Heavy Freezing Spray Warning 138, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
540 AM AKDT Tue Apr 6 2021

.ANALYSIS AND UPPER LEVELS...

A mid-to-upper level shortwave from the northern Bering extends
across the Kuskokwim Coast through the western Gulf, which
continues to bring steady light snow over Kuskokwim coastal area
through Bristol Bay this morning. Meanwhile, a developing upper
level ridge forms over the Bering today, causing the front and
strong jet streak to gradually exit Southwest Alaska by late this
afternoon. Over Southcentral, mostly clear skies and colder
temperatures fills the area.

.MODEL DISCUSSION...

Models are relatively close in agreement through at least Saturday
morning. Large scale pattern seems to transition effectively from
west to east across the Bering through the Gulf of Alaska in the
coming days, with the first Bering low focusing on Kuskokwim Bay
and Bristol Bay between Wednesday and Thursday. The main question
following this low is whether it maintains an eastward motion or
will it dip slightly southeastward when it enters the western Gulf
by Wednesday night? The future location of the low plays an
important role on the distribution of snowfall amounts once it
shifts over the western Gulf.

Farther north, an upper-level trough forces its way southward
past Kuskokwim Mountains through Thursday morning which could
increase the potential for additional snowfall over AKPEN and
Southcentral on Thursday and Thursday night. Meanwhile, colder air
temperatures will continue across Southcentral through the middle
of the week. A second low over the western Aleutians and its
associated front will elevate winds and seas into the weekend. All
in all, the two expected lows from the Bering will be the
highlight of this week's weather events. The forecast confidence
remains moderate to high until Thursday.

.AVIATION...

PANC...VFR conditions and light northerly winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Tuesday
through Thursday)

With the exception of lingering mountain snow showers over parts
of the Copper River Basin and Wrangell Mountains, mostly clear
skies prevailed over much of the Southern Mainland, allowing
temperatures to drop down anywhere in the teens to the single
digits. With little to no significant changes in sight with the
colder air aloft holding firmly in place, expect temperatures to
remain below normal for this time of the year. Meanwhile,
pressure gradient driven northerly winds are expected to subside
briefly and then peak again this afternoon and into late Wednesday
afternoon, with a reinforcement of colder air in tandem with
upper level support. These winds will funnel through the usual
gaps and passes, including Seward, Whittier, Valdez, and the
Copper River Delta, including the potential of freezing spray over
the outer coastal waters.

A weak shortwave and associated surface low situated underneath
the left exit region of a 160+ knot jet streak, will quickly pass
over Kodiak Island, bringing a chance of snow showers this morning
then taper off late this afternoon. Looking ahead, our next
impactful event will be a potent shortwave from the Northern
Bering that will dive southeastward over the southern AKPEN late
Wednesday night. A triple point low will develop along a frontal
boundary near the Western Gulf and Kodiak Island, with strong
winds and widespread snow over much of coastal and inland areas on
late Wednesday night and into Thursday. An upper-level polar low
moving south from the northern half of the state will follow on
the heels of the aforementioned Gulf low as it exits to the east.
These two distinct synoptic features will usher in very cold,
arctic air aloft, with 850 mb temps (~4,500 ft) dropping between
-20C and -30C by Friday morning (temperatures usually seen in the
middle of winter). There is a very high likelihood of northerly winds
developing along with colder temperatures all across the Southern
Mainland in addition to the usual gaps and passes. More details
to come on this impactful event, so stay tuned!

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Today through Thursday)...

A weakening low is currently bringing light snow to the Kuskokwim
Delta and portions of Bristol Bay. This will gradually taper off
through the afternoon. Partly cloudy skies and calm winds are
expected into this evening under a weak upper ridge with patchy
fog possible ahead of it along the Bristol Bay coast.

A front will move onshore early Wednesday morning. Snow will
begin along the Kuskokwim Delta coast and be accompanied by gusty
easterly winds. This will lead to blizzard conditions and as such
a Blizzard Warning has been issued from Wednesday morning through
the evening. Winds will increase and become more northerly through
the day. Gusts up to 50 mph are possible with very strong cold
advection developing. The 850 mb temperatures over this area will
drop nearly 35 degrees between Wednesday evening and Thursday
morning. Although the snow will end, blowing snow leading to
reduced visibilities will be possible into Thursday afternoon.
Accumulating snow is also likely over Bristol Bay, although winds
will not be as strong. Skies will clear into Friday. Temperatures
will be very cold, with highs only in the single digits across
much of Southwest Alaska.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Today through Thursday)...

Snow will gradually taper off through this morning over the
eastern Aleutians and Alaska Peninsula through this morning as a
low weakens over the Kuskokwim Delta. Quiet conditions will
persist elsewhere today under an upper level ridge. A front will
stretch across the Aleutians Wednesday morning bringing light
precipitation to the Chain. Its associated low will lead to
moderate rain over the Pribilofs with strong westerly winds
developing by the evening on its backside. Another weaker front
will track across the western and central Aleutians Thursday and
Friday.

&&

.MARINE (Days 3 through 5: Thursday through Saturday)...

A very active weather pattern ends the week over the Gulf of
Alaska, but the break will be short as another strong system
drives its way across the Bering through Sat. Adding to the
pattern is a blast of Arctic air pushing southward over Western
Alaska, reinforcing the Gulf weather system. Confidence is good
with the pattern, but timing and strength require more attention.

A well developed low and front over the Gulf of Alaska has
widespread high end gale force winds covering the Gulf through
late Fri. With the Arctic reinforcement, areas of storm force gap
winds develop from the Shumagin Islands eastward around Kodiak
Island up to the Barren Islands and into the Gulf through late
Friday. Heavy freezing spray with the winds and waves will be an
issue. Winds diminish to small craft across the Gulf on Sat.

In the west, a new low and front tracks across the Bering through
Sat. As the low moves across the Bering, the winds with the front
wrap the low and brings widespread high end gale force winds with
areas of storm force winds, and heavy freezing spray in advance
of the front. The front reaches Western Alaska and Bristol Bay by
Sat.

&&

.Long Term Forecast (Days 4 through 7: Friday through Monday)...

The upper level pattern for the beginning of the long term
forecast period for the most part is in agreement. Beyond Saturday
though, model guidance diverges drastically. The GEFS is showing
the upper level trough lifting out of southern Alaska and placing
it over the Yukon Territory and northeastern Alaska, with a weak
upper level ridge moving across the southern Mainland. This would
lead to a warmer pattern, with temperatures possibly returning to
more normal levels for this time of year. The EC and Canadian are
closer in agreement with each other with the upper level trough
weakening some but then quickly reestablishing itself over the
mainland and extending out over the Bering Sea. This would lead
to a continuation of below normal temperatures. Basically, well
below normal temperatures are expected for the forecast period
with slowly warming temperatures each day. Temperatures may be
possibly back above freezing for highs by the end of the period,
though, still remaining below normal. As a result, expect strong
gap winds to continue for the beginning of the forecast period
with these easing each day.

Due to the continued disagreement between the global models at
the upper levels, timing and intensity of individual features
remains highly uncertain. However, there is some agreement that
a more active weather pattern (precipitation wise) may be
returning to much of Alaska. A frontal system is expected to
move across the Bering on Friday, though, model guidance varies
widely on what happens with this frontal system and the energy
associated with it once it reaches the eastern Bering Friday
night.

&&
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 155.
		 Winter Weather Advisory 161.
MARINE...Gale Warning 155 165 170 179 181 185
		 Heavy Freezing Spray 180 181 185
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...CMB
SOUTHCENTRAL ALASKA...MF/TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AP
MARINE/LONG TERM...MK/MV