Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
501 PM akst Sat Dec 27 2014
Analysis and upper levels...
the fundamentals of the new longwave pattern are now largely in
place...featuring a highly amplified ridge over the Gulf of Alaska
and into the Mainland...and a deep longwave trough over the Bering
Sea region. A strengthening 135 knots jet streak between these two
features is blasting straight northward up the West Coast of
Alaska. Much of southcentral Alaska is seeing a fairly nice day
under strengthening high pressure...though The Fly in the ointment
is low-level moisture/clouds trapped near the surface around Cook
Inlet and also in the Copper River basin. High pressure at the
surface is building over eastern interior Alaska...which has
switched the overall flow at the surface east of the Alaska Range
to a more northerly wind direction. This should slowly erode the
low-level moisture around Cook Inlet. Meanwhile out west...most
of the Bering Sea is under the influence of a large and stationary
low sitting northwest of Shemya...with southerly flow and
widespread shower activity the Standard for most areas. With a
tight pressure gradient now nearly in place across southwest
Alaska...all eyes turn to the series of disturbances that will be
moving northward along this stationary boundary in the coming
not surprisingly the models are still in good synoptic agreement
on the new longwave pattern that is now upon US. The primary
forecast challenge remains the timing and strength of gap winds
around Bristol Bay...Kodiak Island...and Turnagain Arm beginning
this evening and continuing through early next week. The models
have also come into reasonably good agreement on the timing of the
first disturbance moving up along the stationary front tomorrow.
The 12z GFS was notably stronger than the other models with the
support shortwave aloft as well as the associated surface
feature...and would represent the strongest "squeezing" of the
pressure gradient and resulting increase in winds across the area
during the day tomorrow. This would be a bit of a departure from
the existing forecast...and the preference remains for a weaker
feature more in the line with the European model (ecmwf)/NAM solution throughout
the short term forecast period.
Short term forecast southcentral Alaska (days 1 and 2)...
subsidence and decreasing higher clouds with the high amplitude
ridge setting up over the area from the west will continue to
produce some patchy fog over parts of the Copper River basin and
around the northern Cook Inlet/Knik Arm area overnight tonight.
Drier northerly flow will begin to become more evident over the
western part of the southcentral Mainland tomorrow...making it
harder for fog or stratus to persist over the area. A weather
front moving north toward the Gulf will bring periods of heavy
rainfall and very gusty winds to Kodiak Island by tomorrow morning.
The precipitation and gusty winds will spread to the Kenai
Peninsula tomorrow afternoon and then to the north Gulf Coast by
Monday. Except for upslope conditions along the Alaska Range
(west Cook Inlet/west Susitna valley)...strong southeast cross
barrier flow should minimize precipitation west of the Chugach
Mountains. Windy conditions will also accompany the northward
advancing frontal system. Gales with local storms (barren islands)
will spread across the coastal marines tonight through sun. Strong
gap flows across the Kenai Peninsula and along Turnagain
Arm/higher elevations in Anchorage are also expected to develop
late tonight and continue into Monday.
Short term forecast southwest Alaska (days 1 and 2)...a very
active weather pattern is unfolding across the area. A robust 140
knots jet is streaming up from about 40n and aimed directly at the
Alaska Peninsula and Bristol Bay area. This is bringing a surge of
warm air advection in the middle levels which is building an a
strong ridge over the Mainland. This is reflected at the surface
in the form of a tightening pressure gradient. The core of the
strongest winds has just pushed through the eastern Aleutians and
is now shifting towards the western convective available potential energy of Bristol Bay. This
core will move into the Aleutian Range through the night tonight
and produce gusty winds through higher passes and kamishak gap.
The saving Grace in this scenario is that the highest winds should
stay mostly elevated with strong warm air advection. We did
slightly back off on the strength of the winds with this forecast
package as it does not look like there is any good dynamic
mechanism to mix those stronger winds down to the surface.
The other concern in this pattern is precipitation type. While
warm air is surging up...some places are still at or just above freezing
at the surface. Models are showing a weak deformation axis
stretching between the low entering the Bering Strait and the one
taking shape south of the Alaska Peninsula. So essentially areas
from Dillingham to Bethel could actually see some weak cold air
advection through much of the day Sunday before the warm air comes
up with that next low and turns everything over to rain. This
makes for a very tricky forecast and places in that corridor could
actually see the precipitation type change back and forth from
rain to snow back to rain again several times. There is also the
potential for some freezing rain in this area. At this point...it
does not look like advisory level snows would accumulate as any
snow that does fall would be very wet. But certainly the higher
terrain of the Kuskokwim and Ahklun Mountains could see a foot
of snow or more.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...most of
the area is caught up between the decaying low north of Shemya and
the active jet pushing systems up the West Coast. However...an
extremely impressive low on satellite is about to disrupt this
pattern. This low is tightly wound just south of the central
Aleutians and has been producing hundreds of lightning strikes the
past few hours. This low will be steered north by the meridional
jet later this evening and should punch right through the Adak and
Atka areas. Tightly closed lows like this are hard for the models
to resolve...so it may contain stronger winds than 55 kts they are
depicting. Either way...when combining the tight circulation and
decent forward momentum...expect some strong winds through this
area into Sunday.
After this system...continued active weather will continue to
start the week. Strong southerly flow will continue to bring warm
moist air to the eastern third of the Bering while further west keeps
weak northernly flow embedded in a trough.
Long term forecast (monday night through friday)...
Low pressure moves rapidly north grazing the southwest coastline
Monday into Tuesday and pushes a weather front to its east north
across the Gulf and into southcentral Alaska. As the low passes
Nunivak Island Tuesday afternoon the down-inlet pressure gradient
will reverse and allow for gusty south to southeasterly winds to
move out of Turnagain Arm into Anchorage...with strong onshore
flow conditions over the rest of southern Alaska. Onshore conditions
will remain through much of early Wednesday...then a potential
disturbance moving across the southwest Alaska toward southcentral
middle-week which will represent the leading edge of cooler air
returning to southern Alaska. With this push of cooler air from
west to east will come the Anchorage area's next best chance of
snow later Wednesday into Wednesday night.
Strong upper level and surface ridging builds over Alaska
immediately behind this disturbance and will likely bring a
cooling trend...with northerly outflow winds likely the most
impactful threat...but with no immediate concerns for any warning
level event as it appears right now for Thursday into Friday.
Friday into the weekend...high pressure remains over the state
with more seasonal temperatures for early January over much of
the southern half of the state. Deep southerly flow continuing
over the Bering and grazing the southwest Mainland will keep
conditions warmer in these areas.
Marine...gales 119 120 132 136 137 138 139 141 150 155 160 173 174
175 176 178 180
storms 130 131.
Synopsis and model discussion...ad
southwest Alaska/Bering Sea/Aleutians...mso