Small Craft Advisory 134, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
450 AM AKST Sat Feb 10 2018

.ANALYSIS AND UPPER LEVELS...
A stacked low pressure system located just south of the eastern
Aleutians is slowly tracking north early this morning. The
associated occluded surface front is spreading rain from the
central Aleutians to the northern Bering. This front extends
eastward across the southern mainland to the central Gulf with
light rain and snow developing across the Southwest and light rain
over the Gulf. Winds along the frontal boundary tapered down
significantly overnight across the Bering with the strongest gusts
now mainly across the Southwest. Ahead of the front, a cold air
mass remains in place over the Copper River Basin with 33 below
reported in Glennallen this morning. Elsewhere across Southcentral,
temperatures have moderated slightly warmer than previous mornings
as cloud cover streams overhead and the trough edges further east.

&&

.MODEL DISCUSSION...
Models are generally in good agreement with the synoptic pattern
through the start of next week, however there is still some
uncertainty in how they handle the surface low that tracks into
the Bering today. The GFS and ECMWF track this feature slightly
faster north than the NAM, but the main details that bring
forecast uncertainty is how far north the warm air mass is pulled.
The GFS was mainly used to update the Southwest and Bering with a
combined model approach across Southcentral and the Gulf.

&&

.AVIATION...
PANC...VFR conditions are expected today with increasing cloud
cover through the evening as a front moves up from the Southwest.
MVFR ceilings were introduced after midnight as the boundary
moves up from the Kenai. Light snow is expected to develop along
this boundary; visibilities may need to be adjusted if the system
remains better organized than currently expected. Light winds
today increase out of the southeast Sunday morning with the
pressure gradient shifting in response to the surface low lifting
north.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A gale force front over the southwest Gulf will continue to slowly
weaken as it lifts northward today, with light precipitation
spreading to the Kenai Peninsula coastal areas and clouds moving
inland across the remainder of Southcentral. An approaching upper
level trough will split into two, with one short-wave shooting
eastward across the northeast Pacific (just south of the Gulf of
Alaska) and the other short-wave tracking northward across the
southern mainland of Alaska tonight through Sunday. This will
really serve to elongate and weaken the surface front as it
approaches the north Gulf coast tonight. The upper trough does
look a little stronger than previous guidance indicated, so have
upped the forecast to likely to categorical probability of snow
tonight for everywhere but the Copper River Basin. Vertical motion
will be weak, so generally expect an inch or less of snow.

There will be very little break between this system and the next,
with an amplifying pattern over the North Pacific leading
development of a strong southerly jet which will head right for
the Alaska Peninsula Sunday then curve northeastward into
Southcentral Alaska on Monday. This jet will ultimately steer
a weakening low northward to Southwest Alaska Sunday night.
However, isentropic lift will spread way out ahead of the low
center, with precipitation spreading northward across the western
Gulf Sunday afternoon and up to Southcentral Sunday evening. It
looks cold enough for precipitation to start as mostly snow across
Southcentral, with the exception of the Gulf coast, but much
warmer air will advect northward Sunday night into Monday, leading
to a transition to all rain for areas near the coast. Precipitation
intensity will begin to increase Sunday night as the upper jet
arrives and the upper trough and surface low make northward
progress.

A low track to the west of Southcentral often leads to downslope
winds and drying along the west side of mountain ranges. However,
in this case, the southeast flow will be fairly shallow, with
south to southwest flow through most of the atmosphere. In addition,
the dynamic lift and abundance of moisture with this system are
impressive. Thus, have forecasted high PoPs area-wide. Though the
southeast flow will not lead to complete drying in the lee of the
mountains, it will serve to cut down on precipitation amounts in
these areas.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

A highly progressive pattern is on tap for this forecast period. The
remnants of an occluded front will continue to push into
Southwest Alaska. Generally expecting rain along the coast with
the possibility of transitioning over to snow for interior
locations. As of 12z or 3 AM AKST, the radar at Bethel (PABC) has
showers from Russian Mission to Bethel and Eek. The models
continue to lift this moisture north-northeastward as the
attendant shortwave pushes through the area. All the while,
another low will be churning near the Central Aleutians and
rapidly move into Southwest. The leading edge of the
precipitation shield will be in Bristol Bay Sunday morning. This
second system looks to be even warmer. The way the models are
depicting the thickness values and the jet placement, it looks as
though the actual warm front, not just the occlusion will move
across the AKPEN. A few locations can expect a max temp of 40F or
better if the models stay on track. This will be a stratiform
event.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

A very active pattern is anticipated this forecast period. The
synoptic pattern features troughing from Kamchatka to the North
Pacific along with surface lows near the Central Aleutians and
southwest at 35N/175W. While one front is currently pushing away
from the area and into the Southwest Mainland today, the next
storm will rapidly follow in its wake. This second system will
have a strong jet driving it and will continue to monitor the wind
potential at the surface. Generally expecting the seas to swell.
There is a good sub-tropical moisture fetch associated with this
second system but the most notable sensible weather high light
might actually be the warm temperatures along the Chain. A third
system will rear its head near the end of this forecast period.
The models have been struggling to get in sync with the exact
track of the low but all of them are pinging into a mature cyclone
for the Western and Central Bering.

&&

.LONG TERM FORECAST (Days 3 through 7)...
A change to a warmer pattern remains in store for the beginning
of next week. On Monday, a low pressure system approaches the AK
Pen from the North Pacific and then moves into the eastern Bering
Sea. This system is accompanied by a lot of warm air, which makes
precipitation type a challenge for Monday's forecast. Models have
been somewhat inconsistent with the track of this low as well,
finally coming into some agreement with this morning's 18Z run.
Current forecast keeps a more southerly flow aloft, which
indicates a greater chance for precip reaching the lee side of the
mountains. This will also prevent a true snow-eater Chinook in
Southcentral, but some liquid precipitation is expected with
temperatures rising into the 30s at the surface.

On Tuesday and Wednesday, a short wave ridge will move through the
region, temporarily clearing skies first in the eastern Bering,
then over the mainland. This ridge is followed by a series of low
pressure systems that move into the central and western Bering.
Models forecasts have been very inconsistent on these next
systems, so there is little confidence in specific storm tracks
and timing past Wednesday. Overall though, an active pattern
looks to persist over the Bering and Western AK while high
pressure builds over Southcentral leading to quieter weather by
the middle of next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gales 120 130 131 139 155 178.

&&

SYNOPSIS AND MODEL DISCUSSION...KH
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
LONG TERM...LF