Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 201420
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
520 AM AKST Wed Feb 20 2019

.ANALYSIS AND UPPER LEVELS...

The vertically stacked low that once encompassed the Bering is
pushing into the northern Bering, still bringing active weather
to the southwest and central Bering. This low sits in the left
exit region of the meridional jet where winds are approaching 140
kts at 30,000 ft. The jet, combined with the flow around the low
is promoting southerly flow aloft over much of the eastern Bering
and western mainland. Cold air is wrapping around the low and
advecting over the western Bering and Aleutians. A shortwave
is present over the western Aleutians. The other synoptic feature
is a persistent ridge over the Gulf of Alaska and Southcentral.

At the surface, a tight gradient within the isoheights around the
surface low is creating strong winds throughout the Bering,
notably near the Pribilofs. A cold front ahead of the low, is
moving eastward over the western mainland. Satellite imagery
depicts this textbook system as a comma cloud. The central
rotation is in the northern Bering, and the front is depicted as a
swath of clouds over the western mainland. This front is bringing
heavy snow and strong winds to the Kuskokwim Delta. Ageostrophic
flow around the low is creating easterly winds through the western
Gulf while southerly flow around the surface ridge is causing
more of a westerly flow in the northern Gulf. Over mainland
Alaska, the ridge is keeping skies clear, winds calm and
conditions dry throughout Southcentral. However, this pattern is
changing soon as the next system develops in the northern Gulf.

&&

.MODEL DISCUSSION...

Synoptically, models are in good agreement through Wednesday
afternoon, but they quickly diverge at the surface level. The
first sign is in regards to a developing surface low in the
western Gulf on Wednesday afternoon. Between model runs, there has
been general consensus of a low along the coast forming however
newer runs are showing a slightly more southeasterly track, yet
still the exact location is undecided between the models.

The next system, a Kamchatka low advancing towards the western
Aleutians on Thursday afternoon, is also becoming a struggle for
the models to handle. While all note this feature, they have yet
to lock onto a track and intensity. The American models tend to
agree with placement but the GFS is depicting a deeper low. While
they start out of step with each other, by Saturday, they come
into agreement with the placement, while the strength remains an
issue.

&&

.AVIATION...

PANC... Other than some scattered low clouds mainly south of the
area this morning, the weather starts out VFR and quiet. This
afternoon, snow moves in and will gradually increase in intensity
through the overnight, with the heaviest snow expected from around
midnight through the early morning hours on Thursday, during which
IFR conditions will be common with potential for periods of LIFR
conditions. The snow ends from west to east Thursday morning. Fog
will become an increasingly frequent issue going into the weekend.

&&

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

The main aspects of the incoming system remain on track. However,
there are two nuances that will have a large impact on the
duration and amount of snowfall over the region. The first will be
the track of the surface low and whether it pokes into western
Prince William Sound or stays just south of the Sound. In
general, if the low reaches into the Sound it will bring more
snowfall over the Cook Inlet region and Susitna Valley. The
Susitna Valley is more susceptible to heavier snowfall as the
system moves in this afternoon and evening due to it being in move
southwest flow aloft at that time whereas upper level flow will
become slack even before the upper level low passes through late
Wednesday night.

This upper level low is the other important factor which will
largely dictate when the snowfall ends. Usually the snowfall will
shut off like a light when the 500 mb trough passes through. The
upper level pattern has been consistent showing not just a trough,
but a cut-off low developing in the trough as it passes over
Southcentral late Wednesday night and moves into the northern
Gulf. While a secondary short wave will traverse the area behind
it late Thursday morning, them main snowfall should end with this
initial upper level trough/low.

The 6Z recent model runs did generally scale back a little on the
QPF values expected over Southcentral and this may be in part due
to these two features mentioned above. However, the Winter Weather
Advisories remain in effect and still look to be in the ballpark
for expected snowfall amounts through the Susitna Valley one was
scaled back a little on the amounts.

One other area to watch with this storm will be the Cordova area.
There will be some good lifting along the front as well as gusty
Southeast winds. Temperatures should remain near 30 for the low
which will allow for snow and blowing snow. Since the temperatures
are just below freezing, there is less of a chance for blowing
snow to reduce visibilities to blizzard conditions. Due to snow
intensity and the gusty winds, some blowing snow is likely in the
Cordova area overnight tonight.

&&

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

There is a surface low near St Matthew Island with an occlusion
and a warm front moving into Southwest Alaska. A decent amount of
moisture will work its way into SW Alaska bringing another batch
of snow to the region. The models have the surface low moving
almost due north this forecast period, and the occlusion getting
sheared apart from the remnants of the baroclinic zone. As for
snow amounts, the dendritic growth zone looks pretty healthy with
this system. Expect 2 to 4 inches in the Y-K Delta, and a few
isolated places may receive 6 inches. The models bring a minor
ridge into SW AK on Thursday which will be the lull before the
next storm. The precipitation shield with this second storm is
handled slightly different depending which model solution you look
at. The odds are good that the next wave of moisture will begin
to move into the region late Thursday and early Friday for the
coastal communities of Southwest Alaska.

&&

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

An active pattern is on deck for this forecast period. The latest
ASCAT/Advanced Scatterometer pass has detected gale force winds
over the Central and Eastern Bering. This isn`t much of a
surprise given how tight the pressure gradient forces were in the
models runs leading up to this event. This low will move almost
due north this forecast period. In its wake, another cyclone will
follow and can currently be located on satellite imagery Southeast
of Kamchatka and southwest of Attu Island. This storm will deepen
20 millibars in 24 hrs, not exactly a bomb, but formidable in its
own right. Expect strong winds, rough seas and another round of
precipitation with this storm. This storm is projected to reach
its peak intensity near Attu and Shemya early Thursday and track
north. A secondary wave will form along the baroclinic zone and
move towards the Central Aleutians early Friday. The models are
already spinning up a third cyclone in the Sea Of Okhotsk on
Friday.

&&

.MARINE (Days 3 through 5)...
Thursday through Saturday continue to look windy across the
Bering Sea as a strong low moves up the Kamchatka Peninsula. Winds
ahead of the low in the central Bering are still expected to be
around gale force, but the forecast winds around the low center
have increased in strength and storm force winds are expected for
the Bering Sea and just north of the western Aleutians through
Friday. Saturday appears to be a bit of a break as most of the
energy will go into a low moving up into the central Aleutians,
but gales can be expected to persist across the central Bering.

In the Gulf of Alaska, a low moving into the Prince William Sound
and sliding off to the east on Thursday will increase
northwesterly winds through the gaps. These are expected to be
gale force through the Barren Islands and into the Gulf of Alaska
but higher gusts are possible especially along the coast in the
gap areas.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long term forecast continues to be on track from yesterday,
with a ridge of high pressure building in over mainland Alaska,
pushing the main storm track westward over the Bering Sea. Models
are trending towards a stronger high pressure system, which means
that it looks less and less likely for precip to make its way
inland from any lingering fronts over the weekend. This high is an
unusually warm high pressure system for this time of year,
looking more like a spring time high pressure system than the cold
winter highs we see in late January. Solar heating is still
marginal, so spring is not here yet...but temperature forecasts
for the weekend and early next week across the mainland look to be
a bit of a challenge.

As for the Bering Sea, the usual active storm track of the winter
will continue. The storms moving through aren`t expected to be
unusually strong, but the amount of warm air aloft will bring rain
much further north than we normally expect at this time of year.
Warm and wet conditions are expected to continue for the eastern
half of the Bering through early next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory: 101 111 121 141 145.
MARINE...Gales: 119 120 125 130 131 132 173-176 179 181 351 352
                411-414.
         Storms: 177 178 185.
FIRE WEATHER...NONE.

&&

$$

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


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