Small Craft Advisory 29, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
422 PM AKST Tue Jan 7 2020

.ANALYSIS AND UPPER LEVELS...
A deep 500 mb closed low continues to remain centered over the
interior of mainland Alaska, with a negatively-tilted trough axis
extending southeastward toward Haida Gwaii. Fairbanks, being the
closest sounding site to the center of this low recorded a 500 mb
height of 497 Dm at 12z. The upper-level flow remains weak across
the Southcentral Alaskan mainland due to the low center being in
close proximity to the region. Upper-level flow is also weak
across much of the Bering and over the Pribilof Islands due to a
combination of 500 mb ridging with a weak 500 mb height gradient.
Upper-level flow is strongest over the central Aleutians, across
the AKPEN and into the southern Gulf where the 500 mb height
gradient is steepest. Water vapor imagery shows an occluded front
southwest of the western Aleutians. Both GOES-17 visible satellite
imagery and radar imagery show a low off-shore from Cordova due
to winds through the channel between Hinchinbrook and Montague
Islands combined with the cold airmass in place. Snow shower
activity associated with this low is slowly moving eastward toward
Kayak Island.

&&

.MODEL DISCUSSION...
Models are in good agreement with the synoptic pattern across the
region. The GFS, ECMWF, NAM and the Canadian GEM all have similar
solutions with respect to the placement and intensity of the low
that is expected to move through the western Aleutians/Bering on
Wednesday afternoon through Thursday. The NAMNest was favored for
the wind forecast across the Copper River Delta for the next
couple of days as it has the best handle with resolving the
current mesoscale features across the region when comparing to
real-time observations, satellite and radar.

&&

.AVIATION...
PANC...IFR to LIFR visibilities and/or ceilings are expected through
the afternoon hours Wednesday.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
An upper level low currently over the Interior will move to the
southeast by Wednesday afternoon. While gap winds along the
northern Gulf coast are currently occurring, they are expected to
strengthen overnight tonight through Wednesday morning. The
strongest winds can be expected for Seward/Resurrection Bay,
Whittier/Passage Canal, the Valdez area including Thompson Pass,
and the Copper River Delta. A Wind Chill Advisory has been issued
for Thompson Pass, as wind chills will drop to 45 below zero
through the Pass beginning tonight.

Looking ahead to Thursday, another low spins up south of the
Alaska Peninsula. This low looks to track across the Gulf,
remaining well south of the southern Mainland. A brief shot of
accumulating snow is possible for Kodiak Island Thursday
afternoon as the low quickly passes over before departing the area
by Thursday night.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
An Arctic high remains oriented over the mainland, with very cold
conditions continuing in the Kuskokwim Valley/Delta and Bristol
Bay. A wind chill advisory is in effect until noon Wednesday for
the Kuskokwim Delta, and for interior Bristol Bay (New Stuyahok,
Koliganek) for wind chills down to 50 below. Expect a transition
to southerly flow ahead of an approaching warm front by midday
Wednesday, and conditions will start trending warmer. Temperatures
will generally still be below zero Wednesday, but will cross into
the positives Thursday as the front nears the coast and Arctic
air recedes northward. The front will shear apart as it reaches
the mainland, but should still bring some precipitation to the
AKPEN, and coastal areas of Bristol Bay and the Kuskokwim Delta
Wednesday night into Thursday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A gale force low south of the western Aleutians will push a front
northward through the Bering, bringing rain or a rain/snow mix to
the southern Bering and Aleutians Wednesday...reaching the
Pribilof Islands Wednesday afternoon. Expect another round of
gales on the backside of the parent low, over the central and
western Aleutians late Wednesday into Thursday.

&&

.MARINE (Days 3 through 5: Friday through Sunday)...
A storm in the western Bering Sea has a front that drapes across
the Bering Sea to the Alaska Peninsula on Friday. This storm will
track from St Lawrence Island to Nunivak Island Sunday. The main
impact is gale and storm force winds associated with this storm.
Gales in advance of the front with Gale and storm force wind south
of the low as it traverses across the Bering Sea offshore zones.
The gale and storm force wind should drop off south of the low
through the day Sunday.

A strong storm is exiting the southeast Gulf of Alaska Friday.
North to northwest wind wrapping around the backside of this low
should be below gale force Friday. Wind in the Gulf of Alaska and
along the coast of Gulf of Alaska will remain below gale force
Friday through Sunday with the exception of the Copper River
Delta. The Delta may still have gale force wind blowing out of it
Friday.

&&

.LONG TERM FORECAST (Days 3 through 7: Friday through Tuesday)...
The models are on track with a pattern shift toward this weekend.
A large surface and upper level low will move across the Bering
Sea to the northeast Bering Sea by Sunday. The low then weakens to
a trough aloft as it rapidly moves across northern Alaska through
Monday. This will result in dramatic warming across southwest
Alaska through the weekend as temperatures rise to the 20s to near
30 above. South central Alaska will also see temperatures warm
through the weekend to the teens and 20s. The exception is in the
Copper River Basin where the cold air appears to be firmly
entrenched, and the coldest air will move over the area. Some of
the model guidance is forecasting low temperatures to as low as
-60 F for Gulkana Saturday and Sunday night. This is significantly
colder than what has been in most of the model solutions up to
this point. If this big of a temperature difference does form
between the Copper River Basin and Gulf of Alaska, we could see
very strong gap wind through the Chugach Mountains that would impact
Prince William Sound and the Copper River Delta.

A surface low will form along the front of this Bering low near
Kodiak Island or the Gulf of Alaska Thursday. Here the models
have not come into good agreement and the difference in the
location of the low, while small, has a significant difference in
the sensible weather, in particular for Kodiak Island. If the low
is further south and east, Kodiak could get little if any snow or
a mix of rain and snow, or if the low is closer to Kodiak, there
could be 4 to 10 inches of snow. This will bear watching through
the week.

Another upper trough follows behind this trough Monday and will
extend south across the Alaska Peninsula into the north Pacific. A
surface low will likely form in the base of this trough south of
Kodiak Island Monday night or Tuesday morning. This will primarily
impact the Gulf of Alaska offshore region.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Wind Chill Advisory 131 155 161.
MARINE...Gale Warning 121 127 129 170 173>180 185.
 Heavy Freezing Spray 119 121 127>130 139 141 160 180 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...ED/RC
SOUTHCENTRAL ALASKA...KO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MM
MARINE/LONG TERM...SA