Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 161450
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
550 AM AKST Thu Feb 16 2017

.ANALYSIS AND UPPER LEVELS...

An upper level trough extends from the North Pole to the North
Pacific this morning, with a cut-off low over Bristol Bay and the
Akpen. A wave is moving around the latter feature, taking aim at
Southcentral AK later today. Upper ridges are over the western
Bering and much of western North America, including the Canadian
Rockies.

At the surface, a large low is centered over the Akpen,
with a trough extending southwest along the Aleutians, and a weak
ridge over the Copper River Basin. A front is approaching the
north Gulf Coast. Snow showers are moving across the eastern Kenai
Peninsula, and around the AKpen low, with downsloping across the
rest of the Kenai as well as the Anchorage Bowl. Snow is
redeveloping along the eastern slopes of the AK and Aleutian
Ranges. Northeast flow across the Bering is spreading a few snow
showers over the region.

&&

.MODEL DISCUSSION...

Models are in agreement that the AKpen low will move slowly
northeast through Sat. First the Gulf front and then the low
itself will move into Southcentral during that period, bringing
varying amounts and types of precipitation to the coast, with snow
to inland areas. On the backside of this low, northerly flow will
persist across the Bering with snow showers. The forecast
utilizes GFS in the west with mesoscale models in the east to help
pinpoint precipitation amounts.

&&

.AVIATION...
PANC...Strong 850 mb flow perpendicular to the Kenai and Chugach
mountains will cause some low level wind shear through midnight.
This same flow will downslope the airport and keep precipitation
out of the forecast until a few showers develop in the vicinity this
afternoon. Although VFR conditions will continue into Friday, snow
showers will develop by midnight tonight and lower ceilings to
5000 feet.

&&

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

The next round of wintry precipitation for Southcentral on the
conveyor belt of storms looks underwhelming to say the least.
Let`s break it down...

An upper level low that has been rotating around Bristol Bay and
the Alaska Peninsula for the past several days has been the
primary driver of the precipitation seen across Southcentral. The low
is acting like the center of a wheel around which these numerous
spokes of energy are rotating. The next wave of energy is expected
to impact Southcentral today into tonight.

One factor favorable for snow with this next wave includes the
wave`s movement, which helps locally enhance the lift needed to
generate snow. The model agreement in the timing, orientation of
the wave, and relative strength is good enough to lend confidence
to the forecast. The wave looks to move slowly northward during
the day today, then turn and move more quickly towards the
northeast overnight. Another is the very unstable atmosphere shown
in the Anchorage sounding. Other than the usual nighttime
inversion near the surface, the rest of the sounding up to the
tropopause is very unstable. Finally, much of the sounding below
the tropopause is near saturation, except in the lowest levels,
where downsloping is warming and drying the atmosphere.

Another positive (for some) is location, location, location. In
this case, the favorable locations will be the southern Kenai
Peninsula, the Gulf Coast, and Prince William Sound. For the
latter two, the rationale is very simple. We are taking a moist,
very unstable air mass and pushing it into a "wall" of sorts in
the form of the Chugach and Kenai Mountains. This upslope adds
lift which enhances precipitation production. For the southern
Kenai Peninsula, less downslope due to fewer mountains in the way,
and proximity to the upper level forcing before it accelerates
northeastward will lengthen the time all variables for producing
snow will be present at any single location. A significant negative
will be the warm temperatures today as highs get into the mid to
upper 30s. This will certainly keep total accumulations down.

Now for the negatives from Anchorage north...and they too are
significant. First, thus far overnight, snow reports along the
Kenai Peninsula have been lower than expected, with little
expected to drastically change over the next 24 hours. Second,
downsloping so far has been eliminating any and all convective
shower activity except along the far southern Kenai Peninsula.
Southeasterly winds are prevalent throughout the lower atmosphere,
with the exception of a shallow layer right at the surface. Those
winds are moving perpendicular to the axis of the Chugach and
Kenai Mountains. When that happens, the flow must go up and over
the mountains, as going around them would divert the flow hundreds
of miles, which never happens. Further, with the unstable
atmosphere, little forcing is needed to get the air to rise upon
reaching the upwind side of the mountains. The convective snow
shower activity does fine up until this point, but once the air
begins descending the lee side of the mountains, it warms up and
dries out. That`s why if one ever looks at a radar loop when
downsloping is prevalent (like today), the convective shower
activity moving from off the Gulf to the northwest looks like it
gets erased as soon as it emerges from off the peaks of the Kenai
Mountains. Throughout this upcoming event, the winds are forecast
to hold out of the southeast throughout the lower atmosphere
except possibly right near the ground. Only during a brief window
right as the wave passes by does the airflow possibly turn
southerly, before turning back to the southeast once again. The
timing of this to occur just as the wave is passing is not ideal
either, but may allow a short time when some snow showers should
pass through. However, little more than that is expected.

Third, there is a complete lack of temperature advection. Now
that the upper level low and attendant surface lows have been
spiraling around the same area for multiple days, any temperature
differences between air masses that led to the formation of the
lows have been thoroughly mixed out. Thus, when this next wave
moves through today and tonight, there is no cold air nor warm air
moving in to replace the air mass that is already present. It`s
just one and the same. This matters because temperature advection,
specifically for Anchorage, cold air advection, is effective at
wedging the prefrontal warm, moist air upwards due to density
differences between the air masses. Without this lift, only
upslope or local convergence boundaries can sustain precipitation
generating processes for a long enough time to produce appreciable
snow. Neither are forecast to be present to a non-negligible
degree.

In summary, it looks like at some point Thursday night, the
negative factors will reduce enough to allow for some snow showers
and flurries even in Anchorage and the Mat-Su Valleys, but
significant accumulations are no longer expected.

But wait! There`s more! On Friday afternoon and evening, the
upper level low will transfer its energy to and become absorbed by
another upper level low moving south along the west coast of
Alaska. Before this process is complete, however, the closed low
near Bristol Bay will open up into a wave and track northeastward
around the periphery the West Coast upper low before it`s absorbed.
Here are some details on how that looks at this point:

The computer models are in far less agreement on the evolution of
the wave. This is both in timing and intensity. Small differences
in the orientation of the wave, it`s strength, and forward speed
translate to large differences in potential precipitation at the
ground. Thus uncertainty is very high. Right now though, it
appears that downslope may not be as great with that wave,
however, like most transient features, a significant limiting
factor will be its fast movement. Thus only a small amount of
snow is expected, though once again, with such high uncertainty,
a lot can change. Stay tuned!

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Having made one more circuit around the Alaska Peninsula, the
rather persistent surface low is currently curving back to the
north and tracking onshore over the Alaska Peninsula just to the
east of Pilot Point. The low will continue slowly north to become
vertically stacked with its accompanying upper level low and stall
over the greater Bristol Bay area this afternoon through tonight.
As the surface low weakens and the upper low opens to a trough the
entire system will start drifting to the northeast Friday as an
arctic upper level low digs south to become center over the
western Alaska coast in the vicinity of the Seward Peninsula where
it will remain into the weekend.

Snow will continue to spread further north into the Bristol Bay
area today and Kuskokwim Valley this afternoon and evening
accompanying the surface low and frontal system north. Steadier snow
will transition to snow showers as the low weakens then taper off
to scattered and then isolated snow showers through Saturday.
Temperatures will cool through the end of the week as the
reinforcing upper level trough digs south bringing additional
arctic air to Southwest Alaska. Strong northerly winds along the
Kuskokwim Delta coast and easterlies through gaps in the Aleutian
Range will diminish tonight through Friday as the low weakens.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
As the vertically stacked low in Southwest Alaska weakens to a
trough through Friday, northerly flow across the Bering will
weaken somewhat. An upper low south of the Aleutians tracks east
allowing an upper level ridge to build over the western then
central Bering and Aleutians Friday and Saturday. A frontal system
pushes across the western Aleutians Saturday.

&&

.LONG TERM FORECAST (Days 3 through 7)...

Beginning the long term forecast Friday evening, an upper trough
will be centered over northwest Alaska with a shortwave trough
along the southeast flank of the aforementioned trough moving into
the central interior. Bands of light snow will be ongoing from
interior southwest Alaska through Southcentral. Over the Bering,
there will be continued cold advection snow showers. Heading into
the weekend the upper trough will sharpen and dig south, bringing
in rather cold air into southwest Alaska (850 hpa temperatures -15
to -25C). Cold temperatures will last into Tuesday and will bleed
east into Southcentral as the trough finally weakens and moves
east. However, before that time, there will be continued light
snow chances for most of Southcentral Saturday through Monday
night mainly from the Chugach west.

By Monday, a deep low will move into the Western Bering Sea, and
this will result in a rather dramatic change in the pattern as a a
series of powerful shortwaves eject off a powerful North pacific
jet stream in a general southwest flow. Beginning Wednesday into
the end of the week, it is looking increasingly likely that a
barrage of lows and fronts will impact southern Alaska with
widespread rain and snow events. Temps will be tough as some
inland areas will remain snow while more prone coastal locations
may mix at times. Either way, expect very active weather by
midweek.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale warning 131 181 Heavy Freezing Spray 179.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...DS
SOUTHCENTRAL ALASKA...JPW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
LONG TERM...JA



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