Small Craft Advisory 633, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
610 PM AKDT Tue Aug 25 2020

.ANALYSIS AND UPPER LEVELS...

Not much change in the overall pattern this morning. The upper
level low remains parked over the northern Bering Sea, with a
surface low in the northern Gulf of Alaska. West of the Alaska
Range, precipitation is mostly showery under the influence of the
upper level low. East of the Alaska Range, a deformation band has
set up over Southcentral, with steady rain falling from the
northern Kenai Peninsula, across the Anchorage Bowl and into the
southern Susitna Valley. Half an inch to an inch of rain has
fallen in the last 24 hours with this event. The 12Z sounding
shows moderate instability between the surface and about 600 mb,
which is evident in the heavier showers embedded in the overall
band of precipitation.

&&

.MODEL DISCUSSION...

Models remain in good synoptic agreement on low positioning, but
timing of individual features varies between runs. Of the high-res
models, currently Canadian Regional has the best handle on the
band of precip over Southcentral. This model also moves the rain
band further south over the Kenai Peninsula later this afternoon,
instead of the early afternoon solution preferred by the NAM.

&&

.AVIATION...

PANC...Persistent rain has brought MVFR tempo ceilings to the
Anchorage area. The main band is expected to move southward this
evening, with precipitation expected to diminish to off-and-on
very light nuisance rain across the TAF area. Another disturbance
brings another round of steady rain during the day Wednesday,
with MVFR to IFR conditions returning.

&&

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

The longwave pattern features broad troughing from the Gulf of
Alaska to the Central Bering. Cyclonic flow aloft will continue
this forecast period. There are multiple embedded shortwaves aloft
but the actual surface low is by Montague Island. This low has
already dumped an estimated 3/4s of an inch of rain in the past
24hrs over the Anchorage Bowl. Merrill Field measured .73 and Lake
Hood .72 and the radar at Kenai (PAHG) continues to show
additional moisture for portions of Southcentral. This moisture is
being wrapped around the low as the low with continue to weaken.
Wednesday Prince William Sound and the northern Gulf of Alaska
will get additional moisture tomorrow and Thursday. Upper level
energy that is currently over the Bering will push eastward
ushering another plume of moisture into Southcentral. Overall, the
pattern looks cloudy and wet for the majority of the region the
next 3 days. Friday is looking relatively dry at this time as the
models are advertising a weak ridge building over Kodiak.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2: Tonight
through Thursday)...

A low drifting south along the coast of Southwest Alaska will
continue to keep scattered rain shower activity ongoing into the
overnight hours tonight. Except in any heavier showers, most areas
should stay around one tenth of an inch of rain through tonight.
A brief dry period is possible Thursday morning before more rain
shower activity develops Thursday afternoon. Since the low stays
relatively weak, winds should remain light in most inland areas,
with somewhat gustier conditions along the coast.

&&

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

Widespread cumulus to stratocumulus over the eastern two-thirds of
the Bering is producing shower activity over that area into the
Aleutians. High pressure with northwesterly offshore flow off
Siberia is allowing much of the cloud cover to dissipate over the
western Bering, so unusually sunny conditions are ongoing over the
Western Aleutians and into Shemya. A low will approach the western
Aleutians ending the relatively clear weather for that area on
Wednesday, with rain developing late in the day along with small
craft southeasterly winds. The low will not be moving much, but
will be slowly strengthening with time, with gales possible north
of the chain Thursday afternoon and evening. The low center will
drift east along the western Aleutians into Friday, with gales
continuing north of the center over the open Bering. Associated
small craft conditions will move into the eastern Aleutians by
Friday morning.

&&

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

A North Pacific low moves across the Aleutians through Saturday.
Model confidence during this timeframe has increased and we have
introduced gale force winds along the front in the western to
central Bering Sea. Confidence begins to waiver Sunday morning.
This complex system spins up another low somewhere between 953 and
970 mb. Depending on the strength of this low will determine if
winds could potentially reach storm force or even hurricane force
per some guidance. Placement varies from outside of the Aleutian
waters south of Adak, to north of Adak in the Bering or somewhere
in between. The track of this storm by Sunday night remains highly
variable landing somewhere between the northeastern Bering or
over towards the Alaska Peninsula. All in all, there will be
plenty of refinement associated with this system as we go through
the week, but mariners in the area should pay close attention.

For the Western Gulf, timing, placement and strength of a front
associated with a low over the Aleutians is expected Saturday
morning sometime into the afternoon. Confidence is limited whether
this front will produce small craft or gale force winds as it
propagates towards Prince William Sound late Saturday into Sunday.

&&

.LONG TERM FORECAST (Days 4 through 7/ (Friday afternoon through
Tuesday)...

Starting Friday afternoon, weak upper level ridging will be
present across much of the southern Mainland of Alaska. This will
lead to a brief reprieve from the wet work week the area has been
experiencing.

Starting Saturday through the remainder of the long term forecast
period, a very interesting situation is unfolding. A tropical
cyclone currently to the south of South Korea will continue moving
northward while weakening. The energy from this weakened tropical
cyclone will get caught in the westerly flow aloft as an upper
trough over Siberia dives southeastward Friday through Saturday.
All model guidance is showing this storm system for the weekend.
However, the differences emerge on how each piece of model guidance
deals with the energy from the remains of the aforementioned
tropical cyclone. One model has a very strong low in the Bering
Sea with the other models not as strong with this low. This will
become fine tuned in the coming days as we near the event and
models get a better handle on exactly how the energy from the
remains of the aforementioned tropical cyclone pans out. For now,
we expect an increase in unsettled weather with rainy conditions
(potential for heavy rainfall) especially for Southwest Alaska
over the weekend into early next week along with small crafts to
gale force winds across the Bering. However, we cannot rule out
the potential for a storm force or even hurricane force low in
the Bering if the strongest solution plays out. The front from
this system will also set up southwesterly flow aloft on Saturday
through the remainder of the forecast period bringing rain back
into the forecast for Southcentral Alaska. Depending on the track
of the low and its associated front, a barrier jet and strong
higher elevation Turnagain Arm winds cannot be ruled out for
Saturday afternoon into Sunday.

&&

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

&&

SYNOPSIS AND MODEL DISCUSSION...LF
SOUTHCENTRAL ALASKA...PJS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JPW
MARINE/LONG TERM...CK/MV