Small Craft Advisory 239, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
420 AM AKDT Sat Mar 23 2019

.ANALYSIS AND UPPER LEVELS...
An active pattern remains over the western Aleutians and Bering as
a front associated with a mature low moves eastward towards
Shemya, which has seen strong winds and snow overnight.
The latest ASCAT/advanced scatterometer pass has detected gale
force winds near Attu and Shemya with this new storm. A weakening
low over Bristol Bay that is bringing a few rain showers to areas
across the Southwest Alaskan coast as temperatures remain above
freezing. The front over the Gulf Of Alaska remains to be the main
weather driver for Southcentral this morning. The radar at Kenai
(PAHG) and Middleton Island (PAIH) have been active overnight and
early this morning. Areas along the north Gulf coast continue to
be influenced by the warm influx of moisture lifting northward
from the Gulf resulting in rain showers stretching from Homer to
Cordova. The warm trend continues, yesterday the highs at
Birchwood, Wasilla, Talkeetna and Kenai were 51F.

&&

.MODEL DISCUSSION...
The global models are in good agreement with the synoptic scale
features through 00z Monday. Moreover, the GFS ensemble members
are tightly grouped with the high over the Arctic Ocean, the low
near the AKPEN and the low south of Kamchatka. There are some
minor differences with the areal coverage of the precipitation
shield over Southwest AK and over portions of Southcentral AK.
The NAM was used to for high resolution winds for portions of
Southcentral, but otherwise, the model of choice for the short
term package was the GFS.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist in the morning.
The cigs/ceilings will vary between 5,000 to 7,000 today at PANC.
By 6z, gusty winds will move out of Turnagain Arm and into the
PANC aerodrome.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A frontal system will remain across the area today, stretching
from the Alaska Range west of Skwentna east-southeast across the
Kenai Peninsula through the north Gulf coast. This is bringing
precipitation to the area, mainly along the eastern Kenai and
towards the western Alaska Range. The air mass is quite warm and
the precipitation at low elevation sites is in the form of rain. A
surface low developing to the southwest of Yakutat will slide
west-northwest today along the front, crossing the Kenai Peninsula
this afternoon and then moving across the Susitna Valley. This
will bring some gusty winds into the Knik Valley this afternoon
and into Anchorage in the evening. The front will then push
northward across Southcentral tonight. Another front associated
with a low south of the Gulf will move to the north Gulf coast
Sunday morning. As this front moves inland Sunday afternoon, the
flow aloft will switch from southeasterly to southwesterly. On
Sunday night a ridge of high pressure will begin to build into the
area, with conditions drying out.

&&

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

A 990 mb surface low just to the west of Kuskokwim Bay will
continue to track north while weakening, leading to subsiding
southeasterly winds and diminishing areas of precipitation today.
The next system approaches the area by early Sunday morning as a
front pushes across the SW coast. Most of the energy looks to
remain north over the Kuskokwim Delta, with strong southeasterly
winds and snowfall developing Sunday morning. The pattern begins
to change by Sunday night as a high amplitude longwave ridge
builds north over the mainland bringing another surge of warm
temperatures with persistent southerly flow at the surface.
Temperatures look to climb well into the 40s with some locations
reaching into the low 50s during the day on Monday. A strong low
approaches from the west by Monday with southeasterly winds
ramping up again along the coast. There were some concerns
regarding possible storm surge issues along the coast but the
track of the low is trending further west, keeping the wind
direction unfavorable for building surge high enough to be
impactful to coastal communities.

&&

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

Northerly winds will continue to diminish today over the eastern
Bering as a surface low to the east weakens. Fairly benign weather
will be present over the eastern Bering/Aleutians as a short wave
ridge moves over the area out ahead of the next system pushing
east across the Bering. Out west, a front pushing out ahead of a
surface low near Kamchatka will bring easterly gale force winds switching
to southwesterly as the front passes today. Gales look to
continue over the western Bering through early Sunday morning
before briefly diminishing later in the morning.

Attention turns to a very strong low rapidly approaching the west
Aleutians from the North Pacific by Sunday afternoon. The low
looks to rapidly intensify as it approaches, with the EC deepening
the central pressure from 970 mb to 946 mb between Sunday and
Monday morning (24 mb in 24 hrs). Models are mostly locked on to
the low, with the center tracking north across the Rat Islands and
into the western Bering Sunday night. Ahead of the low, a warm
front will push into the Bering Sunday evening bringing gale force
southerly winds and heavy rain. The strongest winds are expected
with the proceeding cold frontal passage across the western
Aleutians, with strong cold air advection (850 mb temps dropping
from +6c to -8C Sunday night) wrapping in around the backside of
the low. This will likely lead to hurricane force wind gusts near
Adak Sunday night into Monday morning before the narrow core of
strong winds pushes north into the western Bering through Monday
afternoon.

&&

.MARINE (Days 3 through 5)...

Gulf of Alaska... Expect generally benign conditions, with winds
diminishing and seas subsiding as pressure builds.

Bering Sea... Confidence is high that a storm force low will
track into the Bering Sea from the western Aleutians Monday.
This will bring heavy seas and widespread gales to the central
Bering and Aleutians, with a swath of storm force winds southeast
of the low. The low will weaken and shift northward towards
eastern Siberia by Tuesday and winds will diminish.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Confidence remains strong that a pattern change to drier
weather will begin over Southcentral Monday, as a longwave ridge
begins building over the area. Daytime conditions will trend at
least as warm as observed this week in most areas, though valley
locations will likely see colder nights driven by clear skies.
With ample surface moisture and atmospheric stability, there will
also be potential for morning fog which could decrease the
diurnal temperature range. The main question is when the ridge
will degrade, and model agreement is poor on that decision. Some
solutions show the ridge breaking down as early as midweek, with a
return to wetter weather following. That said, there is
indication that a high will rebuild and return the warm, dry
conditions into next weekend.

Looking farther west, a potent low will track from the western
Aleutians to the central Bering Monday. With an expected moisture
fetch almost to the tropics, expect heavy precipitation ahead of
the low. As the low shifts northward, the front and associated
precipitation and winds will progress eastward over the Aleutians
and Bering, then stall over Southwest Tuesday. A ridge in place
over Southcentral will likely diminish precipitation potential
over the Southwest mainland. Showers in cold air advection will
persist behind the front in the Bering Tuesday into Wednesday.
Forecast confidence drops substantially Wednesday
onward...based on model consistency, this forecast package is
leaning more towards quiet weather by midweek. Conditions over
the mainland will depend on the progression of the upper ridge.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 413 411 177 178 181.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...PAMBER
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KVP
MARINE/LONG TERM...MM