Area Forecast Discussion
Issued by NWS Anchorage, AK

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

705
FXAK68 PAFC 111419
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
519 AM AKST Sun Nov 11 2018

.ANALYSIS AND UPPER LEVELS...

A very active morning weatherwise is unfolding across much of
southern mainland Alaska. A strong low centered over the Bering is
producing storm force winds along the Southwest Alaska coastline,
with widespread gales ongoing over much of the rest of Bering, as
shown by an 0808 UTC ASCAT pass over the area. A secondary low is
developing over the North Pacific, which is helping move a strong
atmospheric river of moisture eastward. The dry slot from the
first low is moving over Southwest Alaska, which is temporarily
allowing the precipitation to diminish over Bristol Bay, while a
secondary dry slot is evident on satellite over the southern Gulf.
This second dry slot will help slide the plume of moisture
eastward, keeping the best moisture and heaviest rain over eastern
Prince William Sound and the central and eastern Gulf.

Numerous Winter Weather Advisories for freezing rain are in
effect across the area, as warm air associated with the plume of
moisture and dry slots is expected to begin transitioning the
ongoing snow to freezing rain and then plain rain during the day
today. However, thus far most stations have either reported plain
rain, snow, or a mix of the two. The 12Z Anchorage sounding shows
there has in fact been considerable warming over the lower
atmosphere from just 12 hours before then, but the "warm" air is
only now just starting to get above freezing at about 1500-2000
feet above the ground. Radar shows steady moderate to heavy rain
ongoing along the Gulf Coast. Bands of precipitation, which for
now are mostly falling as snow, are moving inland across the
western Kenai Peninsula, Anchorage, and the Mat-Su Valleys. Some
bright banding is evident approaching the Kenai-Soldotna area,
which may be the start of Southcentral`s transition from snow to
rain.

Southwest Alaska radars for now are much quieter relatively
speaking due to the aforementioned dry slot moving over the area.
The leading edge of that dry air is pushing one last band of snow
northward across the Lower Kuskokwim Valley and the Kuskokwim
Delta. Warm air at the ground is roughly correlated with where the
dry slot air is across Southwest Alaska, as every station in
Bristol Bay is above freezing, and Sparrevohn in the Lower
Kuskokwim Valley has been reporting plain rain for much of the
night. With colder temperatures likely in the valleys, this is a
good setup for freezing rain in the Lower Kuskokwim Valley as
frequently the cold air gets trapped in the valleys with nowhere
to go.

&&

.MODEL DISCUSSION...

The models have different areas of agreement and disagreement this
morning. They are all still consistently showing warm air
spreading northward across all of southern mainland Alaska during
the day today, which should change the precipitation over to all
rain. Beyond that, disagreements are significant and numerous.
Among the many challenges this morning continues to be timing of
when (or if) any of the precipitation changes over to freezing
rain on its way over to a plain rain. Many sites around the area
are within a few degrees of freezing, and the warming caused by
freezing rain should help to boost temperatures above freezing in
a hurry, which then reduces the threat of significant ice
accumulations. So far, all the stations that started out as snow
and then changed to rain did not have a period of freezing rain
in between, but warm air moving in aloft is in most of the models,
which means it may just be delayed. Precipitation type is by far
the greatest point of uncertainty with the forecast for this
morning.

Synoptically, the low currently affecting the area is expected to
interact with multiple other waves of energy, which themselves are
likely to develop into areas of low pressure over the next two
days. All of the models show these multiple areas of low pressure
are likely to interact with one another, leading to spiraling
paths. Since much of this activity is expected to occur over the
North Pacific, where there are very few, if any, observations, the
model solutions on how and where each of these lows will be, even
as soon as 36-48 hours from now remain highly divergent. This
introduces large uncertainty into the forecast as regards wind
speed and direction in any one coastal location, in addition to
timing any areas of rain associated with them. As a result,
forecast confidence is very low, with one of the only points of
agreement being that temperatures are very likely to warm over the
next few days such that most areas are well above freezing by this
time tomorrow.

&&

.AVIATION...

PANC...The main point of uncertainty with the TAF continues to be
timing the transition of snow over to rain over the airport. The
snow thus far has been more robust than expected, and with warm
air just getting into the area as seen on the sounding, it is
likely to occur later this morning. Until then, MVFR to IFR
conditions are expected in the light snow. Rain or freezing rain
will follow the snow as temperatures warm throughout the
atmosphere. The rain will continue through the day, becoming a
bit more scattered by tonight. Winds are likely to remain
relatively uniform throughout the TAF period out of the north,
with potential for gusts from time to time.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
An atmospheric river continues to bring a stream of moisture and
warm air through the Gulf and into Southcentral Alaska. The
heaviest of the precip is being hung up on the north Gulf coast
but areas all throughout Southcentral Alaska are or will be
getting some precip. Conditions are warm enough for areas along
the north Gulf coast to see rain through Monday. Areas beyond the
coastal ranges have started as snow but should transition over to
rain throughout the day. The challenge will be if areas around
Cook Inlet through the Susitna Valley will be able to hold onto
freezing temperatures at the surface allowing for freezing rain as
warmer air overrides the cold surface airmass. At this point it
does look likely that there will be a brief period of light
freezing rain this morning transitioning from south to north over
the area.

There is higher confidence that the Copper River Basin will see
freezing rain today with a better setup to hold onto freezing
temps at the surface as warm air moves in aloft from the south.
This area will also see the transition from snow to freezing rain
to rain move from south to north today. Further south, Thompson
Pass blizzard conditions look to continue through the early
morning before temps increase and winds weaken.

Precip will lessen somewhat Monday morning before another trough
swings through bringing another round of rain to the north Gulf
coast. With this trough, areas beyond the coastal mountains will
stay out of much of the heavier precip.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Sun and Mon)...
The pattern will stay very active to start the week. The first in
the series of disturbances has cleared through most of the area.
We were watching this very closely overnight to see how
temperatures and visibilities responded to some warm air advection
and the potential for blowing snow. There was enough warm air
across the area to push most of Bristol Bay over to rain after a
few hours of snow. There was also enough warm air to keep
visibilities (so far) from dropping much below 2 miles along the
coast of the YK-Delta. We used all of this information as the
jumping off point for the forecast.

As the next wave takes shape today and starts to trek from south
to north across the area, expect a very similar day to Sat. Gusty
northeast to southeast winds will resume as precipitation
overspreads the area once again this afternoon and evening. There
will still be some potential for blowing snow across the area, but
the best bet remains along the coast of the YK-Delta. The biggest
change to the overnight forecast was the issuance of a Winter
Weather Advisory for snow and freezing rain across the Lower
Kuskokwim. Warm air will move over the region just above the
surface this morning into the afternoon. As it does so, it will
create an environment where the precipitation can melt into
liquid. But then, as it falls back through the below-freezing
temperatures near the surface, it will have the potential to
refreeze on contact. This, combined with several inches of snow,
will lead to a hazardous weather day in the valley.

By late Mon, most of the area will see drying as the system starts
to move into the Bering. Another weaker system will queue up to
move into Bristol Bay on Tue.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Sun and
Mon)...
The Bering will be divided into two weather patterns to start the
week. First, over the Western Bering, expect strong northerly
winds, snow showers, and cold temperatures. Out over the Eastern
Bering, look for several low pressure systems spinning through the
area. Each of these will bring rapidly shifting wind directions,
warm air, and mixed precipitation. The interaction between this
two Worms`s will lead to a fairly healthy swath of storm-force
(initially) winds driving from north to south from the Pribilof
Islands down through Dutch Harbor. The duration and fetch under
these winds will lead to seas climbing to near 30 feet. Conditions
will generally improve late Mon as some weak ridging slides in
from the east. But this will be short lived as another system
slides into the Eastern Aleutians from the Gulf by Tue. In
general, northerly flow will persist over the Western Bering. This
will continue to allow cold air to trickle down from the Chukchi
Sea/Bering Strait.

&&

.LONG TERM FORECAST (Days 3 through 7)...

The long-term forecast begins Monday night with an active pattern
remaining across much of the forecast domain. The main feature is
a large upper-level low centered south of the AKPEN and
stretching from the central Aleutians and southern Bering through
the Gulf. Multiple embedded shortwaves are pinwheeling around the
main trough, with the first well-defined feature situated south
of Kodiak Island, the second over the North Pacific well south of
the AKPEN, and the third moving across the eastern Aleutians from
the north. With the trough axis primarily over the AKPEN and two
surface lows in the western Gulf, southcentral Alaska will be
under the influence of a southerly fetch bringing a mild,
subtropical airmass from the Pacific into the region. Moving
through the week, both the deterministic guidance as well as
their ensemble counterparts are in relatively good agreement in
keeping this broad trough in place. The differences are in the
timing and position of the shortwaves and the surface features.
In terms of the big picture, the EC, NAM, and GFS all kick the
low in the northern Gulf toward the Alaska Panhandle on Tuesday
while the low in the southern Bering weakens and becomes absorbed
into the main surface low near Kodiak Island. This low then
weakens as it drifts southwest on Wednesday. Farther south, both
the EC and GFS develop a new north Pacific low, moving it toward
the Gulf on Thursday. Here the models diverge greatly, especially
with the timing of the low, as the GFS brings the low into the
northern Gulf by Thursday night while the EC delays the northward
progression by a good 12 hours or so. Either way, it will remain
fairly active across southcentral through next week with wind and
rain along the coast and mixed precipitation for interior
locations.

Farther west, both the GFS and EC develop a ridge over eastern
Russia, keeping a cold, northerly flow over the Bering. Guidance
also develops a low west of Shemya midweek with a front swinging
north across the Aleutians by late week.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Blizzard Warning 131 (Thompson Pass).
 Winter Weater Advisory 121 141 145 152.
MARINE...Gales 119 120 125 130 131 138 139 160 165
 170 173 174 176 177 180 351 352 411.
 Storm 175 179 181 185 412 418 414.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...DWK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MSO
LONG TERM...TM



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.