Graphical Hydro Meteorological Discussion

381
FXAK68 PAFC 050037
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
437 PM AKDT Sat Apr 4 2020

.ANALYSIS AND UPPER LEVELS...

There are two defining weather features for our forecast area
today. This first is a high pressure system in the North Pacific
whose ridge is pushing north into the eastern AKPEN. The other
feature is a low centered just southwest of St. Lawrence Island
and moving slowly northeast. The ridge of high pressure is just
strong enough to force most of the energy north of our forecast
area, but southwest AK is not escaping this system. In the upper
levels, all of this is supported by a longwave trough digging
south through AK and pushing high pressure south into the North
Pacific. A band of energy is trapped between these two features
and is moving through southwest AK. This setup is also creating
on-shore flow into southwest Alaska and enhanced wind speeds for
both southwest AK and the eastern Aleutians.

The ridge of high pressure is also keeping southcentral AK out of
most of the weather. However as with SW AK, it is not pushing
north far enough to keep weather entirely out. Therefore, cloud
cover and small pockets of instability producing snow showers have
been able to exist in Southcentral AK today.

&&

.MODEL DISCUSSION...

Models continue their run of good agreement in the short term.
This is keeping forecast confidence high. One area of difficulty
is in southwest Alaska, where they are having trouble picking up
on precipitation coming off the Bering Sea. The forecast is being
manually adjusted to account. Otherwise, models are performing well.

&&

.AVIATION...
PANC...Some low level cloud cover will linger into the overnight
hours, but VFR conditions are expected to persist. Winds will be
picking up from the south overnight and be gusting 25-30 kts early
tomorrow morning. Winds should subside midday tomorrow.

&&

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

The upper-level long wave pattern will begin to transition tonight
as the ridge situated over the Gulf moves east and gives way to
an upper-level low tracking from the northern Bering to interior
Alaska. As it does, a moist, southwesterly flow will develop out
ahead of an upper-level wave and associated cold front that will
enter the western periphery of Southcentral Sunday. Gusty
southwesterly winds will develop overnight through early Sunday
from Cook Inlet north to the Susitna Valley in advance of the
front. In addition to gusty winds, areas of snow will develop
along the front, with accumulating snow likely for areas north of
Talkeetna (Broad Pass), Hatcher Pass, and the higher elevations
around Prince William Sound. Enough warmer air will still be
around on Sunday for precipitation to be a mix of rain/snow for
lower elevations and areas along the coast. The front will
continue east through Sunday with gusty west-southwesterly winds
persisting across Cook Inlet, Kachemak Bay, and the Barren
Islands. Strong westerly winds will also develop across Prince
William Sound with winds gusting through favored bays and passes
and rain changing back to snow as the front pushes through.
Although winds are expected to diminish on Monday as the initial
trough exits the region, a second, trailing shortwave should
provide enough instability for a few lingering snow showers over
the higher terrain.

Conditions trend clearer, drier, and cooler Monday afternoon
through Tuesday as the upper-level trough axis slides east of the
area and a weak ridge attempts to build over the region. The next
frontal system then moves into the Gulf late Tuesday with the
potential for precipitation and strong winds for Kodiak Island and
the immediate Southcentral coast through Wednesday.

&&

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

Over the next 12 hours, there will be a significant pattern shift
across Southwest Alaska. The relatively warm conditions will
rapidly end as a powerful cold front sweeps across the area from
west to east. Ahead of the front, temperatures are just above
freezing in most locations across the Kuskokwim Delta. Conversely,
highs got well into the 50s today south of King Salmon. Once the
front moves through, temperatures will stay in the 20s across the
Kuskokwim Delta with low to mid 30s expected further east.

Another effect behind the front will be a significant increase in
the winds along with snow bands setting up in the very unstable
atmosphere. This will begin tonight, and persist through the day
on Sunday. Winds gusting as high as 65 mph are expected late
tonight into Sunday morning. This is the result of a strong and
powerful low moving into Norton Sound, with the wraparound behind
the front accelerating as the low deepens. The combination of the
strong winds and the snow bands that develop will lead to rapidly
deteriorating conditions in portions of the Kuskokwim Delta,
particularly north of Kipnuk. The snow bands will be localized.
Thus, outside of them, conditions will remain very windy, with any
previously fallen snow being blown around, but otherwise expect
mostly cloudy conditions. Inside the bands, expect near whiteout
conditions at times. Since the bands will be moving around, do not
expect long periods of whiteout conditions, but persistent bands
could cause visibilities to drop to a half mile or less. Thus, a
Winter Weather Advisory has been issued from late tonight until
Sunday evening. The worst conditions are expected at the coast,
but the bands will be able to move well inland, so areas such as
Bethel will also see periods of reduced visibilities in strong,
gusty winds through the day Sunday.

The winds will begin diminishing Sunday night as a small area of
low pressure moves near the Pribilofs. There is still significant
uncertainty as to the eventual track of the low, but its
relatively close proximity to the coast should keep most of the
area mainly cloudy Monday and potentially into Tuesday, depending
on how long it persists. Otherwise, expect well-below-normal
temperatures to persist through the day Tuesday, with highs in the
20s and 30s and lows in the single digits and teens.

&&

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

A strong cold front near the Pribilofs this afternoon will rapidly
move into Southwest Alaska this evening. Behind the front strong
northwesterly flow of much colder air is producing widespread snow
showers, which will persist over nearly the entire Bering Sea
through Sunday. A low moving off the Kamchatka Peninsula moves
into the western Bering Sunday afternoon, and strengthens as it
moves over the Pribilofs Monday afternoon. There is significant
uncertainty where it moves from there, but generally a slow
eastward drift towards Southwest Alaska is expected. A large area
of high pressure moves over the western Aleutians Monday, then
gradually shifts east across the central Aleutians on Tuesday. A
weak low moves over the Northern Bering Tuesday afternoon as
a separate front moves into the western Aleutians on Tuesday.

&&

.MARINE (Days 3 through 5/Tuesday through Thursday)...

Bering Sea/Aleutians:

High pressure centered southeast of Adak Tuesday afternoon and low
pressure to the northwest of Savoonga will create a tight
pressure gradient across the northern Bering, where some sustained
gales are possible. Further south and west, a front will approach
the western Aleutians ahead of an incoming area of low pressure.
The center of high pressure then moves to Dutch Harbor on
Wednesday and into western Alaska for Thursday. This allows the
front to trek eastward into the central Aleutians and Bering, with
sustained gales possibly developing.

Gulf of Alaska:

A strengthening area of low pressure approaches the Gulf Tuesday evening,
with a sustained gale force front preceding its arrival. As this
low deepens further across the northern Gulf Wednesday morning,
high-end sustained gales, with storm force gusts, are anticipated
to develop. This low then moves ashore between Cordova and Yakutat
Wednesday evening, with sustained gales continuing near the
northern Gulf Coast. An offshore flow pattern develops on Thursday.

&&

.LONG TERM FORECAST (Days 3 through 7/Wednesday through Saturday)...

Guidance is in fairly good agreement leading to high confidence on
the long term upper-level pattern in the middle to later parts of
next week. There will be an upper level trough over the mainland
and a modest upper level ridge over the Bering. This will lead to
cold temperatures over the mainland of Alaska with warmer
temperatures over the Bering and Aleutian Chain. The upper level
ridge over the Bering will slowly work towards the mainland
Thursday leading to a warming trend. Beyond Thursday night is when
the models begin to diverge on the upper level pattern and the
placement of features. While the trend is there on both models for
a warming trend, the degree to how strong this warming trend will
be is still uncertain. This is leading to only low to moderate
confidence in the forecast for temperatures Friday into Saturday
of next week. The placement of features in the upper level flow is
in question as one model has the upper level ridge further west
and more amplified while another is less amplified.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 155.
MARINE...Gale 119 120 130 131 139 141 179 180 181 185
Heavy Freezing Spray 180 185
FIRE WEATHER...NONE.

&&
$$

SYNOPSIS AND MODEL DISCUSSION...BB
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
MARINE/LONG TERM...PD/MV

000
FXAK69 PAFG 042320
AFDAFG

Northern Alaska Forecast Discussion
National Weather Service Fairbanks AK
320 PM AKDT Sat Apr 4 2020

.SYNOPSIS...
The active weather pattern over Northern continues through early
next week, as a series of weather systems move across the area.
Across the West Coast and Western Interior, heavy snowfall,
especially in the favored upslope areas, is expected, with rain or
freezing rain mixing in with the snow from Kotzebue south. Strong
and gusty winds, are expected to develop on the back side of the
low tonight and into Sunday causing blowing snow. Across the
Central and Eastern Interior, widespread snowfall is continues
with rain expected to mix in with the snow across the Southern
Interior. Winds increase across the Interior by Sunday, causing
blowing snow over summits. A coastal surge will impact the West
Coast with the possibility of water moving overtop the ice and
then inland during the weekend.

&&

.DISCUSSION...
Models...
The 12Z model suite initialized well against the 12Z surface
analysis and verified well against the 18z surface analysis.
Models continue to struggle with the evolution, track and strength
of a strong low in the north Bering Sea and subsequent triple
point that develops over the Seward Peninsula and mover over the
Central Interior and Brooks Range. Significant differences emerge
in the model solutions on the strength and track of the triple
point low as early as tomorrow afternoon. The GFS remains the
outlier as it moves the upper low across northern Alaska,
maintaining more energy. We continued to lean towards the NAM for
winds, with local edits to increase winds across the Y-K Delta and
across the Interior. Blended in the SREF for pops to capture
higher values.

Aloft at 500 mb, a high pressure ridge over Central and Eastern
Alaska will merge with a 555 dam high pressure ridge centered just
east of Banks Island as a 496 dam low centered near the pole with
a deep longwave trough extending south through the Bering Strait
and Central Bering Sea will continue to deepen and push slowly
east into the Eastern Bering and over the West Coast of Alsaka
through early next week. A strong shortwave troughs will move
north and east through the longwave and will push a 504 dam low
east over the Brooks Range with a trailing strong shortwave south
of the low pushing over the West Coast and Western Interior on
Sunday and over the Central and Eastern Interior on Tuesday. Much
colder air aloft will push eastward over Alaska with the 850 mb
temperature over Fairbanks dropping from around 4 below this
evening to 7 below Sunday morning and near 20 below by Monday
morning. Strong and gusty winds at the surface will accompany the
cold air aloft and will produce blowing and drifting snow over
much of the West Coast and Interior especially the highway Summits
in the Interior.

Surface...A 985 mb low currently 250 nm southwest of the Bering
Strait will continue to push north and east to be over the Bering
Strait Sunday morning and along the Northwest Coast and Western
North Slope on Sunday morning. The low will deepen again as it
taps some cold air as it pushes just south of Point Barrow
tomorrow evening. As the low continues to push east expect strong
winds and falling snow to produce reduced visibilities in blowing
snow to the East of Prudhoe Bay. A Winter Weather Advisory for
blowing snow has been issued for zone 204 beginning at midnight
tonight. A triple point low develops over the Western Seward
Peninsula this evening and will and will push east into the
Eastern Seward Peninsula Sunday and the Western Interior Monday
Morning. The low will continue to weaken into a trough over the
Central Brooks Range Monday afternoon and will continue to move
north east over the the Eastern Brooks Range Monday night.

North Slope and Brooks Range: A series of weather systems moving
across the region will bring more snow through the weekend. The
heaviest snowfall will occur on the southern facing slopes of the
Brooks Range. From Utqiagvik west, 3 to 5 inches of snow are
possible through weekend. East of Utqiagvik snowfall totals will
generally be less than an inch. Gusty south winds continue in the
passes of the eastern Brooks Range tonight, with gusts to 40 mph
possible. Maintained the Winter Weather Advisory for zone 206 for
blowing snow in the passes of the eastern Brooks Range and added a
Winter Weather Advisory for zone 204 where strong westerly winds
gusting to 55 mph wrapping around a deepening low north of the
Eastern Arctic Coast will produce reduced visibiltiy in blowing
snow east of Prudhoe Bay.

West Coast and Western Interior: The latest in a series of weather
fronts is moving north and east across the West Coast and Western
Interior this afternoon, bringing a mix of rain and snow to the
region. A strong low will move over St Lawrence Island this
evening to Kotzebue Sound by Sunday and then weaken Sunday night.
This system will produce heavy snowfall, especially in the favored
upslope areas, strong winds, and blowing snow. Rain or freezing
rain may mix in with snow at times this evening, primarily from
Kotzebue south, before precip transitions to all snow by late
tonight. Strong winds will develop tonight and Sunday on the back
side of the low, with gusts to near 60 mph possible across the Y-K
Delta and Norton Sound. This will cause areas of blowing snow
likely reducing visibility to less than half a mile at times.

Central and Eastern Interior: A weather front moving in from the
west continues to bring widespread snowfall to the Interior
this afternoon and will continue into Monday. The heaviest
snowfall is expected to occur from Tanana north. The Middle Tanana
Valley could see an additional 1 to 3 inches of snow tonight and
an additional 1 to 2 inches on Sunday, with the highest amounts in
the hills north of Fairbanks. Rain may mix with snow at times
across the southern Interior and could result in lower snowfall
totals. Southwest winds increase late tonight and will continue
into Monday, with gusts to 30 mph. This will result in blowing and
drifting snow, especially across the summits of the Interior. In
the passes of the Alaska Range, southerly gap winds continue to
increase this evening, peaking overnight with gusts to 50 mph in
the western Alaska Range passes and 45 mph in the eastern Alaska
Range passes. The winds are expected to diminish Sunday morning an
are expected to remain below wind advisory criteria.

Coastal Hazard Potential...The ongoing strong weather system that
is currently affecting the west coast will continue through this
weekend. This storm has the potential to cause elevated water
levels along the coast. As water levels rise there is the
potential to push water on top of the ice through open leads and
along the coast where shorefast ice is breaking up. Areas around
Norton Sound and along the Yukon Delta will be most susceptible as
the low moves inland.

&&

.FIRE WEATHER...None.


&&

.AFG WATCHES/WARNINGS/ADVISORIES...
Coastal Flood Advisory for AKZ211-AKZ212.

Winter Storm Warning for AKZ207-AKZ208-AKZ209-AKZ210-AKZ211-
AKZ213-AKZ217-AKZ218-AKZ219.

Winter Weather Advisory for AKZ204-AKZ206-AKZ212-AKZ214-AKZ215-
AKZ216-AKZ220-AKZ221-AKZ222.

Gale Warning for PKZ200-PKZ210-PKZ220-PKZ225.

Brisk Wind Advisory for PKZ215-PKZ230-PKZ235-PKZ240.
&&

$$

CCC APR 20

200
FXAK67 PAJK 050051 CCA
AFDAJK

Southeast Alaska Forecast Discussion...CORRECTED
National Weather Service Juneau AK
451 PM AKDT Sat Apr 4 2020

.SHORT TERM.../Saturday evening through Monday/ The upper ridge
over the gulf responsible for the dry weather Saturday will shift
eastward on Sunday. Tranquil weather is expected to continue for
most of the day tomorrow; however a pattern change begins to take
shape Sunday evening. As the ridge flattens through the day
tomorrow, a strong 100+ kt jet streak will approach the Panhandle
from the west. At the surface, a low will develop over the far
northern gulf and switch flow around from the west. Strong PVA
aloft and onshore flow at all levels of the atmosphere will cause
showers to develop Sunday evening across the northern Panhandle
and persist through the short term period. Shower activity will
spread southward Sunday night however the bulk of the showers are
expected to be along the outer coast and across the northern
portion of the region.

Aloft, much colder and drier air will be advected into the region
Monday. Temperatures at 850mb will fall throughout the day from
around -5C in the morning to -11C in the afternoon. Looking at
NAM model soundings over the northeast gulf and eastern gulf
coast, steep 700-850mb lapse rates and elevated CAPE up to 300
J/kg are present. Added a slight chance for thunder from around
Dry bay southward to Cross Sound to around Port Alexander late
Monday morning through the afternoon hours.

Cold air advection pushing into the Panhandle should change most
of the precipitation over to snow from north to south Monday
morning. Due to the convective nature of the precipitation,
stronger showers may be able to produce brief periods of heavy
snow. Surface temperatures are expected to be in the mid 30s
across the north to the upper 30s to low 40s across the south,
therefore little to no accumulations are expected. With convective
showers in a colder environment, precipitation is able to stay
snow even with surface temperatures well above freezing as rates
are usually heavier and snow melt cooling aloft cools much of the
column down to near freezing.

Winds across the Gulf and inner channels increase Sunday night
with the development of the low in the far northern gulf and the
increased pressure gradient. SCA and gale force winds should
develop across the outside waters, especially in the north on
Monday. SCA southerly and westerly winds are expected to develop
in the inner channels during this time frame.


.LONG TERM.../Monday to Saturday as of 10 pm Friday/...A ridge
over the Gulf of Alaska will slide E out of the panhandle. More
zonal flow behind it will enable impulses to move across the
Northern Gulf, interacting with the panhandle and bringing a
renewed chance of precipitation next week.

In regards to surface features, the first system to impact the
panhandle will be a shortwave trough which will interact with the
area on early Monday morning. Though a lack of substantial
moisture support should prevent substantial snow amounts it still
looks possible that Juneau could see measurable snowfall on the
ground before joining the rest of the panhandle in a predominantly
rain shower regime which will be driven by cold air advection.
Following the passage of this system, westerly flow will help
continue to encourage the propagation of these showers throughout
the panhandle, and there is a chance of follow-up snow showers
Monday night in areas where the cloud deck proves insufficient for
keeping temperatures above freezing.

The long range model solutions and ensembles have continued to
indicate the development of a low in the North Pacific which will
enter the Gulf of Alaska and impact the panhandle sometime
between Tuesday and Wednesday. There still remains considerable
disagreement on what the exact track of the system will be, though
in a reversal from yesterday, the most recent model runs have
trended further to the east, bringing the low closer to Yakutat.
The exact track and timing of the system will need to be monitored
closely, as depending on the strength and timing of the warm
sector, overrunning could lead to an initial period of
accumulating snow in parts of the northern and central panhandle,
especially in Yakutat. As it stands currently, outside of Yakutat,
it looks like the majority of the panhandle will see mainly rain
out of this event, barring a possible period of snow for the
northern panhandle and Juneau initially. Ensembles have indicated
that ample moisture support will be present, but operational
models continue to run the gamut on how much QPF the system will
ultimately bring to SE Alaska, and so opted to choose a middle
ground for the time being.

After the system pulls out on Thursday, ensembles lean towards
drier weather ahead for the remainder of the forecast period.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...None.

&&

$$

CM/GFS

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