Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 181345
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
445 AM AKST Mon Feb 18 2019

.ANALYSIS AND UPPER LEVELS...

A trough extends from the northern Bering down to the central
Aleutians this morning. A vertically stacked upper level low is
positioned within the flow of the trough over the Pribilof
Islands. Cold air is advecting over the western Bering, and
wrapping around the low. On the southern and eastern edge of the
trough, the south-southwesterly flow is bringing warm, moist air
from the North Pacific into Southcentral and the Gulf of Alaska.
Upper air support is waning as the jet stream becomes zonal and
shifts southward, skirting the Aleutian chain.

At the surface, a warm front ahead of the surface low in the
Bering, pushes inland. Satellite imagery shows the extent of this
front as it drapes from interior Alaska across the western Gulf,
and into the North Pacific. The front is bringing precipitation to
the western mainland, AK-Pen and Southcentral which can be
observed by radar. As the front progresses northeastward, several
short waves are moving over Southcentral. With the frontal
passage, temperatures are rising above freezing for the eastern
Aleutians, Southcentral coastline areas as well as coastal areas
of the AK- Pen. However, interior and western locations remain
cooler and any precipitation is falling as snow.

&&

.MODEL DISCUSSION...

Models continue to be in good synoptic agreement through Tuesday.
Tuesday morning, a North Pacific low approaches the western
Aleutians and this is where models begin to struggle. As the low
enters the Bering Tuesday night, the EC is the fastest solution
moving to a location between the chain and St. Paul. The NAM in
comparison is the slowest model with the low stalling over the
chain. The GFS and Canadian models sit somewhere between the two.
In addition to the position of this system, models struggle to
come to consensus on the intensity of the storm.

&&

.AVIATION...

PANC... Snow showers have resumed across the Anchorage Bowl,
particularly along the Hillside and East Anchorage. The snow
showers should continue building westward this morning and will
persist around the area for much of the day. A cold front will
swing through this evening which will clear out the snow until the
next round of snow arrives Wednesday afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

Today`s weather front is associated with a strong low in the
Bering Sea located near the Pribilof Islands early this morning.
Precipitation type is the main challenge again today. However it
should not be as widespread of an issue in comparison to
yesterday due to the atmosphere cooling aloft behind the front.
Southerly winds are occurring across the region which is keeping
air at the surface mixed well and above freezing in many
locations...especially near the coast. In spite of the warmer air
right at the surface, the cooling aloft will be the main driver of
precipitation type and as the day progresses the all-snow line
will progress eastward.

The low near the Pribilof Islands will move quickly eastward over
the next day and end up near Yakutat by Tuesday morning. This will
turn the winds to gusty north-to-northwest over the region and
usher in colder air. Expect gusty outflow winds through channeled
terrain and along the Gulf coast for Tuesday. This northwesterly
direction and dry, well-mixed air should keep fog from developing
and leave some widespread areas of clear skies for Tuesday into
Tuesday night before the next front arrives Wednesday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

The radars at Bethel/PABC and King Salmon/PAKC are still very active
as of 3:45 AM AKST. Which is par for the course considering that
a warm front has moved into the region bringing a rich fetch of
Pacific moisture into the region. The latest AMSU/advanced
microwave sounding unit with is a passive satellite instrument has
detected above normal levels of precipitable water from the
Western Alaska Range to Bristol Bay, and the AK-Pen. Lingering
showers across the Lower Kuskokwim Valley will continue today and
begin to taper off tonight. There will be a lull between storms,
expect Tuesday to be dry. By Wednesday morning, the next
precipitation shield will amble into Southwest Alaska. Expect a
complex wintry mix.

The other item of significance is the coastal flood advisory
which is in effect until 6 PM AKST this evening. Given the long
duration of westerly fetch, a coastal flood advisory along the
coast from Levelock to Port Heiden was issued for today. While
there are areas of shorefast ice along the northern coast of
Bristol Bay, there is uncertainty with how storm surge will
interact with the ice. As the low continues to track inland,
conditions will generally improve for Tuesday before the next
front associated with a North Pacific low nears the coast Tuesday
night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

The latest ASCAT/Advanced Scatterometer has detected gale force
winds near the Pribilof Islands, north and south of the Aleutians.
The surface low anchoring the main frontal boundary is near St.
George Island. Looking at satellite imagery, there is an occlusion
wrapped around the low near the Pribilofs, with the warm front
already in southwest Alaska and the cold front extending southwest
of the AK-Pen well into the northern Pacific. A new low will move
towards Adak with the leading edge of the precipitation shield on
track to make landfall 12z Tuesday or 3 AM AKST over the Central
Aleutians. This low will push east-northeast this forecast
period. Early Wednesday, the models develop a "thermal ridge" in
the 1000 mb to 850 mb thickness parameter which signifies the
development of another occlusion in the Bering. Expect gale force
winds to materialize in multiple marine zones with this second
low.

&&

.MARINE (Days 3 through 5)...

On Wednesday, a 990 mb low tracks north along the eastern Bering
bringing gusty gales as it slightly deepens before it crosses over
the Bering Strait and fills. During this same time frame, the
next low organizes across the North Pacific and tracks to the
southern Bering by early Thursday morning. There is uncertainty on
the strength and track of this storm as a more powerful system
develops south of it and races toward the western Bering on
Friday. Both systems have the potential to bring storm force winds
across the southern Bering.

&&

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

A wetter pattern is expected to continue through the end of the
work week across the southern mainland and surrounding waters. A
drier trend is still expected for the southern mainland at the end
of the week as a ridge amplifies across the mainland. Models are
in good agreement with the general synoptic pattern across the
Bering as low pressure systems continue to develop across the
North Pacific and track toward Alaska. However, models are lacking
in run to run consistency on the individual track and strength of
several low pressure systems that will move to the area.

Only minor changes were made for the Wednesday forecast with
the main concern surrounding a low tracking north across the
eastern Bering. This system is expected to bring gusty winds and
snow across the Kuskokwim Delta with snow spreading eastward
across to Southcentral and the Gulf for Thursday. This solution is
highly dependent on the development and evolution of a strong
wave at the base of the Bering trough, developing near the Alaska
Peninsula.


&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Coastal Flood Advisory: 161.
MARINE...Gale Warning: 120 130 131 132 137 138 150 155 160 165 170
                       171 172 173 174 178 351 352 411 414.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...RJA
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
MARINE/LONG TERM...KH


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