Small Craft Advisory 90, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
459 AM AKST Sun Jan 26 2020

.ANALYSIS AND UPPER LEVELS...
Arctic air remains steadfast over mainland Alaska. Northerly flow
between low pressure in the Gulf Of Alaska and high pressure over
the Bering Sea is steering the arctic air southward across
Southwest Alaska and across the Alaska Peninsula. This set up is
resulting in dangerously low wind chills over southwest Alaska.
Cyclonic flow persists over the Gulf Of Alaska. The radar at
Middleton Island has pinged into showers south of Chenega while
ASCAT (Advanced Scatterometer) data has multiple discernible
circulations of low pressure at the surface. ASCAT has also
detected a fetch of strong winds coming out of the Copper River
Delta, over the Barren Islands and out of bays and passes along
the south side of the Alaska Peninsula.

Looking farther west, the remnants of a frontal boundary spans
from Unalaska to Northern Kamchatka. There is second distinct
cloud shield which is an occluded front over Attu Island.

&&

.MODEL DISCUSSION...
Models are in decent agreement with the longwave pattern bringing
the high amplitude trough over most of the mainland/Gulf and
placing a dome of high pressure over the Central Bering. However,
they struggle with the placement of the surface lows and honing
into the tracks of these lows as they emerge from the north
Pacific to the Gulf Of Alaska. Even in the first 24hrs, the north
Pacific is very sloppy. Despite the model divergence, the
GFS/ECMWF/NAM/Canadian all ping into moisture working its way into
Southcentral this forecast package.

&&

.AVIATION...
PANC...Expect VFR conditions this TAF package. There is a low
spinning over the Gulf Of Alaska and this feature is expected to
bring clouds into the aerodrome later today. At this point the
winds look like they will remain light and out of the north.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Sun and
Mon)...
The pattern is going to undergo some fairly significant changes
over the next few days bringing increasing changes for snow and
warming temperatures. First, the Arctic trough continues to hold
court right over the southern half of AK. On the west side of the
trough, another very cold morning will continue to unfold with
clear skies and calming winds. Campbell Creek Science Center in
east Anchorage is already down to -20 as of 5am. Winds have
started to come down some over inland areas (Anchorage,
Talkeetna), but they will continue to gust through the typical
north to south gaps (Resurrection Bay, Thompson Pass, Valdez, etc)
today. Over the NE Gulf Coast and the Copper River Basin, a
rapidly moving area of low pressure is pushing onshore. This is
bringing light snow from Thompson Pass up through Glennallen with
just a few inches of accumulation expected. However, a Blizzard
Warning remains in effect this morning through Thompson Pass given
the strength of the winds and the powdery nature of the snow
combining to reduce visibilities.

By Mon morning, the trough will slowly prepare to depart the
Northern Gulf as a jet streak strengthens over its eastern
periphery. There are many fragmented pieces of energy and weak
surface features associated with this complex system, so the
models vary widely in how they handle it. However, most advertise
a consolidated shortwave towards Western Prince William Sound by
Mon morning. This could bring the first in a series of snow
threats to that area and inland towards Portage Valley and the
Turnagain Pass. By Mon night, the entire trough begins to lift
north from Seward over Anchorage. This could bring a chance of
snow back into the Anchorage area. However, the best chances for
snow accumulation will certainly be over the NE Gulf Coast and SE
Copper River Basin (McCarthy) with stout onshore flow ferrying
plenty of QPF northwards. This southerly flow will also allow
temperatures to start to moderate across all of Southcentral,
although, they should remain well below freezing still.

&&

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

Southwest Alaska remains situated between an upper trough centered
near Kodiak and a strong upper high over the northern Bering.
The trough has shifted slightly farther west over the past 24
hours and as heights have fallen, surface temperatures have
subsequently dropped. This combined with gusty northerly surface
winds are leading to very cold wind chills, with a Wind Chill
Advisory remaining in effect for the Kuskokwim Delta and Bristol
Bay zones. Blowing snow is also possible in these areas. While the
upper level pressure gradient will gradually weaken, this overall
pattern will persist until a strong system over the Gulf finally
causes the southerly departure of the current low by Wednesday
morning. This will also likely bring the development of snow
across Bristol Bay along its associated front.

&&

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

A series of North Pacific lows will bring scattered rain/snow
showers to the western Aleutians though Monday. Widespread snow
showers will develop across the central and eastern
Bering/Aleutians by Tuesday in a cold, northwest flow.

&&

.MARINE (Days 3 through 5: Tuesday through Thursday)...
Gulf of Alaska: Northerly outflow winds will continue through
bays and passes with areas of gales and heavy freezing spray
across Kamishak Bay and the Barren Islands early Tuesday. On
Tuesday, these winds will relax as a weak low and associated front
pushes inland along the Southcentral Coast. There is increasing
confidence that beginning late Tuesday a ~950mb low will develop
and lift north into the southern Gulf. This system will produce
widespread precipitation with easterly gales (perhaps some storm-
force winds as well) across the Gulf ahead of the low, reaching
the Southcentral Coast by Wednesday. Westerly gales will wrap
around the back side of the low. Waves in excess of 20 feet are
expected with this system.

Bering Sea and Aleutians: Gusty northerly winds will continue
across the eastern Bering through Tuesday with areas of gales and
heavy freezing spray along the Pacific side of the eastern
Aleutians and AKPEN. Gusts will increase along the AKPEN again
Wednesday as a low in the western Gulf strengthens.

&&

.LONG TERM FORECAST (Days 4 through 7)...
There is increasing confidence that a developing storm-force low
will push toward coastal Southcentral Alaska Wednesday. This will
usher in copious amounts of moisture in the form of snow over
much of the coast along with a southeasterly flow and warmer
temperatures. This system will weaken rapidly by late Wednesday as
it pushes onshore and runs into the arctic air entrenched over
interior portions of the Southern Mainland. Given the depth of the
colder air in place, there is still a fair bit of uncertainty
regarding how much of the precipitation makes it over interior
locations. There also remains the possibility that the
southeasterly flow initiates some downsloping over the Kenai and
Chugach and limits the potential for snow in the lee of these
mountains. Snow chances then increase for late Wednesday into
Thursday for the Kuskokwim Valley as the remaining energy from
this feature continues to trek inland and overruns the cold air in
place.

By the end of the week, confidence in the guidance wanes as
solutions diverge. Overall, the pattern remains an active and
unsettled one over Southcentral. The GFS keeps the upper-level
trough axis over the western Gulf, allowing a second deep low to
track north into the Gulf and toward the Southcentral Coast by
late Thursday into Friday. The EC, on the other hand, shifts the
trough axis to the east, shifting the brunt of this second surface
low toward the Alaska Panhandle.

Farther west, arctic air will remain in place over much of
Southwest Alaska, although there is the potential for a brief
period of moderation in the cold temperatures on Thursday as
clouds and light snow move across the region. For the Bering and
Aleutians, an area of low pressure will slide across the Bering
from west to east beginning late Thursday, reaching the AKPEN by
early Saturday. Models are then keying in on a deepening low
southwest of Shemya that looks to swing a front across the western
Bering for Saturday.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 131 Wind Chill Advisory 155 161.
MARINE...Gale 121 127 130 411 412 413 414 150 155 173 175 176 177
178 Heavy Freezing Spray 150 155 165 170 121 126 127 128 130 131
137 138 139 141 129 412 414 179 185.

&&

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