Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
454 PM akst Wednesday Nov 25 2015
Analysis and upper levels...
A potent upper level jet streak stretching from southwest Alaska
well into the interior of the state has left southcentral Alaska
in a very moist warm sector currently...with a slug of total
precipitable water anomalies well over 200 percent of normal
moving north toward the area. In the lower levels...an unusually
strong low-level jet persists which has continued to raise
temperatures well into the 30s and 40s over the entire
region. As a result...the rain/snow line has been pushed well
above 1000-feet elevation as far north as the northern Susitna
Valley...with only Chulitna and Cantwell reporting significant
snowfall through the overnight into this morning.
Additionally...strong gap and Chinook winds through the region
impacting western slopes including Anchorage and Palmer have
largely melted the existing snowpack over the area.
To the west...a surface front remains draped along the Aleutian
Range...with disturbances moving north out of the North Pacific
along the front. One such disturbance is currently bringing light
snow to the eastern parts of Bristol Bay and the Kuskokwim valley
along the Alaska Range. On the west side of the jet
stream...subsidence under the left exit jet region has trapped
low-level moisture near the surface and resulted in fog formation
over the Kuskokwim Delta Region.
Further west...the next potent system with storm-force winds is
impacting the western and central Aleutians...with snow at onset
of precipitation quickly changing to rain. This system will
continue to trek east along the island chain through tonight.
Models are in great agreement synoptically through about Friday
evening...then begin to diverge slightly with timing of
disturbances moving south of the upper level low/trough over the
Bering Sea that will largely affect the Aleutians...as well as
disturbances moving north from the North Pacific toward
southcentral Alaska through early next week.
In the near term...models agree that low pressure will move north
from the North Pacific tonight up along the eastern slopes of the
Aleutian Range and western Cook Inlet into southern Alaska before
dissipating Thursday evening. As the low approaches...the warm
low-level jet over the Susitna Valley will weaken substantially
and allow for what we think will be enough cooler air to turn the
rain over to snow for the northern and western Susitna Valley.
An additional disturbance will move north through the western
Gulf into southern Alaska Thursday night into Friday.
Meanwhile...the storm-force low over the western and central
Aleutians moves east to near the akpen Friday before moving north
and dissipating over southwest Alaska Saturday.
Short term forecast southcentral Alaska (days 1 and 2)...
the warm and wet pattern continues for most areas this evening...except
for the far north Susitna Valley which continues to receive snowfall.
A disturbance and slug of moisture is making its way overhead tonight
bringing periods of heavier rain to most areas of the north Gulf Coast
and Cook Inlet region. The rain/snow line looks to remain in the far
north Susitna Valley. Rain will continue through the morning hours
before tapering off in the afternoon hours. The precipitation shifts
eastward toward the southeastern Copper River basin before shifting
back westward tomorrow evening. For this second round most of the
moisture will stay confined to the north Gulf Coast. Turnagain Arm
and higher elevation winds will continue to blow as they have been
through the overnight hours before relaxing in the afternoon...
however...this will be short lived as the gradient tightens back
up Thursday night.
Short term forecast southwest Alaska (days 1 and 2)...
a stalled front will be the focus for snow tonight across the
eastern areas of the Kuskokwim valley and Bristol Bay. A colder
and drier air mass stays in place across the Kuskokwim Delta to
the western convective available potential energy through Thanksgiving. Another front moves into
the Alaska Peninsula Thursday morning bringing gusty southeasterly
winds along with a mix of rain and snow. This front pushes into
the southwest Mainland on Friday spreading precipitation northward.
Snow develops initially along the frontal boundary...however a
warmer air mass moves inland along southeasterly flow just off the
surface. This warmer and moist air mass will slowly change precipitation
over to rain with a chance for freezing rain as surface temperatures
are slower to modify toward freezing.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
the storm force low centered near Shemya is expected to track
slowly east overnight. Winds are expected to be strongest along
the core tonight through Thursday with east flow ahead of the
system becoming northwesterly behind the system. A mix of snow and
rain can be expected along the front with the warmer air mass
making it to the Pribilof Islands and eastern Bering on Thursday.
Northwesterly flow persists through Friday as the surface low over
the Alaska Peninsula tracks to the north. Precipitation tapers off
on Friday however weak disturbances in the upper flow keep showery
conditions across the Bering and Aleutians through the end of the
Long term forecast (days 3 through 7)...
Models are in excellent agreement that by Friday evening the low
moving along the Aleutians will reach the akpen...and then move
into the eastern Bering where it will dissipate along the coast
Sat afternoon. The next low...however...contains abundant tropical
moisture from the western Pacific and as a result is increasing
model uncertainty. The GFS and ec bring the low to the eastern
Aleutians on Sat...and then they hint at moving the low to the
eastern Bering/Aleutians with different solutions. Wpc utilized
their ensemble approach...keeping the low south of the chain as it
travels to near Kodiak Island and eastern Bristol Bay Tuesday through
Thursday. The forecast uses this solution...especially confident
in the eastward progression of the low.
The east domain will be under the influence of strong, warm,
southerly flow through Sat with the first low...and then again
late Sat into Wednesday with the second. This will bring rain to the
coast changing to snow over much of the Susitna Valley and Copper
River basin through the middle of next week.
The west will have small craft winds associated with the initial
low and then potential gales early next week on the backside of
the second low. Rain along the eastern flank of the second low
will spread across the chain through the weekend...with a mix of
rain and snow on the backside beginning sun and lasting through
public...Snow Advisory 145.
Marine...storms 170 172 173 174 175 176 177 178 413.
Gales 119 120 125 131 132 155 165 171 179 185 351 352 411 412 414.
Synopsis and model discussion...cirrocumulus
southwest Alaska/Bering Sea/Aleutians...kh