Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
352 PM akst sun Feb 1 2015
Analysis and upper levels...
A quasi-Rex block pattern continues over the northeast Pacific and
Alaska as a huge but weakening storm continues to spin well south
of the Gulf of Alaska. An impressive closed upper high formerly
over the interior of the state is in the process of weakening and
shifting into the northern Gulf...with increasing amounts of
subsidence/dry air evident on water vapor satellite imagery. With
a very dry airmass in place (the precipitable water value on the
12z Anchorage sounding was a very low 0.12 inches) clear skies and
no precipitation is occurring over the Mainland...though steep inversions
continue to Foster the development of some fog/stratus mainly
around Turnagain Arm and the Copper River basin. The typical gap
winds around southcentral continue to be active as an offshore
pressure gradient continues...though as the surface high slowly
weakens and shifts into northwest Canada this evening these gap
winds will diminish significantly.
The models remain in very good synoptic agreement through the
first few days of the forecast. The weakening upper high over
southern Alaska will continue to weaken as it encounters the
large storm over the North Pacific through tonight. Additional
weakening will occur as the first in a series of cold Arctic
troughs dive down from northeast Russia into Mainland Alaska
tomorrow morning and shears out/weakens. A second trough moves in
by midweek...and it is the evolution of this second trough where
the 12z NAM diverges from all of the global models. The
GFS/ECMWF/gemglb solutions all take the midweek shortwave quickly
into northwest Canada...whereas the NAM amplifies the trough and
plunges it across Cook Inlet. The 12z NAM solution seems a clear
outlier with this feature...especially as affects the mass/wind
fields out toward the end of the 84 hour period(wed afternoon)...and
is the only model rejected for forecast preparation today.
Short term forecast southcentral Alaska (days 1 and 2)...
Upper ridging over the Mainland will continue to dominate the
weather over southcentral through Tuesday. Gap winds will begin
to diminish over the coast tonight as low pressure over the North
Pacific weakens. The biggest challenge will be the development of
fog tonight and again Monday night as largely clear skies and
weakening winds promote strong radiational inversions.
Short term forecast southwest Alaska (tonight through wed)...
A relatively benign weather pattern is in store for southwest
Alaska as high pressure is expected to build from the western
Bering Sea over into the interior of Alaska. Onshore flow along
the Kuskokwim Delta will couple with a weakening pressure
gradient which will be favorable for fog developing after midnight
tonight and lingering through the morning hours on Monday. Prior
to the ridge building over southwest Alaska...a potent shortwave
ahead of an Arctic trough will push across the Kuskokwim Delta
Monday afternoon into Monday evening and bring a slight chance for
a few flurries. Once that wave pushes through...colder air will
begin to filter in from the Arctic behind the aforementioned
trough and bring another round of below normal temperatures and
clear skies to the region.
Short term forecast Bering Sea/Aleutians (tonight through wed)...
The two play-makers for the Bering Sea will be a building ridge
and an area of low pressure in the North Pacific. The low in the
North Pacific will push a weak warm front that will stall near the
Aleutians chain and bring isolated and scattered rain/snow showers
in the forecast through midweek. As the ridge builds across the
Bering Sea...the pressure gradient across the southern Bering Sea
will be enhanced and allow for winds just shy of gale force to
develop by midweek. Temperatures will begin a steady downward
trend by Tuesday as colder air filters across the region behind an
Long term forecast (wednesday through sunday)...
an upper level shortwave will move from the north through the
southwest Alaska extending into the Mainland by Wednesday with
cold air advection starting to filter into the region. This
feature will produce very little precipitation as it moves
through the area. The models have come into better agreement while
previously the NAM was the outlier. High pressure will remain over
the Bering Sea extending through southwest Alaska into the
Mainland. This will continue to produce patchy fog in the region.
This high pressure over the Bering Sea will start to retrograde
toward the Kamchatka peninsula by Friday evening. This will
coincide with another shortwave from the north tracking through
southwest Alaska into the Mainland by Saturday morning. This will
bring more Arctic air resulting in even colder temperatures
throughout the region. This will potentially be the coldest
temperatures of the season to southwest Alaska and the Anchorage
Marine...heavy freezing spray 180 and 414.
Synopsis and model discussion...ad
southwest Alaska/Bering Sea/Aleutians...mc