Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
427 PM akdt Tuesday Jul 28 2015
Analysis and upper levels...
The upper level low moving across the northern Gulf has moved less
than 100 miles east since yesterday. This is indicative of how
stagnant the pattern has become. The low is filling...so that
will gradually allow for an upper ridge over the western Bering
to begin building eastward. The jet stream is currently flowing
nearly north to south over the central Bering. A shortwave
disturbance that moved over the eastern Aleutians early this
morning is now racing into the North Pacific along the
southwestern periphery of the upper level trough. A weak but very
moist deformation zone persists over the Cook Inlet region and
the Susitna Valley between the upper low over the Gulf and weak
ridging over the Kuskokwim valley. The deformation continues to
generate some showers over that area.
At the surface...the slowly weakening upper trough and building
upper ridge will favor a gradual warming and drying trend for
southern Alaska over the next few days. Other than a broad surface
low over the interior...the surface continues to show a distinct
lack of definable features which will keep the winds light. The
00z soundings continue to show a very deep moist air mass over
all of southern Alaska. Therefore cloudy skies and seasonable
temperatures will remain the norm.
The models remain in excellent agreement regarding the evolution
of the large scale pattern...showing a gradual break-down of the
upper trough over Mainland Alaska. This will be replaced by weak
upper ridging over interior Alaska and an upper level low centered
over the southwestern Gulf. Due to the lack of strong features in
this pattern...the models struggle defining the movement of
smaller disturbances moving through the broad pattern. With the
very moist air mass in place... showers are likely to be
associated with these features...though timing is very difficult.
Thus forecast confidence that there will be scattered shower
activity over much of southern Alaska is high...but confidence on
where and when those showers will impact any particular area is
lower than normal.
Fire weather concerns are minimal again today...with a very moist
air mass in place across much of southern Alaska keeping relative humidity values
high with extensive cloud cover and light winds. Wet thunderstorms
are possible again today over the Alaska and Aleutian ranges...northern
Susitna Valley and the wrangells where there is more sunshine
increasing instability. A gradual warming and drying trend is
expected over all of southern Alaska in the coming days...though
warming temperatures aloft will increase stability and reduce the
possibility of thunderstorms.
Short term forecast southcentral Alaska (days 1 and 2)...
the upper level low over the northern Gulf will continue to
gradually fill and shift inland...to be replaced by weak surface
ridging along the coast. Southeasterly gap winds will increase
slightly this afternoon and evening through Turnagain Arm and the
Copper River valley under the weak onshore flow...and increase a
touch more on Wednesday. Winds will become quite light Thursday as
the surface high becomes centered over the Kenai Peninsula.
Abundant moisture and a somewhat unstable atmosphere will continue
to keep scattered to numerous showers going for the next several
days. An upper trough over southwest Alaska will dig south into
the western Gulf on Wednesday with weak ridging aloft building
over southcentral Alaska. This weak ridging will act to suppress
convection slightly...however an isolated thunderstorm or two is
still possible along the mountains surrounding the Susitna Valley
or the far northwestern Copper River basin Wednesday afternoon
and evening. The potential for isolated thunderstorms will
increase again Thursday afternoon and evening as the interior
thermal trough edges towards northern south central Alaska.
Short term forecast southwest Alaska (days 1 through 3)...
A strong shortwave moves into the southwestern Mainland this
evening bringing increased chances for showers and thunderstorms
across the Kuskokwim valley and much of Bristol Bay. The environment
across the region was strongly heated this afternoon...due to a
long break in the clouds...and shower activity is beginning to
quickly develop across a large swath of the southwest region. The
afternoon forecast package was updated to increase thunderstorm
coverage for this evening. The strongest storms are expected to
initiate along a trough axis that stretches from McGrath to
Dillingham...with another focus along a line from near King
Salmon to Iliamna...as a new surface low forms close to
Dillingham. Storm activity tapers off after midnight with
isolated to scattered showers continuing through Wednesday day as
a new upper low center forms across the eastern Bering.
Thunderstorm activity will redevelop on Wednesday afternoon and
evening...however activity will not be as intense as anticyclonic
flow over southcentral works against the Bering low.
Short term forecast Bering Sea/Aleutians (days 1 through 3)...
An organized storm system in the northwest Pacific makes slow
northward progress to the western half of the Aleutians. At the
same time the upper level ridge over Kamchatka moves into the
western Bering. This pattern set up...with a stacked low just
south of Shemya and ridging over the Bering...becomes stationary
through Wednesday. The occluded front will bring periods of
moderate to heavy rainfall at times across the western and central
Aleutians through middle week. Otherwise...low clouds and fog persist
across the rest of the Bering and Aleutians.
Long term forecast (days 3 through 7)...
Deterministic and ensemble guidance is in much better agreement
today in regards to a bit of a pattern change taking place late
this week and into the weekend. The longwave trough that has been
centered across Alaska will amplify southward a little bit and
move into the North Pacific on Friday...and multiple upper level
low centers begin to close off the upper level pattern. At the
same time...high pressure will be building across the Bering Sea
and will begin to nudge into the interior of Alaska before
overspreading the state early over the weekend. The upper level
pattern becomes a little complicated and hard to figure out as the
interaction of two of the aforementioned upper level lows may or
may not interact with one another. While there has been a bit of
change from the 00z and 12z runs of the ECMWF/GFS/gefs...it
appears the lows will ultimately phase into one concentric low
over the North Pacific on Sunday before either pushing into the
northern Gulf of Alaska or further east toward the Alaska
Panhandle. This upper low and its surface feature will play a huge
role in determining rainfall as well as winds (outflow or gap)
through channeled terrain of southcentral Alaska. Details will
become clearer in coming days...as the phasing of two lows tends
to be poorly handled by models especially 5-6 days out.
Synopsis and model discussion...jw/rmc
southwest Alaska/Bering Sea/Aleutians...kh