Scientific Forecaster Discussion
fxak68 pafc 221331
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
431 am akst Fri Nov 22 2019
Analysis and upper levels...
there is a surface low near King Salmon and the occluded front
extends from southwest Alaska to Prince William Sound. The warm
front is already in Southeast Alaska. On ascat (advanced
scatterometer) data, a low level jet of 50 knots has been
detected south of Cordova. Additionally gales and a few storm
forces winds have been detected south of the chain and akpen. The
radars at King Salmon, Kenai and Middleton Island have been in
precipitation Mode showcasing the extent of the precipitation
shield across the southern tier of Alaska. The jet core remains
impressive and is south of the Aleutian chain with the nose
pushing into Southeast Alaska.
the run to run continuity with the models in the short term has
been decent. The models continue to be in good agreement with the
major features through the weekend.
panc...gusty winds will rapidly diminish this morning, but remain
from a southerly direction through late tonight. An upper level
wave may bring some rain shower to the terminal area this
afternoon into the evening. Ceilings will likely drop below 5000
ft at that time and may drop into MVFR conditions as well through
the night tonight.
Short term forecast southcentral Alaska (days 1 and 2)...
The surface low center is near King Salmon in the Bristol Bay
region as of early this morning. The main front is moving rapidly
northward through the northern Susitna Valley and Copper River
basin. The strongest winds have been from the southeast just ahead
of the front with some brief gusty southerly winds occurring
after frontal passage. These winds have been diminishing within a
few hours after the front so it should be overall much less windy
today compared to yesterday for most of the southcentral area.
With the front through the area, southcentral will get into the
showers associated with the colder air aloft that is beginning to
move into the region today. The surface temperatures will still be
warm, but should remain steady or slowly drop through the course
of the day as winds subside. However, significant cooling will
not occur until the surface low moves through the area. This low
should move into Cook Inlet this afternoon and then stall as
another low undercuts it and moves into Prince William Sound
Saturday. This double-low with one in Cook Inlet and the stronger
one in Prince William Sound is not overly conducive for snow
development so any snow potential over the Kenai though Matanuska
Valley (including anchorage) will be more due upper level waves
than anything else.
The notable exception will be in the Susitna Valley north of
Talkeetna, and especially toward Broad Pass. Persistent southerly
flow aloft and colder air in that area will keep snow in this
area through this evening and the Winter Weather Advisory for
this snowfall was kept in place until midnight Friday night.
By Saturday night, temperatures will quickly drop over the region
as the cold core of the upper level low moves into southcentral
Short term forecast southwest Alaska (days 1 and 2)...
Showery conditions will persist across southwest Alaska today as a
surface low weakens over the Mainland. Below freezing temperatures
across the Kuskokwim Delta and valley will keep snow the dominant
p-type, while areas along the akpen and southern Bristol Bay will
start off as rain today. The transition line between rain and snow
is still a bit uncertain, but could leave a rain/snow mix for
areas near Cape Newenham, Dillingham, and Iliamna, before
switching over to all snow by later this evening. A cooling trend
over the next few days will keep any new precipitation falling as
snow, as things start to clear out on Sunday across the southwest.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
Northerly flow and small craft winds persist over the central and
eastern Bering today through Saturday as a surface low continues
inland. Areas south of the eastern Aleutians and akpen will also
continue to see these strong winds as well with west-southwesterly
flow. Stronger winds out of gaps along the Aleutians and akpen
are possible if winds become more northerly over these areas,
which will continue to be monitored. Freezing spray will continue
to be a concern stretching from the northern Bering to along the
Kuskokwim Delta coast with a colder air mass situated over the
warmer water temperatures. This pattern will weaken through
Saturday night into Sunday when the next strong front approaches
the western Aleutians and Bering.
Marine (days 3 through 5)...
The period begins Sunday evening with weak ridging over the
eastern Bering and a 1006 mb surface low in the vicinity of Prince
William Sound. Further out west in the western Aleutians a gale
force front associated with a 990 mb Kamchatka low is entering the
forecast area. Model agreement is okay for day 3, as GFS and NAM
are in line with some gales in the gaps along the northwest Gulf
Coast, NAM has a bit weaker solution to winds, but it also puts
the surface low a little further to the east. By Monday afternoon,
gales have pushed offshore into the central Gulf and the low in
the Bering has deepened to 965 mb with a wide area of gale force
to sub-storm force winds in the western half of the Bering and
near gales in the remaining half of the eastern Bering. Models are
pushing a strong high into the southern Gulf Monday into Tuesday
afternoon with values of 1040 mb, this feature is also acting as a
block on the Bering low which appears to track northeast into
Siberia. Its front will still produce storm force winds in the
northern Bering Tuesday morning with strong gales in the coastal
waters of southwest Alaska. By Tuesday evening winds subside in
the eastern Bering and we're left with an area of high pressure
over much of southcentral Alaska. In the beyond it will be of
interest how much influence the high pressure in the North Pacific
exerts and whether we see a blocking pattern set up like we had
in previous weeks.
Long term forecast (days 3 through 7)...
The upper level trough over southwest Alaska will shift east into
southcentral Alaska Sunday through Sunday night spreading a
significantly colder air mass over southern Alaska. In response to
this and a leeside trough forming in the northern Gulf offshore
flow will increase rapidly over southcentral Alaska Sunday night
and continue through Monday. Winds will be strongest through the
typical favored terrain gaps with northerly winds through Seward
and Resurrection Bay, westerly winds through Whittier and Passage
Canal and north to northerly winds through Thompson Pass, the
Valdez area and across the Copper River Delta.
The next frontal system associated with a Kamchatka low will push
into the western Aleutians Sunday morning followed quickly by a
second stronger storm force front Sunday night. This second very
large front will progress across the western and central Bering
Monday as the initial front moves through the eastern Bering to
reach the southwest Alaska coastline by Monday afternoon as it
merges with the warm front of the larger system. The warm front
will move onshore spreading abundant precipitation across
southwest Alaska Monday night through Tuesday while the cold
frontal portion of the storm tracks from the central Bering into
the eastern Bering. With a warm, rather moist air mass overrunning
the low level cold air in place in southwest Alaska this is going
to be another system to watch closely for significant winter
weather impacts and a wide variety of precipitation types
A North Pacific frontal wave approaching the eastern Aleutians
and Alaska Peninsula from the south on Tuesday will phase with the
upper level trough associated with the cold front in the Bering.
This interaction will spin up a new closed low south of the Alaska
Peninsula on Wednesday as the leading edge of the front pushes
pushes across Kodiak Island and the western Gulf. As the frontal
system spreads abundant precipitation across southcentral Alaska
Wednesday afternoon through Wednesday night, the surface will
continue to the northeast to reach Prince William Sound by early
Thursday morning. While model confidence remains low with today's
runs tracking this surface low significantly further to the north
than yesterday's, if more northerly track holds this storm will
bear very close watching for potential mixed wintery impacts to
southcentral Alaska. An initial weaker overrunning front Tuesday
night or Wednesday following the rather cold temperatures Monday
and Tuesday is a possible freezing rain concern with the stronger
and moister front arriving Wednesday or Wednesday night bringing
the possibility of heavy rain to the coast and heavy snow inland.
public...Winter Weather Advisory 145.
Marine...gale 131 132 150 165 185.