National Weather Service United States Department of Commerce


722
FXAK68 PAFC 220124
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
524 PM AKDT Wed Oct 21 2020

.ANALYSIS AND UPPER LEVELS...
A large upper level ridge extends from the Gulf of Alaska and
into the northern Bering Sea. The upper level trough to the west
of the ridge has developed a cut-off low just south of Attu today
with a surface low underneath it. This low shows up well on both
visible and IR satellite with the front associated with the low
extending across the Aleutian Islands and out to the southeast of
the Alaska Peninsula. In this area near the Alaska Peninsula, a
wave is enhancing the gradient along the front and is bringing in
Storm force winds. On satellite, this feature looks like it could
turn into a triple point low at the surface, but models are
consistent in maintaining it as an open wave for the time being.

The strong outflow winds out of the Copper River Delta can be
seen on visible satellite transporting silt into the central Gulf.
This narrow area of stronger winds is also causing an eddy to
develop in the shear just east of the delta. This has caused a
meso-low to develop just south of Cape Suckling that is now strong
enough to produce some showers in an otherwise clear region.

&&

.MODEL DISCUSSION...

Models remain in good synoptic agreement through the weekend. One
of the main issues will be watching the 850 mb temperature field
on different models and from run-to-run to diagnose precipitation
type later this week over the southern mainland. Models are
differing some on low level temperatures, and precip type may
become the main forecast challenge going into the weekend.

&&

.AVIATION...
PANC...VFR conditions and northerly winds will persist.

&&

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

Clear and dry conditions will continue across mainland
Southcentral from Friday night to Saturday morning as a ridge of
high pressure extends from southwest Alaska to the Yukon
Territory. Gusty northerly gap winds along the Southcentral coast
have diminished over the eastern Kenai Peninsula, but persist from
Prince William Sound to the Copper River Delta. These winds are
likely peaking, however, and will begin to trend downward in
intensity tonight. A northeasterly gap wind will also persist over
the Matanuska Valley through Friday, aided in part by the
tightening pressure gradient between the aforementioned ridge and
an area of low pressure advancing toward the southern Bering.

The low entering the Bering will push a front over the western
Gulf tonight. The front will then lift slowly north and east,
stalling over the northern Kenai Peninsula by Saturday. This
system will bring gales to the western Gulf along with rain to
Kodiak Island through late Friday. Gales will push over the Barren
Islands Thursday and linger through early Saturday. Winds will
also increase through Turnagain Arm, but these winds will bend
down inlet, keeping Anchorage and the Mat-Su Valleys in a colder,
northerly flow.

Moisture is expected to overrun the colder air at the surface as
the front lifts north. This will bring a chance for light snow
(less than half an inch) from Kenai north to Anchorage and the
Mat Valley late Friday evening into early Saturday. The challenge
will be how much snow reaches the ground, as it will remain quite
dry in the lowest levels of the atmosphere. There is a
possibility that any precipitation may stay light enough and fall
through a deep enough column of dry air to evaporate before
reaching the ground. However, as the front nears the Anchorage
area, the southeasterly cross-barrier flow will increase and
likely limit the chance for any additional precipitation along the
lee of the Kenai and Chugach Mountains Saturday afternoon into
Sunday.

&&

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

The main weather producer will be the frontal system moving into
Southwest Alaska from the Aleutians and Bering Sea starting
Thursday afternoon. This will bring increased moisture, rainfall
and elevated winds to the region for the next couple days. Warm
temperatures aloft will reduce the chance of snow near lower
elevations. Nevertheless, there`s still a good chance of snow
over higher elevations should temperatures decline aloft in the
coming days. Thus, anticipate a seasonably cold, wet pattern
especially over the coastal areas through the forecast period.

&&

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

Storm force winds over the central Bering and over much of the
eastern Aleutians will continue through Thursday, then diminish
to gales by Thursday night into Friday. Meanwhile, strong gales
over central Aleutians westward will persist through at least
Thursday evening. Two separate storm lows south of the Aleutians
will continue to influence local conditions tonight. Models
indicated the merging of these two lows by Thursday afternoon,
then keep the merged low moving north-northeastward toward the
central Bering starting Thursday night into Saturday morning.
Additionally, the storm lows transitioning into one parent low
will generate hazardous waves and winds along with increased
rainfall over much of the Bering and Aleutians. Overall, the wet
pattern and cooler temperatures will stick around for the next few
days.

&&

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

Gulf of Alaska:

There is high confidence that a slow moving front will exist over
the northern and central Gulf through Sunday. Winds should peak
near the coastline on Sunday as the front turns into a barrier
jet. After this time forecast confidence decreases considerably
with the arrival of the next system from the north Pacific late
Sunday afternoon. At this time the majority of model guidance
indicates broad southwesterly flow from the southern Gulf into
Southeast Alaska on Monday.

Bering Sea:

There is high confidence that the low pressure system present in
the central Bering will continue to weaken as it moves onshore
over western Alaska on Monday. Winds will be the strongest on
Saturday with a small area of potential gales west of St. Matthew
due to cold air. There is moderate confidence that a weak front
will persist over the western Aleutians Sunday and Monday.
Forecast confidence on Monday is fair with most model guidance
indicating calm conditions.

&&

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

There is a fair amount of uncertainty in the long term forecast,
particularly Sunday and Monday. Synoptically on Sunday a high
amplitude trough and weakening low pressure system will exist over
the Bering while a ridge builds in over the north Pacific. The
main forecast question for the Sunday/Monday time period will be
the extent that the warm air and potentially a low pressure system
will move into Southcentral as it competes with the cool air
moving southeast out of the Bering. This could lead to some
precipitation type issues for Southcentral with warm air
potentially overrunning cold air at the surface. The GFS solution
is the most aggressive of the deterministic model solutions in
bringing a large warm air mass and strong low pressure system over
the AKPEN and western Alaska on Monday. The GFS remains the
outlier with this solution and is moves that low even farther to
the west than solutions indicated yesterday. Other deterministic
and ensemble guidance say that the bulk of the warm air and
potential weak low will be shunted off to the east upon arriving
in the Gulf. Warm air and isentropic lift could still lead to some
precipitation type issues as well for Monday.

Synoptically models return to better agreement on Tuesday as a
trough of cold air moves southeast and encompasses the entire
mainland. Temperatures at 850 mb should drop between -10C and -15C
so precipitation should be snow. The trough will also return cool
temperatures to the AOR through Wednesday.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning: 150-170, 179, 180, 412, 414.
Gale Warning: 130-138, 171-175, 177, 178, 181, 185, 352, 411, 413.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
MARINE/LONG TERM...CJ



643
FXAK69 PAFG 220044
AFDAFG

Northern Alaska Forecast Discussion
National Weather Service Fairbanks AK
444 PM AKDT Wed Oct 21 2020

.SYNOPSIS...
Low stratus clouds and accompanying fog will persist over much of
the Interior tonight, with freezing drizzle in the Fairbanks area
continuing into this evening. This is causing a light ice glaze
and slippery conditions in the Fairbanks area today. This will
keep temperatures near or warmer than normal most of the Interior
tonight.

A warm front over Northwest Alaska will move east across the North
Slope tonight bringing a mix of rain and snow to NW Alaska from
Utqiagvik west, and 1-2 inches of snow to the North Slope east of
Utqiagvik tonight and Thu. There will be west winds of 20-30 kt
along and following this front over the North Slope tonight and
Thu.

A strong low moving into the southern Being Sea tonight and Thu
will cause east winds to increase across the YK Delta Thu night
and to the West Coast and the drainage areas of the Interior on
Thu.

A second and stronger low will move into the Southern Bering Sea
Thu night and Fri. This will bring rain north over the West Coast
and across the YK Delta Thu night, and to the West Coast and SW
Interior south of the Bering Strait Fri, and to the remainder of
the West Coast and Western Interior Fri night and Sat.
This feature will also cause east winds to increase to25-35kt
across most of the Weest Coast by Fri, and cause the Tanana Valley
jet and other easterly drainage winds to blow 20-30 kt Thu night
into Fri. Gap winds will develop through Alaska Range Passes thu
night and Fri as well.



Aloft...
The long wave pattern consists of a trough over Western Canada, a ridge
over Mainland Alaska, and another trough over the Bering Sea and
North Central Pacific. The pattern will slowly progress with the
Bering Sea trough moving east over the West Coast on Fri and Sat,
then continuing east over Mainland Alaska by Mon.

The ridge over Mainland Alaska has allowed stratus and fog to
form over much of the Interior, which will continue through
tonight, and then diminish from the south on Thu and Fri as
easterly surface winds develop and mix out the fog. This stratus
will keep low temperatures warmer than normal in many areas, but
clear areas of the Interior will see cold temps tonight and Thu
night.

As the long wave trough moves east over Western Alaska Fri and Sat
this will cause weak downslope conditions over the SE Interior.

Several short wave trough will move east over the top of this long
wave ridge and east across the North Slope through Sat. Weather
associated with these will be addressed below in the surface
section.

850 MB temperatures range from -4C along the West Coast and
Western North Slope to -12C along the ALCAN Border. Temperatures
warm 2-4C over most areas through Thu, then warm another 1-2C Fri,
and remain warm into Sat. See weak to moderate southerly chinook
winds across the Alaska Range Fri and Sat.

Surface...
A 1040 MB high over the Interior will persist into Thu, then
retreat slowly east. Low stratus clouds and accompanying fog
will persist under this high over much of the Interior tonight,
with freezing drizzle in the Fairbanks area continuing into this
evening. This will keep temperatures near or warmer than normal
most of the Interior tonight. Stratus and fog will dissipate over
the southern Interior Thu as winds develop, and over the Northern
Interior Fri and Sat as winds develop there.

A 1009 mb low 600 NM northwest of Utqiagvik will move to 400 NM
north of Demarcation Point as a 1010 mb low by 4pm Thu, then
continue moving east. A warm front from this low to Utqiagvik to
Kotzebue will move east across the North Slope tonight bringing a
mix of rain and snow to NW Alaska from Utqiagvik west, and 1-2
inches of snow to the North Slope east of Utqiagvik tonight and
Thu. There will be west winds of 20-30 kt along and following this
front over the North Slope tonight and Thu. There will also be fog
with visibility less than 1NM. Flurries and fog will linger after
the front passes Thu night and winds will decrease.

A low will move into the Southern Being Sea tonight and Thu. A
stronger low will move into the Southern Bering Sea Thu night and
Fri.

A strong low moving into the southern Being Sea tonight and Thu
will cause east winds to increase across the YK Delta Thu night
and to the West Coast and the drainage areas of the Interior on
Thu.

A second and stronger low will move into the Southern Bering Sea
Thu night and Fri. This will bring rain north over the West Coast
and across the YK Delta Thu night, and to the West Coast and SW
Interior south of the Bering Strait Fri, and to the remainder of
the West Coast and Western Interior Fri night and Sat.
This feature will also cause east winds to increase to25-35kt
across most of the Weest Coast by Fri, and cause the Tanana Valley
jet and other easterly drainage winds to blow 20-30 kt Thu night
into Fri. Gap winds will develop through Alaska Range Passes thu
night and Fri as well.

&&

.DISCUSSION...
At 12Z, models initialize well aloft. Models show similar
solutions aloft through 4pm Sat. Next week, there is increasing
divergence in models solutions regarding the strength and speed
of movement the short waves and even the long wave trough moving
over Alaska next week. Models decrease moisture levels under the
building ridge tonight and Thu, but since the low stratus under
this ridge has been quite persistent, will keep clouds over the
Interior until east winds mix it out.

Precip is similar in models through 4AM Fri. They area all a bit
slow moving precip into Deadhorse today, so will add precip
likely over the North Slope from Utqiagvik east through tonight.
After 4am Fri, the ECMF is a bit fast moving precip north over the
West Coast and Western Interior Fri into Sat. Given the strength
of the storm and southerly flow associated with it, favor the
faster ECMF for Fri through Sat.

At the surface at 15Z, models all initialize well most features,
except that they are all 3-4 mb too weak on the low NW of Wrangel
Island. Models show similar solutions through 4pm Sat. Will use a
blend of models for surface features and winds, but boost winds a
few kt from the Arctic Coast north tonight and Thu to acknowledge
the deeper than forecast low moving east north of the Coast.

Bottom line for models is that we will use a blend of the
GFS/NAM/ECMF for temps, clouds, winds through Sat, but boost
winds a few kt Arctic Coast North tonight and Thu, and keep
stratus in place over the Interior tonight until east winds
increase and mix it out on Thu. Will use a blend of models for
precip through 4am Fri, then use mainly the faster ECMF for Fri
into Sat.

Coastal Hazard Potential Days 3 and 4...None.

&&

.AFG WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory for AKZ222.

Small Craft Advisory for PKZ200-PKZ210-PKZ225-PKZ230-PKZ235-
PKZ240.

Gale Warning for PKZ210.

Brisk Wind Advisory for PKZ245.
&&

$$

JB OCT 20



000
FXAK67 PAJK 212240
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
240 PM AKDT Wed Oct 21 2020

.SHORT TERM.../Through Fri night/ Outflow is the main rule of the
day today. Arctic front has ventured as far south as Sumner Strait
based on the lower dew point temperatures that are being observed
across the northern 2/3rds of the panhandle. Skies have also
cleared out over most of the panhandle except for a few places in
the southern half. Outflow winds have not been that strong so
far. Strongest winds have been in northern Lynn Canal and out of
Taku Inlet where 30 kt winds have been blowing all day. There has
also been a minor mountain wave ongoing in downtown Juneau and
Douglas today (gusts to 30 to 40 mph).

Into Thu and Fri, outflow winds are expected to continue with 25
to 30 kt winds blowing down Lynn Canal, Cross Sound, and out of
Taku Inlet. Likewise gusty winds will also be likely around
downtown Juneau and Douglas over the next few days as well.
Highest winds will likely be on Thu as cross barrier flow reaches
a max of around 30 to 40 kt. Wind gusts will likely reach 40 to
45 mph after the low level inversion breaks in the morning. Main
forecast changes were mainly local effect driven for wind speeds
in various areas.

With the outflow, conditions around the panhandle will stay mostly
clear and dry. Though with the clearer skies and cold air outflow,
temperatures will be noticeably cooler especially at night.
Expect highs in the 30s and lows in the 20s, if not colder in wind
sheltered areas. Little in the way of changes for this part of
the forecast.

.LONG TERM.../Fri through Wed as of 10 pm Mon/ We begin the long
term with fairly quiet and cool conditions across the panhandle.
Ridging over the region will be slowly breaking down to allow for
a low pressure system to advance on the region from the west. This
ridging should provide sunny skies for everyone on Friday, and
also looks to carry on into Saturday for most locations within the
inner channels. The aforementioned low brings the first chances of
precip to the Gulf coastal locations late Saturday night and will
push chances for precip into the remainder of the panhandle
through the day Sunday. Sunday night, many locations within the
northern inner channels will see the potential for temperatures to
drop low enough to see a change to snow, or at least a mix,
however with rising 850 temperatures arriving with the warm front,
any snow at sea level right now looks to be more conversational
than anything else. At the higher elevations of the Haines Hwy and
White Pass, an inch or two Sunday night looks like a good bet.
After the warm front passes over on Monday, rain chances will
continue to be elevated through the end of the forecast period as
the cold front encroaches on the region as the low begins to take
a trajectory to the northwest. After cold frontal passage, precip
chances begin to diminish, however still exist due to onshore
flow. Look for slightly cooler temperatures Wednesday.

In regards to winds, at the onset of the forecast period, the
pressure gradient still looks strong enough to support SCA type
winds through Cross Sound and Lynn Canal. These winds should ease
up over the course of the day Friday. Things get a bit quiet until
the low moves in. Sunday into Sunday night, the low deepens enough
that winds will be picking up out in the Gulf. Solutions between
models differ as to how strong these winds will become, with some
solutions pointing toward gales. Opted to raise the winds to
higher SCA criteria and will evaluate future guidance to determine
if winds will in fact need to be raised more.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Strong Wind until 4 PM AKDT this afternoon for AKZ025.
Strong Wind from Thursday morning through Thursday afternoon for
AKZ025.
MARINE...Small Craft Advisory for PKZ012-013-022-031-041-043.

&&

$$

EAL/JDR

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