Flood in Alaska, December 21 to 31, 2019

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
535 PM AKST Fri Dec 20 2019

.ANALYSIS AND UPPER LEVELS...
Arctic air continues to drain southward across the forecast area
this afternoon; however, the core of the coldest air remains
locked north of the Alaska Range. Temperatures across the region
range from -20F in Paxson and Sleetmute to the upper 20s to mid
30s across the immediate coast and Aleutian Chain. A tight
pressure gradient between the interior ridge and the persistent
trough over the Gulf continues to produce gusty northerly outflow
winds through the passes and along the coast. Wind gusts through
Thompson pass have been consistently near 45 mph today with gale-
force winds through Passage Canal, Resurrection Bay, and the
Valdez Narrows.

Out west, a decaying occluded front is producing a band of light
snow from the Y-K Delta south to Bristol Bay. Winds of up to 25
mph have reduced the visibility to less than 1 mile at times
through the morning and afternoon hours around the Kuskokwim
Delta, including Bethel. Some of the higher cloud cover from this
front is also beginning to push over the Alaska Range and filter
over the western half of Southcentral.

Farther west, gusty northwesterly winds and scattered showers
also continue along the southern AKPEN and eastern Aleutians. The
main weather feature, however, is an area of low pressure
southwest of Shemya. This system is deepening rapidly as it
tracks northeast toward the Chain. Clouds associated with its warm
front are approaching the western Aleutians this afternoon. This
feature has plenty of upper-level support, as it is situated in
the left exit region of a 120 kt jet streak and ahead of a
digging and deepening 500 mb trough.

&&

.MODEL DISCUSSION...
Models remain in good synoptic agreement through Monday. After
that, guidance diverges on the evolution of a Bristol Bay low as
it weakens and moves inland. The movement of this low will have
subtle differences in impacts downstream across Southcentral.
However, even with the noted differences, the GFS ensemble members
continue to cluster well together, trending toward the EC and
away from its own deterministic GFS run. This also means that
sensible weather impacts are not wildly varied between models.
Moving into next week, the GFS ensemble and EC solutions both
indicate a major pattern change toward more active weather across
the Southern Mainland beginning with the initial low moving across
the Bering this weekend.

&&

.AVIATION...

PANC...VFR conditions and light winds are likely to persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Southcentral will continue with an offshore flow pattern, with
dry conditions across much of the region through Saturday. The
exception is the southern Copper River Basin, which could see
snow showers moving in from the northeast Gulf coast as an upper
level trough moves through the area. Another round of cold air
advection will bring strong gap winds to the western Gulf
beginning this evening through Saturday.

The next big pattern change enters the southern Gulf Saturday
night into Sunday morning when a frontal system brings widespread
precipitation and strong winds. Kodiak will see snow initially as
the front lifts north over the island by late Sunday morning,
which will transition to rain as temperatures warm up. This
frontal system will eventually reach Southcentral Alaska by Sunday
afternoon, bringing snowfall to the Kenai Peninsula and coastal
areas.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
A weakening front will continue to spread some snow showers
across Southwest this evening. Showers will eventually taper
overnight and northwesterly flow will allow cold air to settle in
across the area for Saturday morning. Generally clear conditions
are expected for Saturday ahead of a strong front approaching the
area by Sunday. Strong southeasterly winds will pick up across the
southern Alaska Peninsula beginning Sunday morning. Snowfall is
also expected across the Alaska Peninsula and interior areas of
Bristol Bay with this front, which when combined with the winds,
may produce blowing snow and possibly low visibility. As the low
center tracks eastward toward Bristol Bay late Sunday into early
Monday, there is the possibility for elevated water levels along
the Alaska Peninsula coast. While confidence on this surge
potential is low right now, we will continue to monitor this
possibility. By Monday, the low center moves eastward and
eventually inland. With a significant amount of cold air on the
backside of the low, enhanced northerly flow is expected along the
Kuskokwim Delta Coast late Monday into early Tuesday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
Active weather continues across the Bering as a strong low tracks
across the area through the weekend. A High Wind Warning has been
issued for Adak and Atka late Saturday through Sunday afternoon
and a High Wind Watch has been issued for the eastern Aleutians
for Sunday, including Nikolski, Unalaska, and Akutan.

As the leading front arrives over the western Aleutians Saturday
morning, expect strong southeasterly flow and rain to progress
eastward. As the low center moves into the Bering by Saturday
afternoon, cold air will begin to wrap around the backside of the
low, allowing for strong winds and snow to spread across the
Aleutians late Sunday. The strongest winds will be on the backside
of the low, and hurricane force winds are expected. As the low
tracks eastward, strong winds on the south side of the low may
reach the central Aleutians Saturday night into early Sunday.
There are slight discrepancies in the models regarding where the
low center is at this time, and there is the possibility of the
low center shifting slightly northward which would keep the
strongest winds north of the Aleutians. Therefore, a High Wind
Watch has been issued for the eastern Aleutians for Sunday, which
includes Nikolski, Unalaska, and Akutan. Stay tuned to the
forecast for updates as we continue to track this low.

&&

.MARINE (Days 3 through 5)...
Beginning the extended period marine forecast on Monday
afternoon, a weakening and departing gale force low in Bristol Bay
will fall apart by Tuesday morning. The Pacific storm track will
shift south of the Aleutian Chain through Thursday and move into
the Gulf of Alaska as a series of lows moves into the Gulf.
Currently the first will move into the Gulf by Tue and last
through Wednesday, possibly as a gale force low as it tracks north
to the Gulf Coast. A second low, possibly stronger, moves in
sometimes Wednesday/Wednesday night. Over the Bering, this pattern
will keep a cold, northerly or northeast flow over that area with
areas of freezing spray.

&&

.LONG TERM FORECAST (Days 4 through 7)...
The long term will kick off Tuesday with a deep pool of cold air
over the majority of mainland Alaska. A Bering low looks to track
across the northern Gulf Tuesday along the boundary of cold arctic
air spilling out of the state. Model guidance suggests a winter
precipitation event along the southern coast as the low moves
east. Another low in the Western Aleutians Tuesday is expected to
track across the Aleutian chain and into the northern Gulf on
Wednesday through Thursday, adding reinforcements with another
round of winter precipitation. Guidance is mixed on the exact
track of these two storms, but model guidance suggests that we
will be covered in cold air and odds for some wintry weather are
strong.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...High Wind Warning 187.
 High Wind Watch 185.
MARINE...Hurricane Force Wind Warning 177 178.
 Storm Warning 173>176.
 Gale Warning 121 130>132 138 155 160 165 170>172 179>181 185.
 Heavy Freezing Spray 129.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...AH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...JA/LB