecaster preferred a primarily-NAM solution
today, with just a hint of GFS.

&&

.AVIATION...
PANC...VFR conditions will persist under a persistent broken deck.
Late-day and evening winds along the Turnagain Arm will be
probable the next few days.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

Widespread rain and cloud cover will continue from the Kenai
Peninsula to the Copper River Delta as a cross- barrier flow
continues over the coastal marine zones through Tuesday afternoon.
This is in response to a nearly stationary low pressure system in
the southern Gulf pushing yet another front northward overnight.
Precipitation and cloud cover will be more widely scattered for
interior portions of Southcentral Alaska, including Cook Inlet,
as the general southeasterly flow creates downsloping winds off
the Chugach Mountains. A strengthening thermal trough over the
interior and high pressure building along the coastal ranges will
generate gusty south to southeast winds in Turnagain Arm, the Knik
River Valley, and along portions of the Copper River during the
afternoon and evening hours Tuesday and Wednesday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The thermal trough stretched across Southwest Alaska today will
continue to bring a good potential for isolated to widely
scattered thunderstorms this evening. The best chance for
thunderstorms looks to be along and just to the west of the
thermal trough axis stretching from the middle Kuskokwim Valley
south into the Greater Bristol Bay area extending south to the
coast just to the east of Dillingham. With instability diminished
on Tuesday, precipitation should remain showery and limited in
coverage. The same general pattern will continue on Wednesday,
although with inland shower coverage more isolated, and a
dissipating frontal system approaching the coast by Wednesday
night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
An upper level ridge will track from west to east across the
Aleutians and Bering tonight, with a frontal system currently
tracking eastward over the western Aleutians. The front will slow
and shear apart as it continues east Tuesday and Tuesday night
before stalling over the eastern Bering and Alaska Peninsula
Wednesday and then dissipating Wednesday night. The large
vertically stacked parent low will follow, crossing the western
Aleutians Tuesday afternoon and then continuing slowly east to the
south central Bering through Wednesday night.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long term forecast begins Thursday with a weak surface low
rotating in the Southern Gulf spreading clouds and rain showers across
the Gulf Coast and most of Southcentral. This system will slowly
fill in and die out, though persistent onshore flow will lead to
clouds and rain, especially along coastal areas on Friday. Another
low will rapidly move in from the west on Saturday providing no
real break in the weather for the Gulf Coast and no prolonged
periods of sun and warm weather for the inland areas. This low
will remain in the Gulf through most of the long holiday weekend
as a stubborn long wave ridge over the Western US will continue
to direct systems north into the Alaskan mainland. The pattern
doesn`t look to be a total washout though, especially for more
inland areas, with breaks in between individual waves bringing
periodic dry weather, though coastal areas look to remain rather
cloudy and wet. Models do show this system slowly moving off to
the east on Memorial Day (Monday) providing a better chance of
decent weather for the holiday. High pressure looks to build in
over Western AK for the later half of the weekend, though
afternoon showers/thunderstorms should develop over the higher
terrain and may drift over more populated areas.

Out West, a long wave ridge will build in over the Western Bering
by the weekend bringing drier northerly flow to the western two-
thirds of the Bering/Aleutians. There is some disagreement between
models with the next system coming into the Western Aleutians but
it appears the ridge looks to hold through most of the weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...None.

&&
$$

SYNOPSIS AND MODEL DISCUSSION...RA/DS
SOUTHCENTRAL ALASKA...TM/DK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP
LONG TERM...KVP



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