Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
113 PM akdt sun Oct 19 2014
Analysis and upper levels...a high amplitude trough extends all
the way from the Chukchi Sea southeastward across Mainland Alaska
and down into the northeast Pacific. With a weak polar jet on the
back side of the trough over the central to eastern Bering Sea
there is little movement over Mainland Alaska. The subtropical jet
is nearly zonal south of the Aleutian chain and feeds into the
trough south of the Alaska Peninsula...helping to steer it north
and eastward toward the Gulf of Alaska. This is the primary feature
of concern for the weather over southcentral Alaska tonight through
Monday. As the trough becomes more negatively tilted an upper
short-wave and surface low over the eastern Gulf will be pulled
northwestward toward southcentral. The front edge of precipitation out
ahead of the warm front is making its way into eastern Prince
William Sound this morning.
Meanwhile...an upper low and short-wave over the western Aleutians
is about to get kicked out by a sharp short-wave moving out of
Kamchatka this morning. At the surface a couple of lows along the
Aleutians will follow suit and begin to exit to the east-southeast
Model discussion...there are some key differences in track of
surface low across the northern Gulf tonight and speed of upper
trough as it swings northward toward southcentral Alaska tonight
through Monday. The NAM had been a large outlier with much slower
progression of the upper trough. However...all solutions have
trended that direction on the 12z model cycle. Even with the trend
toward the NAM solution...the NAM remains the slowest. As a result
after an initial period of precipitation in Anchorage tonight with
arrival of the warm front the NAM then keeps precipitation off to the
south until Monday afternoon. The Gem-reg is not too far off from
this. Not quite ready to buy into this solution. Will go with a
slightly faster upper trough (as per the global models) and a
much shorter break in precipitation.
The other big challenge for precipitation in the Anchorage area is
precip-type. Conceptually...this is not a pattern that one would
expect snow accumulation in Anchorage. Low level thicknesses in
the models along with experimental snow level guidance from the
NAM and GFS all indicate a mostly rain event. However...when you
look at forecast soundings most of the solutions indicate a near
freezing isothermal layer...with potential for precipitation to fall as
snow. The 12z Anchorage sounding indicates precipitation would be all
rain. However...there has been some cooling of the column since
then. The other factor for precipitation-type will be the intensity of
precipitation. The band moving across Cordova this morning is moderate
(of not heavy in places)...but Anchorage will be on the northern
and western edge of this band as it rotates through...so precipitation
won't be as heavy. Also...if there is in fact a break in precipitation
temperatures will be more likely to rebound a bit in support of rain.
When you put it all together it seems the best chance for snow or
rain and snow mixed is the initial warm front band. After that
expect mainly rain. Current forecast actually depicts something
similar to this with the only chance of minor snow accumulation
at higher elevations.
Due to the continued uncertainties in timing and placement of key
features forecast confidence over southcentral is low for tonight
through Monday morning. Elsewhere across the forecast area
forecast confidence is moderate to high.
Short term forecast...
Southcentral Alaska...warm frontal band will continue to rotate
westward across Prince William Sound this afternoon and evening.
Arrival of the surface low along the north Gulf Coast late tonight
and then upper trough Monday morning will then cause precipitation to
become more widespread. Precipitation will be all rain along the
coast...with potential for rain and snow for Anchorage (as
discussed above) as well as the western Kenai Peninsula and mat-su.
Accumulating snow looks like it will be confined to the mountains
and mountain passes (such as Thompson pass). Overall... this looks
like a minor event with little impact ot travel. The heaviest
precipitation will be along the coast with much lighter precipitation inland...
especially as you head northward through the Copper Basin and
Susitna Valley. Conditions will dry out behind the trough Monday
night into Tuesday. With a new storm system expected to make its
way into the eastern Gulf late Tuesday attention will shift to
increasing gap winds along the north Gulf Coast.
Southwest Alaska...as the southern portion of the upper trough lifts
northward across the Gulf the northern portion will drop southward
across the Bering. Southwest Alaska will be near the Pivot
Point... leading to development of a low over Bristol Bay. A
series of short-waves rotating around this low will produce
periods of light snow for most of the region through Monday night.
Expect minor snow accumulation...on the order of one inch for any
given location. The upper trough/low will exit to the south as a
ridge builds in from the west early next week...leading to dry and
seasonably cool weather.
Bering Sea/Aleutians...the system along the Aleutians will exit to
the south tonight while the trough over western Alaska drops southward
across the central to eastern Bering. This will bring dry conditions
to the western Aleutians while the rest of the region will enter a
classic winter-type pattern with cold advection showers and gusty
northerly winds. With sea surface temperatures still very warm expect
rain and snow...especially from the central Aleutians to the southern
Alaska Peninsula. The air mass is cold enough to support all snow
as you head farther north across the Bering (including the
Long term forecast...
another low moving into the Gulf on Tuesday will cause cold air
to spread into the southern Mainland through middle week...producing
below normal temperatures and gusty gap winds along the Gulf
Coast. This however appears to be a short lived event...as a low
skirting the eastern siberian coast on Friday will bring
increased cloudcover and warmer temperatures by next weekend. The
front associated with this system will bring rain and gusty winds
to the western Aleutians starting Thursday....then spread onto the
Mainland by the weekend. The upper trough from this system then
looks to slowly dissipate over the Mainland through next
weekend...leaving the Mainland generally cloudy and showery with
near normal temperatures.
Marine...Gale Warning 173 174 185.
Seb/dek Oct 14