Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
433 PM akdt Tuesday may 5 2015
Analysis and upper levels...
there is a large upper level low centered just south of the
central Aleutians. A dissipating front associated with this low
stretches from near King Salmon southeast into the Gulf of
Alaska. There is a frontal system also associated with this low
that is near 45 degrees north latitude and 160 degrees west
longitude and moving to the northeast. There is an "open wave"
trough in interior Alaska moving eastward...and is just clipping
the northern Susitna Valley and Copper River basin. There is an
upper level ridge over the northern Gulf that extends across the
Kenai Peninsula and northwestward to near the Seward Peninsula.
the models are in fair agreement through the short term (through
thursday). The main disagreements concern the central location of
several surface lows that work their way into the Gulf. Admittedly
the first two of these lows are fairly weak (1005-1010 mb range)
the third low that will move into the Gulf Thursday night is a bit
stronger (around 990 mb). The forecast confidence is thus near
Short term forecast southcentral Alaska (days 1 and 2)...
areas of light rain are showing up on the radar this afternoon as
a middle-level front weakens from the Kenai Peninsula into the Gulf.
This front will continue to fall apart through the overnight hours
as the surface front begins to reorganize underneath an approaching
Pacific disturbance. The front will strengthen as this happens
bringing gale force winds to the Barren Islands region before
moving up to the north Gulf Coast during the day Wednesday. Steady
rain will spread over the Gulf Coast during the day while the Cook
Inlet region will remain mostly dry from downsloping flow. The
front will hang along the coast and weaken on Thursday keeping the
chance for showers and light rain along the coast the a chance for
light rain elsewhere. Surface ridging out ahead of the front will
begin to increase gap winds through Turnagain Arm and Portage
Valley...along the north Gulf Coast...Knik River Valley and along
the Copper River.
Short term forecast southwest Alaska (days 1 and 2...Wednesday and
thu)...a fairly quiet pattern will hold over the area through the
remainder of the week. The front along the coast brought King
Salmon and surrounding areas between one and two tenths of an inch
of rain. But that front has run its course and will continue to
fall apart tonight. In its wake it will leave a weak boundary
that another system will develop along on Wednesday. This system can
probably best be described as an inverted trough coming off of the
parent low currently over the central Aleutians. It will bring
another steady round of rain to most of the area...with the
heaviest amounts being confined to the eastern slopes of the
Aleutian Range. That parent low will be quite slow to move into
the Gulf of Alaska...so expect a rather soggy end to the week.
Short term forecast Bering Sea/Aleutians (days 1 and 2...Wednesday and
thu)...the upper-level parent low continues to spin within the
base of the broad longwave trough centered over the Aleutians. A
jet streak will enter the west side of the trough tonight and
dive towards the base of it Wednesday morning. This will help to keep
the weakening low progressing ever so slowly to the east-
southeast. Showers and winds will continue to diminish as it moves
away. By Thursday...some riding moves into the western Bering and
starts to slide east. This will create some gusty northwesterly
from the central Aleutians East by Thursday afternoon. With just a
little cold air advection associated with this flow...expect light
showers to move over the Bering side of the chain.
Long term forecast (thursday night through monday)...
just as the front over the Gulf dissipates on Thursday
evening...another front associated with a North Pacific low will
quickly move in to replace it on Friday. The jet will slide to
the east and weaken throughout the day on Friday with this
system...allowing for it to become a semi-permanent feature under
a cutoff low that will continue through this weekend. This pattern
will bring continued rain to much of the Gulf Coast into this
weekend...with rain being more showery in nature for inland
southcentral. This system will then begin to dissipate on
Sunday...allowing for clearing skies for late this weekend and
early next week.
Rainy weather over the southwest Mainland on Thursday and Friday
will begin to clear on Saturday as weak offshore flow develops
behind the Gulf low. This will bring a period of benign and calm
weather to the southwest Mainland for Sunday and Monday as another
Bering low passes south of the area and weak ridging builds in.
Over the Bering...there decent agreement that a North Pacific low
will enter the Aleutians and possibly the Bering Sea late this
week...but there is considerable model discrepancy as to the
strength and location of this low. There is good agreement this
this will be a fairly weak system...but otherwise confidence is
low in the overall forecast over the Bering this weekend into
early next week.
Marine...gale 130 131
Synopsis and model discussion...bl
southwest Alaska/Bering Sea/Aleutians...MO