Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 200209
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
509 PM AKST Tue Feb 19 2019

.ANALYSIS AND UPPER LEVELS...
The overall synoptic pattern remains much the same as the last
forecast package with an upper level ridge positioned over
mainland Alaska with a trough to the west over the Bering. The
main weather driver is the vertically stacked low that moved up
from the North Pacific yesterday to the central Aleutians and
eastern Bering this afternoon. The front arcs through Bristol Bay
then south across the Alaska Peninsula and Southwest. There is a
150-160 knot jet along this front supporting this low along with a
strong plume of moisture advecting along this frontal boundary
from 25 north latitude. Currently along the front and in advance
of the low we are seeing strong gusts and primarily snow showers
for across the eastern Aleutians into the Alaska Peninsula.
Conditions are clearing from the west across Southcentral Alaska
as the upper trough that moved through last night continues to
push east. This is bringing dry conditions, gusty outflow winds, and
even blue skies to the Anchorage Bowl, before the pattern shifts
for tomorrow afternoon.

&&

.MODEL DISCUSSION...
Models are in good consensus with the overall synoptic setup
through the forecast package, including the low lifting north from
the central to northern Bering through tomorrow afternoon. The
high resolution models are also in strong agreement bringing ample
moisture supporting the higher QPF amounts across the west side of
the Alaska Range tonight that migrate to the Cook Inlet and
Southcentral Alaska regions through Wednesday. The models are
also in good agreement with the placement and intensity of the
second low approaching the western Aleutians tomorrow afternoon.

Since there is strong model run to run consistency, this is
leading to high confidence in model solutions for snow returning
to Southcentral Alaska starting tomorrow afternoon. At this point,
the NAM is slightly further north with the surface low and thus,
the most bullish with the QPF. More details for the evolution of
this snow event will follow in the short term south central Alaska
discussion.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through the
night with some patchy fog possible after midnight into the
morning hours. Snow is expected to return to the airport late
tomorrow afternoon ushering in IFR CIGS and VSBY tomorrow evening.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
(Tonight through Thursday evening)

The forecast remains on track as model consensus is good, leading
a higher than average forecast. A strong mid-level disturbance,
currently in the eastern Bering, will cross the Alaska Range and
into our neck of the woods tomorrow. This system has abundant
moisture with it, as it has tapped the tropics as a moisture
feed. A strong jet to our south will place Southcentral in the
left exit region of the mid and upper-level jet streams leading to
abundant lift, aided by strong vorticity advection. All of these
features will help induce cyclogenesis (low pressure development
at the surface) over Prince William Sound, with the deformation
zone (an area of lift where winds tend to converge as you increase
in height) looking to set up along the the western Kenai
Peninsula, through the west side of Anchorage, into the western
Matanuska Valley.

As the surface low develops and low level winds change directions,
this should allow this band to move further inland towards the
upper Hillside and eastern portions of the Matanuska Valley. As
this occurs, orographic lifting will further enhance precipitation
efficiency, with a pretty uniform distribution of snowfall
accumulation expected, which looks to be in the 4 to 8 inch range
for these areas. Further south along the western Kenai, both of
these processes will help boost snowfall totals by a couple
inches. If the latest trends in the models depicting the surface
low being slower to depart to our east end up verifying, the
current snowfall forecast will likely need to be increased.

&&

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

In the wake of the recent frontal passage a surface ridge has
moved into Southwest Alaska. The latest nighttime microphysics
satellite image shows an area of fog around Bethel and extending
northward. But the fog hasn`t been able to make it south of the
Kuskokwim mountains. With that surface ridge in place, today will
be the respite between storms. Looking down range, there is a
mature low south of Adak. The latest AMSU (Advanced Microwave
Sounding Unit) has the richest portion of the moisture plume south
of Adak, Unalaska and Sand Point. Expect the edge of the
precipitation shield to continue to push north and east today but
it won`t actually reach SW AK until tomorrow morning. The thermal
profiles of the atmosphere suggest that this will be a mixed mode
event meaning, expect some snow and potentially a change in
precipitation type at least for the Bristol Bay area and the
AKPEN. Portions of SW AK that are further inland look
substantially colder and those communities (such as Bethel, Aniak
and Sleetmute) can expect a fresh blanket of snow.

&&

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

The latest ASCAT/Advanced Scatterometer pass has detected gale
force winds south of the Aleutians and in the Western Pacific.
There is a mature low south of Adak with a well-defined occlusion
and warm front. The latest GOES Airmass image already has the
associated cirrus shield over the Pribilof Islands and clouds
wrapped behind the low near Shemya with the cold front extending
into the North Pacific. The storm is expected to intensify as it
moves northeastward bring gale and storm force winds to the
Bering, as well as rough seas and another round of precipitation.
All of the major operational models suggest that there is a
thermal ridge that will develop bringing a wedge of warm air into
the Bering. There is no doubt that cold air will continue to
advect on the back side of this frontal boundary. However, this
storm absolutely has a warm sector. Rain and snow will accompany
this system.

&&

.MARINE (Days 3 through 5)...
Thursday through Saturday continue to look windy across the
Bering Sea as a strong low moves up the Kamchatka Peninsula. Winds
ahead of the low in the central Bering are still expected to be
around gale force, but the forecast winds around the low center
have increased in strength and storm force winds are expected for
the Bering Sea and just north of the western Aleutians through
Friday. Saturday appears to be a bit of a break as most of the
energy will go into a low moving up into the central Aleutians,
but gales can be expected to persist across the central Bering.

In the Gulf of Alaska, a low moving into the Prince William Sound
and sliding off to the east on Thursday will increase
northwesterly winds through the gaps. These are expected to be
gale force through the Barren Islands and into the Gulf of Alaska
but higher gusts are possible, especially along the coast in the
gap areas.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long term forecast continues to be on track from yesterday,
with a ridge of high pressure building in over mainland Alaska,
pushing the main storm track westward over the Bering Sea. Models
are trending towards a stronger high pressure system, which means
that it looks less and less likely for precip to make its way
inland from any lingering fronts over the weekend. This high is an
unusually warm high pressure system for this time of year,
looking more like a spring time high pressure system than the cold
winter highs we see in late January. Solar heating is still
marginal, so spring is not here yet...but temperature forecasts
for the weekend and early next week across the mainland look to be
a bit of a challenge. Strong high pressure like this can also
bring ample valley fog the longer it persists. If this occurs, it
will further complicate the temperature forecasts.

As for the Bering Sea, the usual active storm track of the winter
will continue. The storms moving through aren`t expected to be
unusually strong, but the amount of warm air aloft will bring rain
much further north than we normally expect at this time of year.
Warm and wet conditions are expected to continue for the eastern
half of the Bering through early next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisories: 101, 111, 121, 145, 141.
MARINE...Storm Warning 177-185 412 414
 Gale Warning 119 125 170 173 175 176 411 413
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...AH/SA
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MM
MARINE/LONG TERM...LF



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