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Area Forecast Discussion (AFD)

Issued by Anchorage, AK (AFC)

                            
000
FXAK68 PAFC 210020
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
420 PM AKDT Sat Apr 20 2019

.ANALYSIS AND UPPER LEVELS...
A trough that has been digging south through the eastern Bering
now stretches through Bristol Bay and across the Alaska Peninsula.
This trough is helping to pull down colder air from the north. To
the southeast a low is approaching the Gulf from the south
bringing in a strong push of warm air. Out west, high pressure has
built in over the western Aleutians.

&&

.MODEL DISCUSSION...
There are two main areas of model concern in the short term. The
first is inconsistency with a low moving across Kenai Peninsula 
Sunday. As it tracks across the peninsula and then Cook Inlet the
models are struggling to handle its track resulting in challenges
with winds in the Southcentral Alaska area and snowfall for much 
of the southern mainland. The other issue is with a low moving 
into the eastern Gulf late Sunday. The models are continuing to 
have significant problems with the placement of this system and 
that will have an impact for winds across the Gulf and potential 
snow for eastern portions of Southcentral Alaska.

&&

.AVIATION...
PANC...The main challenge is with the timing of a front moving
through the area tonight. There is reasonable agreement that it
will happen between 3-6Z tonight and will bring conditions down 
to MVFR. MVFR conditions are still possible through tonight and
Sunday morning, but at this time VFR conditions are expected to be
most likely. Gusty southerly winds are expected to develop Sunday
afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A low is strengthening over the southern Gulf of Alaska this 
evening with warm air advection already spreading rain/snow along 
the coast. Some snow is beginning to move over the Chugach 
Mountains as well. As the evening progresses cross-barrier flow 
will increase and largely end any snow chances for inland low 
elevation areas with the exception being the Southern Kenai where 
snow and blowing snow will continue into Sunday morning. The low 
will eventually reach low end storm force along the N Gulf coast 
with strong gales moving through Prince William Sound. Easterly 
winds will also increase through Portage Valley/Turnagain 
Arm/Anchorage Hillside and along the Chugach Mtns Sunday before 
the low moves inland over Cook Inlet. A brief shot of gusty winds 
will move through the Mat Valley and Anchorage with the passage of
the low; temps will likely spike up as that occurs Sunday 
evening.

The forecast gets more complicated Sun night as another low 
tracks across the eastern Gulf of AK and merges with the parent 
low center over SW Alaska. Some moisture may move westward far 
enough to support snow as far west as Anchorage. This will be 
occurring during the cold air advection stage, but for now, this 
forecast will keep the best threats along the mountains until 
there is better consensus with the second low. The better 
agreement is for snow to move into the Copper Basin and along the 
gulf coast as rain transitions back to snow Sun night and Mon 
morning.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2/Tonight
through Monday)...
Winter is on the way back! Starting with tonight, a short-wave 
trough over Bristol Bay and another short-wave headed toward 
the Kuskokwim Delta will maintain considerable cloudiness across 
Southwest Alaska. Expect isolated to scattered snow showers, 
mainly along and near the coast. On Sunday, the Bristol Bay trough
will become negatively tilted, helping to pull a low over the 
Gulf of Alaska (and associated moisture) westward into SW Alaska.
Snow will develop along and just west of the Alaska Range late 
tonight, then spread westward across Interior Bristol Bay and the 
Kuskokwim Valley during the day Sunday. Snow will then continue 
westward to Bethel and the Kuskokwim Delta Sunday night through 
Monday. The heaviest snowfall will be along a deformation band 
which sets up across the Kuskokwim Valley. For the lowlands, the 
highest snow accumulation is expected between Stoney River and 
Aniak. The negatively tilted trough will come full circle, 
rotating back westward across the Kuskokwim Valley Sunday night. 
This will shift the best deformation (and heavier snow) north of 
the Kuskokwim Mountains, which should keep snow amounts just below
winter weather advisory criteria. 

Meanwhile, Arctic air will dive southward across the eastern
Bering Sea and into Southwest Alaska. This will lead to
strengthening winds and an increase in snow showers along the
coast. As steadier snow arrives along the Kuskokwim Delta coast 
on Monday, there is potential for very low visibilities in snow 
and blowing snow. There is still some uncertainty in where steady
snow will set up, so do not planning on issuing an advisory for
this quite yet. 

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2/Tonight
through Monday)...
A couple weak upper level short-waves dropping south across the
Bering Sea will focus showers along the eastern Aleutians and
southern Alaska Peninsula tonight through Sunday. Arctic air will
then dive south across the central to eastern Bering Sea Sunday
through Monday, along with a strong upper level short-wave. This 
will result in strengthening winds region-wide and an increase in 
snow showers and freezing spray. The lone exception to this will 
be the western Aleutians (west of Adak), which will remain quiet 
underneath the influence of an upper level ridge. 

&&

.MARINE (Days 3 through 5)...
Tuesday and Wednesday across the Gulf and eastern half of the
Bering...A broad low pressure over the southern mainland keeps 
gusty north to west flow through the middle of next week. 
Thursday night into Friday a new surface low pushes toward the 
western Bering bringing a front to the Pribilof Islands by Friday
afternoon. Gales may develop along the boundary of this system, 
however there is still uncertainty on the timing on the track of 
this system. 

&&

.LONG TERM FORECAST (Days 3 through 7)...
The forecast beginning on Tuesday continues with the most active 
weather across the southern mainland and nearby waters as an upper
low centers near the Alaska Peninsula. Strong vorticity on the 
backside of the trough swings around the base which helps to set 
the upper low in motion with the it centering over the Gulf on 
Thursday. The ridge over the western Bering kicks into eastward 
motion with the next front advancing to the western Aleutians 
Thursday morning. There is much higher uncertainty with how this 
overall pattern evolves through the end of the week as models 
greatly differ on solutions. Only conservative edits were made for
the extended forecast with the ECMWF and its ensembles favored 
today as it maintains the best forecast consistency. 

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Storm Warning 119.
 Gale Warning 125 131.
 Heavy Freezing Spray 180 181 185.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...DK
SOUTHCENTRAL ALASKA...JA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...SB
MARINE/LONG TERM...KH