NWS Anchorage, AK wrote its last area forecast discussion before the first event at Mar 20, 3:46 PM AKST, 12 hours 28 minutes earlier.
Area forecast discussion, Mar 20, 3:46 PM AKST, 12 hours 28 minutes before
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
446 PM AKDT Mon Mar 20 2023
.ANALYSIS AND UPPER LEVELS...
A deep long-wave trough extends from the Kamchatka Peninsula
across the western half of Alaska. This trough is anchored by a
strong upper-level low centered just south of Saint Lawrence
Island. At the surface, this translates to an occluded and nearly
vertically stacked surface low that is extending a front across
Southwest Alaska and Kodiak Island. Widespread snow and gusty
southerly winds persist this afternoon across the southwest ahead
of the front as it moves east. Temperatures across Southwest are
also fairly uniform, ranging in the upper 20s to lower 30s, a
result of the eroding warmer airmass as the system occludes as
colder air wraps back in underneath the main low.
Gusty westerly winds and scattered showers continue in the wake of
the low over the Bering and central Aleutians. Clouds and
precipitation are on the increase over the western Aleutians where
a developing surface low is advancing from the Kamchatka
Peninsula.
For Southcentral, conditions are generally clear and dry. A weak
trough over the northern Gulf, combined with an increasing
southerly flow aloft ahead of the aforementioned front, is
pushing some clouds and scattered snow showers across the
Southcentral coast as well as across portions of Cook Inlet and
the Susitna Valley. Temperatures across Southcentral range from
the lower to mid 20s in the Copper River Basin to the mid to
upper 30s for coastal locations.
&&
.MODEL DISCUSSION...
Overall, models are in good agreement through the short term. The
NAM solution, as well as other hi-res models, indicate a
splitting of the low over the Kenai Mountains late Tuesday night
as it forms along the triple point. This solution would have some
effect on the wind field, but overall the difference is minor.
Models also denote a progressive pattern with the long-wave trough
moving east through Wednesday before becoming negatively tilted
with an anchoring upper-level low and axis oriented northwest to
southeast just west of Anchorage. While there are differences in
the position of the upper-level shortwaves and attendant surface
lows, the evolution of the pattern suggests the potential for
precipitation along the coast to eventually move farther inland
across interior Southwest and Southcentral Alaska.
&&
.AVIATION...
PANC...VFR conditions will persist. A southerly Turnagain Arm
wind continues through this evening before transitioning to a
light northerly wind overnight. Winds are expected to switch back
to the south Tuesday morning. Light snow and MVFR ceilings may
also develop over the terminal by mid-morning Tuesday.
&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Tonight
through Thursday)...
An amplified pattern persists as a deep upper-level trough is
situated over the western half of the Mainland. At surface levels,
an occluded low is extending a front across Southwest Alaska and
Kodiak Island. The aforementioned front will continue to push
eastward across Kodiak and then spin up a triple point low near
the southern Kenai Peninsula tonight. The thinking for this
forecast package is that warmer air will work into Kodiak and turn
precipitation to a rain or rain/snow mix. However, the
aforementioned low will aide in keeping warmer air away from the
Mainland areas, so snow is expected for the Prince William Sound
region overnight into Tuesday morning. Southeasterly winds off of
Turnagain Arm are expected to clip Anchorage this evening into
tonight, with the Matanuska Valley seeing an increase in easterly
winds as well. As upper-level waves move across the region Tuesday
morning, these winds should decrease. Also, a chance of light
snowfall over Anchorage and the Matanuska Valley is possible
Tuesday morning as this energy moves overhead, but any snowfall
accumulations should not amount to much. Another low will then
move into the Gulf from the southwest Tuesday night into
Wednesday. This will bring back Gale force winds to the Gulf and
precipitation to areas on the Gulf coast. The aforementioned low
and its impacts for the region heading into Thursday, will be
largely dependent on the orientation that the upper-level trough
takes and how close it gets to Southcentral. Be sure to check back
with the forecast as we monitor this system's potential and as
models come into better agreement.
&&
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Tonight
through Thursday)...
A continued unsettled pattern is anticipated to continue for the
next few days, but with much fewer impacts compared to what we saw
with the powerful front coming ashore this morning. This same
front associated with the stacked, occluded low drifting near St
Lawrence Island is still continuing to push inland over Bristol
Bay and up into the Middle Kuskokwim Valley, with the edge of the
light to moderate snow along and ahead of the boundary now pushing
past Dillingham. The main axis of snowfall will continue to move
into eastern parts of Southwest as the front weakens tonight, with
falling snow lingering the longest towards the Alaska Range.
There won't be much of a break before the next system comes into
play, with another low expected to slide southeast from the
eastern Bering across the southern Alaska Peninsula (AKPen) on
Tuesday. Areas of light snow will spread back into the Kuskokwim
Delta and Bristol Bay north of the main center on Tuesday, with
perhaps an inch or two at most falling through Tuesday evening.
It's looking likely a band of slightly heavier and steady snowfall
will set up northeast of the low as it heads into the Gulf between
Tuesday night and Wednesday morning, mainly over Bristol Bay along
an axis roughly between Koliganek and Pilot Point. A few more
inches of new snowfall will be possible in this vicinity until the
upper support moves east into Southcentral by Wednesday evening.
The next low in the series will arrive with another round of light
snow spreading in from west to east on Thursday. This will
coincide with a batch of much colder, Arctic air arriving, pushing
lows back into the 0s to 10s by Thursday night.
&&
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3:
Tonight through Thursday)...
The brunt of the strong low that brought high winds to much of the
western Aleutian Chain and Pribilofs over the past day or so is
finally exiting the picture, with the now occluded and filling
center drifting up into St Lawrence Island. A cool and showery
pattern is setting up in the wake of this system, with most of the
Bering still under the influence of a deep longwave trough and
cyclonic upper level flow aloft. An embedded shortwave trough is
now approaching the western Aleutians, and this low is poised to
skirt quickly along the Aleutian Chain and towards the southern
Alaska Peninsula between tonight and Tuesday. A quick-hitting but
strong corridor of winds is expected to develop on the trailing
portion of this system, with northwest winds spiking up to high
end gale to low end storm force from west to east just behind the
low. The strongest winds are still expected to materialize across
the eastern Aleutians, where gusts will likely peak between 55 and
65 mph for just a few hours late Tuesday morning or early
afternoon.
Snow showers and winds at or near gale force will persist across
the eastern Aleutians and southern Alaska Peninsula on Wednesday
as another shortwave embedded in the main trough pushes across
eastern parts of the Chain. Quieter conditions will return by
Thursday as a very flat upper ridge moves over the Bering, with
winds dropping back to small craft range or lower. Still, snow
showers will likely persist up towards the Pribilofs near a weak
frontal zone sliding across the central Bering.
&&
.LONG TERM FORECAST (Days 4 through 7: Friday through Monday)
The long-term opens up with a negatively tilted trough extending
from Siberia to the southern mainland. There is a lot of
disagreement on the ultimate evolution of the pattern through
Friday for Southcentral. The main disagreement is how guidance is
handling a secondary southeast-moving shortwave within this trough
and how strong it is and where it tracks. Right now the 12z GFS
is the most gung-ho with a stronger shortwave tracking into the
northern Gulf Friday morning, while the 12z Canadian and 12z ECMWF
are less amplified with this feature. All guidance is showing a
period of precipitation for interior Southcentral Alaska with this
feature; it remains up for debate with how amplified the
precipitation could be, especially for Anchorage and the Mat-Su
Valleys on Friday. A transient, but amplified ridge looks to build
into Southcentral Saturday bringing quieter weather Saturday and
much of Sunday before another Bering Sea front approaches the area
the latter half of Sunday and into Monday.
Out west, conditions along the southern half of Bristol Bay and
the Alaska Peninsula should remain fairly quiet through Saturday.
Northern parts of Bristol Bay into the Kuskokwim Delta region
could get clipped by a Bering low pressure system's warm-front the
latter half of Friday into the day on Saturday, which paves the way
for light precipitation chances. The cold-front of this Bering
low will sweep the western Bering and western Aleutians beginning
on Friday making it to the Southwest coast by Saturday night into
Sunday morning. Accompanying this front will be gale-force winds
with possible storm-force gusts, especially for the western
Aleutian Chain. As the front progresses eastward throughout the
central and eastern Bering, the winds will weaken mostly to low-
end gales to high-end small craft through Sunday morning.
Southwest looks to become unsettled late Saturday night into the
day on Monday as the front will cross the southern mainland then.
There are still a few timing differences regarding the progression
of the front through Southwest. Namely, the 12z ECMWF has the
quickest progression of the front with it moving into Southcentral
by early Monday morning. Meanwhile, the 12z GFS and 12z Canadian
both linger the front in Southwest longer, which yields more
precipitation. All- in-all, the progressive pattern looks to
continue with no real sustained breaks in activity.
&&
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning 770-774, 776, 778.
...Gale Warning 411-716, 731, 733, 751-755,
758-759, 765-767, 775, 777, 780-787.
...Heavy Freezing Spray 767
FIRE WEATHER...NONE.
&&
SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...NS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AS
LONG TERM...DN