Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
446 am akdt Sat Sep 20 2014
Analysis and upper levels...
the high amplitude longwave trough continues to sit centered between
the southwest and southcentral Mainland. Impressively the trough
stretches from the Brooks range well into the sub-tropics with
some cyclonic curvature to the wind field aloft just east of the
Hawaiian islands around 20n latitude. On the downstream side of
said longwave several baroclinic Leaf structures are apparent on
water vapor imagery...a telltale sign of developing low pressure.
One is just west of the Panhandle...the other down in the middle-
latitudes. Closer to home...the previous closed low center aloft
in the Gulf is quickly moving inland and deamplifying as an open
wave. The steady rain associated with it is now dissipating while
a more showery regime is setting up in the northern Gulf. Pressure
rises behind the departing system have brought a wave of stronger
southerly winds across the north Gulf Coast and through the gaps
in the chugach as well. Further out west...an upper ridge is
translating eastward across the Bering Sea ahead of a strong
Kamchatka low pressure system.
in the upper levels...guidance is in great agreement through the
short term forecast. The two developing systems in the eastern
Gulf will effectively refocus the upper trough to the east while
both the Kamchatka low and upper ridge in the Bering Sea continue
to slide eastward. At the surface there is a little more
variability...mainly with the strong low developing in the eastern
Gulf...but in general all guidance has a strong compact system
moving west of the Panhandle through the weekend before weakening
in the northern Gulf.
Short term forecast...
Southcentral Alaska...mostly cloudy conditions a more showery
pattern is in the works today. The best chances will remain along
the north Gulf Coast and over Prince William Sound. Diurnal
showers moving off the terrain will be in store for the afternoon
and evening hours for the chugach. After today a strong system
moving up toward the Panhandle will begin to switch the flow
around aloft in conjunction with ridging building over the
southwest Mainland. This flow reversal will bring some colder air
across the Alaska Range over the region. Combined with a weak
surface pressure pattern and decreasing cloud cover...some of the
coldest overnight temperatures of the season can be expected into
early next week.
Southwest Alaska...the upper level is finally starting to exit the
area but it will keep some "parting" showers in the forecast
through today. Later tonight and into Sunday...ridging from the
Bering will nudge into the Mainland and finally shove the low into
the longwave trough over the Gulf. This will bring a drying
offshore flow to most of the Mainland. Over the Alaska
Peninsula...it will bring Stout northerly flow which results in a chance
of showers....mainly on the Bering side... through the weekend.
The offshore flow over the Mainland will increase through the day
Sunday and should help to limit fog development. However it could
make for gusty conditions...especially in the afternoons with
Bountiful sunshine and a well mixed atmosphere. The offshore flow
will also bring cooling conditions. Many spots are likely to
approach the freezing mark by Monday morning. The flow will relax
Monday and Tuesday as the ridge axis moves overhead. Clouds won't
start to increase again until late Tuesday.
Bering Sea/Aleutians...high pressure building eastward will begin
to taper the northwest flow and showers over the eastern Bering
and Alaska Peninsula today. A weakening front and low pressure
system over the western Aleutians will linger through the weekend.
Long term forecast...
beginning the extended period on Tuesday...a trough of low
pressure will exist across the western Bering...with a weakening
upper low in the vicinity of the Copper Basin stretching southward to
the central Gulf of Alaska. High pressure aloft will be centered
over the eastern Bering and southwest Alaska. Lingering clouds and
showers over the Copper Basin and along the Chugach Mountains will
rapidly diminish as offshore flow develops through midweek. This
will set the stage for a somewhat unusual extended period of dry
weather...and cooling temperatures...especially during the night
time periods. The dry and pleasant weather will likely persist
into and possibly through next weekend as the main storm track
remains across the North Pacific.
Mtl/MO/ja Sep 14