Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
505 PM akdt Wednesday may 6 2015
Analysis and upper levels...
the atmosphere over the northern Pacific is in the process of a
pattern change. The main upper low that has been near to just
south of the central Aleutians is working its way eastward towards
the Gulf of Alaska and is currently south of the eastern Aleutians
and Alaska Peninsula. An front and upper trough associated with
this low currently stretches from the Bristol Bay area eastwards
across Kodiak Island into the Gulf of Alaska and is moving to the
north. An upper level ridge over southern and central Alaska is
retreating in the face of this front. Another system associated
with the main low is spinning up over the northeast Pacific about
1000 miles south of Kodiak Island. There is a weak ridge over the
western and central Aleutians and a negatively tilted trough
stretching from extreme northeast Russia across the eastern Bering
and loosely connects to the main northeast Pacific low.
the numerical models are only in fair agreement today for the
short term (through friday). They are struggling to come to a
consensus on any of the surface lows. The GFS and European model (ecmwf) are in
good enough agreement with each other on lows in the Gulf to give
some confidence to the forecast in that area. While the European model (ecmwf) and
Canadian global show good agreement on a low near the central
Aleutians late Friday. Over-all though...the forecast confidence
is a bit below normal due to the impacts on the sensible weather
of the differences in the models.
Short term forecast southcentral Alaska (days 1 and 2)...
the frontal system in the Gulf will continue to spread rain to
coastal regions tonight while downsloping keeps inland regions
somewhat drier. A shortwave trough will rotate onshore tomorrow
spreading showers further inland. One potential area of concern
tomorrow will be the Copper River basin where the incoming frontal
moisture will be the slowest to spread into. Surface ridging
along the coast shifting inland will bring some rather strong
south to southeast wind through the Copper River in the late
afternoon and early evening time period. While relative humidities
will climb as the Gulf moisture spreads inland there may still be
several hours of dry and windy conditions before they increase
much. Right now thinking that with the increasing cloud cover and
cooler temperatures they should remain above red flag thresholds.
The next frontal system will reach the Gulf Coast on Friday
reinforcing the moisture there and onshore flow.
Short term forecast southwest Alaska (days 1 and 2...Thursday and
fri)...a closed surface low has developed just south of Kodiak
Island per ascat satellite imagery. This weak feature will move
into Bristol Bay tonight and then proceed to fall apart there into
early Friday. Showers are already quite widespread on the King
Salmon radar and we only expect them to increase in coverage and
spread north as this low brings better instability and more
moisture to the area. The gusty southeast winds...mainly found
along the Aleutian Range...will subside Thursday morning as upper level
ridging slides off to the east and the gradient relaxes. The low
will start to dissipate Thursday and should allow some breaks in the
clouds...especially over parts of the Alaska Range. Models are
showing just a hint of instability east of Sleetmute...so there
will be potential for some heavier showers there. The pattern will
remain much the same for Friday until a ridge arrives from the
Bering late in the day and starts to dry the area out in earnest.
Short term forecast Bering Sea/Aleutians (days 1 and 2...Thursday and
fri)...weather will remain quite quiet for the end of the week.
The upper-level low that has controlled much of the area for the
past few days is finally moving into the Gulf of Alaska. In its
wake...a broad and dirty ridge is starting to move into the
western Aleutians. This feature will work with the low near
Bristol Bay to create a fairly wide swath of increasing northwest
winds from Adak to the east. Slightly cooler air will work its way
southward with this flow and should help to create some
showers...especially on the Bering side. Temperatures could cool
enough to see a rain and snow mix for the pribilofs. The ridge
slides quickly to the east on Friday. This will open the door for
a new strengthening low pressure system to move through the
central Aleutians and into the Bering late Friday night. This system
will bring widespread rain and some minimum force gales.
Long term forecast (friday through Monday night)...
This weekend is a challenging forecast as two storm systems move
into the forecast area...one in the Gulf and another in the
Bering. Confidence is lower than average in the extended time
frame as model solutions lack agreement beyond Friday night. The
general outlook across the southern Mainland and Gulf is focused
around a stacked low over the Gulf that becomes stationary Friday
night through Saturday. A blocking ridge across the alcan border
works against the low preventing an eastward progression and
northward movement is blocked by a stronger low digging toward the
Bering Strait. The most likely outcome to this pattern is the low
in the Gulf weakens and eventually becomes an open wave as the
stronger system near the Bering Strait kicks it east.
Expect gusty winds in the vicinity of the low centers with
periods of rain across area waters this weekend. Also gusty winds
are expected through channeled terrain along Prince William Sound
and the eastern Kenai which taper down as the low weakens.
Ridging between the low pressure systems keep the the southern
Mainland mostly dry...with isolated to scattered showers still
possible inland as moisture increases and short waves move from
the two upper level lows.
Marine...gale 119 120 130 131.