Storm Warning 111, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
455 AM AKDT Thu Aug 29 2019

.ANALYSIS AND UPPER LEVELS...
Abundant moisture was observed streaming across the Alaska
Peninsula to the west coast on Water Vapor Satellite imagery early
this morning...between low pressure over the Bering and a ridge
across the mainland. A stable air mass over the southern mainland
is allowing patchy fog to develop with areas of smoke also
reducing visibilities. The parent low over the Bering is a stacked
system with the surface low pushing a front across the eastern
Aleutians to the Pribilof Islands. The front is currently slow
moving while the zonal jet stream noses from the Kamchatka
Peninsula to the Central Aleutians.

&&

.MODEL DISCUSSION...
Models are in overall good agreement with the synoptic pattern
through the end of the week. Only minor changes were made to
the morning package using a blend of the GFS and NAM. Rain chances
were generally increased Friday night and Saturday...and coverage
was also expanded across the Kenai, Gulf and Mat-Su Valleys.

&&

.AVIATION...
PANC...VFR conditions may drop to MVFR during the morning
today...used a tempo group for visibilities as there is too much
uncertainty to add a prevailing condition. A ridge over the southern
mainland is keeping a quiet wind pattern across the terminal this
morning. The ridge shifts east this afternoon bringing a westerly
wind shift; however, winds remain light through Friday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Wednesday
night through Friday)...

An upper ridge continues over Southcentral today. The main impact
through tonight for Southcentral will be smoke from wild fires,
the biggest contributor being the Swan Lake fire. Except in the
immediate area of the fires, the lowest visibilities in smoke
will coincide with the strongest inversions during the
overnight/morning hours. A dense smoke advisory will continue for
the western Kenai Peninsula through the early afternoon hours
Friday. A surface low and associated front head toward Kodiak
Island Friday bringing rain to the western Gulf. This system
phases with the upper trough and lifts north late Friday. Rain
should move over the Kenai Peninsula, Cook Inlet, and Anchorage
Bowl to MAT-SU Valleys overnight Friday. The brunt of the rain
will fall on the eastern Kenai Peninsula and western Cook Inlet,
but the lack of significant downslope drying will allow for
measurable rain in traditionally downsloped areas to the lee of
the Kenai, Chugach, and Talkeetna Mountains.

&&

.FIRE WEATHER...

Southcentral...Continued warm and dry conditions will persist
through Friday. The next chance for rain is looking increasingly
likely for Friday evening and overnight as a weak front moves
toward Kodiak Island and lifts north. The best chances for light
wetting rain will be along the Kenai Peninsula and coast with much
lighter rain and lower chances inland as the front weakens.

The next likely time for rain will be Sunday as a powerful front
moves inland, which will likely be accompanied by strong mountain
gap winds. Wetting rain chances will once again be most likely
along the coast and the Kenai Peninsula with somewhat of a
downslope drying for inland locations, although those details are
still not clear this far out.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

The ridge is slowly being displaced towards the east. A pattern
change is in store for Southwest Alaska as back to back fronts
push into the area. The first front is expected Friday. Its
already a little past its prime by the time it reaches Southwest
but a re-enforcing shortwave moves about the base of the longwave
trough and helps give the precipitation shield some forward
momentum. Late in the weekend a new low will push north of the
Aleutian Chain bringing a southerly fetch into Bristol Bay. A
hefty amount of moisture is expected with this second storm but
will fall just outside the time frame of this forecast period.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

A front over the central Bering and Aleutians will continue to
push eastward and gradually weaken. A second storm is brewing near
northern Japan and this system will intensify and lift to the
northeast. The surface low is projected to bottom out somewhere
between 968 mb and 970 mb, which is an order of magnitude greater
than the region has seen in a while. Robust winds, rough seas and
copious amounts of precipitation expected with this storm.

&&

.MARINE (Days 3 through 5: Saturday through Monday)...

By far the primary weather story in the marine forecast is a
strong low near the Aleutians Saturday morning producing
widespread gales across most of the central and eastern Aleutians,
with possible storm force winds south of Adak. Gales will move
north and east with time, moving into Bristol Bay and much of the
eastern Bering by Saturday evening. Any storm force winds on the
Pacific side of the Aleutians should diminish below those
thresholds Sunday morning. The gales will persist over the central
Bering through the Pribilofs southeastward to the southern Alaska
Peninsula into the day Monday. The models are in good agreement
with the low and widespread gales, with a bit less certainty on
exact track and timing of those winds, and also how widespread any
storm force winds will be.

Over the Gulf, any hazardous winds and seas will be associated
with the eastern side of the aforementioned Bering low. Thus,
Saturday looks quiet. Southeasterly winds will gradually increase
with localized gales through southern Shelikof Strait and the
Barren Islands to Kamishak Gap during the day Sunday. When the
leading front reaches the north Gulf Coast on Monday, gale force
easterly winds associated with the barrier jet are possible. Those
winds will diminish Monday night.

&&

.LONG TERM FORECAST (Days 3 through 7: Saturday through Wednesday)...

Previous Discussion:

The weather pattern looks to finally take a turn for the wetter
across Southcentral through the long-term forecast period. The
first chance of rain comes Friday night into Saturday morning.
Unfortunately there is considerable model uncertainty with this
feature. The upper level forcing for the rain, which will be the
primary driver, will be weakening. Whether it weakens as a wave,
or splits into a cutoff low in the Gulf will determine how much
rain gets into Anchorage and points north. For the most part,
rains of 0.05 inches to one quarter inch is decently agreed upon
for the Kenai Peninsula, including over the Swan Lake fire and
other drought-stricken communities. By late Saturday morning, the
rain will shift east to the Gulf coast.

On Sunday, a strong front associated with a strong Bering low
(more on the Bering low below), will move into the Gulf. This will
bring another round of wetting rains to portions of Southcentral.
Light rain will begin very early Sunday morning at Kodiak,
spreading north to the southern Kenai Peninsula by mid-morning.
Heavier rain will spread north and east through the day across
all of Southcentral except the Copper River Basin. Strong
Turnagain Arm winds may make for some downsloping, but at least
initially, some light rain should get into both Anchorage and the
Mat-Su Valleys Sunday afternoon and evening. Downsloping and
splitting energy will end the rain for Anchorage and the Mat
Valley for the day Monday, but some shower activity will still be
around. Meanwhile, the stronger winds and rains will have
competing effects at the Swan Lake fire. A trailing trough behind
the low moves through Tuesday, with more widespread shower
activity across much of Southcentral expected.

Out west, a strong low will be moving into the Aleutians Saturday
morning. The low will produce widespread gales, with potential for
storm force winds to the west and south of the low center on the
Pacific side of the islands. The center will cross the chain west
of Dutch Harbor Saturday evening and slow down as it moves
northeastward to near Kuskokwim Bay over to Nunivak Island by
Monday morning. Associated wetting rain will move into
Southwest Alaska Saturday night, overspreading much of the area on
Sunday. Gusty southeast winds are also expected with the rain.
Thus, in the usual downslope areas, from King Salmon to Iliamna
and the west-facing slopes of the Kuskokwim Mountains, less rain
is expected. Once the winds turn more southerly on Monday, all of
Southwest Alaska will get into light to moderate rain. The winds
turn southwesterly to westerly on Tuesday as the rain generally
diminishes from west to east.

The strong southeasterly winds begin Saturday night in Southwest
Alaska, and will persist into the day on Sunday. Coastal villages
with exposure to the southeast such as Kwigillingok should be on
alert for potential coastal flooding from Saturday night through
Sunday. Strong south to southwest winds and onshore flow in
Bristol Bay may also cause coastal flooding concerns during the
day on Monday. There still remains some uncertainty as to the
exact storm track. If the center moves further west, towards
Nunivak Island, there is potential for a longer period of onshore
winds and potential for coastal flooding concerns in Kuskokwim Bay
and the Kuskokwim Delta coast. In that scenario, concerns in
Bristol Bay would be lessened.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Dense Smoke Advisory 121.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...KH
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
MARINE/LONG TERM...JW