Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
402 PM akdt Sat Jul 4 2015
Analysis and upper levels...
a warm front moving north in Tangent with an upper level shortwave
is currently bringing mainly light precipitation to much of
southern Alaska. The shortwave is rotating around a strong upper
level low centered over the Aleutians. In addition...a very strong
upper level ridge is nearly stationary over the northeast Pacific.
The ridge is effectively blocking any longitudinal progression of
the Aleutian low. Thus the low is expected to remain anchored in
place for the foreseeable future. Weather-wise...this pattern
means that continued cool...wet...and cloudy conditions will
remain the norm for much of Mainland Alaska and the Aleutians. The
one exception is for the Copper River basin where warm and dry
conditions will return under the ridge once the front pushing
through today dissipates.
as has been the case the past few days...on the large scale the
models remain in good agreement on the overall pattern described
above. Differences begin to rear their ugly heads when looking at
the timing of the precipitation associated with the next front on
Sunday night. The NAM is the slow outlier at that time. Beyond
that the models largely agree on precipitation placement and
timing...with subtle differences in intensity.
a fairly robust upper trough with abundant moisture transport from
the Pacific favors a cool and wet pattern over southwest Alaska
with fairly widespread wetting rains. Strong southeasterly flow
will accompany each front which will keep the west side of mountain
ranges marginally warmer and drier.
After the warm front moves through today southcentral will briefly
dry out on Sunday. A stronger front will enter the region from the
west on Monday. With much more significant upper level forcing
expect a good chance of wetting rains as this front traverses
southcentral Monday through Tuesday. Meanwhile a strong ridge will
build out ahead of the front on Monday from the eastern Gulf to
the Copper River basin...likely leading to warm dry and windy
conditions. There is some uncertainty in the speed of the front.
If it remains west of the Copper Basin into the afternoon hours
on Monday then there is a chance of reaching red flag thresholds.
Will re-evaluate this as confidence in handling of the front
Short term forecast southcentral Alaska (days 1 and 2)...
As the front moves inland and dissipates tonight...showers will
persist over coastal sections and taper off inland of the coastal
mountains. Channeled flow will also diminish. Clouds will begin
to thin sun and Sunday night. The next front will spread moisture
over all zones except the Copper River basin...where temperatures
are expected to rise well into the 70s. Channeled flow will
increase through Turnagain Arm and the Copper River. In the
vicinity of the Copper River the combination of sunshine and
channeled wind will create very warm and dry conditions Monday
Short term forecast southwest Alaska (tonight through Monday night)...
Rain associated with a warm front continues to fall across southwest
Alaska this afternoon and will gradually taper off through the
evening hours as the front continues to slowly slide northward.
Gusty winds have been seen across the Bristol Bay area as the
front is favorably oriented to allow gusty winds to blow through
the mountain passes of the Alaska and Aleutian ranges. A brief
break in the rainfall will be observed on Sunday as some breaks
will develop in the clouds...which will result in temperatures
warming into the middle 60s to middle 70s.
A shortwave currently located over North Pacific is helping to
amplify the upper level trough located over the Bering Sea. In
addition to this amplification...it appears the shortwave is
helping to tap into some subtropical moisture and will transport
this moisture toward southwest Alaska an the Alaska Panhandle
Sunday night and on Monday. This new air mass will be moisture
rich (over 200% of the average precipitable water) as it impacts
southwest Alaska and will lead to heavy precipitation at times
along the typical upslope locations of the Alaska Peninsula. The
second front will likely bring initial downsloping conditions to
southwest Alaska on the Lee side of the Alaska and Aleutians
ranges Sunday night as a stable layer couples with southeasterly
winds of 20 to 35 knots at mountain top. The downsloping should come
to an end Monday morning as flow becomes weaker and less favorably
Short term forecast Bering Sea/Aleutians (tonight through Monday night)...
A series of upper level lows are currently scattered across the
North Pacific and Bering Sea. These lows are expected to phase
together starting Sunday afternoon and develop into an upper low
that encompasses the majority of the Bering Sea. A pattern like
this typically leads to a favorable regime for precipitation due
to the synoptic lift associated with the upper low. At the
surface...a series of lows in the North Pacific will continue to
phase during the overnight hours and move into the Bering Sea
Sunday evening. This will push a potent surface low (with support
from the aforementioned upper low) and its associated front into
the Bering Sea Sunday evening and will produce widespread
rainfall that could be heavy at times as it upslopes along the
Pacific side of the Aleutian chain.
Long term forecast (days 3 through 7)...
a large area of low pressure will remain centered over the Bering
through at least next weekend while the upper level ridge over Mainland
Alaska shifts east into Canada and flattens. The net result will
put the area into a cooler and moister fairly steady state
pattern with a surface ridge in the Gulf supporting low level
onshore flow. Shortwave troughs will periodically rotate up from
the western Gulf and eastern Bering keeping skies mostly cloudy
and bringing scattered showers...periods of rain and mild
temperatures to much of southwest and south central Alaska.
Marine...Gale Warning 131 155 160 165 170 172 180.
Synopsis and model discussion...jw
southwest Alaska/Bering Sea/Aleutians...mc