Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
400 am akst Sat Feb 28 2015
Analysis and upper levels...one weak low is working its way
through the Cook Inlet region this morning while it falls apart.
A massive dry layer up top 18,000' was evident on the afternoon
sounding from Anchorage and thus...anything falling through that
layer is evaporating before it reaches the ground. The bigger
story is the cold front out along the West Coast. This front is
supported by a fairly sharp upper-level wave in conjunction with
an 80-90 knots jet. Snow has started at Bethel and stretches back
through the Kuskokwim Delta. This upper-level trough will sweep
quickly across the state through the day and will provide the lift
for snow across much of south central. Over the western
Bering...an aggressive 140 knots piece of the polar front jet is
supporting a 980 mb low just west of our domain. This low is
driving a front through the extreme Western Islands with a small
area of storm force winds.
Model discussion...models continue to be in fantastic agreement
in the short term leading to overall fairly high confidence in the
forecast. Most models were a little slow with the timing of the
precipitation that moved through Bristol Bay last night. So as we
start to shift our focus to the cold front over the yk Delta...we
will need to watch to see if this trend re-emerges. Mainly the
high-resolution models were used for this portion of the forecast.
After the trough pushes through south central today and the
western Aleutian low drives through extreme eastern Russia on
Sunday...the next feature of interest will be a low complex
emerging just south of Dutch Harbor. This low will result from
phasing between some middle-level energy caught up in the fast zonal
flow south of the Kamchatka peninsula and a vertically stacked low
over the North Pacific. Models are still trying to sort out the
details of this complicated scenario...so a broad brushed model
approached was applied to this portion of the forecast.
Short term forecast southcentral Alaska (days 1 and 2)...
forecast is on track for a quick shot of snow across much of
southcentral...with rain and snow along the Gulf Coast and across
the Kenai Peninsula. Due to the rapid movement of the upper trough
generally expect very light accumulation (of an inch or less). The
one exception will be areas along the west side of the Talkeetna
and Chugach Mountains. Strong low level southwest flow ahead of
the trough followed by weak cold advection in westerly flow behind
the trough will enhance snow amounts and likely produce several
inches in the upslope areas. A look at forecast soundings for this
area shows the favored snow growth region will be fairly high in
the atmosphere at 9000 feet or greater. Thus snow ratios will
remain fairly low somewhere between 10 to 1 and 15 to 1. This
gives additional support to the idea that snow accumulations will
be light despite the enhanced upslope flow.
Precipitation will quickly come to an end across the region this
evening as the upper wave exits to the east and a large ridge of
high pressure builds in. The passage of the trough combined with
weak cold advection and deep offshore flow will produce the usual
gap winds along the north Gulf Coast and Prince William Sound.
They will be fairly short-lived with the ridge building overhead
by late Sunday.
The largest forecast challenge is determining extent of fog and
stratus in the wake of the snowfall. The cold advection is short-
lived and low level winds will quickly die down in northern Cook
Inlet. Meanwhile the middle to upper levels will dry out as soon as
the upper trough passes. This seems like a great scenario for fog
and stratus formation. Thus...will introduce this into the
forecast for the entire Cook Inlet to Susitna Valley corridor.
Conditions will also become favorable for fog in the Copper River
Short term forecast southwest Alaska (days 1 and 2)...
Scattered snow showers will become steady snow through this morning
for the Kuskokwim valley and taper off from west to east through
the day. High pressure builds overhead tomorrow evening into
Sunday drying out the atmosphere.
Low pressure pushing north from the Pacific will begin to impact
the Bristol Bay zone toward Monday morning with a chance of rain
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
A weather front from the Kuskokwim Delta extending south through
the southeastern Bering will move quickly onshore through this
afternoon. Winds will quickly veer from northwest to southeast
ahead of a strong low in the western and central Bering.
This low will push a front across the western Aleutians today and
then begin weakening steadily through Sunday night as it moves
through the northern Bering. Enough cold air is in place for
precipitation to begin as snow before transitioning to rain. A
broad pair of low pressure systems will form south of the central
and eastern Aleutians Sunday into Sunday night and develop a
deformation band of wet snow and rain across the central Bering
and periods of rain mainly along the Pacific side of the eastern
Aleutians and akpen.
Monday highlights some of the greatest uncertainty associated
with the long term forecast. The challenge is determining how a
cut off low in the North Pacific merges with an upper level trough
pushing across the Bering Sea. This impacts how a weak surface low
will develop in the south-central Bering Sea and move toward the
southwest Mainland on Tuesday. Precipitation chances will be on
the rise across the Pribilof Islands and southwest Alaska but the
exact track of the low will determine the precipitation pattern.
It appears that much of southwest Alaska will see a mix of rain
and snow on Monday and the first half of Tuesday. Weak ridging
building across the western Bering Sea on Monday will shift
eastward through Wednesday while the ridge breaks down...a low
coming off Kamchatka will move into the western Bering Sea where
it is expected to gradually weaken. A second Kamchatka low will
move into the western Bering Sea on Friday. These two Kamchatka
lows will increase precipitation chances across the Aleutian
Islands with much of the Mainland staying on the dry side as weak
ridging resides and keeps the lows to the north and west of the
Southcentral Alaska will remain largely on the dry side on Monday
and Tuesday as high pressure shifts eastward toward the Alaska
Panhandle. An area of low pressure in the Bering Sea will create a
weak low that will push into the northern Gulf and increase
precipitation chances across the northern Gulf Coast Tuesday
before spreading inland by Wednesday. This would result in a mix
of rain and snow across much of southcentral Alaska from
Wednesday through Friday.
gale 177 185
heavy freezing spray 180
Synopsis and model discussion...MO
southwest Alaska/Bering Sea/Aleutians...cirrocumulus