Small Craft Advisory 739, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
507 AM AKDT Mon Sep 19 2022

.ANALYSIS AND UPPER LEVELS...

The longwave pattern is an amplified one that features a ridge
axis over Kamchatka with a deep trough situated over the central
Bering Sea and extending into the North Pacific. Within the
northern portion of this trough, over the southern Chukchi Sea,
reside the remnants of the powerful Bering Sea storm that brought
a wide range of impacts to western Alaska the last few days. This
low continues to fill and weaken as it slowly drifts northward.
Also within this trough, as seen on water vapor imagery, are a few
weak shortwaves tracking across the Aleutian Chain. Further to
the east, a ridge axis extends from the North Pacific to the ALCAN
border. The combination of the Bering trough and ridge to the
east is helping to pull a band of moist Pacific air northward.
CIMSS MIMIC satellite-based Total Precipitable Water (TPW) product
depicts a narrow band of TPW valued at around 1.5" streaming out
of the North Pacific to the southwest Gulf of Alaska.

Looking closer to the surface, current observations reveal low
clouds over Shemya and Dutch Harbor stemming from the series of
weak shortwaves traversing over the Aleutian Chain. Meanwhile,
the Bethel radar shows areas of showers moving over Kuskokwim Bay,
Kuskokwim Delta, as well as around Bethel itself. King Salmon and
Middleton Island radars both show areas of rain moving over the
Alaska Peninsula, Kenai Peninsula, Prince William Sound, and up
Cook Inlet in association with moisture from the atmospheric
river. This atmospheric river looks to impact the sensible weather
over the next few days, especially for southcentral Alaska.

&&

.MODEL DISCUSSION...

The primary forecast challenge for the short term period will be
the progression of the atmospheric river, particularly in the
Tuesday/Wednesday window. The 00z NAM continues, to this point,
to be the most progressive of the models with moving a shortwave
eastward quicker with for the Tuesday/Wednesday period. The 00z
Canadian, is not as progressive as the NAM with eastward movement
of the shortwave. The 00z GFS/EURO continue to be consistent with
a slower movement of this feature to the east. These model
differences will have implications on who sees how much rain and
the exact timing of the precipitation. While confidence is
increasing that Prince William Sound communities (Valdez/Cordova
areas) and portions of the Copper River Basin (McCarthy and other
communities along the southern to southwestern facing slopes of
the Wrangell Mountains) will see the most precipitation of the
atmospheric river, details still remain uncertain as to the exact
progression of the system. To that point, a Flood Watch has been
issued for those areas.

&&

.AVIATION...

PANC...Rain continues this morning with MVFR ceilings and periods
of MVFR visibilities. Winds should continue to be lightly out of
the north until the surface low over southwest Alaska moves north
of the area this morning. After the low passes to the north, the
pressure gradient will be aligned to allow gusty southeast
Turnagain winds to bend into the terminal from early morning to
the evening. This pressure gradient alignment should also
discontinue the threat for low-level windshear as flow on the
Anchorage Hillside should also be from the southeast. However,
this will be actively monitored. From there, flow looks to shift
to a more south to south-southwest component as the Turnagain jet
relaxes during the evening. As precipitation ends late tonight,
there is uncertainty if ceilings will return to VFR conditions due
to south to southwesterly flow which tends to favor lower
ceilings.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3; Monday
through Wednesday night)...

A shortwave trough and associated surface low are moving from the
AKPen this morning northeast toward Southcentral. A warm front
attached to the low is lifting to the Southcentral coast with
widespread rainfall. As the front gets closer to the coast this
morning, the gap winds through Turnagain Arm and across the Knik
and Copper River Valleys will increase. Currently, the pressure
gradient is aligned in a manner that is bending the winds around
Anchorage and Palmer down Cook Inlet. However, as the surface low
lifts into interior Alaska later this morning, the winds will
bend back over these locations, with gusts approaching 30 to 40
mph, especially through Turnagain Arm, the Anchorage Hillside, and
along the Copper River. The advancing front will increase the
cross-barrier flow along the Chugach this morning, allowing the
heavier rain dissipate as weak downsloping takes place. The rain
fills back in as the low and upper-level wave lift north and the
resultant flow in the wake of these features turns southwesterly.

As this first wave moves into the Copper River Basin, another
upper-level wave south of the Aleutians and anchoring the longwave
trough will dig south amplifying the longwave pattern and lifting
yet another wave north from the Pacific into the Gulf. This wave
will spin up a surface low along the trailing cold front of the
first system for Tuesday. The cyclonic flow will bring the front
out ahead of the new low back across the Gulf from east to west as
a warm front, with an enhancement of precipitation across the
northern Gulf coast. The resultant decrease in pressure along the
north Gulf will also allow for the development of gusty northerly
winds out of the Valdez Narrows and Resurrection Bay, and westerly
winds out of Passage Canal Tuesday afternoon through Wednesday.

The low then deepens and tracks to Prince William Sound for
Wednesday as its trailing cold front begins to sweep toward the
Southcentral Coast. The resultant southerly fetch of moisture
from the North Pacific to the Gulf Coast will usher in an
atmospheric river that will prolong bouts of heavy rain from
Valdez and Cordova northward to the Copper River Basin. Flood
Watches remain in place for these locations through Wednesday
afternoon. Valdez could potentially see 3 to 5 inches of rain with
5 to 8 inches of rain for Cordova through the time period.
Additionally, significant rainfall is also possible across the
Wrangell Mountains, with 2 to 3 inches of rain possible from
Chitina to McCarthy. Similar amounts are also possible along
northern portions of the Copper River Basin, especially from
Paxson east along the Denali Highway. For locations east, there is
still some uncertainty as to the exact amount of precipitation
given the uncertainty in the exact track of the low. The GFS
remains the farthest west solution, bringing an area of heavier
rain from Kenai to Palmer for Wednesday. Still, 1 to 2 inches of
rain is possible for the Mat-Su Valleys (and Seward), with the
bulk of the rain falling today.

&&

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

An unsettled but benign pattern is expected to continue over the
next few days. Intermittent showers are expected for Monday and
Tuesday. A front will move through on Wednesday and bring rain to
the area.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

An unsettled pattern continues over the next few days. A front
will move across the Bering/Aleutians Tuesday through Thursday.
This front will bring rain and gale force southerly winds across
the area. Otherwise, no significant weather expected.

&&

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

Gulf of Alaska: A low in the Gulf will have the potential to
drive northwesterly gale-force winds through the Barren Islands on
Wednesday. Confidence in the strength of these winds is moderate,
though they diminish quickly by Wednesday night. Another low
tracking eastward from the Bering is likely to be accompanied by
southerly gale-force winds along its front beginning Thursday
morning. The front sweeps eastward quickly, reaching the eastern
Gulf by Thursday night. Seas remain less than 20 ft.

Bering Sea/Aleutians: South to southwesterly winds reach gale
force along a front across the eastern Bering/Aleutians on
Wednesday. This quickly moves eastward into the Gulf of Alaska by
Thursday morning, with high confidence. Behind the front,
westerly winds develop across much of the Bering for Thursday and
Friday. There will be some potential for localized gale force
winds to develop on the backside of this system. Seas may approach
20 ft in the central Bering Thursday night into Friday.

&&

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

Unsettled weather pattern is expected to continue for much of
Southern Alaska and the Bering Sea/Aleutian Islands heading into
the weekend. An upper-level longwave trough is expected to remain
in place through the long term period, with its axis situated over
the Bering Sea and Western Alaska. This will lead to a string of
shortwaves tracking through the region on roughly zonal flow
toward the end of the work week before the longwave trough
amplifies for Saturday and Sunday. For Thursday, a front sweeps
eastward across the Gulf, with a triple point low developing
near Kodiak Island and tracking into the northern Gulf. Steady
rainfall can be expected for the north Gulf coast, with more
intermittent rainfall over inland Southcentral. Expect a more
enhanced period of rain Thursday night into Friday as the front
lifts north and east.

A more showery regime present over Southwest Alaska takes over
across the southern mainland on westerly flow behind the front.
These westerly winds will also contribute to gusty gap flow out of
the Aleutian Range across the Alaska Peninsula and into the
western Gulf through Friday night. Embedded shortwave troughs,
whose timing and placement depend on complex interactions that are
difficult to forecast at long lead times, will lead to bouts of
heavier rain through Saturday for the southern mainland.
Meanwhile, onshore flow will continue to support steady upslope
rainfall along the north Gulf coast. As the upper-level trough
begins to amplify, a broad surface low dives south from Bering
Strait into the Gulf by Sunday. This will decrease the onshore
flow component and allow for some drying of inland Southcentral
and Southwest Alaska. On the back side of the low, northwesterly
flow across the Bering Sea leads to further gap winds through the
central and eastern Aleutians Saturday into Sunday. In tandem,
a cooler airmass advected on this flow will allow for temperatures
across the Bering/Aleutians to decrease a bit into the weekend.
Finally, another weak surface low or low complex moves out of the
North Pacific and reaches the western Aleutians, bringing the
heaviest rain that region will see during the period by Sunday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Watch: 141, 135, 131.

MARINE...Gale Warning: 411, 178.

FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...DAN
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...ED
MARINE/LONG TERM...CQ