Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
457 am akdt Monday Aug 31 2015
Analysis and upper levels...
A long wave trough spans across the Alaska Mainland and western
Canada with the central axis over the alcan border. A strong
shortwave over the central interior is moving toward the Gulf this
morning and providing enough lift to generate a few showers along
the southern Susitna and Kenai. Elsewhere across the southern
Mainland...it is relatively dry with northerly flow gusting along
channeled terrain and across the western Gulf. To the west...an
upper ridge is shifting east across the Bering with a closed off
low spinning just west of Shemya. Moisture is streaming into the
Bering along southwesterly flow on the west side of the ridge. A
disturbance is evident on WV/infrared satellite imagery along 45n in the
North Pacific and it is moving toward the central Aleutians.
Models are relatively close in agreement on the 00z solutions and
therefore only minor edits were needed in the short term. The
higher resolution of the NAM and Gem were preferred for winds
through the short term into Wednesday. One notable change on this
forecast package was for increased precipitation chances for the
front moving into the the Aleutians/Bering tonight with a faster
progression into middle week. This front moves along with the disturbance
that is currently in the North Pacific. Models are not going to have
much accuracy on the track and timing of this system initially as
it is originating across a vastly data void area. The GFS and NAM
were in best alignment with this system and therefore were used to
speed up the front as they are showing a quicker approach.
Short term forecast southcentral Alaska (days 1 and 2)...
winds diminished across much of southcentral overnight but will
quickly ratchet back up during the morning hours as an upper level
low overhead (centered near anchorage) drops into the Gulf. The
strongest corridor of winds will be west of the low center across
the western Gulf/Kodiak Island as well as the Cook Inlet/Kenai
Peninsula area. A potential vorticity anomaly will help mix very
strong winds from aloft down to the surface. The axis of strongest
low level cold advection is also through this area...so denser
colder air will contribute to the downward motion. Although upper
level forcing is not nearly impressive farther east...tight
pressure gradients and an unstable boundary layer will still lead
to gusty winds...which will be enhanced through mountain gaps.
Winds will begin to diminish tonight as the upper level low
quickly exits taking the surface low over the eastern Gulf with
it. However...all models have now locked on to one more upper
level wave dropping down from interior Alaska and across
southcentral late tonight through early Tuesday. This will bring
one more surge of cold air advection and likely kick winds back
up...especially through gaps along Prince William Sound and the
northern Gulf...which includes Seward...Whittier...and Valdez.
Some inland locations will likely decouple during the evening...
allowing for light winds and more effective radiational cooling.
Thus...expect freezing temperatures to be a little more widespread.
The upper level ridge sitting to the west will build eastward
toward southcentral Tuesday through Tuesday night bringing an end
to the cool and windy pattern which has been in place over the
past week. With much lighter winds region-wide...Tuesday night
should be the coldest night thus far with widespread freezing
Short term forecast southwest Alaska (days 1 through 3)...
as the shortwave trough swings through eastern southwest Alaska
this morning the cold upper level low over interior Alaska will
follow...dropping to the south and east today and tonight. An
amplifying upper level ridge over the eastern Aleutians will
shift east over the Alaska Peninsula today and western Alaska
tonight causing scattered showers left over from the shortwave
this morning to quickly taper off and end through the day. Also
the strengthening ridge will begin a warming trend which will last
through the first half of the week. Drier weather will prevail
Tuesday with the next frontal system approaching the West Coast
Tuesday night and spreading rain inland Wednesday and Wednesday
Short term forecast Bering Sea/Aleutians (days 1 through 3)...
as the upper level ridge over the Bering shifts east a large
vertically stacked low off of the Kamchatka peninsula will track
northeast into the western Bering. A North Pacific surface low
will cross the central and eastern Aleutians Tuesday morning with
an associated frontal system then lifting north and east across
the Bering Tuesday and Tuesday night bringing southeast winds in
the 30 to 35 knots range. Meanwhile an upper level trough to the
south of the main western Bering upper low will swing east
dragging showers across the Aleutians from west to east Tuesday
night through Wednesday night.
Long term forecast (days 3 through 7)...
Models remain in good agreement through Friday with high pressure
over the eastern Bering Sea shifting east across the Alaska
Mainland on Wednesday and another series of upper level lows
across the western Bering Sea and North Pacific. There is some
discrepancy on how an upper low sitting east of the Kamchatka
interacts...if at all...with a second upper low pushing off the
southern tip of the Kamchatka peninsula on Wednesday. This
creates some placement issues as the NAM tries to keep some
interaction between the two systems while global models hint very
little interaction. The net result of this interaction will be
seen as the upper low falls apart and becomes more of an open wave
heading into Saturday. Significant deviations in the models
develop in the shortwave placement of this wave as it enters the
western Gulf of Alaska and therefore precipitation along the Gulf
Coast. In addition....the upper level trough looks to sharply
amplify late Saturday with the injection of a potent positive
vorticity anomaly. The GFS wants to undercut the building ridge
and lead to a wet zonal flow pattern across Alaska while the European model (ecmwf)
uses the cold air dropping out of Siberia to amplify the trough
and therefore the ridge building back across the Alaska Mainland.
The European model (ecmwf) solution appears to be the more probable one based off
of pattern recognition...especially with abundant cold air
dropping out of Siberia/the Arctic.
The Bering Sea will likely remain under a large upper level trough
leading to yet another period of showers as cold air aloft keeps
middle-level lapse rates favorable for convective showers. Southwest
and southcentral Alaska should remain mostly dry with a few
showers through much of the extended...especially if the European model (ecmwf)
solution verifies with the re-amplification of the ridge over the
Mainland. The one hiccup comes on Saturday as the upper level low
mentioned early begins pushing into the western Gulf of Alaska and
may clip the Alaska Panhandle. This would bring some rain mainly
to the Bristol Bay area and coastal locations. The period of
normal to slightly cooler than average temperatures looks to
persist through the majority of the extended.
Marine...gales 120 121 126 127 128 130 131 132 136 137 138 139 150.
Synopsis and model discussion...kh
southwest Alaska/Bering Sea/Aleutians...Jr