Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
455 am akdt sun Aug 30 2015
Analysis and upper levels...
An upper trough is amplified over the state this morning with two
centers moving toward each other. The one center is spinning near
Yakutat and the other near Barrow. This upper level trough and
another west of Shemya are pushing the dominant Bering ridge to
the south. The pattern shift is allowing cold air advection across
the Mainland as northerly flow is tapping into the Arctic air.
Scattered low clouds are filtering into the southern Mainland with
isolated showers across the area. The Copper River basin is mostly
cloudy as the low center in the Gulf drifts north. Across the
Bering...rain is developing along a nearly stationary front just
south of the Pribilof Islands. General wind flow is out of the
north across the north and eastern Bering with southerly winds
across the western waters as a low approaches the western chain.
models have a good handle on the pattern change into early this week.
Only minor changes were needed with the morning forecast package
using the NAM and Gem to refine the gusty winds along the
tightening pressure gradient across the southwest and western Gulf
through Monday. The general pattern into Tuesday will be cooler
with passing clouds along the trough over the Mainland and chances
for precipitation mainly along the mountains as shortwaves move
through the upper flow. To the west...a trough edges slowly into
the western Bering with the the ridge moving back into the
northern Pacific. This next trough will bring a warmer air mass
into the Bering along with increasing chances for rain.
Short term forecast southcentral Alaska (days 1 and 2)...
an upper level short-wave crossing the Kenai Peninsula will continue
into the Gulf this morning leading to strengthening of a surface
low sitting over the northeast Gulf. The combination of low level
cold advection...tightening pressure gradients...and mixing down
of stronger winds aloft in an unstable well mixed air mass will
lead to increasing winds region-wide. The strongest winds will be
along the polar jet over the western Gulf and Kodiak Island...as
well as through the usual coastal gaps of Seward...Whittier...and
Winds will slacken just a bit this afternoon through evening as
all forcing mechanisms weaken. However...an upper level low and
associated short-wave along with another polar jet streak will
drop down across southcentral Alaska tonight and into the Gulf
Monday leading to even stronger winds. Winds will peak most places
late Sunday night through Monday morning then gradually diminish
Monday through Monday night.
Isolated to scattered convection is focused out ahead of the
short-waves dropping out of the Arctic. It is difficult to
pinpoint the timing and location of specific showers so have used
a fairly broad-brush approach to the precipitation forecast. Snow levels
have dropped to approximately 2000 to 3500 feet across
southcentral...so expect to see some snow covered mountains.
Short term forecast southwest Alaska (days 1 through 3)...
northwesterly flow between a ridge over the Bering and an
elongated cold upper level low over Mainland Alaska will continue
to bring brisk northwest winds and isolated to scattered showers
to southwest Alaska through Monday morning. A weak shortwave
trough will graze the western convective available potential energy and Bristol Bay area this
morning with a stronger trough digging south further inland late
tonight and early Monday morning. Showers will be more numerous
over the lower Kuskokwim valley especially as the second stronger
shortwave swings through. Given the timing of the second wave and
the colder air aloft accompanying it think there is a good chance
of seeing some snow mixed in with the showers late Sunday night
through Monday morning especially in more eastern locations
towards the Alaska Range. Upper level ridging will shift east
over the the Alaska Peninsula Monday and Monday night ending
showers and starting a gradual warming trend for the first half of
the work week.
Short term forecast Bering Sea/Aleutians (days 1 through 3)...
upper level ridging over the central Aleutians will shift east and
amplify Monday and Tuesday as a vertically stacked upper low
becomes established over the western Aleutians. An accompanying
frontal system will push slowly east to be located over the
eastern Bering by Tuesday night.
Long term forecast (days 3 through 7...Tuesday through sat)...biggest
change for this time period from the previous days is that it
looks like the northern energy will keep the ridge suppressed a
bit longer. Rather than seeing the ridge be over the southwest and
southcentral portions of the state on Tuesday...there will still
be some pieces of energy moving through from the northwest. This
will keep some gusty winds...especially through favored areas like
Seward...Whittier...and the Barren Islands. It will also keep
temperatures on the cooler side for another day with 850 mb
temperatures right around 0c. Places further inland where the
winds die off first will likely be the coldest spots on Tuesday
morning. By Wednesday...models still indicate that an extension
of the North Pacific ridge should build over southwest and
southcentral. This will result in abundant sunshine and should
allow temperatures to moderate a bit going through the week.
Over the Bering...typhoon atsani remnants will stay anchored in a
broad trough over the western Bering. This will keep warm moist
air flowing into the eastern Aleutians and Pribilof Islands
leading to widespread fog and the chance for rain. By
Wednesday...a weak surface low develops over the central Aleutians
along an old boundary. Models are now in much better agreement in
carrying this low quickly northward toward the Bering Strait.
While not a deep low by any means...it could be just enough to
help to tick winds up to the small craft level. It will also bring
a fairly wide swath of rain from the eastern Bering to the West
Coast by Wednesday morning.
The latter half of the week will feature the warm front over
southwest Alaska falling apart as it tries to make it across the Alaska
Range. But the ridge and dry over the Mainland could be enough to
keep things dry across much of the area. Even if a few showers do
develop across the Kenai and Chugach Mountains into Friday...no
one system looks impressive at this time. The flow becomes more
zonal by next weekend...which could open the door for more weak systems
from the Bering.
public...gales 121 127 130 131 132 136 137 138 139.
Synopsis and model discussion...kh
southwest Alaska/Bering Sea/Aleutians...Jr