Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 290045
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
445 PM AKDT Tue Mar 28 2017
.ANALYSIS AND UPPER LEVELS...
A high amplitude upper level trough extending across western
Alaska down into the western Gulf of Alaska is slowly becoming
negatively tilted as the southern end of the trough begins to lift
northeast. A low level baroclinic zone sits right on the front
edge of the trough (to the east of Kodiak Island) with a steady
stream of moisture advecting north toward south central Alaska.
Radar imagery shows widespread snow falling along the Gulf coast
and spreading inland across the Kenai Peninsula to Anchorage and
the Mat-Su as a surface low moves onshore near Seward. Another
distinct surface low is tracking north across the Gulf of Alaska
out ahead of the baroclinic zone. This low will become a key
player in producing additional snow across south central Alaska
High pressure centered over the central Bering Sea and Aleutians
continues to produce cool and dry conditions across the eastern
half of the Bering Sea and southwest Alaska. Cold air advection
behind the upper trough axis is producing some localized gap winds
on the south side of the Alaska Peninsula. To the west of the
upper high a weak system is lifting north across the far
southwestern Bering Sea, bringing some light rain to the western
Aleutians. To the south, a large deep low over the north Pacific
is tracking east along the subtropical jet stream. This system
will become a major player in the forecast for later this week.
Although latest models are in much better agreement with the track
of lows across the Gulf of Alaska, there are subtle but critical
differences with the track of lows inland along with waves moving
along the front edge of the upper trough. This is leading to
significant differences in the precipitation fields. Thus, timing
and location of heaviest snowfall remains the biggest forecast
challenge, especially for the Kenai Peninsula up to Anchorage and
the Mat-Su. Conditions for heavier snow do become much more likely
overnight tonight into Wednesday morning with a more well defined
low backing into western Prince William Sound and a strongly
negatively tilted upper level trough. Thus, feel more confident in
upping snow amounts for this time period.
PANC...Periods of snow will continue through at least midday
Wednesday with MVFR conditions prevailing. The challenge is
determining the timing of heavier snowfall at which time
conditions will drop to IFR or possible even briefly LIFR. Right
now there is quite a bit of uncertainty in snow intensity for
late this afternoon through evening. Confidence is higher for a
period of heavier snow overnight tonight through early Wednesday
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Snow will increase in intensity through this evening from
Anchorage north into the Mat-Su region as the upper level trough
becomes more negatively tilted and begins to lift north into the
Cook Inlet region. This will coincide with a fast moving surface
low tracking into western Prince William Sound and spreading ample
moisture into the region. The heaviest snow will occur during the
overnight hours into mid Wednesday morning, with the dynamic
setup appearing to favor the highest accumulations across the
Anchorage Bowl as mid level deformation develops overhead.
Therefore, we have issued a Winter Weather Advisory for the
Anchorage Bowl. The Mat-Su region is expected to remain below
advisory criteria, although several inches of snow is likely to
fall along the Parks Highway corridor from Wasilla to Willow, with
slightly lower amounts toward Palmer. Elsewhere, occasionally
heavy precipitation will continue to fall through Wednesday
afternoon along the gulf coast, with snow continuing to pile up
from Whittier to Valdez and through Thompson/Turnagain passes.
Snow will quickly diminish area-wide on Wednesday afternoon as the
trough lifts through south central Alaska.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The high pressure and northerly flow that had persisted over the
southwest Mainland over the past few weeks has given way to a
slightly more active pattern as an upper level trough has set up
over the western interior. This trough is however, very dry
throughout the atmosphere, so anything that does develop should be
light and limited to the Alaska Range. As a result we will remain
generally dry with seasonable temperatures through Wednesday.
Starting Thursday the incoming pattern change will become much
more apparent as a gale force front approaches from the south.
This will spread gusty easterly winds along with rain and snow
throughout the area for Thursday and Friday.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
Weakening high pressure will keep much of the Bering Sea region
quiet for one more night before the next storm force low moves in
beginning on Wednesday. This system is currently well south of the
Aleutians, but will quickly track towards the eastern Aleutians
and Alaska Peninsula as it strengthens to storm force on
Wednesday. With this track, the worst conditions should occur over
the Alaska Peninsula on Thursday with snow mixed with rain and
gale force easterly winds. Depending on how much snowfall reaches
of Bering side of the Alaska Peninsula this may produce some
areas of blowing snow. This was however left out of the forecast
as there is currently no transportable snow in the area with
uncertainty in how much snow will fall on the Bering coast. This
low will then move over southwest Alaska on Friday, leaving
widespread northerly winds and a chance of showers in its wake.
.LONG TERM FORECAST (Days 3 through 7)...
Thursday will be the day that the upcoming pattern change will
begin to be felt across much of southern Alaska. An upper level
trough is still expected to amplify in the coming days as strong
jet support traverses the western periphery of the trough. This
will help to advect a warmer and wetter air mass up towards
Alaska. On Thursday, a surface low pressure south of Dutch Harbor
will begin to weaken. However, the frontal boundary associated
with this low and a surplus of upper level energy will help a
triple point low to develop south of the Alaska Peninsula. This
low is expected to move inland sometime late Friday morning into
Friday afternoon but a a bit of model divergence has made it quite
difficult to pinpoint all of the big details at this time.
With certainty, southern Alaska will see a pattern shift that
will entail temperatures warming a good bit above normal and
periods of rain and snow that will largely be confined to the
northern Gulf Coast. As it appears now, Anchorage and other areas
on the lee side of the Chugach Mountains will be downsloped from
any precipitation with impressive cross barrier flow developing.
Confidence is increasing that a chinook type pattern will set up
over the Anchorage bowl on Thursday and/or Friday which could
allow temperatures to warm into the lower to mid 40s. The
temperature forecast is heavily dependent on the amount of wind
that makes it into town but a warm up is nevertheless inevitable.
A mix of snow and rain will likely be seen along the coast
Thursday afternoon ahead of the frontal boundary but precipitation
across most of south central Alaska will quickly change to all
rain as 850 temperatures warm to near zero degrees Celsius and sfc
to 850 mb thicknesses rise quickly. Any precipitation from Friday
morning onward across south central Alaska will likely be rain,
except for portions of the Talkeetna Mountains, the Alaska Range
and the Aleutian Range where some minor snow accumulations could
Heading into the weekend, chances for rainfall and possibly a mix
of snow at times cannot be ruled out thanks to an active synoptic
pattern. A series of lows will pass through the Gulf of Alaska
heading into next week but trying to come up with any low position
is near impossible this far out as the models are quite volatile
past 4 or 5 days. The weekend looks warm as temperatures will
likely remain above normal and no reprieve is seen through the
first half of next week.
MARINE...Storm 172. Gale 130 150 155 165 170 173 175 175 176.
Heavy Freezing Spray 179.
SYNOPSIS AND MODEL DISCUSSION...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PS/DEK