Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 220057
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
357 PM AKST Mon Jan 21 2019

.ANALYSIS AND UPPER LEVELS...
The strong low in the central Aleutians has become vertically
stacked today signaling it going into a weakening phase. The 250
mb jet stream is pushing 200 kt as it moves off the coast of Japan
to south of this low. The jet then curves to the northeast toward
northern Vancouver Island. This is setting up an area of
favorable development of a surface low at the inflection point of
the jet south of the Central Aleutian low. It is also rapidly
developing a surface low to the south of the Gulf of Alaska. The
front associated with the main Aleutian low has made significant
headway northward through all of southern mainland Alaska today
and is bringing some areas of precipitation with it. This change
in pattern has helped to clear out the persistent fog that as been
in northern Cook Inlet the past few days.

&&

.MODEL DISCUSSION...
Models continue to do well in the short term with the major points
of concern resolved. However, when the Wednesday night to Thursday
timeframe arrives, models start to show a significant difference
in the low structure. The NAM and GFS are splitting the low into
two centers about 600 miles apart while the ECMWF and Canadian
models consolidate the low into one center in between the other
centers. The main impact of this will be Thursday and beyond so
more about this will be discussed in the long term section.

&&

.AVIATION...
PANC...VFR conditions and light northerly winds will persist into
Tuesday. There may be some wind shear develop this evening as
southeasterly winds aloft increase and surface winds remain
northerly. At this time the 2000 to 3000 ft level looks like the
most likely level for this shear to occur.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A weakening front continues to cross Kodiak Island into the Gulf,
and this front will wash out tonight before reaching the Kenai
Peninsula. Southwesterly flow aloft has allowed the atmosphere to
moisten from the top down, with minor shortwaves crossing the
region in the flow bringing just enough ascent for areas of light
snow to develop across portions of the area through the early
morning. This afternoon`s Anchorage sounding shows this rapid
increase in moisture depth since this morning, with a strong
inversion just above the surface. With this in mind, we wouldn`t
be surprised to see some flurries continue through the noon hour
tomorrow, but, confidence in this was insufficient to insert into
the forecast at this time.

Our attention then turns towards a more significant front,
associated with a system lifting north across the Bering. This
front will be much stronger in nature, aided by a strong
vorticity ribbon crossing the region. With the increased lift
associated with this feature and widespread precipitation
development will likely occur. Given the low position in the
Bering and the strength of the front, look for the usual downslope
precipitation shadows to develop across the western Kenai ,
through the Anchorage Bowl, into the Matanuska Valley. Although
some drizzle wouldn`t be out of the question given the deep
warm/moist southerly flow above 4000 feet. The increased strength
of the warm advection may lead to some precipitation type concerns
for a few hours during the initial onset, but areal coverage and
intensity remain in question. For now, have introduces some
mention of this into the forecast for the Kenai Peninsula into the
Anchorage metro.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
An occluded boundary is currently pushing through southwest
Alaska. Low level flow is still out of the northeast over much of
southwest AK, keeping colder temperatures in place for most
locations. We should see winds beginning to shift tonight and
start to increase in speed, especially along the coast. This is
expected to induce some blowing snow along coastal areas.
Temperatures will still stay on the cool side in interior areas,
as the strongest push of warmer air does not arrive until
Wednesday. This is when the low center associated with the
occluded boundary moves north into the central Bering Sea, which
will shift winds to a southerly direction in southwest Alaska.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The North Pacific low impacting the Aleutian Islands continues to
track slowly north, with the low center now directly over the
Central Aleutian Islands and well depicted on satellite. The
occluded boundary pushing northward through the Bering Sea has
considerably reduced visibility for the Pribilof Islands.
Conditions did get a little worse than expected on the Pribilofs,
as visibility was occasionally reduced as low as one quarter mile.
With the event mostly over at this point, we chose not to issue
any hazard products as conditions there have been slowly
improving.

As the low works north through Wednesday, rain and snow will
continue as waves of energy wrap around the low pressure center.
The low is also bringing warm air out of the north Pacific, which
will carry the freezing line north for the central and eastern
Aleutians as well. This will transition snow to rain for the
Pribilofs tomorrow. The western Aleutians will be on the backside
of the low and will continue to see precipitation in the form of
snow.

&&

.MARINE (Days 3 through 5 Thursday through Saturday)...
The marine outlook remains quite challenging with poor run to run
model consistency. The general trend continues to be the
development of a ridge of high pressure extending across the AK
Panhandle and a broad trough over the central Aleutians and
Bering. This pattern will allow a series of lows to track from the
central and eastern Aleutians north into the Bering through
Saturday. How fast this occurs, along with the strength and
position of the low pressure centers remains unclear due to
guidance uncertainty.

Given this synoptic setup, periods of westerly and southerly
gales are possible over the Pacific side of central and eastern
Aleutians, respectively, from Thursday through Saturday as a
series of lows parallel the Aleutian Chain before lifting north
into the eastern Bering. Easterly gales are also possible from
Bristol Bay to the Pribilofs Thursday night into early Saturday.
The trend through the weekend is then for winds to weaken to a
broad area of cyclonic small-crafts across almost all of the
Bering as the main low weakens and lifts north. For the Gulf, the
biggest issue is the possibility of widespread southerly gales
developing on Friday as a low lifts north toward the southcentral
coast. These should begin to weaken by Saturday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Model uncertainty with the shortwave features between both
deterministic and ensemble runs remains quite high in the long
term with guidance diverging quite quickly by day three.
Specifically, the NAM and GFS begin day three solutions in rather
good agreement with a trough over the northern Gulf and a closed
surface low south of the eastern Aleutians with a triple point low
spinning up well south of Kodiak Island off of an occluded front
draped east from the main low. The EC, on the other hand, broad-
brushes a larger, weaker low south of the AKPEN. All three models
do build a ridge along the AK Panhandle during this time, so
guidance produces a northerly track for the low(s) to the west.
However, with the GFS and NAM developing a closed low south of
Kodiak, this center of circulation lifts into the western and
northern Gulf and eventually along the southcentral coast Friday
as a stronger, more organized system than what is depicted by the
EC.

The bigger picture, however, is that guidance is holding onto the
longwave solution of a developing ridge along the west coast of
Canada and a deep trough over the western Bering. This will favor
a trend for moderating temperatures as we head through the long
term with prolonged southerly flow from the central Bering east
to the Gulf allowing for various shortwaves bringing
precipitation across the forecast area from Friday through early
next week. Precipitation type issues for interior sections of
southcentral and southwest may also be a concern due to the
influx of warmer air aloft with the increased southerly flow.
However, as each wave moves through, more of the colder air at the
surface will scour out. Areas along the coast may see a mix of
rain and snow with the system on Friday, but mostly rain for
subsequent waves over the weekend.

Guidance is also hinting that the main low over the Bering may
begin to lift north beginning early next week. If this holds
true, expect a return of cooler temperatures from west to east by
the start of next week as colder air from the northern Bering and
eastern Siberia begins to wrap in behind the departing low.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 130>132 150 173 175 177>179 185.
 Heavy Frz Spray 185.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
MARINE/LONG TERM...TM


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