Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 151519 AAA
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
454 AM AKDT Mon Apr 15 2019

.ANALYSIS AND UPPER LEVELS...
Three parent lows are spinning about the state this morning. The
low across the Gulf is nearly stationary as it centers near
Middleton Island. A surface low is organizing along the eastern
waters as a strong short wave swings around the base of the upper
level trough. Another mature low is centered over the Central
Aleutians and is also stationary while a strong short wave moving
off Kamchatka wraps around its core. The most noteworthy low is
beginning to track southward along the northwest coast. This
system is expected to bring much colder temperatures and chances
for snow across the southern mainland through mid week.

At the surface, relatively benign conditions are noted across the
southern mainland with the most active weather across the western
Bering. Gusty gale force northwesterly winds started Sunday
evening across the western Aleutian Chain, and continue this
morning with light snow reporting since midnight.

&&

.MODEL DISCUSSION...
Models continue to be in good agreement with the overall synoptic
pattern through mid week. However, there are details that evolve
with each new model run. There is increasing confidence that the
arctic front will bring light snow across the region Tuesday night
and Wednesday, but only minor changes were made to reflect this
as models continue to differ on the frontal boundary passage.

&&

.AVIATION...
PANC...VFR conditions continue through the next TAF package. Light
winds this morning are expected to increase briefly out of the
southeast this afternoon and evening.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A weak upper level ridge will build into most of the area today. A
weak upper level trough remains over the Copper River basin, so
convective showers will still be possible but will be less
widespread and less active than yesterday given that the
instability is weaker. Above normal temperatures will continue
with another otherwise pleasant spring day in store. On Tuesday,
an upper level low will move into Southwest Alaska, nudging the
ridge eastward. This will bring increasing clouds to the western
portion of Southcentral, but decreasing clouds to the Copper River
basin. Chances of precipitation will begin moving from west to
east later in the day Tuesday through Tuesday night.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Today
through Wednesday Night)...

The theme of the forecast across Southwest Alaska the next three
days will be a turn towards much colder temperatures. A strong
cold front is currently moving southward across the Seward
Peninsula. It will reach Southwest Alaska this evening,
overspreading all of the region by Tuesday evening. This will
effectively end the well-above normal temperatures we have been
seeing in favor of far more seasonable values. Forecast highs are
not expected to crack the freezing mark by Tuesday and Wednesday
along the Kuskokwim River, which again is where the temperatures
should be this time of year.

The much colder air will also prompt the return of snow showers
across the Lower Kuskokwim Valley and Bristol Bay tonight into
Tuesday. Areas that see persistent snow showers may see a few
inches of accumulation, but limited moisture should keep any one
location outside of the mountains from getting large amounts of
snow. With that said, it will still be quite the shock for many
folks who have grown accustomed to the warmer temperatures and
very early breakup this Spring. The cold air will deepen through
Wednesday, persisting right into Wednesday night. However, most of
the organized snow shower activity will move eastward into
Southcentral by then. Mostly dry weather is expected Tuesday
night and Wednesday, though ocean-effect snow may impact portions
of the Kuskokwim Delta through Wednesday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3: Today
through Wednesday Night)...

As with Southwest Alaska, the primary forecast theme will be a
semi-return to winter as cold air spreads across the Bering from
northeast to southwest, eventually covering all of the Bering Sea
by Wednesday night. The cold air will cause a return of ocean-
effect snow showers. While temperatures along the Aleutians and the
Pribilofs will get at or slightly above freezing during the day,
helping to limit any accumulations, the snow showers should be
scattered in nature.

The low across the southern Bering now will move south back across
the Aleutians this afternoon. In its wake will be the surge of
Arctic air moving southward on small-craft strength northerly
winds. The cold air will also allow for redevelopment of freezing
spray conditions across the northern Bering. The snow shower
activity should keep fog from becoming too big a problem anywhere
through Wednesday night.

&&

.MARINE (Days 3 through 5)...
(Wednesday through Friday)

...Gulf of Alaska...

The main item of interest will be a low lifting north into the
Gulf in the Wednesday through Thursday time frame. There are a
lot of questions and uncertainty about low track and strength.
Some model guidance indicates potential for gale force winds,
especially from around Cordova eastward. For now, will stick
with small craft winds in the marine forecast until get a
better lock on storm track. Cold air will move in behind the
low on Friday, with small craft or less winds likely.

...Bering Sea and Aleutian Coastal Waters...


Freezing spray looks to be the main concern through the period,
as cold air dives south for Wednesday. This will persist through
Friday with just a slight modification/warming. Luckily, sustained
winds will remain below gales for the Bering and below small
craft criteria for the coastal waters, with waves generally less
than 10 feet area-wide. At this time, the best chance of seeing
any freezing spray or heavy freezing spray looks to be north of a
St. Matthew Island to Goodnews Bay line.

Given how cold this air will be, a large vertical temperature
gradient will likely result in snow showers/snow squalls, which
may enhance winds and reduce visibilities locally.

&&

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

The weather certainly looks to become more active during the
middle to end of the week, with the Alaska Range helping divide
the two air masses. First out west, the models have come into
better agreement regarding a pair of disturbances dropping south
across the Bering Sea, Southwest Alaska, and the Aleutian
Islands. The first disturbance is a strong mid-level low that
drops south from the Arctic, reaching Norton Sound by Wednesday.
Then, another disturbance (albeit smaller and weaker) drops into
the Bering from eastern Siberia, helping to pull the Norton Sound
low southwestward toward the Pribilof Islands Thursday night.

The net effect from these systems is abundant cold air dumping
into the region, with 850 mb temps dropping as low as -17 deg
Celsius in some locations. This results in steep mid-level lapse
rates of 8 to 9 deg C/km, and near dry adiabatic surface to 900
mb lapse rates. This, aided by a higher sun angle which may allow
for pockets of differential heating over land, and the warm sea
temps (compared to the air aloft), widespread snow shower/snow
squall development is a good bet. The limiting factor for and more
robust convection is strong dry advection above 850 mb, which
limits the cloud height growth. Still, even shallow convection can
put out good bursts of precipitation, similar to what was seen
today at Iliamna when a convective shower dropped the visibility
to 4 miles with moderate rainfall.

Meanwhile, to the east of the Alaska Range the weather looks to be
influenced more by a storm moving northward into the Gulf for
Wednesday into Thursday. The numerical models have trended a few
hundred miles west with this system over the last 24 hours, as
they now portray a low moving towards the Copper River Delta, then
curling it to the northwest towards Cordova. This is a stark
difference than a Panhandle low forecast yesterday. One reason for
this was the western Alaska low dropping south, helping to pull
this storm west as if some of the energy aloft was trying to
phase. While this is a distinct possibility, we`re not totally on
board with the storm retrograding that much.

In any event, will stick close to current forecast, which matches
well with the 12Z runs of the GFS/ECMWF as well as WPC guidance.
All solutions eventually track a weak low inland across Southcentral
sometime Friday. Thus, the forecast looks favorable for
cloudy/showery weather as well as gusty southerly winds behind the
low.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gales 177 178.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...KH
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
MARINE/LONG TERM...PD



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