Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
457 am akst Thursday Nov 27 2014
Analysis and upper levels...
a Stout upper level ridge over Mainland Alaska is promoting
mainly clear skies across the southcentral part of the state this
morning...with patchy fog developing over parts of the Matanuska
Valley and Anchorage bowl in the vicinity of the Knik Arm. Gusty
outflow winds continue through favored areas along the Gulf Coast
as the pressure gradient tightens with the eastward advance of the
ridge. Stratus and patchy fog prevail across much of the
southwest Mainland as the ridge nudges eastward and onshore flow
develops over the area. A broad upper trough remains over the
Bering Sea with an embedded low center and associated 983 mb
surface low tracking rapidly northward through the central Bering
approximately 200 miles west of the Pribilof Islands...spreading
rain and gale force winds over much of the Bering.
model agreement remains very good into Saturday as the ridge over
the Mainland weakens and moves off to the south and east...
allowing an upper trough to move across southcentral Alaska from west
to east on Saturday. Forecast confidence remains above average
given the strong model agreement.
Short term forecast southcentral Alaska (days 1 and 2)...
pressure gradients along the north Gulf Coast will begin to slacken
today as the upper ridge shifts eastward and northerly flow
weakens. However...models have all picked up on development of an
upper level wave and associated jet streak rounding the top of the
ridge and moving overhead of Prince William Sound this afternoon.
This will provide enhanced subsidence and will likely keep gap
winds going a little longer from the Valdez area to the Copper
River Delta. Have extended the stronger gap winds through early
evening and then dropped them off significantly tonight as low
level flow begins to shift with arrival of the ridge axis.
According to satellite imagery and web cams fog and stratus
overnight was largely confined to the north end of Knik Arm
(to around palmer) and along Eagle River valley. Low level
flow is very light and variable and that will not change today.
Thus...there may be some weak drift but fog should largely remain
in those areas. Pressure gradients will begin to tighten tonight
ahead of a frontal system approaching from the Bering Sea which
may be enough for some of the fog to drift down to Anchorage.
As always...this is very difficult to forecast and confidence is
low. Any remaining fog should dissipate Friday as clouds begin
to overspread the region.
Forecast confidence is high that snow will fall across much of
southcentral Alaska Friday night and Saturday. A neutrally tilted
upper level trough will move through quickly in progressive upper
level flow...which means snowfall amounts will generally be light.
Short term forecast southwest Alaska (days 1 and 2)...
The widespread fog seen Wednesday morning has lifted in low
stratus over the inland Bristol Bay and Kuskokwim valley areas as
the ridge axis has shifted downstream over the Alaska Range and
weak disturbances provide some synoptic scale lift.
Otherwise...the main problem the next couple of days will be with
the incoming weakening front as it moves onshore and moves west to
east slowly across the area. Warm temperatures just above the
surface on the Bethel and King Salmon sounding lend some reasoning
to believe there could be some isolated brief pockets of freezing
rain...but for the most part with a lack of low-level cold
advection precipitation should fall as rain and snow.
By late in the day on Friday...a more vigorous disturbance moving
east toward the Bristol Bay region will help to initiate a steady
band of snow...mainly along eastern Bristol Bay and Kuskokwim
valley zones on the west slopes of the Alaska and Aleutian ranges...
where decent accumulations may be possible into Saturday.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
Strong yet weakening low pressure will spin north across the
Bering Sea today bringing high-end gales impacting areas just west
of the pribilofs this morning. Conditions become much more
tranquil late tonight into Friday before the next strong low moves
north toward the Bering Friday night into the weekend.
Long term forecast (days 3 through 7)...
A pattern change from nearly stationary to progressive will be
occurring on Sat as an upper trough from Japan helps to build a
ridge over the Bering. This in turn will push a trough across
Mainland Alaska that will finally break down the ridge that has
resided over the state for several days. Sat night into sun a
second trough will dive across the Kamchatka peninsula and further
reinforce the Bering ridge. This second trough will then cut off
and spin in the Bering for several days. The ridge will move from
the eastern Bering to British Columbia Monday through Wednesday...finally
allowing the Bering cut-off to move south of the akpen by midweek.
The surface reflection of all this action will be a front that
moves across the Mainland on Sat...a residual low in the northern
Gulf associated with the front that persists into early next weak
as a trough...and a massive low that crosses the Aleutians on sun
and pinwheels in the Bering until finally exiting across the
eastern Aleutians Tuesday into Wednesday.
Confidence is increasing in this solution as all models are
showing the front moving across the Mainland on Sat...bringing some
rain and snow to the region before exiting the state. The low in
the northern Gulf will keep rain showers along the coast into next
week. The ec and GFS are also persistently showing a pinwheeling
low in the Bering...with some question remaining on the exact
trajectory. For now the forecast is of moderate confidence...
leaning on a blend of wpc guidance and the ec. This will bring
rain and at least gale force winds to much of the Aleutians Sat
into Tuesday. On Monday and Tuesday strong winds with snow showers behind the
low will affect the western half of the chain.
Marine...gales 175 176 177 178 179 180 181 185.
Synopsis and model discussion...cumulonimbus
southwest Alaska/Bering Sea/Aleutians...cirrocumulus