NWS Anchorage, AK wrote its last area forecast discussion before the first event at Nov 27, 5:16 PM AKST, 3 hours 54 minutes earlier.
Area forecast discussion, Nov 27, 5:16 PM AKST, 3 hours 54 minutes before
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
516 PM AKST Sat Nov 27 2021
.ANALYSIS AND UPPER LEVELS...
A large upper level low remains centered directly over most of the
AK mainland aloft, with Arctic high pressure and frigid temperatures
anchored in place closer to the surface. As has been the case for
the past few days, the coldest temperatures as low as -20 to -30F
(and even -40F earlier this morning) reside over Southwest Alaska,
with the most extremely cold air gripping the Middle Kuskokwim
Valley. Temperatures are warmer, but still well below climatological
norms over Southcentral, with mainly teens to single digits below
0F observed for interior spots throughout today. Over the
northern Gulf, a mesoscale surface low continues to meander in
place just to the west of Middleton Island. The low coupled with
marine layer instability also is continuing to set off a myriad of
convective snow showers visible on the Middleton Island radar.
Most of the shower activity has pulled away from areas east of
Prince William Sound (including Cordova) for now, with sporadic
snow showers still making it onshore to the west near Portage and
Whittier. Cold drainage/gap winds in this vicinity have continued
to blow around the powdery snow and reduce visibilities. Aside
from a stratus deck and associated snow flurries in the Y-K Delta,
clear and cold conditions persist for the remainder of the
southern mainland.
In the west, sensible conditions are overall much the same for
the eastern Bering/Aleutians, with cold air generally below
freezing spreading out anticyclonically from surface high
pressure. A few snow showers are still reaching the southern
Alaska Peninsula now and then, though they have become less
frequent and intense compared to the past couple days. Farther
west, a roughly 970 mb North Pacific low south of Shemya is
sending a gale force front into the western Bering/Aleutians.
A much warmer air mass is slowly advancing with the frontal
passage, with snow becoming mostly rain for the western Aleutians
as temperatures warm into the upper 30s.
&&
.MODEL DISCUSSION...
Models are, to put it lightly, not in the greatest agreement for
the most significant weather-maker to impact the outlook area over
the next several days, particularly beyond 24 to 36 hours from
initialization. The synoptic setup that models are struggling to
key in on is rather similar to what was occurring almost exactly a
week ago, with a low transiting areas just south of the Aleutians
and then up into the Gulf, running right up against a deeply
entrenched Arctic air mass over the AK mainland and Bering. One
important difference for this setup compared to one week ago is
that the scale of the low in question is much larger, with the
wind fields diameter spanning some 2,000 miles from west to east
across the Bering and North Pacific by tomorrow afternoon.
Even with the difference between the previous and upcoming
scenario mentioned above, the most important interaction models
are struggling with is still similar this time around. It remains
unclear how much the dense, Arctic air in place will resist being
displaced by the strong low pressure attempting to make northward
progress as it moves from south of the Aleutians on Sunday to the
Gulf on Monday and Tuesday. The American (GFS/NAM) models are
favoring a scenario where the cold air in place gives way more
easily, allowing the low to track farther northwest and close to
Kodiak Island and the Kenai Peninsula from Monday to Tuesday. The
Canadian and particularly the ECMWF models are essentially showing
the cold air win out for longer, and take the center farther east
across the Gulf before eventually recurving it as it weakens back
towards the Kenai Peninsula and Prince William Sound on Tuesday
afternoon. The most important difference for these diverging
scenarios will be the degree of impactful weather for areas
including the Alaska Peninsula, Kodiak Island, Prince William
Sound and Cook Inlet areas, with a more east track favoring lower
impacts to all of the above areas as well as a more eastward
shift for the stronger winds and blowing/falling snow possible
from Monday to Tuesday. This package will represent a middle
ground between a more west (mostly higher impacts) and east track,
with a lean closer to the ECMWF/Canadian. In addition, the GFS
has mostly been thrown out due to its apparent struggle with
its wild and inconsistent depiction of rapidly spinning up
multiple triple point lows.
&&
.AVIATION...
PANC...VFR conditions and light winds will persist. A low end VFR
ceiling could develop as a weak short-wave rotates through Sunday
afternoon, but confidence is low.
&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 to 2: Tonight
through Monday)...
A mesolow near Montague Island will continue to bring snow
showers to the Prince William Sound region and north Gulf coast
through Monday. As the mesolow moves eastward across the Gulf and
high pressure persists over land, the resulting pressure gradient
will drive stronger outflow winds through Kamishak Bay,
Resurrection Bay, and the Copper River Delta into Monday evening.
Upper level forcing will favor the strongest gap winds across the
western Gulf coastal waters, a with a series of short-waves
tracking from Southwest Alaska into the Gulf and driving the
deep Arctic air in place over Southwest through the gaps. These
winds will cross southern Cook Inlet into the outer portion of
Kachemak Bay. Thus, expect a sharp gradient of wind and waves as
you head up Kachemak Bay.
Beginning Sunday evening, a complex low south of the AKPEN
moves towards the Gulf. Much uncertainty remains with how the
complex low will evolve, where it will move, and where triple
point lows may form. For now, the low looks to track just south
of Kodiak by Monday afternoon. Its associated front will bring
gale-force winds throughout the Gulf and potential winter storm
conditions to Kodiak. Significant snowfall is possible on Kodiak
Island from late Sunday night to Monday morning before warmer air
likely causes a transition to rain. Uncertainty regarding the
timing of the transition could result in lesser or greater snow
amounts. There will also be quite a bit of wind, so blowing snow
will likely be an issue, especially early Sunday night before
temperatures begin to warm. While the forecast track of the low
close to Kodiak will result in stronger winds and blowing
snow/visibility concerns, the low could also remain further east.
In that scenario, precipitation may remain all snow and winds will
be lighter. The forecast for Kodiak will continue to be refined
with subsequent forecast packages. Beyond Kodiak, areas of
moderate to heavy snow will occur along the eastern Kenai
Peninsula and in Prince William Sound on Monday as the low and
its front move northward. Gale-force winds are likely along the
north Gulf coast with the formation of a barrier jet.
&&
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Tonight
through Tuesday)...
A general troughing pattern extending from the Alaska mainland to
the eastern Bering Sea will remain in place for the weekend and
eject weak shortwaves with minimal impact expected. A persistent
northerly flow will help maintain a cold airmass with temperatures
still well below zero at least through Sunday night. A low south
of AKPEN will help draw moisture northwestward and light snow into
portions of the Y-K Delta, Bristol Bay, and the Lower Kuskokwim
Valley beginning Monday and continuing through Tuesday.
&&
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3:
Tonight through Tuesday)...
A broad frontal boundary extending from a strong low pressure
system south of Shemya will continue to dominate the forecast
area over the next couple of days. Meanwhile, the front will
work in concert with another low developing south of Nikolski on
Sunday morning. In addition to rain and snow mix, high end gales
to low end storm force winds and elevated seas will spread across
the Aleutians, Bering Sea, and AKPEN through Monday evening. A
Winter Storm Watch has been updated for the Eastern Aleutians for
blizzard conditions remaining possible starting early Sunday
morning into Monday morning. Additionally, the latest operational
model runs are still not in agreement yet with the exact track
and position of the complex low as it migrates south of the
Aleutians and into the Gulf. Suffice to say that an active weather
pattern will persist through early next week.
&&
.MARINE (Days 3 through 5: Tuesday through Thursday)...
Gulf of Alaska: A complex area of low pressure will move from the
western Gulf to the northern Gulf for Tuesday. There is a great
deal of uncertainty in the track of any feature that enters the
Gulf through mid-week. However, it is looking more likely that any
gales or elevated seas earlier in the week will be diminishing by
Tuesday. Increasing westerly gale-force winds along with seas 10
to 15 feet are possible from the Barren Islands south to Kodiak
Island Wednesday and Thursday.
Bering Sea/Southwest: Northerly gales and elevated seas will
persist across the eastern half of the Bering and Aleutians
storm-force gusts are also possible across the eastern Aleutians
through Wednesday morning. Heavy freezing spray is also likely
during this time. A storm-force low will move from the western
Bering Tuesday morning to the eastern Aleutians Wednesday evening.
Southerly gales develop along a front over the western Bering by
Thursday.
&&
.LONG TERM FORECAST (Days 4 through 7: Wednesday through Saturday)...
The longwave upper-level trough that has been the dominant weather
feature across the state of recent will re-establish itself by
mid-week, as a strong surface low and associated upper-level
shortwave move from the eastern Aleutians to the Gulf of Alaska.
The movement of these features, along with a developing upper-
level ridge upstream, will enhance the northerly flow, allowing
another round of arctic air to drop south. This arctic airmass
will spill into Southwest Alaska for Wednesday with snow showers
persisting across the AKPen and windward side of the Alaska Range.
The shortwave associated with this cold airmass will then track
east over Southcentral for Thursday. This feature will interact
with an area of weak low pressure in the northern Gulf, helping to
potentially draw enough moisture northward for areas of light
snow over parts of interior Southcentral.
Also by Thursday, guidance is indicating development of a strong
Kamchatka low that looks to lift into eastern Russia. There is
increasing confidence that this feature will drive a warm front
across the Bering as the core of the arctic airmass over the state
shifts east. This system may be the catalyst in changing the
overall upper-level pattern as it becomes more progressive with a
westerly steering flow. The front drives to the west coast of the
state by Friday as it begins to occlude. Nonetheless, there may
be some precipitation type concerns around the AKPen and Bristol
Bay as warmer air advects northward. Both the GFS and EC then show
the potential for a triple point low to form in the western Gulf
as the front moves to the interior of the state. This new low
would likely help draw moisture northward as the front weakens and
its associated shortwave moves east, bringing the potential for
light snow from the Kuskokwim Valley east to the Copper River
Basin through the day Saturday.
&&
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Watch 185.
MARINE...Storm Warning 155 170 172 173 174 175.
Gale Warning 130 131 132 138 139 150 160 165 171 176 177
178 179 180.
Heavy Freezing Spray Warning 121 127 128 129 130 131 136
137 138 139 140 141 150 160
165 179 180 181 185.
FIRE WEATHER...NONE.
&&
SYNOPSIS AND MODEL DISCUSSION...AS
SOUTHCENTRAL ALASKA...KC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MF
MARINE/LONG TERM...TM