Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
422 PM akdt Tuesday may 26 2015
Analysis and upper levels...
a large amplitude pattern persists over Alaska with a large low
over the western Bering Sea and ridge that extends through British
Columbia into the Beaufort Sea. In between the two large features
is an upper level low over the western Gulf of Alaska and a front
making its way to the West Coast of Alaska. The upper level low
over the Gulf of Alaska is pushing moisture into south central
Alaska. The air mass is a bit unstable but with the cloud cover
convective initiation will be limited to the outskirts of the
cloud cover this afternoon and evening. This pushes much of the
convection north of the Alaska and Wrangell mountain ranges. High
pressure at the surface over the Gulf of Alaska is bringing low
level moisture to the Gulf Coast locations as evident by stratus
at many of the locations this morning. With the front across the
eastern Bering Sea gale force winds and moderate precipitation is
accompanying the front. The front is stretching and weakening
this evening as it runs against the ridge over the eastern
Mainland. An upper level disturbance coupled with a 150 knots jet
streak on the back side of the upper low in the Bering Sea has
increased winds in the far western Aleutians today and will weaken
tonight as the low continues to wrap up in the western Bering Sea.
due to the fact the atmosphere remains in a strong long wave
pattern model agreement is high through Thursday in the upper
levels and through Friday at the surface. Therefore forecast
confidence is high for the next 48 to 72 hours.
The low in the western Bering Sea will continue to wrap up into
tonight and retrograde to the Kamchatka peninsula before
dissipating on Wednesday. A strong short wave trough coupled with
a 130 knot jet will dive to the south of the western Bering low
Wednesday and eventually become the dominant low Wednesday night
as it deepens into the 970s mb near Attu Island. This stronger low
builds up the ridge over the Mainland. As the low builds in the
western Bering a front will push through to the central Bering
The front in the eastern Bering pushes to the coast this
afternoon/evening but weakens as it hits the ridge tonight. A
secondary upper level disturbance moves around the big low in the
Bering Sea and is expected to push the weakened front inland
Thursday across the Mainland. This upper level feature is enough
to spark convection but likely will be limited to along the
Alaska Range. The short wave ridge behind the upper level feature
keeps southwest Alaska Thursday afternoon.
Over south central any convection...thunderstorms...will be
limited to the Alaska Range and north toward Tok. Expect showers
over the Copper Basin where the air mass is unstable enough but
cloud cover will limit the chance of thunderstorms this afternoon
and evening. Shower activity will diminish overnight. High
pressure at the surface will have low level moisture present
through Thursday morning. Afternoon heating is breaking that up
today and will again tomorrow but overnight stratus is expected
again tonight. Stratus in most areas will be shorter lived
tomorrow as an upper level trough moves through the area mixing
the air mass. This upper level trough will also initiate showers
along the Talkeetna Mountains and Alaska Range Thursday afternoon.
High pressure builds back Friday for clear skies and warmer
Short term forecast southcentral Alaska...
as the upper level low pushes off to the southeast high pressure
builds in from the southwest. Rain showers will continue to
diminish as the low moves away with cloud cover north of the Kenai
Peninsula also starting to lighten up. Underneath the high
pressure stratus and fog is expected to move in over the western
Kenai Peninsula tonight but should clear up by tomorrow afternoon.
Conditions will continue to improve through Thursday with
relatively light winds around the southcentral region with
afternoon/evening sea breezes.
Short term forecast southwest Alaska (days 1 and 2)...
a frontal system will reach the West Coast of Alaska this evening
where it will weaken and remain stationary through Wednesday. An
upper level trough will swing the remnants of the frontal system
inland in the form of scattered showers Wednesday night and
Thursday morning. A building ridge will push the showers northeast
and out of the area Thursday afternoon and evening with drier and
more stable conditions building in Thursday night.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
the area of low pressure in the western Bering will continue to
fill as it drifts northwest tonight and Wednesday. As the parent
low pulls further away and the advancing Bering frontal system
runs into the large blocking ridge inland it will stall and
weaken along the West Coast as it is gradually sheared apart.
The next rather potent low pressure system will rapidly deepen
Wednesday as it approaches the Aleutians from the south reaching
its deepest point late Wednesday night. Current model runs track
the center of the low over Kiska Island Thursday morning then lift
it north through the Bering following a very similar path to the
last system. Also like the previous system this looks to be
another unseasonably strong low pressure system with good thermal
and upper level support. It will likely bring a broad area of
gales with it to both the Aleutians and Bering with the potential
for stronger winds mixing down in the area of cold advection to
the lows south.
Long term forecast (days 4 through 7)...
A pattern change may be underway starting Friday as an upper
level trough exits the southcentral Mainland to the north and
pushes further into the interior by the weekend. Models are in
fair agreement with positioning and strength of this feature but
begin to struggle as time progresses through the weekend. The ec
and Canadian models show this feature join up with a trough
associated with a low over northern Canada splitting the ridge
into a closed high over the Chukchi Sea and a broad area of high
pressure south of the Gulf of Alaska. This would essentially
result in showery conditions developing over parts of the
southcentral interior Saturday through early next week. On the
other hand the GFS shows this energy track eastward over
northwest Canada Saturday with a separate wave over the interior
pushing southward over the southcentral interior by Sunday
afternoon. This would result in drier conditions over the
southcentral interior through much of the weekend with showery
conditions developing Sunday afternoon through early next week. At
the surface models are in fair agreement with the high over the
Gulf of Alaska retreating to the south as lower pressure noses in
from northwestern Canada through early next week. However with
the model discrepancies in the upper levels forecast confidence
regarding the above mentioned features remains below average.
Further west...as the low over the far western Bering and the
associated front over the southwest coast weakens Friday
night higher pressure will briefly build over the Bering and
Aleutians. This will give the the southwest Mainland and much of
the Bering/Aleutians a quick break in the rainy and windy
conditions through Sunday before another frontal system tracks
into the western Aleutians from the west Sunday. Wet and windy
conditions will pick back up over the western Bering/Aleutians
Sunday afternoon as the front moves through then weaken as it
spreads eastward over the central Aleutians into early next week.
Models continue to struggle with the timing and strength of this
feature leading to lower than normal forecast confidence in the
arrival time of the winds and precipitation into the western and
Marine...gale 175 176 177 180 180 413.
Synopsis and model discussion...jn
southwest Alaska/Bering Sea/Aleutians...Jr