Graphical Hydro Meteorological Discussion

000
FXAK68 PAFC 181257
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
457 AM AKDT Mon Oct 18 2021

.ANALYSIS AND UPPER LEVELS...

A broad mid to upper trough wrapped to a diffused upper low near
Kodiak brings steady light to moderate rain over Kodiak Island and
the northern Gulf, including Western Prince William Sound. A
barrier jet meandering over the northern Gulf has generated strong
easterly winds with high end gales near Middleton Island and
mainly south of Cordova and Valdez overnight. Meanwhile, a vast
upper ridge runs across the Bering to Southwest Alaska then turns
east through Southcentral to beyond Wrangell Mountains. Multiple
shortwaves over the Alaska Peninsula and across the Gulf enhances
the potential for increased rain, clouds and gusty winds in those
areas this morning.

&&

.MODEL DISCUSSION...

Models handle the synoptic systems over the Bering Sea much
better than the revolving and emerging lows over the Gulf and
Southcentral days ahead. Two separate lows over the Bering will
bring active weather across the Aleutians and Southwest Coast, and
models are really straight forward until Thursday afternoon when
the solutions begin to diverge. ECMWF takes the Northwest Pacific
low slightly closer to the north of the eastern Aleutians while
GFS positions its low right over Pribilof Islands which is about
150 nm difference in position. At the Gulf, developing lows from
the North Pacific are absorbed toward a robust frontal system in
the eastern Gulf leading up to a 270 nm variation in the placement
of the lows among NAM, GFS and ECMWF by Wednesday night. Clearly,
lows associated with the front could be a bit difficult to follow
as some could either stay on track or detour from the northern
Gulf in the coming days. For this reason, the forecast confidence
is low beyond Wednesday.

&&

.AVIATION...

PANC...VFR conditions will persist, with gentle northerly winds
continuing through today and Tue morning.

&&

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

The main weather feature in the area today is the very strong
front in the Gulf that is wrapped around an elongated low with a
center near Kodiak and another about 300 miles due east of the
first low. There will be multiple lows that develop and roll along
the front the next few days as the front will remain rather
stationary from the eastern Gulf to near Prince William Sound.

Kodiak remains under the strong easterly winds and persistent
rainfall. Winds should start to slowly diminish this afternoon,
but it will remain windy through tonight around the Kodiak area.
This pattern is also pushing rain toward the north Gulf coast, but
the way the front is bent around the lows, it is not pushing
inland. Therefore the rain will be a bit muted even along the
north Gulf coast today until one of those lows along the front
that was mentioned earlier is able to push inland a little bit.
This looks to be most likely to happen Tuesday night as an upper
level wave moves northward through the Gulf and into Southcentral.
This is moving in response to the main upper level trough digging
southeastward across the eastern Bering Sea at that time.

The biggest uncertainty is where this low will move. Models are
basically moving it across the Kenai Peninsula from southeast to
northwest. However, whether it track over Seward and across the
northern part of the Peninsula or crosses just the southern tip of
the Peninsula will have a bit of impacts as to where there will be
the most precipitation and how the wind field behaves. At this
point, going with the more northerly solution, but this will need
to be watched closely until it happens, or at least tips it`s
hand as to which route it will take.

&&

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

A transient ridge moves over the Southwest Monday, bringing
clearer skies to start the workweek. Temperatures this morning
will start in the 20s before climbing into the 30s throughout the
day.

Unsettled weather returns as a low moves across the Bering
Sea, pushing its front into Southwest Alaska tonight. Snow will
fall initially, then daytime heating and continued warm air
advection will transition the precipitation into a rain/snow mix
or rain through Tuesday afternoon. Further south, at locations
like Dillingham and King Salmon, warm air aloft will keep
precipitation as rain even as temperatures Tuesday morning hover
near freezing. As temperatures decrease into the evening,
precipitation may transition back into a mix or snow before
slowly tapering off. Overall, precipitation is expected to be
light with minimal snow accumulations.

&&

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

A ridge continues to move across the Aleutian Chain, bringing
light winds and drier conditions. Over the Eastern Aleutians and
AKPEN, lingering rain and gusty gap winds will diminish through
Monday morning as the ridge moves overhead.

Behind the ridge, wind and precipitation chances increase as
fronts track across the Aleutians from a low in the northern
Bering Sea. A warm front moves across the Western Aleutians
beginning Monday morning, bringing gale force winds and light
rain. Rain remains likely with this front as it tracks across the
Central and Eastern Aleutians, though wind speeds diminish as the
front weakens. A cold front follows on the heels of the warm
front, bringing a chance for showers across much of the Aleutians
from Monday evening through Tuesday evening. Snow or rain/snow
showers are possible over the Pribilofs on Tuesday with the cold
front. However, rain showers are expected for the remainder of the
Aleutians as heat from the warmer ocean waters modifies the air
mass.

&&

.MARINE (Days 3 through 5/Wednesday through Friday)...

Bering/Aleutians

Forecast confidence remains above average on Wednesday as guidance
shows similar solutions with the placement and intensity of
synoptic scale features. However, forecast confidence
significantly decreases on Thursday as the next low pressure
system enters the western Bering/Aleutians as guidance shows
significant differences with respect to the placement and
intensity of this low. One common theme is gale force winds are
likely in the southern Bering Sea and along the Aleutian Chain
Thursday and Friday.

Gulf of Alaska

Forecast confidence remains above average for Wednesday as
guidance shows similar solutions with respect to the placement and
intensity of synoptic scale features. Gale force winds across the
northern Gulf will ease on Wednesday with the weakening of a
barrier jet. Confidence is high that winds will remain below gale
force on Thursday as a transient ridge of high pressure moves
across the Gulf. Confidence decreases during the forecast period
on Friday as the next low moves into the vicinity.

&&

.LONG TERM FORECAST (Days 4 through 7/Thursday through Sunday)...

A cloudy, cool and wet pattern is expected for Southcentral
during the extended period. A positively tilted upper trough will
extend from the northern Bering Sea through the central Gulf of
Alaska. This pattern will allow for moisture to advect from the
Gulf into Southcentral as the trough-axis will remain southwest of
the Kenai Peninsula and thus result in southeasterly onshore
flow. Lower confidence exists with respect to precipitation type.
Snow chances increase as elevation increases.

Southwest will also remain cloudy and cool during the extended
period. Showers and intermittent precipitation is likely, though
steady precipitation is less likely due to the southeasterly flow
downsloping on the Alaska Range.

The Bering/Aleutians will remain in a cloudy and cool pattern as
broad upper troughing will continue to extend across the area. We
are monitoring the potential for a large low pressure system
moving from the North Pacific into the Bering Sea on
Thursday/Friday. Forecast confidence remains low with respect to
the placement and intensity of the low center. However, there are
more similarities between the GFS/GEFS and the ECMWF/EPS with
respect to the placement and intensity of this low, meaning for
now a GFS/ECMWF ensemble blend is the preferred solution.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning: 119, 120, 130-132, 136, 138, 150, 155,
173-178, 185, 351, 411-413.
FIRE WEATHER...NONE.

&&

$$

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

000
FXAK69 PAFG 181304
AFDAFG

Northern Alaska Forecast Discussion
National Weather Service Fairbanks AK
504 AM AKDT Mon Oct 18 2021

.SYNOPSIS...Strong low moving to St Lawrence Island will be
primary influence for most of the forecast area through Wednesday.
Weather front will bring snow to St Lawrence Island today, and
the Lower Yukon Delta tonight. Snow will spread north to the
Bering Strait on Tuesday. Warming temperatures will bring a mix of
rain and snow to areas south of Nome Tuesday. Strong pressure
gradients developing in the Interior and Arctic will bring strong
gusty winds locally. Winds gusting around 60 mph on St Lawrence
Island this evening as the front moves to the coast. Winds will
diminish as the front gets to the mainland. Strong gradient
across the Alaska range will produce strong gusty gap winds, and
the Tanana Valley Jet will set up as high pressure pushes west
into the Upper Tanana Valley. Winds will be gusting to around 60
near the Alaska Range Passes, and around 50 mph in the Delta
Junction area Tuesday. Strong east winds developing in the Arctic
will bring blowing and drifting snow that may impact visibility
and travel in the area. Temperatures will be warming slowly the
next few days.

&&

.DISCUSSION...

Models...The 18/00Z solutions are in good agreement in the near
term, but are struggling with the gap winds and Tanana Valley Jet
winds. The NAM is an outlier with precipitation and timing with
the next system and a wave moving into the Interior Tuesday.
Solution winds are a bit stronger aloft with the south flow across
the Alaska Range so will increase the gap winds, and decrease the
Tanana Valley Jet winds as they battle it out for the same space
around Delta Junction. Surface solutions initialized well against
the 18/06Z local analysis. Will only be making some minor
adjustment to the current forecast database to address the issues
noted above.

Aloft...At 500 hpa...Weak ridging across the southern half of
mainland Alaska as a 516 dam low has moved to 300 nm south of
Seward, while low over the Arctic has merged with a 505 dam low
400 nm west of Wrangel Island with troughing extending east over
74N. A 500 dam low has moved over the Northwest Bering Sea. Sunday
afternoon the ridging will be hanging on over the Gulf of Alaska
Coast as the low continue to move southeast to 600 nm south of
Prince William Sound. The low in the Siberian Arctic will be quasi
stationary with a shortwave moving over the Northwest Arctic
Coast to lie over Wainwright to Anaktuvuk Pass as it moves through
the flow. The low over the Bering Sea will move to 400 nm west of
St Matthew Island with a trough lying from the low to just west
of St Matthew Island then southwest over the Central Arctic. A
ridge axis will enhance and lie from Galena over the Western
Chukchi Sea and Northern Chukotka. Tuesday morning ridging will
push west over the Southeast Interior and come into phase with the
ridging over the Northwest Mainland with the axis extending west
over Wrangel Island. The low in the Siberian Arctic will move
north and the trough axis will slide north of 77N. The shortwave
moving through the flow will move over the Beaufort Sea. The low
over the Bering Sea will move to 100 nm west of St Matthew Island
with the shortwave moving to lie from the low over St Lawrence
Island to Nunivak Island to Cold Bay. The low south of Prince
William Sound will start filling and move north over the Gulf of
Alaska at 525 dam, then merge with the low over the Bering Sea by
Tuesday afternoon. A shortwave associated with the filling low
will move over the Alaska Range Tuesday evening and over the
Interior by Wednesday morning. At 850 hpa...-10C isotherm lies
from Old Crow YT to Huslia to Gambell, and will move north to lie
over the Brooks Range by Tuesday morning. A dome of warm air
around 0C will move over the Central and Eastern Interior south of
the Yukon River by Tuesday morning. The -10C isotherm will move
north of the Arctic Coast Tuesday evening as warmer air near 0C is
pushed north to the Eastern Arctic Coast.

Surface...A 977 mb low is just southwest of Kodiak City this
morning with a trough extending north to the Alaska Range, then
northeast. A front associated with the low is moving east across
the Northeast Pacific. Leeside troughing north of the Alaska Range
is weakening. A 1006 mb low is in MacKenzie Bay, with troughing
extending northwest to a 1006 mb low 400 nm northwest of
Utqiagvik. A cold front extending from the low extends southeast
to Prudhoe Bay, then southwest over Bettles. A 1016 mb high has
moved over the Western Brooks Range with high pressure extending
west over the Chukchi Sea, and southwest over the Bering Strait
and the Pribilofs to a high in the Central Pacific. A strong 970
mb low has moved to the Northwest Bering Sea with a front
extending east to the Dateline, then south to the remnants of
Tropical Depression (TD) Namtheun 600 nm south of Attu. Monday
afternoon the weak low pressure will extend from the southern
Beaufort Sea northwest to a 1002 mb low in the Siberian Arctic, a
1014 mb high will be near Utqiagvik, weak troughing will extend
east between the ranges, the low in the Bering Sea will move to
400 nm west of St Matthew Island at 975 mb. The front will move to
lie from the low to St Matthew Island to Adak, while the remnants
of TD Namtheun will be 500 nm south of Adak. Broad area of low
pressure will hold over the Northeast Pacific. High pressure will
be pushing in from the east into the Upper Tanana Valley. Tuesday
morning low pressure will be across much of the state as the low
in the Bering Sea moves to 150 nm northwest of St Matthew Island
with the front moving to lie over St Lawrence Island to Nunivak
Island to Cold Bay then south to the remnants of TD Namtheun 400
nm south of Nikolski. High pressure on the Arctic Coast will slide
over the offshore waters. As the low pressure presses to the east
and high pressure pushes into the Upper Tanana Valley the
pressure gradient will tighten and the Tanana Valley Jet will kick
in with strong gusty winds developing. The gradient will also
tighten across the Alaska Range with strong gusty gap winds
developing near the passes. Leeside troughing developing north of
the Brooks Range Tuesday will combing with high pressure over the
Arctic and pushing west over the south slopes of the Brooks Range
to produce strong gusty winds across the area.

Arctic Coast and Brooks Range...Generally cloudy, but a few breaks
are possible today and tonight as the weather front pulls out to
the northeast. Light snow or flurries mainly east of Utqiagvik
today will taper off. Patchy fog. Along the coast highs will be
in the 20s, with lows in the teens. Inland and in the Brooks Range
highs in the lower teens and lows in the single digits above. Low
pressure pushing into the area from the west will combine with
high pressure over the Arctic and high pressure pushing west over
the south slopes of the Eastern Brooks Range to produce strong
gusty winds across the area Tuesday with areas of blowing snow.

West Coast and Western Interior...High pressure in the Chukchi Sea
extends south through the Bering Strait and across the Pribilof
Island to the North Pacific will slide over mainland Alaska this
morning as the strong low in the Bering Sea moves east. Weather
front will bring snow to St Lawrence Island this afternoon, and to
the Lower Yukon Delta tonight. Snow will spread up the coast to
the Bering Strait Tuesday as snow mixes with rain in the Lower
Yukon Delta Tuesday afternoon. Snow will move into the Middle
Yukon Valley Tuesday night. Snowfall amounts will generally be
less than 4 inches. Winds on St Lawrence Island increasing this
morning to southeast at 20 to 35 mph gusting 45 mph. Winds on St
Lawrence Island gusting to 55 mph this evening. Winds variable to
10 mph this morning along the coast and inland. Strong winds will
spread east tonight south of the Bering Strait, but diminish as
they approach the coast, but still be gusting near 40 mph on the
Bering Strait coast and in Norton Sound. Today highs in the mid
20s to mid 30s, with lows in the teens to mid 20s. Tuesday warming
temperatures will push highs from the Seward Peninsula south
pushing into the upper 30s and lower 40s, while lows will be in
the upper 20s to mid 30s.

Central and Eastern Interior...Quiet across the Interior with high
clouds slowly moving out of the area as weak high pressure moves
in. Winds kicking up with the Tanana Valley Jet setting up Tuesday
so look for winds gusting near 50 mph around Delta Junction. Gap
winds also setting up for Tuesday with the strongest winds kicking
in from the Parks Highway west, spreading east Tuesday night. Gap
winds will be gusting near 60 mph Tuesday. Lows 10 to 20 above
with highs in the mid 20s to mid 30s. Warmer temperatures through
the week.

Coastal Hazard Potential Days 3 and 4...Elevated surf on the
outer coastlines through the week with wave runup 1 to 2 feet
above the normal high tide line.

Extended forecast for days 4 to 7...A low near St Lawrence Island
will weakens with broad low pressure over most of the mainland
and the Arctic nearshore waters. A strong low will move into the
southwestern Bering Sea Wednesday night and to the Pribilofs
Thursday evening while high pressure builds over the Eastern
Interior. Precipitation generally limited to the Lower Yukon
Delta and St Lawrence Island. Temperatures will be slightly above
climatological normals.

&&

.AFG WATCHES/WARNINGS/ADVISORIES...

Heavy Freezing Spray Warning for PKZ215-PKZ220-PKZ225-PKZ230-
PKZ235-PKZ240-PKZ245-PKZ500.

Small Craft Advisory for PKZ200-PKZ215-PKZ220-PKZ225-PKZ230.

Gale Warning for PKZ210.

&&

$$

SDB OCT 21

000
FXAK67 PAJK 181402
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
602 AM AKDT Mon Oct 18 2021

.SHORT TERM...After an absolutely pleasant weekend for the bulk of
the citizens of SEAK, we will be returning to the cloudy
conditions that are more typical. A ridge of high pressure is
currently in place over the region and is blocking the eastward
advance of a very deep low pressure system in the Gulf. While some
cloud cover will be able to advect overhead, precip will be
limited to coastal areas Monday, and even then PoPs are limited to
the 20s until Monday night. More significant to Monday will be
that the winds will be picking up along coastal areas, mainly
Baranof and POW Islands, as a result of a very strong pressure
gradient around the low. Expected wind gusts in locations such as
Sitka and Hydaburg could be in excess of 40 mph.

Monday night into Tuesday, the ridge holds strong so the low
begins to move a bit north. This should allow for chances for
precip to extend a bit further into the inner channels but really
should not impact areas further east such as Juneau and Ketchikan.
Up to the point of the northward turn, rainfall totals will have
been rather unimpressive, however this movement will direct the
moisture plume directly at Yakutat. While not a great amount by
southeast Alaska standards, rainfall totals around an inch appear
to be plausible from early Tuesday morning through Tuesday night.
The main push of the front eastward really won`t be until after
the conclusion of this forecast period, however could potentially
see the beginnings of it happening late Tuesday night as another
low begins moving into the Gulf from the South. The benefit of
this ridge stalling the front out is that it will provide time for
the parent low to weaken, reducing the speed of the winds before
it can push further inland.


Because of the increasing cloud cover, temperatures should be
moderated and there should be a slight warming trend. This will
be particularly noticeable at night as below freezing temperatures
are not anticipated at sea level locations as has been routine
the past couple of evenings.


.LONG TERM.../Wednesday through Sunday/...A fresh round of energy
from the southern Bering Sea splashing east across the Aleutian
Chain into the western Gulf of Alaska should provide sufficient
momentum to push rainfall from the stalled eastern gulf front into
the Panhandle Wednesday. With a strong front stuck in the eastern
gulf for days, the southeasterlies ahead of the system will have
long ago driven out any signs of an early season polar air mass as
850 temperatures largely rise above 0 C. Thus, this precipitation
will fall as rain across Panhandle sea level communities. An
extended breezy period should persist into most of Wednesday, but
as the front pushes in, strength falls markedly, and thus by
Wednesday night winds ease briefly.

With a complex upper low positioned in the Gulf for the
foreseeable future and periodic intrusions of storm inducing
colder air from the Bering and moisture-amping typhoon-fed north
Pacific storms, the pattern becomes ripe for the possibility of a
series of storms riding northward into the southeast gulf. How
strong winds will be, how many storms will arrive, where and when
exactly will they impact are all subject to debate. While this
pattern possesses impactful wind potential, models sadly and
notoriously struggle with specifics for various reasons. Ensembles
tend to agree of the threat of wind from Baranof to the southern
Inner Channels including Prince of Wales, especially the ECMWF
Extreme Forecast Index during this time period, mainly Thursday
through the weekend. We elected to alert partners/public days
ahead with a Special Weather Statement for wind impacting
transportation and causing power outages. Currently we barely
mention gales for the southeast gulf, but this can be ramped up
later as confidence increases. The operational GFS pushes 50 plus
knots of winds at various points late this week. We also
acknowledged the outside chance that one such system (suggested
by the NAM) could develop on Wednesday. But the most likely time
will be Thursday into Sunday. However, the period could last into
early next week.

While not the main focus, the pattern also tends to favor the
highest probability and amounts of rain along the coast and in the
south with much lighter amounts and lower chances in the north.
With the focus of lower pressure deep in the central and
southeastern gulf, winds will largely stay northerly. However,
given warmer temperatures in western Canada, we do not expect
typically strong outflow in response to the storms from the south.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Strong Wind from this afternoon through late tonight for AKZ027.
Strong Wind from 1 PM AKDT this afternoon through late tonight
for AKZ023.
MARINE...Gale Warning for PKZ022-041>043-051-052-310.
Small Craft Advisory for PKZ021-033-035-036-053.

&&

$$

JDR/JWA

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