Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 120159
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
459 PM AKST Mon Dec 11 2017

.ANALYSIS AND UPPER LEVELS...
The deep low pressure system which crossed the Gulf last night
rapidly weakened this morning as it tracked inland across
Southwest Alaska. There are some remaining areas of rain or snow
over Southwest, but precipitation intensity is very light. Strong
pressure rises behind the low kicked up winds across most of
Southcentral this morning, though winds are diminishing during
the afternoon hours. The next in a series of lows being ejected
from the North Pacific long-wave trough is tracking northward
toward the Alaska Peninsula. In contrast to last night`s storm
system, this low has already occluded and reached its peak
intensity. Thus, the low will be in a weakening state as it
approaches.

Meanwhile, back over the Bering Sea and Aleutians widespread
northerly flow and cold air advection are leading to gusty
winds and showers, with precipitation primarily in the form of
snow.

&&

.MODEL DISCUSSION...
Models are in excellent agreement with the well formed low
approaching the Alaska Peninsula. However, they quickly diverge
with the next developing low, with a difference of 18 mb from the
weakest solution to the strongest solution as the low tracks to
south of Kodiak Island Tuesday night. There really isn`t much
run-to-run consistency with this low either. Because there is
some cold air feeding into the trough from the Bering Sea and
there is decent jet support out ahead of the trough, am leaning
toward a deeper low. For now, don`t want to dramatically change
inherited forecast until get a better idea of track and intensity,
so will make a minor trend toward an overall stronger wind field.
Forecast confidence with the first system is high and quickly
drops to low with the second system.

&&

.AVIATION...
PANC...Surface winds will die down quickly late this afternoon and
evening as the next low nears the Alaska Peninsula and pressure
gradients shift back to producing down-Inlet flow. Some wind will
persist along Turnagain Arm and on the mountains and this will
strengthen as a front lifts northward across the Gulf tonight.
Thus, expect low level wind shear to develop. With this next low
tracking farther west than the previous two lows downslope flow
will be stronger, so expect VFR conditions to prevail through the
TAF period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The pattern of ridging over southeast Alaska with multiple waves
moving north through the Gulf into Southcentral Alaska will
continue through Wednesday. This setup brings continued warm air
to Southcentral Alaska making the majority of precip near the
surface fall as rain. However, this setup also leads to most of
the precip falling along the north Gulf coast with inland
locations being protected by the coastal ranges. As various lows
track to the west of the area, pressure gradient winds will
continue for many east-west oriented gaps through Wednesday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
(Tonight through Wednesday evening)

Our next storm system is quickly lifting north towards the AKpen
this even, as the low continues to fill (weaken) from its current
strength of 967 mb, to near 982 mb as it crosses just west of Port
Heiden late tomorrow morning. An occluded front attached to this
low will swing north across the peninsula into southwestern
Alaska during the day tomorrow, with widespread precipitation
developing along it. Warming low-levels will follow with the
frontal passage, allowing snow to change over to rain. This low
will continue to weaken/head north through the evening, with
another low quickly lifting north on its heels.

Similar to its predecessor, this next low will head north to
northwest towards Bristol Bay, sending yet another occluded front
northward into the area. With both of these systems, this fronts
looks to stretch out in a west to east fashion from eastern
Bristol Bay towards the Alaska Range, before continuing to rotate
northwest then westward around the low. Although the fronts will
undergo frontolysis (the weakening of a front), they will have
enough surface convergence with it, aided by bursts of ascent
associated with vort lobes aloft, to produce widespread
precipitation. With the atmosphere having been preconditioned from
the first system, the rain/snow line should progress further north
with the second system.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
(Tonight through Wednesday evening)

A pair of lows will continue to lift north to northwest across
the Alaska Peninsula and through eastern portions of the Bering
Sea, helping provide periods of widespread precipitation. These
lows will be in a weakening state, with only small craft
conditions expected. Meanwhile, a pretty stout ridge is entrenched
along the western Bering. The pressure gradient between it and
the eastern lows is providing abundant cold air advection across
central portions of the being, through the Aleutian chain, into
the north Pacific. As a result, snow showers have been routinely
crossing the region, some of which have been of moderate
intensity.

As a strengthening low moves towards Shemya over the next few
days, the ridge will get shunted to the southeast while it
de-amplifies while pushing the trough axis eastward. This will
allow for widespread precipitation to develop ahead of the
approaching low and its associated cold front, with winds and
waves increasing into at least hi-end gales. This low is then
expected to stall, while the associated front continues to slowly
progress east through the remainder of the area.

&&

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

An active pattern remains through the long range forecast as
multiple low pressure systems track to the southern mainland from
the Gulf and Bering. Wednesday night the persistent ridge pattern
remains amplified over the Southeast, with a new low moving east
across the Southern Bering along a zonal jet, which set up south
of the Aleutians mid week. This system is projected to break down
the ridge anchored over Southeast AK heading into the weekend.
However, there is high uncertainty of how quickly the ridge breaks
down as models continue to diverge in tracks for on the low as it
moves to the eastern Bering on Friday.

The storm system rides north of the jet streak and therefore is
not on a fast track. Current model trends are now showing the
streak running ahead of this system, amplifying a new ridge along
the southeast during the weekend. This projection shunts eastward
progress of this low with all models struggling with the strong
shortwaves rotating around the periphery of its center. The ECMWF
and its ensembles had best run to run consistency and was heavily
weighted in the WPC blends which used a blended approach of all
operational models and ensembles.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 130-136 138 150 173-176 178 179 185.

&&
$$

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHCENTRAL ALASKA...DK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...KH


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