Small Craft Advisory 91, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
500 AM AKST Mon Jan 27 2020

.ANALYSIS AND UPPER LEVELS...

On satellite imagery there is a low spinning southwest of Shemya
with an occluded front wrapped into it. There is a 180+ knot jet
core south of the western Aleutians. Northerly flow continues over
southwest Alaska which is the result of low pressure in the Gulf
Of Alaska and high pressure over the Bering Sea. This pattern has
been persistent and has been ushering arctic air into the region
resulting in dangerously low wind chills over southwest Alaska.
The radar at Middleton Island has detected an uptick in shower
activity compared to 24 hrs ago associated with a low south of
Chenega.

&&

.MODEL DISCUSSION...

For initialization purposes compared the returns at 6z on
Middleton island radar to the projected precipitation with the
GFS/NAM/ECMWF and Canadian. The NAM did well pinging into the
precipitation near Kayak Island while the GFS had the correct
orientation of the precipitation band associated with a weak
inverted trough south of Chenega and Seward. Even the high
resolution of the ECMWF was simply too broad brushed with the
precipitation over the Gulf Of Alaska. At 6z using the cyclonic
curvature as a proxy for the location of the low, the NAM and GFS
did fairly similar with the placement of the surface low south of
Montague Island. This was seconded by the 6z ASCAT (advanced
scatterometer) pass.

Models are in decent agreement with the longwave pattern keeping
the high amplitude trough over most of the mainland/Gulf and
placing the block over the Central Bering. However, they struggle
with the placement of the surface lows even in the near term and
the the divergence amplifies handedly Tuesday as a low moves from
the north Pacific and into the Gulf Of Alaska.

&&

.AVIATION...
PANC...VFR conditions are expected at the onset of this TAF
package. There is a low south of Seward and moisture is getting
wrapped around this feature. The expectations is that cigs will
develop over the aerodrome later today but at this point the cigs
should remain above 5,000 feet until 6z on the 28th. The winds
are expected to remain relatively light.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2; Sunday
night and Tuesday)...

An upper level trough situated across mainland Alaska will keep a
strong north to south pressure gradient and gusty outflow winds
across the region. Coastal locations, including Seward, Whittier,
and Valdez will continue to see these strong winds with the
continuation of polar lows developing over the Gulf of Alaska
today. The Middleton Island radar this morning has already
indicated snow showers moving into the north Gulf coast associated
with the the first polar low, which will move inland and lead to
blizzard conditions throughout the day for Thompson Pass. Areas
near Seward and Whittier will also see blowing snow and reduced
visibilities are expected through this evening with a winter
weather advisory in effect.

Winds begin to diminish across Southcentral late Monday as the
upper level trough begins to shift westward as a North Pacific low
begins to lift northward. By Tuesday morning, this strong storm-
force low will make its way into the southern Gulf. Expect a
strong pressure gradient to redevelop over Southcentral Alaska,
which will lead to another round of gusty outflow winds through
Wednesday. Kodiak City and areas along the north Gulf coast will
be areas to keep an eye on with this next system as it could lead
to advisory level snowfall amounts.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Today
through Wednesday Night)...

The primary weather story continues to be the cold temperatures
and gusty winds causing very low wind chills across Southwest
Alaska. Nearly the entire area is under mainly clear skies, but
some ocean-effect snow is impacting the Kuskokwim Delta coast and
Nunivak Island near Hooper Bay and Mekoryuk. Low stratus clouds
are moving over the area east of King Salmon as well. The mainly
clear skies and gusty north winds continue most everywhere else.
The Wind Chill Advisories remain in effect through noon on
Wednesday. The coldest wind chills will be late tonight into
Tuesday morning. A strong low moving into the Gulf late Tuesday
night into Wednesday will increase the winds, but will also bring
some cloud cover which will modestly increase temperatures. Any
precipitation will be limited to areas right along the Aleutian
and Alaska Ranges.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3: Today
through Wednesday Night)...

A very stagnant weather pattern will persist through Wednesday
night across the Bering. Strong cold air advection on northeast
winds will keep ocean-effect snow bands continuing across the
eastern half of the Bering, and along the Alaska Peninsula and
eastern Aleutians. High pressure over the central Bering this
morning will shift westward by Tuesday morning into the western
Bering. This will allow the cold air to move further west across
the basin. Otherwise, the coldest air will continue to stream
across the eastern Bering through Wednesday night, keeping the
temperatures well below average, with ocean-effect snow causing
briefly reduced visibilities, and large areas of heavy freezing
spray across the eastern Bering.

&&

.MARINE (Days 3 through 5)...
Gulf of Alaska: The leading edge of a storm force low enters the
southern Gulf Tuesday morning, bringing ample precipitation and
easterly winds across the Gulf Tuesday evening. While there is
high confidence in the strength of the low, there is lower
confidence on exactly where the low center tracks. We are
currently expecting the low center to track east of Kodiak Island
Wednesday morning, bringing a swath of easterly storm force winds
to the waters along the northern Gulf coast Wednesday. Westerly
gales will fill in across the Gulf on the backside of the low as
it lifts northward. It is possible that these westerly gales could
reach storm force levels Wednesday night. There is high
confidence that this low pressure remains nearly stationary,
becoming reinforced with additional energy in the upper levels for
the end of the week. Waves in excess of 20 feet are expected with
this low.

Bering Sea and Aleutians: Gusty northerly flow will persist over
the eastern Bering waters midweek. Gales will become more
widespread across the Bering as a low in the Gulf strengthens and
lifts northward. Enhanced winds through the usual gaps and passes
along the Alaska Peninsula will last through Thursday. Heavy
freezing spray is expected across much of the eastern Bering
waters, and the Pacific side of the eastern Aleutians.

&&

.LONG TERM FORECAST (Days 4 through 7)...
A storm force low in the Gulf of Alaska will slowly lift
northward and inland over Southcentral Wednesday night. The
northern Gulf coast will likely see ample precipitation through
Thursday. Model solutions continue to key in on reinforcing
features in the upper levels, leading to the possibility of
prolonged precipitation over the northern Gulf coast through the
end of the week as this system is revived by an upper level wave
Thursday into Friday. The remnants of this low look to finally
exit the Gulf to the east by the weekend, as an upper level ridge
approaches from the west. Meanwhile over the Bering, cold
northerly flow is expected to continue through the end of the
week. By the weekend another low approaches the western
Aleutians, bringing unsettled conditions for the beginning of
February.

&&
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Wind Chill Advisory 155 161. Blizzard Warning 131
(T-Pass). Blowing Snow Advisory 125. MARINE...Gale 411 413 414
150 155 165 170 172 173 174 175 176 177 178 179 119 120 121 127
128 130 132 131 138 139. Heavy Freezing Spray 121 127 129 130 138
139 141 414 150 155 165 171 179 185.

&&

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...MO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
MARINE/LONG TERM...KO