Scientific Forecaster Discussion
fxak68 pafc 200017
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
417 PM akdt Thu Oct 19 2017
Analysis and upper levels...
there is an upper level trough over southcentral Alaska that
extends southeast from the main upper low centered along the
northwest coast of Alaska near Point Lay. At the surface, offshore
flow is evident over much of southcentral with a surface low over
the northeast Gulf. There is a weak ridge over the eastern Bering
Sea. A fairly strong low is over the western Aleutians, with a
981 mb surface low center near Kiska Island. A front associated
with this low stretches east-to-west across the western and
south-central Bering Sea then drops southeast across the
Aleutians just west of Umnak Island.
the numerical models are in good agreement through Friday evening,
then begin to diverge on how they handle a low moving into the
Gulf later Friday night into Saturday. The run-to-run consistency
is not very good, although all the models seem to be trending
eastward with the surface low. This creates a situation with
lower than normal forecast confidence for the weekend.
panc...VFR conditions will persist. Breezy north winds will
diminish this evening.
Short term forecast southcentral Alaska (days 1 and 2)...
clear and cold conditions will continue to prevail across much of
southcentral Alaska through Friday in weak offshore flow. Modest
outflow winds being driven by the surface pressure gradient and
cold air advection from the interior will continue into the
overnight hours along the north Gulf Coast, with the strongest
gusts expected through the typical gap flow areas including
Valdez, Whittier, and Seward. Winds will trend downward through
early Friday as the upper low over northwest Alaska drops into Norton
Sound and upper level flow backs to the southwest, putting an end
to the cold advection for the time being. Elsewhere, light snow
will continue into Friday morning across the far southeast Copper
River basin including the McCarthy area as a weak upper trough
lifts northeast through the region and a surface low in the
northeastern Gulf helps to advect a limited amount of moisture
northward. The surface low will drift westward into the Prince
William Sound area through Friday night and spread light snow over
eastern portions of the sound. A dusting of new accumulation will
be possible in the Cordova area through early Saturday before
rain begins to mix in later in the day.
Short term forecast southwest Alaska (days 1 through 3)...
Northwesterly cold air advection will continue over the
southwest Mainland through Friday. Friday night, the fairly
quiescent pattern will become more active as better dynamics move
over the area. The main pattern changer will be an Arctic low that
will drop southward to the Seward Peninsula Friday night,
bringing a rush of northerly cold air with it. A low level
baroclinic zone will sharpen over the southwest coast Friday as
southerly warm air ahead of a North Pacific low collides with cold
air advecting southward ahead of the Arctic low. Enhanced upper-
level lift associated with the upper-low will move over the low-
level convergent baroclinic zone, which will increase
precipitation along the coast and support the development of a
weak surface low over northern Bristol Bay Friday night.
At this point, models begin to diverge on what happens next. The
baroclinic zone off the southwest coast will strengthen into a
weak front Friday night as low-level convergence along the
boundary increases. This front will move onshore Saturday
morning and weaken as it moves to the upper Kuskokwim valley
Saturday evening. There is a fair amount of discrepancy within
the models in the strength of the front and the amount of
southerly flow advecting moisture northward along the front.
Therefore, confidence in the amount of quantitative precipitation forecast associated with this
weak front is fairly low for now. Behind the front, the center of
the Arctic low will drop to the Kuskokwim coast area by Saturday.
However, the exact track and placement of the low and it's
associated rain remains a bit uncertain.
Short term forecast Bering Sea/Aleutians (days 1 through 3)...
A low pressure system over the western Aleutians will track
eastward tonight and move south of the Alaska Peninsula towards
the Gulf Friday. An associated gale force front currently over the
central Aleutians and southwest Bering will skirt the southern
Alaska Peninsula Friday morning before exiting to the North
Pacific Friday afternoon. Once the low has departed to the east
Friday, widespread northerly flow will resume over the entire
Bering Sea and Aleutian Islands. Fairly strong cold air advection
will steepen lapse rates, leading to widespread rain/snow showers
over the Bering through Saturday.
Long term forecast (days 3 through 7)...
An active pattern is expected through the long range forecast
through the middle of next week. The main challenge surrounds a
storm system which slides east into the Gulf this weekend bringing
the potential for snow across southcentral into early next week.
Confidence remains higher in cold air advection keeping
temperatures below normal across much of the southern Mainland
through the weekend. During this time, the area will remain under
dry offshore flow conditions as low pressure tracks south of the
Alaska Peninsula to the Gulf. For locations along the Bering,
including the southwest coast, precipitation chances increase
through the upcoming weekend, as an upper low pressure system in
the eastern Bering slowly propagates south toward Bristol Bay. A
colder air mass advecting from the northern Bering changes precip
type mainly to snow except along the Aleutians where a mix of rain
and snow prevail under moderate temperatures.
The biggest forecast challenge remains the track and placement of
the low tracking south of the Gulf, and the associated frontal
boundary over the northern Gulf this weekend. Guidance continues
to struggle with the placement and track of this system, as well
as some upper level disturbances peeling off of the upper level
low over the western coast. Confidence in snowfall over the Cook
Inlet region and inland areas of southcentral has decreased with
this forecast package given the drier trend and more southerly
low track. However, due to the inconsistency and variation of the
models recently, we can not count snow completely out of the
forecast just yet. With high uncertainty during the mid to long
range forecast, kept changes minimal through the middle of next
Early next week there is better agreement on the remnants of
typhoon lan tracking into the western Aleutians/Bering Monday
morning, and becoming the dominant weather feature over the
Bering/Aleutians and southern Mainland through the end of next
week. This system is showing the potential to develop into a
strong system, capable of bringing gusty winds, warmer
temperatures, and a fair amount of rain to the area.
Marine...Gale Warning 130 170 172 173 174 175 176 177 178.