Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
512 am akdt Friday Jul 31 2015
Analysis and upper levels...
Very few changes have occurred from yesterday as the stagnant
weather pattern has persisted. An upper low over the southwestern
Gulf is now beginning to cut off as a ridge over the western
Bering and eastern Russia shifts eastward due to subsidence.
Between these features...the jet stream is flowing generally along
the 170w Meridian through the Bering into the North Pacific. A
stationary vertically stacked low persists south of the western
Aleutians...cut off south of the western Bering ridge. That low
remains the strongest surface feature to affect the Anchorage
office area of responsibility. Water vapor imagery indicates much
drier middle- level air moving south toward southern Alaska from the
western and central interior.
The models remain in excellent agreement on the large scale
pattern evolution going into the weekend. The lack
of definable surface features near Mainland Alaska means that
small upper level disturbances interacting with the terrain and
localized convergence boundaries will remain the driving factors
for precipitation through Friday. As an upper level ridge builds
east into Mainland Alaska...warming temperatures in the upper
levels by Saturday will diminish the instability that has
supported diurnal shower and thunderstorm development. Confidence
is high on the overall pattern and moderate on the timing and
location of any showers and isolated thunderstorms through
A moist weather pattern has persisted for the past week and
brought periodic showers to much of southern Alaska. A warmer and
drier pattern is expected with less in the way of cloud cover by
the weekend...with humidity levels dropping below 30 percent this
weekend immediately downstream of the Alaska Range in the Susitna
Valley and Copper River basin. The wet thunderstorm potential
remains through today especially from the Talkeetna Mountains
south to the chugach range...after which warming temperatures
aloft will cap potential convection and significantly reduce the
threat for thunderstorms. The lack of any strong surface features
through the weekend will keep winds in most areas light.
Short term forecast southcentral Alaska (days 1 and 2)...
Downslope drying will clear skies out from the Susitna Valley
down through the Cook Inlet region...including outflow areas along
the north Gulf Coast such as Whittier and Seward. However...low-
level moisture will be much slower to retreat from the mountains.
With sufficient low-level moisture and moderate to strong drying
a few thousand feet above...instability will greatly increase from
the Talkeetna Mountains south to the chugach and Kenai mountain
ranges. However...given storm motions likely to stay north or even
north- northwest...the greatest impact on populated areas will
likely be the Glenn Highway corridor from Palmer to Eureka...which
will lie in the crosshairs of thunderstorm activity that will move
off of the Talkeetna Mountains. These storms will have the
potential to produce heavy rains...small hail...and gusty winds
Increasing stability and warming temperatures aloft will largely
cap convective activity for Saturday...with the possible exception
being areas along the eastern chugach range from the Richardson
Highway east and south of the Edgerton Highway...where isolated
thunderstorms will be still possible.
A middle level ridge will then build into the area by Saturday
morning...allowing for warmer and drier conditions to spread into
the area and persist into early next week.
Short term forecast southwest Alaska (days 1 and 2...Friday and
sat)...another round of convection is in store for today...but it
should be slightly more consolidated further south than days past.
The ridge from the Bering will continue to build in over the next
few days. However...on the leading edge of that ridge...in a
corridor from King Salmon to the Aleutian Range...there will still
be enough instability this afternoon to make for some
thunderstorms. While they will not be terribly widespread...those
that do form will have a rather unstable environment to take
advantage of which could result in some heavy downpours...gusty
winds...and small hail. Northwest to northeast flow will push the
storms through the Alaska Peninsula and into the Gulf of Alaska
during the overnight hours.
Saturday we will start to see the ridge building in a bit
stronger. This should help to cap any thunderstorms from
developing. Although a very weak upper level wave will climb over
the top of the ridge and could trigger a few showers late in the
day...especially over the Kuskokwim Delta. Otherwise...a warming
and drying trend will start to establish itself through the
weekend and persist into early next week.
Short term forecast Bering Sea/Aleutians (days 1 and 2...Friday and
sat)...high pressure will remain firmly in control of the area
for the next several days. With abundant low level moisture...this
means low stratus and fog will likely become more of an issue each
day. It will also keep winds light for most of the Bering Sea. The
exception to these conditions will be the central to western
Aleutians. One weak area of low pressure will drift southward away
from the area today...but it will make for enough gradient with
the high to the north to keep some modest easterly winds in the
forecast. A new low will form along an old boundary south of Dutch
Harbor Saturday into Sunday which will reinforce these easterly
winds. Models are still struggling with the details of this
system...so opted to hold at small craft speeds for now.
Long term forecast (days 3 through 7)...
The 12z deterministic and ensemble guidance are finally in good
agreement with the progression of the synoptic pattern through the
majority of the extended period. An upper level trough south of
the Aleutians islands and extending to the Alaska Panhandle will
begin pinching off from the mean flow as high pressure over the
Bering Sea and North Pacific build and cut off an upper level low
south of the eastern Aleutians on Sunday. This low looks as if it
will stay nearly stationary while tracking slightly north during
the week as building high pressure traps the low in the North
Pacific/Bering Sea. Two other upper lows (one located over the
Kamchatka peninsula and the other over the Alaska panhandle) will
act as anchors in the longwave pattern and prevent the breakdown
of the ridges which would lead to a more progressive upper level
pattern and a pattern change. What this means for Alaska is that
southcentral Alaska will likely remain on the dry side with plenty
of breaks in cloud cover as high pressure remains in full control
of the weather. However...southwest Alaska and the Bering Sea
could see a bit cloudier and wetter weather depending on the exact
track and extent of the upper level low and its associated surface feature.
Synopsis and model discussion...cirrocumulus
southwest Alaska/Bering Sea/Aleutians...MO