Small Craft Advisory 253, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
312 PM AKDT Tue Mar 17 2020

.ANALYSIS AND UPPER LEVELS...
Two low pressure systems are our focus today. The first is a weak
low pressure system moving northeast up Cook Inlet. Surface
observations show the barrier jet and turnagain winds beginning to
set up as expected with this system. RADAR also shows
precipitation moving up the Cook Inlet. The backside of this low
is moving through southwest AK, with RADAR and satellite showing
precipitation coming to an end. This system is being supported in
the upper levels by a 500MB shortwave trough currently over
Southwest AK and diving southeast. It should be noted that this
trough has a push of colder air behind it, depicted well at the
850MB level. The other system is a large low pressure system
just moving into the western Bering Sea. Satellite shows it is a
mature and has a well wrapped occlusion. Its front is currently
pushing through the western Aleutians.

&&

.MODEL DISCUSSION...
Models are in good agreement through the short term forecast. As
mentioned in the previous discussion, there was a change in
driving cold air more southerly behind the passage of the low
currently working its way up the Cook Inlet. However, models all
agree on the change so we have adjusted the forecast. Since models
are agreeing even when trending towards new solutions, we have
high confidence in the forecast going forward.

&&

.AVIATION...
PANC...MVFR to IFR conditions in snow are expected into early
Wednesday morning, then should improve to VFR by late morning.
Light winds are expected.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The upper level trough over southwest Alaska will continue to
track east. This should bring a shot of precipitation to
Southcentral Alaska as it moves east. The main upper low will
drop southeast from the Iliamna area across the Barren Islands to
the Gulf of Alaska overnight. The surface low forming in the base
of this upper trough will be near the southern Kenai Peninsula
this evening and track to the east and southeast overnight.
Conditions are expected to remain cold enough from the Susitna
Valley and Copper River Basin to support snow. South of these
areas enough warm air should move over the Kenai Peninsula and
Prince William Sound to result in a mix of rain and snow or all
rain. Colder air will flow back across the region tonight as the
upper trough moves southeast. An upper level ridge then builds
over the Bering Sea coast by Wednesday morning. How fast
conditions dry out is the concern. It is likely some snow showers
will persist into the early morning for the Cook Inlet to Susitna
Valley region. The Copper River Basin will be slower to dry, thus
we will hold on to snow showers through the morning, though these
should be scattered and have little accumulation.

The outflow returns Wednesday and we should see outflow wind in
the usual places of the Chugach Mountains, Whittier,
Valdez/Thompson Pass and the Copper River Delta. The wind is not
expected to be very strong, generally remaining below 30 mph.

The Ridge will slowly move across the southern mainland Wednesday
and be pushed east Thursday as another front moves into the
Bering Sea coast. As this front moves through, moisture returns
to the southern mainland along with snow or rain and snow likely.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
A front is currently tracking across Southwest Alaska bringing
rain and snow to the Bristol Bay region and all snow to the
Kuskokwim Delta. As temperatures are near freezing, only minimal
accumulations are expected through the remainder of the afternoon.
A ridge will build in this evening in the wake of the front.
Thus, skies will clear overnight and winds will diminish, leading
to the potential of fog developing over much of the area into
Wednesday morning. Another front will move over Wednesday night
and light rain/snow is likely through Thursday.

A series of upper level shortwaves Thursday afternoon through
Friday morning will keep snow showers in the forecast for mainly
the areas of Bethel to Hooper Bay to Kipnuk. Weak upper level
ridging is expected to move into areas along the AKPEN and Bristol
Bay area keeping the area dry with the normal upslope regions of
the Alaska Range experiencing continued snowfall.

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
A strong front is bringing gusty southwesterly winds and rain to
the western Aleutians. This will track west to east through
Wednesday, however as it will be relatively fast moving, impacts
should remain minimal. An upper level shortwave will move across
the Bering and Aleutian Chain Thursday afternoon through Friday
afternoon bringing a mix of rain and snow. An upper level ridge
will begin to move over the western Bering and western Aleutian
Chain Friday afternoon.

&&

.MARINE (Days 3 through 5 Fri through Sun)...
Due to a triple point low in the northern Gulf Friday, expect
gale force winds to occur through channels including Shelikof
Strait, the Barren Islands and through gaps and passes on the
Pacific side of the Alaska Peninsula through Saturday morning.
Following this, there are indications that another triple point
low will spin up. If this plays out, expect similar winds in the
aforementioned channels through Sunday.

A gale force front over the Bering will continue to push
northeastward before it exits the Bering Friday. There is
moderate confidence that another front will enter the western
Bering Friday night. Forecast challenges include timing and
strength. If the faster and stronger solution holds, gale force
winds will be possible in the western Bering through Sunday.

&&

.LONG TERM FORECAST (Days 3 through 7 Fri through Wed)...
On Friday, an upper level ridge looks to move across the Bering
and into southern Alaska. Similar to last week, warm temperatures
and moist conditions will advect into the Aleutians and the Bering
Sea following the backside of the ridge through Saturday. Behind
this, an upper level trough splits this upper level ridge with
another upstream. This looks to bring westerly flow to the
Southwest which will propagate precipitation into the region
through Sunday. This active weather pattern will continue through
the middle of the week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning 177.
 Gale Warning 130>132 170 173>176 178 179 185.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...BB
SOUTHCENTRAL ALASKA...SA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AP/MV
MARINE/LONG TERM...CK