NWS Anchorage, AK wrote its last area forecast discussion before the first event at Mar 23, 4:14 AM AKST, 9 hours 53 minutes earlier.
Area forecast discussion, Mar 23, 4:14 AM AKST, 9 hours 53 minutes before
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
514 AM AKDT Tue Mar 23 2021
.ANALYSIS AND UPPER LEVELS...
An amplified pattern presents itself this morning with a drawn
out and positively-tilted trough working its way across the
mainland and over the Gulf. Upstream, a strong ridge is building
into place over the Bering/Aleutians, and this is bringing mainly
northerly flow into the AKPen and Eastern Aleutians where island
and mountain gaps are generating corridors of stronger winds up to
gale force, mainly on the Pacific side of the eastern Aleutian
Chain. Low stratus persists in the cool northerly flow over the
eastern Bering, while skies are generally clear across the
Southwest with drying and subsidence in the wake of the wave
passage where the ridge is also moving onshore. To the west of
the longwave ridge, a strong front extending from a low over
northeastern Siberia and Kamchatka is approaching the western
Bering. Very strong southerly gales gusting to storm force at
times exist along and ahead of the front along with an axis of
dense stratus dropping rain and snow as it progresses mainly
east.
East of the trough axis over Southcentral, virga in the form of
snow is visible on the Kenai radar reflectivity returns. This snow
is being generated by lift along and just ahead of trough, but
very dry air in the low levels has prevented it from reaching the
ground for much of the early morning thus far. Some of this snow
may begin to reach the surface later this morning across the Mat-
Su Valleys briefly, especially in areas closest to the foothills
of the Chugach and Talkeetna Mountains where upslope enhancement
will be greatest. However, the lack of moisture near the surface
casts doubt on whether the layer will saturate before the trough
axis pushes through this morning, and any accumulation from this
snow should remain virtually negligible.
&&
.MODEL DISCUSSION...
Models display generally good agreement for the first 48 hours of
the forecast period, but a few key details begin to become
murkier by late Wednesday and Thursday. Models show the northern
portion of a frontal system will come ashore over the Y-K Delta by
tonight, while the southern portion becomes sprawled out on a
west-east axis as it stalls in the southern Bering by Wednesday
morning. Subtle differences remain for the exact vicinity where
the boundary will stall, but the larger discrepancies begin as the
pattern becomes increasingly zonal and progressive from Wednesday
evening into Thursday.
Models agree that a shortwave quickly moving along the front
aloft will break into several smaller impulses as it approaches
Southcentral near the crest of a strong but rather flat ridge by
Wednesday night. The exact path of these impulses as they move
across the southern mainland varies across current solutions,
particularly with regards to how far north or south they track.
This is likely due largely to differences in the strength of the
ridge over the gulf, with the solutions showing a stronger ridge
pushing most of the energy with the westerly waves farther north.
The waves tracking through increasingly southwest flow would
favor moderate precipitation near the windward faces of the
Kuskokwim, Alaska and Chugach/Talkeetna Mountains. However, the
aforementioned differences both spatially and from run-to-run are
leading to substantial uncertainty for when and where this precip
will fall between Wednesday and Thursday night. Consequently,
there is degrading forecast confidence for the areas mentioned
above by midweek and beyond. As such, little change was made for
this timeframe over Southcentral with this package.
&&
.AVIATION...
PANC...Mainly VFR conditions are expected throughout the period
with generally light north winds switching to southeast early
Wednesday. Light snow flurries remain possible this morning, but
significant reduction to visibility is generally no longer
anticipated.
&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Thursday night)...
A neutral to slightly positive tilted trough nearing Southcentral
this morning will march quickly eastward across the region today,
phasing with a trough over the Gulf and driving a surface low
eastward toward Southeast Alaska. There could be some flurries in
the mountains, while valleys look to dry in the low levels for
anything to reach the ground. A ridge will build in quickly behind
the trough, tightening surface pressure gradients. This will
combine with low level cold advection to produce strong gusty gap
winds across portions of the western Gulf beginning this morning,
followed shortly later by the north Gulf coast and Prince William
Sound.
Strong zonal flow will drive a weak upper level short-wave
eastward across Southcentral late tonight, serving to reinforce
the gap winds. With deep westerly flow through the atmospheric
column it does not look favorable for precipitation. Beyond this
time, there is uncertainty in the amplitude of an upstream
trough/ridge over the eastern Bering Sea. The amplitude will
determine when the trough reaches Southcentral. Right now, the
best estimate is sometime Wednesday night or Thursday morning. As
the trough nears flow will back from west to southwest. This will
favor enhanced snowfall rates along the western Chugach and
Talkeetna Mountains Thursday through Thursday night. Stay tuned
for details, as small changes in the amplitude and track of this
trough will affect the snowfall forecast.
&&
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Today
through Thu night)...
Quiet weather conditions will continue for Southwest Alaska
especially for the morning hours as an upper level ridge moves
through the area. A strong frontal system approaching from the
Bering is currently spreading high clouds into the Yukon-Kuskokwim
Delta this morning. These clouds will continue to lower and
thicken throughout the day as the atmosphere moistens. Winds will
also be on the increase especially for the Kuskokwim Delta Coast
this morning as the front approaches. Areas in the Middle
Kuskokwim Valley and Bristol Bay will remain dry but cold
throughout much of the day.
A Blizzard Warning remains in effect for snow and blowing snow
mainly for the Kuskokwim Delta Coast and Nunivak Island as the
front moves through the area beginning this afternoon and
continuing into the evening hours. South winds of 20 to 30 mph
with gusts up to 40 mph are expected along the immediate coastal
areas of the Kuskokwim Delta. Winds and snow combined will reduce
visibilities to as low as one quarter mile. The front will
continue spreading eastward into Southwest, weakening as it does
so with snow being the main issue for inland areas and not the
wind. Snow amounts of 2 to 5 inches are expected throughout
Southwest.
The region will remain unsettled over the coming days as the
frontal system currently moving through stalls over southern
Bristol Bay Wednesday. Another piece of upper level energy with an
elongated area of low pressure at the surface over the Bering will
ride northeast along this frontal axis and bring another round of
snow to some of the same areas. Exactly where this low tracks will
determine how much warm air gets into Bristol Bay, and this may
cause precipitation type issues for Wednesday/Wednesday night.
&&
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3:
Today through Thu night)...
A storm force frontal boundary will continue moving across the
Bering today bringing a round of snow and blowing snow to the
Pribilofs with mainly rain for the Aleutian Island Chain as warmer
air is brought northward. While reductions in visibility are
expected in snowfall for the Pribilofs, thinking that it won't be
enough to warrant an advisory. Another wave of upper level energy
with an associated weak, elongated surface low will ride
northeast through Adak (Tuesday night to Wednesday) and through
the Pribilof Islands (Wednesday) as the aforementioned front
weakens and stalls over the southern Bering Tuesday night. Another
frontal system will move into the western Bering and western
Aleutians Wednesday afternoon with more rain/snow showers and
windy conditions.
&&
.MARINE (Days 3 through 5: Thursday through Saturday)...
Gulf of Alaska: There is a dome of high pressure south of the
Aleutian chain and a ridge axis over the GOA Thursday. Friday the
ridge will get pushed to the south and a trough will bring a line
of showers through the GOA. Saturday a new organized low will
move into the eastern Gulf but this storm will predominately
impact Southeast Alaska.
Bering: There are two weak fronts in the Bering Thursday. One is
between the Y-K Delta to the Pribilof Island to Dutch Harbor. The
other is near Kamchatka with the south edge of the boundary
clipping Attu and Shemya. Both of these boundaries push
southeastward with the leading system rapidly decaying.
&&
.LONG TERM FORECAST (Days 4 through 7: Friday through Monday)...
There is a noticeable amount of model divergence from the Bering
to the Gulf Of Alaska. This section of the forecast package
reflects a blend of several models. There are a few items to
glean past 96 hrs. At the onset of the forecast period, expect a
relatively amplified pattern on with a trough over the Eastern
Bering and ridge over the Eastern Pacific and the Gulf Of Alaska.
There is a weak frontal boundary over the Western Alaska Range and
portions of Southcentral. The Canadian, GFS and ECMWF have honed
in on another low near Kamchatka and the associated front will
bring another batch of precipitation from the Western Bering
eastward reaching Bethel by Saturday. The GFS and ECMWF are
spinning up a storm force low in the NW Pacific Saturday. However,
in the past several model runs, the placement of the surface lows
are more than 300 miles apart and the ECMWF is 15 mb deeper with
the surface low by 18z Saturday.
&&
.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning: 155.
MARINE...Gale Warning: 119,120,130,131,165,170,173,175-185,
411-414.
Heavy Freezing Spray: 150,160,180,185,412,414.
FIRE WEATHER...NONE.
&&
SYNOPSIS AND MODEL DISCUSSION...AS
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MV
MARINE/LONG TERM...PJS