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000
FXAK68 PAFC 150141 CCA
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
441 PM AKST Thu Feb 14 2019

.ANALYSIS AND UPPER LEVELS...
A ridge is building aloft over mainland Alaska, and a short wave
trough extends from a vertically stacked low in the northern
Bering. These synoptic patterns combined are creating southerly
flow over much of the Bering, and weak offshore flow over the Gulf
of Alaska. The southerly fetch in the southwest is bringing in
warm and moist air to the Aleutian chain but most of the Bering is
encompassed by cooler air. Ahead of the ridge, there is
southeasterly flow over the western Gulf and northern Gulf coast.
At 30,000 ft, the jet stream is weakly meridional in nature and
maximum wind speeds are reaching 115 kts.

At the surface, a North Pacific low is advancing toward the
western Aleutians. A front extends in front of the low which is
pushing over the western Aleutians and into the southern Bering. A
second front, correlated with the short wave trough aloft, is
moving over the western mainland. This front is creating gusty
conditions and moderate to heavy snowfall. Blowing snow conditions
were reported on the Kuskokwim Delta coastline this morning.
These two fronts can easily be identified in infrared satellite
imagery as a band of increase cloud cover over the western
mainland, eastern Aleutians and into the northern Gulf.
Southcentral remains clear and dry under the ridge, and winds are
beginning to dwindle. Slowing winds in combination with clear
skies will cause temperatures to drop lower overnight throughout
Southcentral.

&&

.MODEL DISCUSSION...
The numerical models are in fairly good agreement for the short
term portion of the forecast (Saturday afternoon). By Saturday
afternoon there are some differences in the surface location near
the Alaska Peninsula. There are also some differences in how they
handle the moisture moving into Southcentral with this system
during the day Saturday. This creates some challenges with Pops
and snow amounts, so confidence in the forecast is below normal
today.

&&

.AVIATION...
PANC...VFR conditions and light winds are expected through Friday
afternoon.

&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through
3/Tonight through Sunday)...
A shortwave trough riding over the top of the ridge currently in
place over Southcentral will increase cloud cover across the
region overnight. As the trough slides east tonight, the
combination of subsidence and increased northerly flow from the
rebuilding of the ridge will allow for a brief uptick in the gusty
outflow winds through tomorrow morning. As the ridge crests over
Southcentral late tomorrow, a southwesterly flow aloft will
develop, driving a series of shortwave troughs across the area.

The first of these troughs and associated weak surface front will
move west to east across Southcentral late Friday night into
Saturday. Isentropic lift is expected to produce an area of light
snow from Cook Inlet north through the Susitna Valley. Snow
amounts, however, are expected to be very light across the region
given both the very dry air at the surface and the trend in recent
model runs of a weaker system. The best chance for measurable
snow with this system will be the upslope areas of the Talkeetna
and Chugach Mountains.

The upper-level ridge exits the region early Saturday as a surface
low east of Kodiak Island transitions into an open wave as it
lifts into the northern Gulf. This system gets absorbed into a
larger frontal system attached to a surface low south of the
AKPEN late Saturday/early Sunday. As both the low and the front
push north, widespread precipitation will develop along it. Snow
will quickly change to rain on Saturday for Kodiak Island with
snow overspreading the Kenai Peninsula and areas of Prince William
Sound late Saturday. An initial southerly push of warmer air
over the top of the colder air at the surface may lead to a brief
period of light snow in the Anchorage Bowl before the flow aloft
transitions to a downsloping southeasterly wind. Given the strong
southerly flow, snow is expected transition to rain along the
coast overnight Saturday into Sunday from Homer east into Prince
William Sound. The entire system then lifts northeast late Sunday
as the upper-level trough moves across Southcentral. Southeasterly
gap winds (Passage Canal, Turnagain Arm, etc.) will also increase
ahead of the frontal system from late Saturday through early
Sunday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Friday and
Saturday)...
The pattern will remain very active heading into the weekend.
There will be three fronts impacting weather across the area. The
first front has just about cleared the YK Delta and will continue
to move east tonight and dissipate. The snow and blowing snow
conditions associated with it earlier today will continue to
rapidly improve. Westerly flow behind this front overnight could
bring some patchy fog, but instability should help to mitigate
that on the whole.

Front #2 is already taking shape over the Central Aleutians. A
rapidly deepening near bomb-force cyclone will push into the
Central Bering overnight. Much like the last system, it will sling
its associated warm front toward the coast. This will bring
another round of strong E to S winds and a couple inches of snow
to most of SW AK. The area of greatest concern is along the YK-
Delta coast. Low-level offshore winds could serve to reinforce
cold air into the area as they feed into the front. Much like
today, the combination of these conditions could lead to snow and
blowing snow making for very low visibilities along the coast and
Nunivak Island. A Winter Storm Watch has been issued to address
the potential for blizzard conditions Fri morning through Fri
evening.

Front #3 will take a completely different track into late Sat.
Instead of a powerful low over the Bering with an elongating warm
front affecting the coast, this system will move in starting from
the North Pacific and pushing north across the Alaska Peninsula.
Once again, with cold air in place, blowing snow will be a
concern. The warm air associated with this system should be
quickly shunted to the southeast (deflected by the Alaska Range),
so more places should pick up a couple inches of snow
accumulation Sat afternoon and evening.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
An active pattern continues across the Bering. An area of gales
over the southwest Bering moves westward as the remnants of a
Siberian low exit the area this evening. Another low quickly
advances on the heels of the first, forming a closed center near
Adak this evening. The associated front pushes across the eastern
Bering, bringing warm southerly flow to the central and eastern
Aleutians by Fri morning. A large area of gales with embedded
pockets of storm force winds will move across the central and
northern Bering through Fri afternoon. Even though the fast moving
low departs the Bering to the north by Fri afternoon, it
continues to strengthen, and leaves a large trail of gale force
winds through Sat morning. West to southwest winds will persist
bringing in relatively colder air and widespread snow showers in
behind the passing front through Saturday.

&&

.MARINE (Days 3 through 5/Sunday through Tuesday)...
A strong ~970mb low will move into the Bering Sea Sunday and
confidence is growing for southwesterly storm force winds and high
seas across the western Aleutians. These winds are expected to
gradually diminish as they track west to east as the associated
front weakens, however gales will be possible as far east Bristol
Bay. Winds will gradually diminish and seas subside through Monday
as the low weakens and departs to the north. Generally light
winds and calm seas are expected Monday evening into early Tuesday
morning under a ridge. Although model agreement begins to degrade
Tuesday, it appears another low will track into the Bering,
though looks much weaker than Sunday`s low and as of now models
are indicating gales for the western Aleutians and sub gale for
eastern regions.

A weak low will track into Bristol Bay Sunday and bring the
potential for marginal gales to the western Gulf, mainly through
the Barren Islands. Generally sub gales expected Monday under a
ridge. The ridge will remain in place Tuesday, however as models
are indicating a strong pressure gradient the gale potential will
return and seas will slightly build through the day, especially
over the southern Gulf.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Strong winds and moderate rain are likely over the Aleutians
Sunday into Monday as a strong low tracks across the Bering. As
this low tracks to the north and east across the Bering, a strong
fetch of southwesterly winds will impact the southwest coast by
Monday afternoon. If current guidance comes to fruition, there may
be potential surge and rough surf issues along the coast,
especially where shorefast ice is absent. A weaker low will
develop over the western Aleutians Tuesday bringing more light
rain over the region. Precipitation will continue moving west to
east Wednesday with partial clearing beginning to occur Wednesday
evening into Thursday as a ridge builds in behind the front.

Most of the latest model runs now indicate a weak low moving
across western mainland Sunday. Given the strong southerly flow
and subsequent moisture advection into the region, significant
snow could occur around the center of circulation near the
Iliamna area. Accumulation is much more uncertain for inland
areas in Southcentral area with some models splitting the low over
Prince William Sound which in turn would affect the flow pattern
over the Chugach Mountains and subsequently how much downsloping
occurs and how much snow makes it into the typical leeside
locations. Model agreement becomes poor into early next week. The
general pattern appears to look mostly benign with party to mostly
cloudy skies, seasonal temperatures and possible scattered show
showers through Wednesday. Both the ECMWF and Canadian model
solutions indicate a relatively strong cold front tracking over
southern Alaska on Thursday which would bring the potential for
snow, though confidence remains low at this time.


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Storm Watch 155.
MARINE...Storm Warning 180 181 185.
 Gale Warning 155 165 170 172>179.
 Heavy Frz Spray 181.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...RA/BL
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...AP