Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
410 PM akdt Tuesday Oct 13 2015
Analysis and upper levels...
a high amplitude long-wave trough remains firmly entrenched over
Alaska and the North Pacific. A strong polar jet streak is tracking
eastward just south of the Aleutians but then dives southward
across the northeast Pacific. This is keeping the main storm track
to our south. There are however a host of relatively weak features
embedded within the weak steering flow across the forecast area.
Focusing on the most significant features...a potent upper level
short-wave digging across the north-central Pacific is combining
with the polar jet streak to induce development of a triple point
low south of the eastern Aleutians. More importantly...a warm
front out ahead of the triple point low is lifting northward
toward the eastern Aleutians and southern Alaska Peninsula.
Meanwhile...a short-wave is dropping southward across the central
Bering Sea advecting low level cold air. As this cold air feeds
into the back side of this storm system this afternoon and evening
it will increase baroclinicity along the warm front leading to
heavier rain and stronger winds.
Downstream of this system a couple of east-west oriented short-waves
from the eastern Bering to Bristol Bay to the south of Kodiak Island
are beginning to phase as they lift northward toward the southern
Alaska Mainland. Right now there is very little precipitation out ahead
of these...but expect more rain to develop as upward vertical
motion increases out ahead of a single consolidated short-wave.
A series of even weaker upper level short-waves are traversing
the southern Mainland today producing some light showers.
models continue to be in excellent agreement with the overall
evolution of larger scale features...with the usual minor
differences with handling of smaller scaler features. Over the
next couple days these model differences lead to diminished
forecast confidence in exact timing/placement of precipitation...but
confidence is pretty high with the bigger picture of expected
One notable feature where confidence is below average is the
movement of the consolidated upper level short-wave northward
across southcentral/southwest Alaska tonight into Wednesday
morning. Each model depicts the structure and track of the
short-wave a little differently. Based on the complex nature
of phasing of multiple short-waves and feeding of additional
energy into the back side of the trough the stronger NAM solution
with hints of an upper level circulation over Cook Inlet seems
to make the most sense. The precipitation forecast will largely be
based off this solution with some buffering to allow for
Short term forecast southcentral Alaska (days 1 and 2)...
southerly flow continues to bring rain along the north Gulf Coast
and areas around the Susitna Valley. Further south a trough is
making its way to Kodiak Island. This trough will track north
overnight and will push through the southcentral by tomorrow
evening. The main vorticity maximum with this trough will move up
over Cook Inlet bringing rain later tonight into tomorrow morning
for areas around Cook Inlet while also bringing rain to the north
Gulf Coast. After the trough passes flow will become
southeasterly keeping rain along the north Gulf Coast but
downsloping areas west of the Kenai Mountains and extending into
the Copper River basin. The next front will move in from the south
and make it to the northern Gulf Thursday afternoon.
Short term forecast southwest Alaska (days 1 and 2)...
a broad longwave trough centered over the western coast of
Alaska will begin to weaken and slowly track northeastward...with
some weaker upper level waves rotating through the southwest
Mainland through Thursday. This will keep mostly cloudy
conditions and a chance for showers throughout the area through
Wednesday morning. Warm moist air associated with a warm front will
begin to round the top of a North Pacific ridge and bank up
against the Alaska Peninsula Wednesday afternoon. This will likely
form a barrier jet along the Aleutian Range...increasing winds
along the range and bringing in some steadier rainfall to the
area through Thursday. Temperatures will remain relatively mild
keeping most of the precipitation type as rain. The exception to
this could possibly be over the Kuskokwim Delta and lower
Kuskokwim valley during the colder overnight hours where a few
snow flakes may mix in.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
a robust 175 knots jet continues to run parallel to the western
Aleutian chain in the North Pacific. The energy along the jet will
continue to Transfer into the North Pacific while developing into
a surface low south of Dutch Harbor this evening. The associated
warm front will slide through parts of the Alaska Peninsula and
eastern Aleutians tonight...bringing gusty southeast to northeast
winds and chances for rain. Much of the moisture associated with
this system will remain over the area through Wednesday. A ridge
will continue to creep into the northwest Bering tonight...which
has started to kick up the north winds once again over the
Pribilof Islands and central/eastern Bering. This ridge will be
short-lived though...as a stronger front starts to push into the
western Aleutians by Wednesday afternoon. The warm front
associated with this system will bring a modest size areas of
gale-force winds and another round of rain to the western and
Long term forecast (days 3 through 7)...
the beginning of the extended forecast focuses on a system
spinning in the North Pacific that extends a warm front north and
eastward into the Gulf. The gale force front will stretch from
Shelikof Strait through the north Gulf Coast. This front will
continue to push inland on Friday...spreading rain over much of
the southern Mainland. The Cook Inlet region will remain
downsloped and dry but some sprinkles or light rain could make it
over the mountains. There will be a slight chance for freezing
rain at this time in the Copper River basin as cold overnight
temperature could linger when the front arrives.
For the weekend into early next week the pattern will be
anchored with a broad upper low pressure center over the Bering
Sea/Aleutians which sets up the southern Mainland for an active
pattern. However...most of the frontal energy has been used up so
any precipitation will driven by upper level energy moving
overhead meaning periods of rain or showers in most areas. A
continuation of cloudy and cooler conditions will linger while
this pattern stays in place.
Synopsis and model discussion...seb
southwest Alaska/Bering Sea/Aleutians...tp