Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 240050
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
350 PM AKST Thu Feb 23 2017
.ANALYSIS AND UPPER LEVELS...
The upper-level pattern across Alaska today is fairly amplified
with a trough over the western Bering and Aleutians and a ridge
over the Mainland. A vertically stacked weakening low over the
western Bering is embedded in the trough, bringing a swath of gale
force winds over much of the central and western Bering. Ahead of
the trough, a long fetch of deep sub-tropical moisture is rapidly
moving northward under a potent 160 kt jet. At the surface, the
warm sub-tropical airmass moving in under the jet has allowed for
temperatures to rise to above freezing over much of the Alaska
Peninsula and the southwestern Mainland. Precipitation moving
inland over the Kuskokwim Delta is associated with an upper-level
wave and a developing surface low north of Nunivak Island. This
precipitation is mainly falling as rain over the Kuskokwim Valley,
however some areas in the northern part of the Valley are
remaining cold enough for snow or a rain/snow mix.
Downstream, quieter weather is in place over Southcentral Alaska
under the effects of upper-level ridging and surface high
pressure. As the ridge axis pushes east and approaches the AlCan
border, subtropical moisture is moving in from the southwest and
keeping skies overcast and temperatures relatively warm. With the
lack of any significant forcing in the area however, Southcentral
is getting a brief break in precipitation today before the next
front approaches from the west tonight into tomorrow morning.
The models are initializing well with the current pattern and
remain in good synoptic agreement through the short term. There is
some uncertainty in regards to how quickly the warm air will move
out of the Southcentral area Friday, which lends to a slightly
lower confidence in precipitation type during the morning. However
by late morning confidence rises that enough cold air will move in
to support predominately snow. There are also some discrepancies
with QPF amounts as the front pushes through Friday. The NAM is
more bullish than the other models, bringing higher snow amounts
to the Anchorage area and the Matanuska Valley than the GFS and
ECMWF. The afternoon forecast trended towards the GFS and the
ECMWF with regards to precipitation amounts in the short term,
however forecast confidence in QPF amounts is slightly lower. The
highest uncertainty with regards to snow amounts exists over the
Matanuska Valley, where model spread is the greatest and favorable
topographic orientation could result in locally heavier snow
PANC...MVFR ceilings will likely return later tonight. Snow will
develop by late Friday morning and bring the potential for IFR
conditions (ceilings and/or visibilities).
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A very challenging forecast for the next day and a half is
setting up as a strong ridge currently over Southcentral flattens
out tomorrow as a shortwave trough tracks in from southwest
Alaska. Deep layer southwest flow is warming the low levels of the
atmosphere as indicated by the evening upper air balloon sounding,
and this will continue through the overnight. Tomorrow, light
showers may mix with some rain and very light freezing rain before
turning to all snow as strong evaporative cooling and dynamic
cooling take over. These patterns are subtle, but they tend to
favor higher snow amounts for locations banked up near mountains
such as Palmer/Wasilla and north toward Talkeetna. There is rather
significant uncertainty with how much moisture upslopes into those
areas, but this forecaster does feel there is sufficient moisture
top support impactful (but sub-advisory) level snow amounts from
the Matanuska Valley northward. Anchorage and Eagle River should
see less snow with the higher amounts close to the mountains. It
should be noted both the morning and afternoon commutes will
likely be affected (see Special Weather Statement).
A quick shot of snow will move through the Copper Basin and
Northern PWS before exiting Friday night. Quiet conditions will
quickly follow Saturday as flat ridging sets up. Snow threats will
continue along the Alaska Range and Northern Talkeetna Mountains
as another low tracks across Northwest Alaska Saturday.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The Southwest Alaska region continues with onshore flow with
remnants of a frontal boundary as it dissipates tonight. The
warming trend continues as temperatures rise above freezing which
precipitation falling as rain. The temperatures in the interior of
the Bristol Bay region will take a little longer before rising
above freezing tonight. Therefore, look for a wintry mix with some
pockets of freezing rain with no accumulation this evening for the
aforementioned region. As we head into the early Friday morning
hours, fog develops along the Kuskokwim/Bristol Bay coastline.
This weather phenomena accompanied marine layer stratus will
spread inland with the aid of the onshore flow. By Friday
afternoon look for some lingering snow showers along the Alaska
Range. Otherwise, the next round of precipitation occurs with a
weather front approaching the Southwest Alaska coastline by
Saturday. The precipitation will spread from west to east as the
frontal boundary moves inland. Therefore, look for snow as we head
into the weekend with the heaviest snowfall amounts occurring along
the Alaska Range due to upslope with the onshore flow.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The occluded storm force low near 59N 175E continues to track
north before being positioned over the Bering Strait by Friday.
The Bering Sea remains with gale force southwesterly flow, and the
majority of the Chain will have weaker southwesterly flow as we
head into Friday. Meanwhile, a storm force North Pacific low
tracks near Kamchatka Peninsula by early Friday morning. The
associated weather front approaches the Western Aleutians during
the same time-frame bringing warmer temperatures with a large
swath of moisture. Look for storm force southerly winds with some
gusts to 70 mph as this weather front moves through the Western
Aleutians into the western Bering by Friday afternoon. This front
will continue to spread eastward, and moves into the Central
Aleutians extending northward into the northern Bering Sea by
early Saturday morning.
.LONG TERM FORECAST (Days 3 through 7)...
The long range forecast transitions late weekend, returning colder
conditions across the southern mainland and much of the Bering as
the upper level jet flattens as it sags southward on Sunday. A new
upper level wave pattern develops on Monday with ridge amplifying
along the west Bering. Cold air advection along the Bering spreads
east across the mainland and gulf through the middle of next
week. Models are in relatively good agreement that a closed low
forms along a deepening wave Monday into Tuesday. This pattern
sets up to bring multiple waves from the Arctic to the gulf
through much of the next week which brings continued chances for
snow for the mainland. To the west...ridging will bring a quieter
weather pattern over the western areas of the Bering dominated by
low clouds and period showers with generally small craft winds
along the western Aleutian Chain.
MARINE...Storm Warning 177 178.
Gale Warning 120 131 170 173 175 176 179 181 185.
SYNOPSIS AND MODEL DISCUSSION...RD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD