Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
451 PM akdt Sat Apr 18 2015
Analysis and upper levels...
The longwave pattern remains highly amplified with a deep longwave
trough over the Bering Sea and North Pacific with a building ridge
over western North America and the Alabama-can border. A 120 knots
continuous Arctic jet is diving into the base of the longwave
trough and coming up out of the trough into the Gulf in a nearly
due south-to-north direction. This highly meridional flow is
bringing significant precipitation to Gulf coastal sites...with
Portage picking up more than 2 1/2 inches of liquid precipitation
overnight and this morning as a front moved onshore the north Gulf
Coast and dissipated.
A low south of the Alaska Peninsula bottomed out in the low to middle
970s mb range this morning and has begun to weaken as it drifts to
the northeast. Satellite imagery confirms the circulation around
this low is beginning an expected elongation to the north...with a
significant deformation band developing along a line from the
Kuskokwim Delta coast southward to Dutch Harbor. This deformation
is enhancing existing precipitation rates...and when combined with
strong north winds is resulting in blizzard conditions along the
Delta coast. Further south...the rain/snow line is located right
around Dutch Harbor...with cold air slowly trying to spread
eastward along the Alaska Peninsula.
The models remain in very good synoptic agreement. Once the
current low south of the Alaska Peninsula transfers energy to a
new and complex low in the northern Bering on Sunday there are
some differences that exist among pretty much all the models with
the strength and timing of individual shortwaves moving around the
overall circulation. These differences will mainly have an impact
on the timing of The Rounds of showers that will be moving onshore
the southwest Mainland Sunday and Monday. Have opted to stick with
the most consistent solution of the European model (ecmwf) at this time...with some
small contribution of the GFS over the Bering Sea.
Short term forecast southcentral Alaska (days 1 and 2)...
widespread precipitation will continue along the Gulf Coast
through Monday as a series of fronts and associated upper waves
continue to track northward through the Gulf in deep southerly
flow. Rain will be heavy at times on the windward slopes...with
snow continuing to mix in at higher elevations. Rain showers will
also begin to increase in coverage on the leeward side of the
mountains tonight as the first upper disturbance moves overhead.
This will be followed by another wave on Sunday night. Colder air
will move back into the Cook Inlet region including Anchorage behind
that system and allow precipitation to change back over to mostly
snow late Sunday into Monday morning...although any accumulation
is expected to be minor. Strong gap winds will also continue to
impact Turnagain Arm and the Matanuska Valley through Sunday as a
strong pressure gradient is maintained by a surface low tracking
over southwest Alaska. A weakening down inlet gradient will begin
to allow some of these winds to make it into Anchorage beginning
this evening...and then become stronger Sunday morning lasting
into Sunday afternoon.
Short term forecast southwest Alaska (tonight through monday)...
the remnants of the warm front that moved over the southwest
Mainland has stalled out over the western Kuskokwim Delta...which
will allow for blizzard conditions along the coast to continue
into this evening. Further to the east...rain will continue in
most areas with gusty easterly winds along the Alaska and
Aleutian ranges. A few isolated thunderstorms may also fire along
the akpen this evening...mainly along the Pacific side due to
strong upslope flow under the upper level low. As the upper trough
stalls to the west tomorrow...rain will then begin to transition
to snow tomorrow afternoon as the column cools and downslope
warming ends. There is even a slight chance of diurnal
thunderstorms tomorrow afternoon with weakly unstable environment
and a shortwave aloft. By late Sunday night...cold air aloft
surging into the southwest Mainland will bring snow to the area
into Monday morning...with a slight chance of a thunderstorms
along the leading edge of the cold air overnight. The upper level
trough will remain over the southwest coast on Monday...keeping
rain and snow with onshore flow over the area.
Short term forecast Bering Sea/Aleutians (tonight through monday)...
behind the low currently just south of the akpen precipitation type will
almost immediately switch over to snow showers as northwesterly
flow and cold air advection takes over. As heavier snow Showers
Pass over eastern Aleutians and akpen tonight...these gusty
northwesterly winds will bring temporary visibilities below one
mile...but these should be short lived when they occur so an
advisory is not expected at this time. The akpen will see mostly
rain this evening before transitioning to snow as well late
tonight as the winds become northwesterly. Starting tomorrow
afternoon...a strong shortwave rotating around the upper level low
will move through the Bering...bringing accumulating snow to the
pribilofs starting in the afternoon before spreading into the
eastern Aleutians late Sunday night.
Long term forecast (tuesday through saturday)...
An overall slower and more amplified pattern is depicted for the
latest model guidance with multiple areas of low pressure across
alaskan waters and the North Pacific. A ridge of high pressure
over the North Pacific will develop across the Gulf of Alaska on
Tuesday as an area of low pressure dives south from the Aleutian
Islands into the North Pacific. This ridge will bring a relatively
benign period of weather to much of southcentral while a cut off
low continues a showery pattern across the eastern half of the
Bering Sea and southwest Alaska. As the low dives into the North
Pacific and amplifies the longwave trough...longwave ridging
behind the trough will also become more amplified. This will slow
down the area of low pressure pushing off the Kamchatka
peninsula...with it entering the Bering Sea late Tuesday. This
will also cause the low to take a more northern track through the
Bering Sea and bring high seas and gale force winds to the
Aleutians before moving into the Gulf of Alaska late Friday.
Models are still hinting at a building North American Pacific
Ridge which could bring much warmer temperatures to the state.
However...models continue to struggle with the strength and timing
of the ridge so temperatures were raised a bit again but not too
high at this time.
marine...gales...119 125 129 132 150 155 165 170-174 179-181 185
heavy freezing Spry...179 180 185
Synopsis and model discussion...ad
southwest Alaska/Bering Sea/Aleutians...dek