Area Forecast Discussion
Issued by NWS Anchorage, AK

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906
FXAK68 PAFC 231428 CCA
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
528 AM AKST Wed Jan 23 2019

.ANALYSIS AND UPPER LEVELS...A substantial pattern shift is in
progress today, as temperatures continue to creep to well above
climo normal in persistent south flow. A vertically stacked low
north of the central Aleutians is gradually shifting westward, and
a decaying boundary associated with this low, and warm air
advection in south flow continues to bring light precipitation to
the Gulf coast and warm conditions to most of Southcentral. A
secondary low rotating around this main low is over the AKPEN
moving northward. This system is bringing increased winds and
precipitation to the eastern Aleutians, AKPEN, and Southwest.
While northeast flow on the north side of this low is continuing
below freezing conditions in the lower Kuskokwim Valley and the
Kuskokwim Delta, south flow prevails on the east side of the low
thus much of the Bristol Bay area is in the upper 30s with rain.
As the low continues north, northeast flow will dissipate and the
rest of Southwest will change over to rain.

&&

.MODEL DISCUSSION...The forecast remains uncertain beyond 36 hours
out with the setup of an elongated shortwave upper trough and its
associated surface lows tracking from the North Pacific towards
the Gulf Thursday. The system will have a strong subtropical or
possibly even tropical moisture fetch, thus convection will play a
significant role in the development of the lows. Models are
struggling to deal with this and have had a wide range of
solutions over the last several runs. The evolution of this
feature is crucial because it will determine what develops
upstream. The varying outcomes have generally been consistent that
a stronger low will develop in the North Pacific behind this
front, and move inland somewhere in Southcentral or Southwest
Friday. This will continue the above freezing conditions over most
of the CWA Friday, and will probably bring strong winds to
Southcentral.

It`s difficult to be more specific at this point. The last three
runs of the GFS, while differing slightly with timing, have
brought this low into Cook Inlet. This would be a very
climatologically unusual situation, and should this verify,
temperatures would near 50 degrees in the Anchorage Bowl and
Turnagain Arm winds could push 100kt. The last two runs of the EC,
on the other hand, bring the low farther west and track the low
north near Bristol Bay. South flow on the east side of the low
would still bring warm conditions to Southcentral, but not nearly
as extreme as the GFS solution. Strong Turnagain Arm winds would
also develop, but again not nearly the magnitude that the GFS
suggests. The last run of the NAM, which gets its boundary
conditions from the GFS, instead of concentrating the energy into
one low like the global models, brings two lows into the picture
from the North Pacific. One crosses the AKPEN and the other heads
toward the Prince William Sound. Should this verify, Turnagain
winds would be much lower than the other solutions.

&&

.AVIATION...
PANC...VFR conditions and light surface winds will
persist. Turnagain Arm winds now are expected to make a brief
appearance at the terminal this evening, but should only last 4-6
hours or so. Before that, low level wind shear will remain.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Thursday night)...
A period of active weather has begun, with a series of storms
poised to track across the region for at least the next few days.
The exact storm track will heavily determine the impacts, but
generally looking for a warm and windy pattern with lots of rain
along the coast. Starting with today, a strong occluded front over
the Gulf will lift northward and push onshore of Southcentral this
afternoon. Southeast winds will continue to ramp up this morning
ahead of the front, especially from Prince William Sound across
the Kenai Peninsula. As usual, Turnagain Arm and higher elevations
of Anchorage will see some of the strongest winds. The latest
forecast continues the trend of increasing these winds, though
have kept them just below warning threshold (75 mph). As the front
heads inland late this afternoon through early evening, will see
some east to southeast winds move into Anchorage and the
Matanuska Valley. This will immediately cause temperatures to jump
into the upper 30s to lower 40s. The strong southeast flow will
initially produce downslope drying in the lee of the Kenai and
Chugach Mountains. If any precipitation were to fall this morning,
it would be in the form of rain or freezing rain, but not
expecting anything more than sprinkles or patchy light
precipitation. As the front moves onshore, there is a better
chance of seeing a brief period of light rain for these lee side
areas. The greatest potential for freezing rain may be the
typically cooler and wind-sheltered areas of the southern Susitna
Valley, from Houston northward to Willow and up toward Talkeetna.
Will need to keep a close eye on temperatures and precipitation
for this area.

As this front dissipates inland tonight, another strong front
with multiple low centers will make its way into the Gulf. This
front will quickly track north and move onshore around midday
Thursday. With little time between fronts to cool down,
temperatures will remain near or above freezing for most of the
region. Thus, precipitation with the second front looks to be
primarily rain. Although the low level flow will remain
southeasterly, it looks like the core of strongest winds will be
above mountain-top, so downslope flow will not be as effective and
may see some "slop- over" rain from the western Kenai to
Anchorage and the Mat Valley.

Behind the second front, the forecast is fraught with uncertainty
due to the huge spread in numerical guidance. However, expect yet
another low or front to make its way to Southcentral Thursday
night or Friday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Wed and Thu)...
The much more active pattern is starting to unfold. But
uncertainty remains high in many of the details. The net effect
will be much warmer temperatures to end the week with gusty
conditions at times. First, a developing closed low will cross the
Alaska Peninsula this morning. Precipitation ahead of this
feature is already spreading over much of Western Bristol Bay and
the YK- Delta. For Bristol Bay, just about all sites are above
freezing now and most of the precipitation is rain with just a few
northern sites (Platinum) reporting some lingering snow. It is a
different story across the Delta with very gusty offshore winds.
These winds are serving to reinforce some cold air and keep the
precipitation all snow for now. The combination of these elements
is also making for some blowing snow, especially along the
coastal areas. Expect this blowing snow to hold on through the
daylight hours.

By late this afternoon/early evening, the low will track along the
coast north of the area. This will allow some gusty southwest
winds to blast in and drive temperatures above freezing. This
should turn any lingering snow showers over to at least a
rain/snow mix. Only the deepest, coldest valleys of the Lower
Kuskokwim Valley will be able to maintain temperatures below
freezing.

After this, each successive feature brings an increasing level of
uncertainty. The first is another warm front by late tonight/early
Thu. The front itself is on track to push through SW AK on Thu.
However, models are still trying to develop various surface lows
along this boundary. We did not try to latch onto any of the
surface lows and instead, just kept it a broad-brushed front with
higher PoPs and some gusty winds. Then immediately on its heels is
potentially a very compact, strong low blasting through the Alaska
Peninsula late Thu. Models vary wildly with how they handle this
system. For now, we did not make many changes to this portion of
the forecast. However, if the NAM were to verify, we would have to
trend winds and waves significantly higher.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Wed and
Thu)...
A broad trough will remain entrenched over the Bering keeping the
weather pattern quite active. The "parent" low is currently
parked in the Central Bering. To the west of it, expect some gale-
force winds and cold air advection snow showers to persist today.
To the east, expect several fast south-north tracking systems to
spin up and move through. Each of these systems will bring some
warmer air and make for a mixed bag of precipitation.

By Thu (and into the end of the week), the parent low starts to
take over as it wraps cold air in over the Bering. This will make
for somewhat calmer conditions with scattered snow showers.

&&

.MARINE (Days 3 through 5)...
An active and challenging pattern will continue through the
weekend. Thursday and Friday will see a strong low lift across the
AKPEN bringing high end gales, with the possibility of storm force
winds. Model guidance is still uncertain and is keeping confidence
levels moderate on the track and timing, but high on intensity. On
the heels of that system is one that will move into the Bering
this weekend. The current track slides a deepening low through
Adak and towards the Pribs. High end gales are expected. Storm
force winds, and possibly higher on Sunday through the central
Aleutians are not out of the question.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The longwave pattern isn`t changing much through the weekend.
Ridging across the Gulf of Alaska will slide east and weaken as a
deepening trough moves into the western Bering. That signal
spells continued active weather for much of the extended forecast.
The first system will slam into the AKPEN Friday and into this
weekend. A second system will track into the Bering and both will
bring strong winds, warm air and unseasonal conditions. Above
freezing temperatures are expected for Southcentral and coastal
locations, while some inland areas may cool below freezing
overnight. The return to winter (temperatures) is looking to occur
early into the next work week, though a shunt of warm may be on
its heels as well.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale: 411 351 352 119 120 125 130 131 132 136 137 138
139 141 150 155 177 178 185
Heavy freezing spray: 185
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...MM
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MO
MARINE/LONG TERM...SS



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