Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
526 am akdt Wednesday Aug 20 2014
Analysis and upper levels...
The large scale upper air pattern is dominated by an active
subtropical jet stream extending from Japan to the West Coast of
Washington state. A nearly stationary upper low centered over
the western Aleutians is impinging upon the subtropical jet
stream...with a depression in the upper level height field
extending south to 35n. Weaker and slow moving migratory features
are dominating the upper height field across the Gulf of Alaska
and the Mainland...all of which are well displaced from the polar
and subtropical jet streams. One weak upper level shortwave trough
is tracking through the y-k Delta...with another tracking through
the southern Gulf of Alaska. These features will weaken and track
east today...with shortwave ridging building into southern Alaska
through Thursday morning before a fast moving shortwave trough
begins to impinge upon southwest Alaska. The bigger transition in
the pattern does not arrive until Sunday as the western Bering
trough shifts east...and an associated weak front moves into
The numerical models are in excellent agreement with the large
scale pattern through late week before they begin to diverge late
Saturday through Monday with the eastward movement of the western
Bering Sea trough...and the overall interaction with the
subtropical jet stream. One distinct trend in the guidance over
the past two days has been slower and weaker with the front moving
into the Gulf of Alaska...and the strength of the associated low
in the Bering. All guidance has trended toward a weaker system as
a whole...with subtropical energy splitting and shearing along the
front as it tracks into the Gulf of Alaska. The NAM and European model (ecmwf) were
favored through the short term...with a blend of the European model (ecmwf) and GFS
Short term forecast...
A weak upper low will track across the northern tier of the region this
morning and early afternoon...with middle and high clouds spreading
over the area. Showers will generally be focused along the
mountains...but a few sprinkles or isolated showers will be
possible across lower elevations areas from Cook Inlet north to
the Susitna Valley. Showers will become focused over the mountains
in the afternoon...although instability will be less than previous
days...with no thunderstorms anticipated. A general repeat is
expected on Thursday...with more diurnal mountain showers and
another generally warm day across most of the region. A weak
disturbance will track west to east across the northern Gulf
Thursday night into Friday...spreading clouds and the possibility
of showers across southcentral Alaska. Winds will generally be
light and variable through Friday with the exception of local
diurnally driven sea breezes.
Generally quiet weather across the region will continue as high
pressure ridging aloft builds in today. Afternoon and early
evening mountain showers will develop with daytime
heating...favoring the Alaska Range...but weak instability will
keep thunderstorm threats at Bay. A subtle upper disturbance will
track in Thursday with more scattered shower threats...focused
along the higher mountains. Quiescent weather and spotty showers
will continue through Saturday...with a general weak offshore flow
pattern keeping low stratus and fog out over the Bering.
A stationary low over the western Aleutians will remain in place
through Thursday before gradually shifting east into the weekend.
Showers and rain will spread into the central and eastern
Aleutians by Saturday. Weak high pressure will build in the wake
of the low as it moves east Saturday and Sunday.
Long term forecast (sunday Aug 24 - Wednesday Aug 27)...
Beginning the extended forecast on Sunday...a large and broad low
over the eastern Bering will be tracking into southwest
Alaska...with a diffuse warm front moving into the Gulf. This will
spread increasing clouds and rain into southwest Alaska
Sunday...and southcentral Alaska late Sunday into Monday.
Showers...clouds...and slightly cooler conditions will persist
into Tuesday with a more active pattern persisting into midweek.
Ahsenmacher Aug 14