High Wind Watch 12, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
516 PM AKST Sun Dec 20 2020

.ANALYSIS AND UPPER LEVELS...

A weak upper level wave is currently pushing into Southcentral
aiding in the development of a surface low just to the south and
east of the Kenai Peninsula. This weak system is generating
another round of snow showers across the area. These snow showers
are expected to produce only some light accumulations with not
nearly as strong of dynamics or moisture compared with yesterday's
storm system.

Further west, our next major storm system is already taking shape
with a storm force surface low moving through the central
Aleutians. This is all in advance of a deep upper level trough
currently moving just south of the western Aleutians, with a
powerful jet rounding its base. Water vapor imagery indicates a
steadily improving connection to tropical moisture with early
indications of a developing atmospheric river that will have major
implications on the forecast over the next couple of days.

This surface low will continue tracking north and east tonight
while deepening below 950 mb. Meanwhile its front will march to
the east, pushing into the eastern Bering/Aleutians/AKPEN by later
tonight.

&&

.MODEL DISCUSSION...

Models are better overall today as compared to yesterday, but some
very major issues remain. To start with, the complex low in the
Bering Sea remains a bit of a mess with the models varying by
hundreds of miles and 20+ mb with where the two low centers will
be by Tuesday afternoon. The good news is that they all pretty
much have whittled it down to there being only two main centers
and not possibly three or four as they showed yesterday. This
actually is an improvement and we hope they can converge on a
common solution soon.

Models are starting to converge on a more common solution for the
Gulf of Alaska and Southcentral Alaska for Monday through
Wednesday. That is unless you are the 12Z GFS model which is
spinning up a strong low (which the GEM and NAM both show as much
much weaker) and running it over the top of Homer Tuesday evening
which would be enough to turn a major storm in Southcentral into
an even more impactful event. The 18Z GFS still has this low but
about 10 mb weaker. The GFS ensemble also shows a number of
members with s fairly strong low tracking over lower Cook Inlet or
Kachemak Bay so, while the other models are still going with a
faster and weaker low, the GFS cannot be completely dismissed.

Precipitation type differences in the models is making the
forecast for the Kenai Peninsula particularly difficult. A
surprising aspect of this strong storm system is that the models
are showing surprisingly cold temperatures through the boundary
layer through 850 mb. This is a big reason for the Blizzard and
Heavy Snow conditions expected for the eastern Kenai Peninsula.
This is one of the biggest questions as just a little bit more
warming will significantly reduce these amounts and that is
something that will need to be watched closely over the next few
days.

&&

.AVIATION...
PANC...Some snow showers will persist through the evening bringing
some occasional IFR visibilities, but conditions will remain
mostly MVFR. The visibility should become all VFR after midnight
with the potential for some MVFR ceilings to persist. More light
snow with MVFR conditions is likely again by Monday afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Sunday
night through Tuesday)...

Very active and highly impactful weather is on the way for
Southcentral AK. Before its arrival, some light snowfall will
continue into the evening tonight. A brief break in the weather
will occur tomorrow during the day - the calm before the storm.

Late tonight, a broad area of low pressure that is currently
straddling the Aleutians will develop a low in the North Pacific.
This low and attached front will move quickly northeast into the
Western Gulf tomorrow, moving past Kodiak. It will arrive along
the Southcentral coast late in the day tomorrow. This system will
intensify as it moves into the gulf, which will create highly
impactful weather for the Southcentral coast when it arrives.
Weather associated with this system includes high winds due to a
hurricane force barrier jet setting up along the Southcentral
coast, including hurricane force winds in Prince William Sound.
Cross barrier flow and local effects means hurricane force winds
are also possible in passes, areas around Turnagain Arm, and the
upper hillside of Anchorage. So, high wind watches have been
issued for several locations, please see those statements for
details.

This system is expected to bring high amounts of moisture, or QPF.
This means heavy rain for areas along the coast that will get
above freezing, and heavy snow for higher elevations that will
stay above snow level. Temperatures will be warming with this
system and this still needs to be honed. However, there is enough
of a threat to issue a winter storm watch for the Eastern Kenai
Peninsula. Please see that statement for details.

Regarding the Kenai Peninsula, there was a significant change to
the forecast for Seward today, as we are now thinking Seward will
transition to rain overnight Monday into Tuesday. With high
precipitation amounts Tuesday, minor flooding is a concern due to
frozen culverts not being able to drain water. Tuesday into
Wednesday there is also a possibility of winter weather returning
to Homer with authority, due to colder air with snow/wind moving
into the southern Kenai. This is still under development so stay
tuned here...it will all depend on the low track and strength,
which is not locked in yet.

It should be noted there is still uncertainty to exactly how this
will play out, as there is considerable model disagreement on the
development of a secondary low along this front, that could
enhance weather impacts. The GFS is swinging for the fences with
the most intense system possible, while all other guidance has
impactful, but less intense weather. The GFS still cannot be
discounted though. Temperatures and precipitation type will also
be a factor in snowlevels, of which there is uncertainty as well.
So, expect further adjustments to the forecast. There is still
uncertainty with the forecast, but we have enough confidence to
say this will be a big event.

Lastly, while almost all focus is along the coast, inland ares
will see weather as well. The upper Susitna Valley can expect to
see up to 6 inches of snow along the Parks highway, but right now
it doesn't look like enough to issue an advisory. All in all, very
impactful weather coming up right as folks will starts to travel
for Christmas. So please stay tuned for forecast updates.

&&

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

The main challenge in this forecast package was to discern the
precipitation type (P-Type) for Bristol Bay in response to the
approaching frontal system from the Bering between Sunday night
and early Monday morning. Though warmer temperatures from the
front should remain near but mainly south of the Aleutians and
AKPEN through at least Monday night, the potential for snow
transitioning to rain for Bristol Bay is credible and it may be
brief too. At 850 mb, very cold temperatures near -16 degrees
Celsius dips right over Kuskokwim Bay from the northern Bering by
this afternoon. This cold air advection advances eastward and
across Southwest Alaska through tonight until the warm boundary of
the frontal system steps across the eastern Bering tonight, which
will cause the departure of the cold temperatures from the teens
to near single digits as early as Monday morning. The southerly
flow mainly over Bristol Bay will also cause warm air to move
farther inland, and downsloping will take place as well over the
leeward side of Chigmit Mountains. In the meantime, steady light
snow continues over much of the coastal area and farther inland,
south of the Kusko Valley and Nushagak Hills. Strong winds
associated with the frontal system will also move across AKPEN and
Bristol Bay between Monday and Tuesday. There's a good chance for
increased blowing snow to occur over portions of Bristol Bay and
Kuskokwim Bay by overnight Sunday night, and should conditions
worsen it could lead to diminishing visibility into the early
hours of Monday morning. Will keep an eye on this situation and
provide updates in the next forecast package.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

The onset of the North Pacific storm over the central
Aleutians/Bering around noon today will elevate winds, seas, and
precipitation for the rest of eastern Aleutians and Bering in the
coming days. This storm and its strong frontal boundary would most
likely boost winds over Cold Bay just a bit higher by Sunday
night and Monday than the forecasted winds. Strong southerly winds
from the front will trend upward especially along the Gaps and
Passes around Cold Bay. For this reason, will monitor the
observations closely and issue updates should winds worsen there.

&&

.Marine (Days 3 through 5: Wednesday through Friday)...

Gulf of Alaska
There is good agreement on steady improvement of conditions across
the Gulf on Wednesday as the major Monday/Tuesday storm begins to
track inland and weaken. Southerly winds will be found across
much of the Gulf, with all areas falling below small craft levels
by late morning. Wednesday night into Thursday there is
significant disagreement on the strength and track of a North
Pacific low lifting into the Gulf. If the stronger and further
west solution indicated by the GFS works out, southerly gales may
return to the western Gulf by late Thursday. By Friday there is
better agreement on all areas once again seeing winds fall to
small craft or weaker.

Bering Sea:
There is good agreement on a weakening broad low across the
Bering on Wednesday which should result in fairly tame winds. On
Thursday there is disagreement on the placement of a compact
surface low near or just south of the central Aleutians. A small
areas of gales or even briefly storm force winds appear possible,
but the exact placement is difficult to pin down at this point. On
Friday uncertainty increases even further with the potential for
a Gale force front pushing into the western Bering/Aleutians, but
the GFS is significantly weaker with this front versus other
models.

&&

.Long Term (Days 4 through 7/: Thursday through Sunday)...

Active weather looks to persist right into the long term. On
Thursday a broad upper level low looks to extend across much of
the Bering. A shortwave rounding the base of this trough looks to
support the development of a compact storm force low somewhere
near the central Aleutians. Further east, a negatively tilted
trough lifting out of the North Pacific will aid in the
development of another surface low that will lift into the western
Gulf of Alaska and spread another round of precipitation across
the AKPEN, Kodiak and eventually much of the Northern Gulf Coast.

For Friday the details become much more murky, but the general
idea is slightly less active weather taking over for Southcentral.
Meanwhile another gale or storm force front looks to push into the
western Bering, bringing wind/rain to the western Aleutians.

For Saturday and Sunday a deep upper trough looks to dig into the
western Bering/Aleutians with the development of a strong system
likely out ahead of it. The placement of said system remains a
huge question mark with solutions split towards this being a
wind/wave generator for the Bering, or yet another strong low
moving into the western Gulf.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WSW 125 161 195. NPW 121 125.
MARINE...STORMS 130 155 170 172 351 352 411-413. GALES 119 120
125 131 132 138 139 150 165 171 173-177 179-181 185 414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BG/EZ
SOUTHCENTRAL ALASKA...BJB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KM
MARINE/LONG TERM...BG