Small Craft Advisory 306, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
522 AM AKDT Sat Apr 14 2018

.ANALYSIS AND UPPER LEVELS...
The "semi-permanent" low pressure in the Northern Gulf of AK is
finally moving away with a front that slides through the
Aleutians. In between these 2 features is a building ridge of high
pressure. The high pressure will keep weather dry and sunny
across most of Southcentral for yet another day. It has allowed
some low stratus and patchy fog to come into parts of Southwest
AK. Some weak lift and some lingering moisture is making for a few
light snow showers over the Copper River Basin this morning. This
lift should only get more pronounced across that area as there is
an impressive shortwave diving south from the Interior on
satellite. Meanwhile, the front over the Aleutians is not
disappointing. Gale-force southerly winds are ushering in plenty
of rain as this system tracks at a moderate clip from west to
east.

&&

.MODEL DISCUSSION...
Models remain in very good agreement and the result is high
forecast confidence in most places. The few areas of uncertainty
surround the potential for showers over the Copper River Basin
this afternoon and what time the stratus/fog lifts over the YK-
Delta and Bristol Bay. Models did trend slightly further east
with the best energy/cold air associated with the trough over the
Copper River Basin, but the forecast remains mostly the same as
the trend was minor. The fog and stratus out west should lift late
this morning with some diurnal heating and the increasing
winds/instability due to the approaching front.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist with a sea
breeze developing this afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
With a weaving low beginning to depart the Gulf, the focus on the
forecast has shifted northward to the shortwave that will descend
through the Eastern Mainland today. A weak shortwave ahead of this
will continue to bring areas of light snow to the Copper River
Basin today, which will be reinforced and become more widespread
as a much stronger shortwave moves overhead for this evening. The
most widespread shower activity should occur near the Wrangell
Mountains, though light snow showers are expected to redevelop in
areas further west. Another impact from this shortwave will be
from cooler air and increased gap winds out of the Copper River
Basin beginning the evening. The strongest winds are expected to
blow out of the Matanuska Valley and through Thompson Pass, though
confidence remains very good that these should remain well below
advisory criteria. The coldest temperatures into the teens from
this system are still expected to occur in the low elevations of
the Copper River Basin where little wind is expected, though
overnight lows in the 20's are also expected in the Susitna Valley
this weekend.

This shortwave will dive south into the Gulf on Sunday, leaving
much of the area under the influence of a shortwave ridge ahead of
an incoming front to the Western Gulf/Kodiak Island into early
next week. This front is expected to primary impact the Western
Gulf and extreme southern Kenai Peninsula, leaving the reminder of
the region with typically dry springtime conditions and seasonable
temperatures.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Today through Monday)

A pretty stout short-wave ridge will cross the region in a
transient nature over the next 24 to 36 hours. This ridge has
allowed some patchy fog to develop, and this should continue
until a little after sunrise. Otherwise, another fairly pleasant
day is in store for the region temperature wise, with clouds
gradually increasing as we head through the day. Our attention
then turns to a front that approaches the coast by midnight
Saturday night/Sunday morning. A strong push of warm advection in
the low levels will precede this front, with the models having
increased 1000-850 mb thicknesses. All critical thickness values
indicate perhaps a brief (hour or so) period of light snow could
develop on the leading edge of the precipitation shield, mainly
north of the Kuskokwim River. However, given the strength of the
warm advection, a rapid transition to rain is likely to occur,
with all rain expected elsewhere for the majority of the area.
This front will slow its eastward progression through Monday, as
it bumps into the afore-mentioned ridge as it undergoes
frontolysis (the weakening of a front).

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Today through Monday)

An area of low pressure will slowly move from the western Bering
Sea to northern portions of it while weakening. The associated
cold front will extend southeast across the Bering, arcing
southwest towards the Aleutians, with it sweeping from west to
east over the next 36 to 48 hours. Widespread precipitation will
accompany this feature, with mainly showers behind it in the
cyclonic flow aloft through Monday, before yet another system
follows quickly on its heels.

As for winds, look for gales to accompany the front near the
Chain, with some higher gusts possible out of bays and passes.
Small craft to periodic gales will also accompany the low
pressure center across the western Bering, with mainly small
craft winds expected elsewhere. Gales will return to western
portions of the Chain through the western Bering late morning
through the afternoon on Monday, as the next system approaches.

&&

.LONG TERM FORECAST (Days 3 through 7: Monday through Friday)...

Starting Monday, a stationary front will be parked over much of
Southwest Alaska, extending from the Yukon Delta south-
southeastward into Bristol Bay and over the Alaska Peninsula. Near
the front, steady precipitation in the form of a mix of rain and
snow depending on where you are, will be ongoing from Sunday. The
precipitation will be invigorated by an increasingly negatively
tilted upper level trough helping enhance precipitation rates. As
the day goes on the upper level energy will shear apart resulting
in rapidly decreasing coverage and intensity of precipitation.
Meanwhile, narrow ridging over Southcentral and the Alaska Range
will keep any associated precipitation on the west side of the
mountains. Thus, sunny, springlike weather will return on Sunday.
A weak, but cold shortwave will track over the Yukon, which will
bring somewhat cooler air into the Copper River Basin. Subsidence
will keep conditions very dry through the Basin, but the cooler
temperatures will keep conditions safely away from Red Flag
criteria. Meanwhile, a low and its associated front will be
rapidly moving eastward into the western Aleutians.

On Tuesday, the low in the western Bering on Monday moves into
the central Bering. It slows down quite a bit by this point.
Meanwhile most of Southcentral remains dry under weak ridging
aloft. Scattered showers may break out Tuesday afternoon in
Southwest ahead of the occluded front, but steadier precipitation,
especially on the east facing slopes holds off until Tuesday night
into Wednesday as the front moves inland.

On Wednesday, there is some model uncertainty as to how far north
and east precipitation gets as southerly onshore flow develops
across the Gulf. The foreign models have rain much of the day
Wednesday, while the GFS keeps the area dry until Thursday. This
is likely a direct result of very different handling of the track
of the weakening parent low across the Bering. Foreign models are
slowly tracking the low into the eastern Aleutians/Alaska
Peninsula, while the GFS turns it back into the northern Bering
where it develops a secondary low. This secondary low delays the
precipitation at the coast until Thursday. The variability
continues into Friday, but the models do largely agree on onshore
flow resulting in decidedly wetter conditions for much of southern
mainland Alaska to end the workweek.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 165 170 172-74 176-81 185 411-14.

&&

SYNOPSIS AND MODEL DISCUSSION...MSO
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...JW