Small Craft Advisory 539, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
526 AM AKDT Fri Jul 15 2022

.ANALYSIS AND UPPER LEVELS...

A vertically stacked low over the western Aleutians is currently
moving northeastward. A second low over southeast Alaska continues
to remain nearly stationary. Weak, transient high pressure
currently extends across Southcentral Alaska. An upper-trough
currently extends southward from the Chukchi Sea into the eastern
Bering Sea and is moving eastward.

Closer to the surface, radar shows rain and showers moving
eastward across the Kuskokwim Delta, lower Kuskokwim Valley and
Bristol Bay.

&&

.MODEL DISCUSSION...

Models remain in good agreement with the placement and intensity
of synoptic scale features. Models are also consistent in showing
the pattern changing to a wetter and cooler pattern over the next
several days. The biggest forecast challenge will be scrutinizing
precipitation amounts due to local orographic effects enhancing
or lowering precipitation amounts. Forecast confidence remains
above average overall.

&&

.AVIATION...
PANC...VFR conditions will persist through 16th/00z. Conditions
will then become IFR after 16th/00z as ceilings will fall between
500ft and 1000ft. Southeasterly Turnagain Arm winds will gust
between 20-25 kts after 16th/00z.

&&

.FIRE WEATHER...

A transition to a cool, cloudy, and rainy, weather pattern is
underway. A low pressure system will move across the interior of
the state today, with clouds and widespread wetting rains for the
northern half of Southwest and all of Southcentral. There may be
a brief respite from the rain for Saturday morning across
Southwest and Saturday afternoon across Southcentral before the
next system moves in with more rainfall. A widespread 1" to 2" of
rain is possible through Monday morning, with heavier amounts
along the coast and at higher elevations.

&&

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

A front is moving into Southcentral from the west today which will
bring rainfall to the Cook Inlet and Susitna Valley regions by
this afternoon. This front will progress eastward bringing rain
to the Copper River Basin beginning this evening. Rainfall will
persist into Saturday but be intermittent after that through the
rest of the weekend for all of Southcentral. The reason for this
is a longwave trough extending from the Interior into Southwest
Alaska is not only bringing in Southwest flow aloft, but is also
tapping into a deep plume of moisture from the North Pacific. This
overall pattern with the longwave trough will persist and, just
as the ridges that brought the warm and sunny weather were slow to
change, this pattern will also be slow to change. This is
especially true since this pattern is a typical late summer one
for Southern Alaska.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3 - Today
through Sunday night)...

A very wet and fall-like pattern is set to become further
established across nearly the entire outlook area over the coming
days. The first of a series of upper shortwaves is working east
across the Kuskokwim Delta and Lower Kuskokwim Valley, ushering a
quick round of light rain pushing from west to east through early
this afternoon. Ample cloud cover will hold down daytime
temperatures quite a bit today, with really nowhere expected to
top out above the upper 50s for highs. There won't be much time
for things to dry out before another westerly shortwave moves in
with another batch of light rain this afternoon into this evening,
particularly across Bristol Bay, along the western Alaska Range
and over the northern Alaska Peninsula. Rainfall totals through
tonight will range from 0.2 to 0.5 inches, with locally as much as
1 inch across the Lower Kuskokwim Valley.

There will be no end in sight for the wet and cool pattern looking
into the weekend, with the next round of rain moving in Saturday
morning from southwest to northeast as an occluding front stemming
from a Bering low shunts north into Bristol Bay and the southern
Kuskokwim Delta. Partial clearing is expected north of this
frontal zone, allowing the northernmost parts of the region to
heat briefly back up into the low to mid 60s for Saturday highs.
By Saturday evening, the entire low over the Bering will shift
east over Southwest as it swings northeast and becomes an open
wave in the upper levels. This will support yet another round of
widespread, drenching rainfall that will linger into early Sunday
as the trough moves east towards Southcentral. Additional rainfall
between early Saturday and early Sunday is again expected to be
rather significant, with totals ranging from around 0.5 inches to
near 1 inch. Although the rain will be welcome drought and
wildfire relief, increased runoff will still be a concern,
especially off of new burn scars associated with larger fire
perimeters.

Looking even farther out into Sunday, more clouds and showers will
spread across the Kuskokwim Delta and parts of Bristol Bay by the
afternoon and evening as another upper level shortwave moves
onshore ahead of an extremely potent and deep polar low diving
out of the Arctic towards the Bering Strait. While uncertainty for
the track of this low remains higher, unseasonably cool air
associated with this system could reach as far south as the
Kuskokwim Delta by late Sunday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3 -
Today through Sunday night)...

An overall wet pattern will continue for a couple more days for
much of the Aleutians and Bering as a low over the western Bering
moves steadily east towards the Alaska Peninsula through Saturday
evening. Rain extending along the low's front will continue to
push across the eastern Aleutians and southern Alaska Peninsula
through early this afternoon, with winds up to small craft
strength blowing through gaps and passes ahead of the front
through this afternoon. A new low is expected to form at the
front's triple point near or just south of the Pribilofs this
evening, where a band of potentially much heavier rainfall may
setup along a deformation zone pivoting across the Pribilofs north
of the new center. Depending on just exactly how this evolves,
St Paul and St George could see as much as 1 to 2 inches of
rainfall before the band exits with the low on Saturday afternoon.

Overall, a drier end to the weekend on Sunday is anticipated as
the low moves over Southwest and rain showers clear out from west
to east through around midday. Strong zonal flow developing behind
the departing system will support winds up to small craft over the
Pribilofs through Sunday evening, with weaker westerlies
developing across the Aleutian Chain.

&&

.MARINE (Days 3 through 5: Sunday through Tuesday)...

Gulf of Alaska: High pressure over Prince William Sound and the
North Gulf Coast weakens through Monday. A front extending across
the Gulf moves to the North Gulf Coast for Tuesday. Forecast
confidence is good. An Eastern barrier jet with areas of gales
near the capes forms along the North Gulf Coast sunday and Monday,
diminishing. Gusty Northwest winds along the Alaska Peninsula
Coast Sunday become Westerly across the Western Gulf through
Tuesday. Seas 10 feet or less.

Aleutians/Bering: A Bristol Bay low and front moves across the
Alaska Peninsula into the Gulf of Alaska for Sunday. Gusty
Northwest winds with gale force gap winds spread over the Alaska
Peninsula on Sunday, diminishing. Seas 10 feet of the Pacific
side. Bering high pressure weakens over the Bering through
Tuesday. Gusty westerly winds over the Bering, diminishing. Seas
10 feet or less. A North Kamchatka low and front moves to the
Eastern Bering by Tuesday. A North Pacific low and front approach
the Central Aleutians for Tuesday. Forecast confidence is fair due
to the timing of the lows.

&&

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

Negative height anomalies at 500 mb are being predicted across
much of the state for the forecast period. The GFS and the ECMWF
have had a run-to-run agreement with bringing arctic energy south
of the Brooks Range. Temperatures are expected to trend down and
many areas will experience below normal temperatures as a result.
The Canadian, GFS, and ECMWF have a broad precipitation shield
spanning from Southwest Alaska to Cook Inlet and Prince William
Sound. Back to back fronts will bring precipitation to Southwest
Alaska and the Aleutian chain. A low from the northern Bering will
move into Southwest Alaska with the help of some moisture from a
previous system, and will impact Southcentral as well later in the
week. A closed low north of the Bering Strait will force several
shortwaves into western and southwestern Alaska. Models are
showing the aforementioned shortwaves from the north phasing with
shortwave energy from the Pacific on Tuesday. This energy will
then move into Southwest Alaska and bring more rounds of rain to
the region through Thursday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...ED
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AS
MARINE/LONG TERM...MK/NS

SPECIAL WEATHER STATEMENT
National Weather Service ANCHORAGE AK
1247 PM AKDT Fri Jul 15 2022

AKZ101-111-121-125-131-135-141-145-161400-
Anchorage-Matanuska Valley-Western Kenai Peninsula-
Western Prince William Sound-Northeast Prince William Sound-
Southeast Prince William Sound-Copper River Basin-Susitna Valley-
Including the cities of Anchorage, Eagle River, Indian, Eklutna,
Palmer, Wasilla, Sutton, Chickaloon, Kenai, Soldotna, Homer,
Cooper Landing, Whittier, Seward, Girdwood, Moose Pass, Valdez,
Thompson Pass, Cordova, Glennallen, Eureka, McCarthy, Paxson,
Slana, Talkeetna, and Willow
1247 PM AKDT Fri Jul 15 2022

...SIGNIFICANT RAINFALL EVENT ACROSS SOUTHCENTRAL FRIDAY THROUGH
SUNDAY...

Widespread rain is expected across Southcentral Alaska through
the weekend as a series of storm systems move across the region.
Rain will overspread Southcentral from west to east through the
day on Friday. Rain is expected to intensify across the Anchorage
Bowl and Mat-Su Valleys by Friday evening with moderate to heavy
rain at times continuing through Saturday morning. The heaviest
rain is then expected to move east over the eastern Kenai
Peninsula and Prince William Sound through Saturday night. A
second storm system is then expected to bring another round of
moderate to heavy rain across the western Kenai Peninsula,
Anchorage Bowl, and Mat-Su Valleys for Sunday morning. Rain is
expected to taper off and transition to showers from west to east
across Southcentral through the day Sunday. More rain is possible
across Southcentral for early next week.

Storm total rainfall amounts will vary across the region through
the weekend. However, most locations will likely see their
heaviest rainfall totals in almost a year.

Expected rainfall amounts through Monday morning:

Anchorage Bowl: 1.5" to 2.0"
Matanuska Valley: 1.5" to 2.0"
Susitna Valley: 2.0" to 3.0"
Western Kenai Peninsula: 1.0" to 1.5"
Eastern Kenai Peninsula: 1.5" to 2.5"
Whittier and Portage Valley: 2.0" to 4.0"
Prince William Sound: 3.0" to 5.0"
Copper River Basin: 0.5" to 1.0"

The prolonged rain event could push small streams and creeks to or
slightly above bankfull. Smaller rivers could see water levels
rise to bankfull. Water levels along larger rivers are expected
to remain below bankfull due to initial low water levels prior to
the event. Ponding of water on area roadways is also possible.
Please use caution when driving or when recreating along or on
area waterways.

For additional information and updates to forecasts, please visit
www.weather.gov/afc