Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
443 am akdt Monday may 4 2015
Analysis and upper levels...
A very compact area of low pressure resides nearly halfway between
Shemya and Adak this morning with a corresponding front now draped
across the central Aleutians and North Pacific. Scatterometer
data from Sunday afternoon showed a broad area of 40 to 45 knots
winds associated with the front but have since weakened to
marginal gale force as seen with more recent scatterometer data.
The weakening front can be attributed to the early signs of an
occlusion and a tiny fragment of warmer air in the middle-levels
getting advected northward...thus creating a bit of a stable layer
in the lower levels preventing the mixing down of stronger winds
aloft. The area of low pressure over the western Aleutians has a
stacked upper level low associated with it that is currently
interacting with a shortwave rounding the base of the upper level
trough. Water vapor imagery shows a beautiful interaction between
these two systems with the shortwave phasing with the upper level
low...which in turn will create a more broad upper low. One jet
streak rounded the base of the trough Sunday afternoon/evening and
a second jet streak shows up well on water vapor which should
round the base of the trough this morning/early afternoon. This
will help amplify the trough over the Bering Sea which will in
turn create ridging ahead of the low over the North Pacific and
Gulf of Alaska.
A second and much weaker upper level low is centered over the
Bristol Bay area this morning and continues to promote a
favorable atmosphere for convective showers with a decent cold
pool at 500 mb enhancing middle-level lapse rates. The cold pool
however has begun to weaken (warm) which has begun to slightly
stabilize the atmosphere and will continue to do so over the next
24 to 36 hours prior to phasing with the upper low over the Bering
Sea. Some fog has begun to develop over the Kuskokwim Delta
overnight as a dry slot well depicted on water vapor imagery moves
overhead and skies clear. How far the fog moves inland is the real
challenge as no onshore flow is expected. Any fog that develops
inland would be a result of radiational cooling under clearing
skies vs advection fog. Southcentral Alaska appears to be on the
tame side with weak high pressure in place leading to mostly clear
skies (except some cirrus associated with the upper low over
While the models did not capture the more westward track that the
low in the western Bering Sea took Sunday afternoon...they have
now locked onto the low and all synoptic models are in great
agreement. The upper levels are quite complex with the upper low
in the western Bering Sea phasing with a shortwave this morning
and additional jet energy being added to the system from Siberia.
However these features are all depicted in such a manner that
leads to high confidence in the overall evolution of the system in
the coming days. With high model confidence...a blend toward the
higher resolution models (nam/Gem regional) was selected.
Short term forecast southcentral Alaska (days 1 and 2)...
the upper low over Bristol Bay will elongate and send a trough
northward up Cook Inlet and the Susitna Valley today. The colder
air aloft will increase middle to upper level lapse rates...but since
conditions are starting out quite stable this will only create
weak instability. Dry air is also a limiting factor. Therefore...
expect nothing more than a few showers to form...primarily over
the mountains. Weak southerly steering flow will keep them there.
Otherwise expect another mostly sunny day across the region with
some afternoon and evening sea breezes along the coast.
Clouds will be on the increase tonight through Wednesday as the
upper ridge slides north and east of southcentral allowing
moisture from the Aleutian low to move in. The remnants of an
occluded front will make slow northward progress across the Gulf
with the bulk of rain remaining south of the Kenai Peninsula. As
this front approaches...the surface high over the Gulf will build
northward over southcentral resulting in development of weak gap
winds in the usual places (such as turnagain and Knik arms).
As far as fire weather conditions...there will be very little
change in relative humidities today. However...expect a gradual
trend toward increasing moisture beginning Tuesday with the
arrival of clouds and slightly cooler temperatures.
Short term forecast southwest Alaska (days 1 and 2)...
the upper low over the southwest Mainland will continue to
support a showery regime through Tuesday...as it tracks northward
over the area and weakens into an open trough by this afternoon.
There remains a slight chance of isolated thunderstorms along the
Alaska Range in the Kuskokwim valley this afternoon...as some breaks
in cloud cover will allow for surface heating and enhanced
instability. Precipitation will then become more focused along
coastal portions of Bristol Bay this afternoon and through
tomorrow night as the weakening front pushes in from the
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
rain and gusty southeast winds associated with the front currently
impacting the eastern Aleutians will continue to weaken and spread
eastward into the akpen and southwest coast through tomorrow
afternoon. Rain will also continue across the western and central
Aleutians through Tuesday but will become more showery in
nature...as the front departs to the east and the parent North
Pacific low remains south of the Aleutian chain. The western
Aleutians will continue to experience gale force gusts on the
back end of this system through tomorrow afternoon.
Long term forecast (tue night through Sat night)...
a series of disturbances/fronts will begin to move north from the
North Pacific through the western Gulf into southern Alaska middle-
to late week...bringing scattered to numerous showers to southwest
Alaska and the Cook Inlet region and rain from Kodiak to Prince
William Sound. Any shower activity over southcentral late
Wednesday will initially have to overcome dry air near the surface
and will likely fall as virga...but as the rest of the week
progresses it is likely many locations from the western Kenai
Peninsula to Anchorage to the mat-su valleys will pick up at
least a hundredth of an inch of rain by the weekend. Southeast
winds out of Turnagain Arm...Knik Arm...and through the Copper
River valley will increase but should not be all that strong as
forcing remains south over the western Gulf.
Most of southern Alaska will continue to see relatively unsettled
weather for may over most of the region late week through early
next week as upper level ridging continues over western North
America with deep troughing over the eastern Bering and western
Gulf regions...which will result in deep southerly to southeasterly
flow from the Gulf into southern Alaska.
Synopsis and model discussion...mc
southwest Alaska/Bering Sea/Aleutians...tp