Graphical Hydro Meteorological Discussion

000
FXAK68 PAFC 200031
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
431 PM AKDT Sat Sep 19 2020

.ANALYSIS AND UPPER LEVELS...

A shortwave ridge axis extends from the north Pacific, through
southwest Alaska, up through the center part of the state. This is
in between a trough that extends from eastern Siberia through the
central Aleutians, and an area of low pressure across the
southern Gulf. Area radar imagery shows showers with this low
moving northward through the Gulf Coast, with other showers trying
to move into coastal locations of southwest Alaska as the
aforementioned ridge breaks down.

&&

.MODEL DISCUSSION...

The models are generally in good agreement at the surface and
aloft over the next few days, leading to higher than average
forecast confidence.

&&

.AVIATION...

PANC...VFR conditions along with southerly winds occasionally
gusting into the 20 to 25 knot range will persist through this
evening. MVFR ceilings are possible after 8z due to an increase in
shower activity around the terminal. There is also some indication
in the forecast guidance that IFR ceilings may develop over the
terminal after 14z. Confidence in this is low, however, and is
currently not included in the current TAF package.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Sat
afternoon through Tue afternoon)...

An upper level trough to our west will move in this evening,
bringing increasing chances for rain across Southcentral. The
greatest amount of precipitation will be along the northern Gulf
coast and the eastern Kenai Peninsula. There are some showers
lifting northward this afternoon over Prince William Sound and
across the Susitna Valley, but as this trough moves in, rain is
expected to become more widespread. Sunrise on Sunday morning may
reveal sights of termination dust across the higher elevations of
the Chugach and Talkeetna Mountains, as cooler temperatures aloft
allow for overnight precipitation to fall as snow.

As the trough exits to the north Sunday afternoon, conditions
should generally improve over the Susitna Valley, western Kenai
Peninsula, and Anchorage Bowl, though a stray mountain shower
cannot be ruled out. Additional rainfall is likely for the Prince
William Sound area as well as across the Copper River Basin
through Sunday night.

By Monday, the attention shifts westward, where a large low
developing over the Bering moves toward the southeast and lifts a
front across Kodiak Island early Monday. Abundant moisture and
gusty easterly winds are likely, with gale force winds expected
along the extent of this front across the Gulf waters. While the
low center remains well south of Southcentral over the Gulf, the
associated front will bring gusty winds and rain to the northern
Gulf coast as it lifts northward Tuesday morning. Once again, the
bulk of precipitation is expected to fall across Kodiak Island and
the northern Gulf coast, while interior areas of Southcentral
including the Susitna Valley and northern Copper River Basin
should remain relatively dry through Tuesday.

&&

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

Generally quiet conditions are expected through the weekend. Weak
shortwave troughs are currently producing isolated showers across
portions of the Kuskokwim Delta and Bristol Bay. These will
gradually taper off overnight under a weak upper level ridge.
Combined with weak southerly flow, patchy fog and/or low stratus
is possible near the Kuskokwim Delta coast. A similar pattern is
expected for Sunday with weak shortwaves propagating over
Southwest Alaska bringing isolated showers. A front associated
with a low over the eastern Aleutians will bring gusty easterly
winds to much of the region by Monday. As such, conditions will
remain dry in the lee of the Alaska Range and temperatures will
drop slightly, with lows well into the 30s over interior areas.

&&

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

A broad low will move southeast across the Bering through the
weekend. Its associated front will bring steady rain and gusty
southerly winds across the Aleutians, while strong northwesterly
winds and showers will develop on its backside. Cold air
advection is starting to become more robust as the fall season
approaches. The low will become vertically stacked near Unalaska
on Sunday evening, thereby weakening. A cool, northwesterly flow
will develop is its wake ahead of a developing ridge over the
western Bering. Thus, showers will continue through early next
week.

&&

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

Gulf of Alaska: A broad and strong storm system is expected to
remain nearly stationary and weaken over the southern Gulf through
Thursday. There is high confidence in Gale force winds over the
northern Gulf coast with potential for storm force gusts if a
secondary low is to develop in the lee of Kodiak Island on Tue. By
late Wednesday night the low is expected to weaken, which will
bring winds down to at or below small craft criteria.

Bering Sea/Aleutian Islands: There is high confidence that the
broad low pressure system over the Gulf will bring moderate to
strong northerly winds over the eastern Bering. The winds will be
strongest (gale force) through the terrain gaps from Nikolski to
Sand Point through Thursday morning. By this time winds should
calm down to well below gale criteria as the low to the east
breaks down. Over the western Bering, there is high confidence
that high pressure and calm winds are expected to persist through
Wed night. On Thursday confidence decreases significantly in
regards to the next system, with models showing a 20 mb spread.
If the stronger solutions comes to fruition, gale force winds are
possible moving east across the Aleutian Chain Thursday night into
Friday.

&&

.LONG TERM FORECAST (Days 4 through 7/Wednesday through
Saturday)...

A broad and strong low pressure system is expected to be centered
over the Gulf at the start of the forecast period. The main impact
from this system will be for the Gulf coast and Kodiak Island,
where strong winds and rain are expected to persist through
Wednesday afternoon. For Southcentral it should be mostly dry,
with seasonal temperatures and the occasional shower as weak
shortwaves move around the low center. On the backside of the
low, strong winds and increasingly cool, northerly wind are
expected to persist over the eastern Bering and coastal areas of
Southwest through Thursday night. By Friday morning, an arctic
trough is expected to move in from the Bering Strait, which will
cool off the Bering considerably through Saturday.

The main forecast question is in regards to the next system
forecast to move over/near the central Aleutians Thursday night.
Forecast confidence decreases considerably due to a large model
spread in both the location and strength of this system. The GFS
and Canadian models have a storm force low clipping the central
Aleutians on Friday as it rapidly moves east into the Gulf by
Saturday morning. The primary difference between these solutions
and the EC, which has a considerably weaker system, is whether or
not jet remains zonal or becomes slightly meridional. If the jet
is to become meridional, then the storm will have a better chance
of tapping into the strong temperature gradient N/S of the
Aleutian Chain to produce a stronger storm. Regardless, model
agreement and forecast confidence increases once again Saturday, with
a strong system moving into the Gulf.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 160 172-175 177 179 411-414.
Storm Warning 176 178.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...PD
SOUTHCENTRAL ALASKA...KO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AP
MARINE/LONG TERM...CJ

800
FXAK69 PAFG 192112
AFDAFG

Northern Alaska Forecast Discussion
National Weather Service Fairbanks AK
112 PM AKDT Sat Sep 19 2020

.SYNOPSIS...
A broad troughing pattern will continue to support showery, cool
conditions across much of northern Alaska through the weekend.
Snow levels will gradually drop into early next week, producing
good chances for accumulating snow in the Brooks Range and Alaska
Range, which could impact travel. Otherwise, weather looks to be
seasonably normal with temperatures near average for most areas.

&&

.DISCUSSION...

Aloft 500 mb...
Longwave troughing extends across the Bering Sea and eastern
Siberia, all the way to western Canada. The primary trough axis
extends southwest from the high Canadian archipelago, across the
Beaufort Sea, and into the western Bering, with closed lows
located just north of Kaktovik and over the western Bering.
Ridging over the Siberian Arctic will strengthen and build
eastward over the next few days, forcing the trough axis
southeastward. By Monday, the low in the western Bering Sea will
have crossed the Aleutians into the north Pacific near Unalaska
with ridging building into northwestern Alaska. By Tuesday, the
trough axis will be oriented north to south from the northern
Beaufort Sea to the Gulf of Alaska with high pressure pinching in
from the eastern Bering Sea and southeastern Interior. The result
will be an elongated deformation zone running northeast to
southwest across the state which will support showery conditions.
However, pleasant weather can be expected west of this boundary.

Models...
No major model discrepancies through 48 hours. There is some
disagreement late Monday in the positioning of the aforementioned
deformation zone. This forecast is tricky, as there will be a
fairly sharp boundary between clear skies to the west and cloudy
skies along the boundary. The GFS and NAM indicate rainfall
occurring along the boundary, but differ in positioning, with the
NAM taking the boundary just south of Fairbanks, and the GFS to
the northwest. The Canadian shows less precipitation, but nearly
mirrors the GFS in positioning the boundary. The ECMWF has lower
resolution, and precipitation rendered farther to the southeast.
At this point, we will blend the NAM and GFS solutions through day
3, keeping chances for rain in the Fairbanks area.

North Slope...
A 990 mb surface low situated roughly 150 nm north of Kaktovik is
the dominant feature with a front stretching southwest to an
adjacent 998 mb low over Kotzebue sound. The latter system will
gradually weaken into Sunday, becoming absorbed into a 980 mb low
in the southeastern Bering Sea on Sunday. Along the front, a mix
of rain and snow showers is expected to continue as the front
drifts gradually southward. Northerly onshore flow with high
pressure building over the Chukchi Sea will support continued
stratus and areas of fog along the coast, and snow for the Brooks
Range. Snow amounts in the Brooks Range will vary by elevation,
with accumulating snow generally expected above 3000 feet today,
dropping to 2000 feet Sunday. Up to two inches of slushy snow is
possible across higher elevations of the Dalton Highway through
Sunday evening. A second occluded front will move onshore along
the Chukchi coast Monday, bringing another round of mixed rain and
snow. Colder air will follow in tow and will push southward into
the mid week as temperatures drop below freezing across the
region.

West Coast and Western Interior...
Overcast skies and showers are expected to persist through Sunday
as low pressure over Kotzebue Sound gradually weakens. Westerly
flow south of the Bering Strait will support upslope rainfall
along the immediate coast with snow levels generally around 3000
feet. Southwesterly flow in the Western Interior will stagnate
Sunday, they reverse from the north Monday as high pressure
reaches east from Siberia. Gradual clearing is expected into
Tuesday for much of the western Interior.

Central and Eastern Interior...
Light south to southwesterly flow across the Interior will support
generally cloudy conditions and a few showers this weekend,
except areas north of the Alaska Range east of Tok. Snow levels
will continue to gradually fall through Monday, facilitating
modest snow accumulation of up to 4 inches in the Alaska Range
generally above 3000 feet, including portions of Denali Park. A
low will move into the northern Gulf of Alaska late Sunday then
track northeast into Canada. An associated weak easterly wave will
supply cloud cover and moisture to the eastern Interior. Ridging
will begin to build from the southeast Monday, with a lingering
front remaining sprawled out across the Interior from Circle to
Fairbanks to McGrath. Showers and cloud cover will remain along
this stationary front, with mainly clear skies to the north and
west.

Coastal Hazard Potential Days 3 and 4...
Elevated surf through the Bering Strait and St. Lawrence Island
Sunday night through Monday with NNE winds 25 to 35 mph.

&&

.HYDROLOGY...
The Kobuk and Koyukuk Rivers are currently trending upward,
supported by continued rainfall across the southern Brooks Range.
At this time, no major impacts are expected as rainfall gradually
diminishes through Monday.

&&

.AFG WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory for PKZ210-PKZ220-PKZ225-PKZ245.
&&

$$

COWMAN SEP 20

000
FXAK67 PAJK 192248
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
248 PM AKDT Sat Sep 19 2020

.SHORT TERM...Saturday has been a break day between systems with
several areas getting some sunny breaks and drying. This makes
fog potential for late tonight/Sunday morning lower, but some
patchy fog remains in the forecast for the southern panhandle.
SWLY flow aloft will cause isolated/scattered showers to move
onshore through Saturday night, primarily for the northern outer
coast.

Low pressure moving into the central gulf from the west can be
seen on satellite imagery with a comma shape frontal band wrapping
around it and numerous showers near the low center. CAPE values
are high enough that some showers near the low and on the back
side of the frontal band could produce lightning, so a slight
chance of thunder remains in the forecast. The main frontal band
is expected to track across the outside waters early Sunday
morning then into the outer coast in the afternoon. The southern
outer coast, south of Sitka, will see the most intense showers out
of this front and this area has the greatest potential for
thunderstorms in the evening. Outside of the southern panhandle,
scattered showers will remain possible, but the northern inner
channels were kept on the low end due to weaker forcing away front
the low.

The low weakens to a trough Sunday night into Monday as it tracks
inland. This timing will be when the Juneau area has the greatest
likelihood of showers. A ridge builds over the inner channels
behind the trough on Monday morning, causing southerly winds.
Remnant showers linger through the day, then are overtaken by a
stronger low tracking into the gulf from the west Monday night.

.LONG TERM.../Tuesday through Saturday night/...As of Friday
evening. The long term forecast remains largely on track. In the
upper levels, a broad area of troughing will dominate the Gulf
from Tuesday onwards. Closer to the surface, a large corresponding
low will move into the Gulf, sending a strong autumn front moving
towards the panhandle. Models still remain in disagreement on the
exact timing of the front. For the time being, am leaning towards
the faster ECMWF solution, but will need to monitor model trends
for possible shortwaves which could slow down the system`s
progression. Gale Force winds associated with the system still
look likely to arrive in the Gulf by Monday night, moving into the
outer coastal waters through Tuesday. Have increased winds over
the coastal areas and much of the southern panhandle, and may
still need to increase them in the future as the system becomes
better defined. Winds over some of the inner channels still
anticipated to reach 25 kt with this system, though winds should
start to relax through Wednesday and Thursday. Along with the
winds will come rain with the southern and central panhandle
currently looking likely to receive the bulk of the precipitation
from this event.

Beyond this main event, forecast confidence degrades as it becomes
difficult to determine just how long the main low will remain in
the Gulf. The general trend is to keep most of the panhandle
under generally rainy conditions through the rest of the week.
Temperature grids were adjusted to better reflect the increased
cloud cover over the panhandle. Overall changes to the grids were
primarily made using the ECMWF and Canadian before switching over
to WPC for extreme long range. Forecaster confidence is average.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Small Craft Advisory for PKZ041>043-051.

&&

$$

Ferrin/GFS

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