Flood Watch 7, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
434 AM AKDT Thu Oct 1 2020

.ANALYSIS AND UPPER LEVELS...

A very active storm pattern remains in place for Alaska. A deep
trough remains anchored in place across the North Pacific with a
strong ridge over south British Columbia south to the Lower 48.
This setup will keep the pattern stagnant as we can already see
the next storm system on water vapor and satellite imagery this
morning moving northward. One 985 mb low just moved into the
Bristol Bay area and will continue moving westward through the
Bering today. This low has been bringing occasional rain showers
to much of Southwest and Southcentral this morning. Looking closer
at the radar returns, you can see the flow is generally
southeasterly across much of the southern Alaska Mainland. This is
leading to downsloping for the Anchorage Bowl, Kenai Peninsula,
and areas west of the Alaska Range and the Kuskokwim Mountains. An
occasional shower or spritz makes it over the mountains into the
aforementioned areas this morning, otherwise many areas are
remaining dry. The next low is seen near 45N and between 150W and
160W. This low has a moisture fetch southward to just north of
Hawaii. This means the already wet pattern will continue to become
even wetter as this system approaches.

Over the Bering and the Aleutian Chain, generally quiet weather
continues as a weak upper level ridge dominates the pattern. Some
cold air is still present over the central Bering this morning
which can be seen in the cumulus clouds that are evident on the
nighttime microphysics imagery this morning. An occasional brief shower
may accompany these cumulus clouds due to the unstable atmospheric
profile that cold air creates moving over relatively warm water.
Lastly, a weak system is riding along the northern periphery of
this weak ridge that is expected to bring increased cloud cover
and showers to the western and central Aleutian Chain.

&&

.MODEL DISCUSSION...

Models remain in very good agreement on the pattern for the next
couple of days. All models are in agreement on another low moving
up from the North Pacific which is already evident on satellite
imagery this morning. By the weekend the models continue the theme
of the jet becoming more zonal. This will change the storm track
from being south to north to more of a southwest to northeast
moving track. Models generally all agree on a storm system south
of Dutch Harbor Saturday morning but differ on the placement,
exact track, and intensity of the low. The Canadian-NH is the
furthest west model and the strongest with the low. The NAM is the
furthest east and much weaker with the low. The EC and GFS are in
the middle with GFS closer to the Canadian and the EC closer to
the NAM leading to moderate forecast confidence for this system
over the weekend.

&&

.AVIATION...

PANC...VFR conditions will persist through the TAF period. A storm
moving northward through the Gulf will lead to northerly winds
Thursday afternoon. Low level wind shear is expected over and
near the terminal this afternoon through the remainder of the TAF
package.

&&

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

A weakening storm center over Bristol Bay will continue generally
windy conditions across Southcentral. Strong easterly winds are
expected to continue across Turnagain Arm and Portage Valley
through today. Pressure rises on the backside of the front which
moved inland have allowed the Turnagain Arm wind to bend into
Anchorage and brought gusty southeast winds out of the Knik River
Valley, and southerly winds over the Copper River Basin. Rain
across most areas have switched to showers with frontal passage,
but still remain pretty steady over the north Gulf/eastern Kenai
Peninsula.

Yet another strong low and associated front head north toward the
western Gulf this evening/tonight bringing another round of storm
force winds and moderate to heavy rain to Kodiak Island and the
eastern Kenai Peninsula. This additional rainfall will increase
the potential for flooding for the Kenai Peninsula streams
(mostly near Seward) and localized flooding and mudslides on
Kodiak Island. The front moves north across Southcentral on Friday
so precipitation will trend toward showers, though still fairly
steady along the Gulf coasts.

As the front heads north the pressure fields tonight will cause
the Turnagain Arm wind to bend south and kill the southerly winds
across the Anchorage Bowl. Additionally, southerly winds across
the Copper River will diminish and there will be shift from a Knik
wind to a Matanuska wind (NE). Pressure gradients will increase
easterly winds across Turnagain Arm and Portage Valley late
tonight/early Friday and give rise to high wind potential which
will likely hold into the afternoon hours on Friday. Friday will
also bring a return to gusty Knik winds and southerly winds across
the Copper RIver Basin with pressure rises on the backside of the
front as it moves inland.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2: Today through
Friday night)...

The remnants of the storm force low that impacted the AKPEN
yesterday have moved over Bristol Bay with the low level center
just off the coast. Cross barrier flow should limit rain to the
windward sides of the major mountain ranges, but as the low
center moves northwest through the day today rain will become more
expansive over the region as the flow shifts to the southwest.
Since this storm was generated in the tropics, expecting well
above averages temperatures again today as the warm, southerly air
moves in. Winds are also expected to be gusty throughout the
region, with the highest winds expected through Kamishak Gap and
into Iliamna.

Tonight, yet another strong low is expected to reach the AKPEN,
so another round of heavier rain is expected on the windward
slopes. Like Thursday, downsloping should keep chances for rain
lower on the leeward sides (northwest) of the major mountain
ranges although showers are possible. By Friday afternoon the low
center will have crossed the AKPEN once again transitioning the
flow to be onshore. As a result, rain should become more
widespread. Temperatures on Friday should be above average once
again due to warm air advection.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2: Today
through Friday night)...

The pattern of bimodal weather for the east and west Bering is
expected to continue for the Bering/Aleutians through Friday
night. Today, a weakening low is expected to move from Bristol
Bay to the northwest before curving back towards the Pribilof
Islands. This will continue the rainy pattern with small craft
advisory criteria winds. Another strong low is expected to impact
the AKPEN tonight through Friday night, which will return rain and
gale force winds from Cold Bay east. Over the western Bering,
northerly flow at or below small craft advisory criteria and rain
will persist through Friday night.

&&

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

Bering/Aleutians

There is a decaying low near St Matthew Island Saturday morning
and a new low south of Dutch Harbor. Widespread northerly flow is
expected across the Bering with showers possible. The surface low
south of the Chain will continue to deepen and track south of the
AKPEN Sunday morning and move into the Gulf.

Gulf of Alaska

At the onset of the forecast period expect widespread southwest
flow and somewhat of a lull between systems. However, an
organized system will move into the southern Gulf on Sunday.
Confidence is below average for Sunday as there are significant
discrepancies between solutions on the placement and intensity of
the low center. Stay tuned.

&&

.LONG TERM FORECAST (Days 3 through 7/Saturday through
Wednesday)...
The final in a series of fall storms will make landfall along the
Gulf Coast on Saturday night through Sunday evening. This will
bring another bout of heavy coastal rain and strong winds to the
coast, with lighter rain inland. This low track will generally
keep Southwest Alaska and the Bering away from the impacts of this
low. Expect rainy and cloudy weather to persist through Tuesday,
with a brief period of high pressure and drying on Tuesday night
before the next Bering front quickly moves into Southwest Alaska
on Wednesday. Inland blocking high is looking more and more
likely, thus the storm track looks to finally shift south of the
Gulf of Alaska by mid to late week, allowing cooler high pressure
to dry out Southcentral and the Gulf Coast for the second half of
the week, although Kodiak will continue to see wetter weather.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning 119 120 125 130 131 132 136 137 138
Gale Warning 127 129 139 140 141 150 155 160
.FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MV
SOUTHCENTRAL ALASKA...RMC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CJ
MARINE/LONG TERM...PS