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Area Forecast Discussion (AFD)

Issued by Anchorage, AK (AFC)

                            
000
FXAK68 PAFC 211700 CCA
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
521 AM AKST Thu Feb 21 2019

.ANALYSIS AND UPPER LEVELS...

The main feature is a long wave trough sitting over the western 
Gulf of Alaska. At the base of the trough, a negatively tilted
vertically stacked low spins near Middleton Island. The jet 
stream is fairly zonal, but satellite imagery depicts a 
developing jet streak northeasterly aligned over the central 
Gulf. In the mid-levels, cold air is wrapping around the low, 
while warmer air is pushing into the Gulf on the leading edge of 
the trough. This cold air is also advecting over the Bering and
western mainland. Within the flow around the low, westerly 
flow dominates the western mainland, while general southeasterly 
flow is present along the northern Gulf coast. A second upper- 
level low is advancing from Kamchatka towards the western 
Aleutians. Between the two lows, a ridge is building over the 
central Aleutians and Bering. 

In the western Gulf, the surface low is moving eastward along the 
coast line. Precipitation is occurring throughout Southcentral, 
mainly concentrated in the northern Gulf coast. The snow that fell 
overnight in the greater Anchorage are and Kenai Peninsula has 
tapered off this morning. In the early stages, southeasterly flow 
through Southcentral created Turnagain Arm winds overnight. However, 
those winds have since ceased and become more northerly.
The Kamchatka surface low, that is just off-shore of Shemya, is 
pushing a strong storm force front over the western Aleutians.

&&

.MODEL DISCUSSION...

Models are in good synoptic agreement through the period. Models
have locked onto a track with the low in the northern Gulf, and 
the track/strength of the Kamchatka low. The main model 
differences arise specifically with wind speeds in the Bering
associated with the Kamchatka low. The American models are 
holding onto stronger winds, while the EC/Canadian are weaker
through Friday. The next feature, a North Pacific low moving over
the Aleutians on Friday morning, is well handled until it moves 
into the central Bering Friday night. 

&&

.AVIATION...

PANC...Northerly winds with a few higher gusts will continue into
the morning, along with VFR conditions as the snow has ended. 
High pressure moving in later this morning will cause skies to
gradually clear.

&&

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

As the last of this underwhelming snowfall event ends from west to
east across Southcentral this morning, the forecast focus will
turn from snow to winds as a modestly colder air mass moves into 
Southcentral. The upper level support for the snow this morning
ended up tracking a bit further south than expected. This resulted
in most of the moisture and heavier snow staying further south, so
snowfall amounts were lighter than anticipated in most areas.

As the aforementioned colder air mass settles into Southcentral 
later on this morning, the cold air will be squeezed through the 
mountain passes and gaps. This will result in stronger westerly and 
northerly winds through the passes. The strongest winds are expected 
through Thompson Pass and Valdez today. Winds could gust up to 55 
mph in the town of Valdez later on this afternoon. Gusts through 
Thompson Pass will be in the 60 to 70 mph range into Friday morning. 
Peak wind gusts of 40 to 50 mph are forecast through Seward and 
Whittier.

Elsewhere across Southcentral, the strong offshore winds will 
help clear out the cloud cover by this afternoon for most areas 
making for sunny skies to end the day. The clear skies will 
persist into Friday, though some additional clouds are possible 
Friday afternoon as a dry front moves through. Fog will become 
increasingly common each night starting Friday night as 
temperatures aloft warm with the approach of a strong upper level 
ridge.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Thursday
through Friday night)...

Lingering snow showers across the Southwest coast will eventually 
taper off today as a weak ridge over the eastern Bering advances 
inland. The weather pattern will change by the evening as an 
advancing front, associated with a Bering low, pushes eastward, 
bringing another chance for precipitation. By Friday morning, 
coastal areas along the northern Kuskokwim Delta may see gusty 
southerly winds resulting in brief periods of blowing snow. 
Initially, temperatures will be cold enough for most precip across 
Southwest to fall as snow. However, a second system moves into the 
Bering Friday night, bringing strong southerly flow. With warmer 
temperatures and ample moisture, this will allow any remaining 
precipitation, especially along the AKPEN and Southwest coast, to 
change over to rain. Gusty southerly winds along the Kuskokwim Delta 
coast, including Nunivak Island, are expected through Friday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Thursday
through Friday night)...

A storm force low centered just west of Attu is bringing strong 
southerly flow across the western and central Aleutians.
While some areas across the Bering will see a rain/snow mix with
this system, most precipitation will fall as rain as warm air is
advected northward. This trend will continue through the 
afternoon as the associated front advances eastward across the 
Bering. A wide swath of south to southwest gales will persist 
until Friday morning. 

Meanwhile, a second system in the North Pacific advances 
northward, bringing another round of gales to the Bering Friday 
afternoon. As the low lifts northward into the central Bering, 
northerly gales will persist over the western Bering while the 
leading front will bring southerly gales to the east side of the 
Bering through Friday night. Due to the warm southerly flow, 
precipitation with this system will fall mostly as rain across the
Aleutians and Pribilofs.

&&

.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)...

.MARINE (Days 3 through 5)...
The Bering Sea remains in an active weather pattern Friday through 
Sunday. The eastern Bering Sea will continue to see gale to storm 
force lows through Saturday with a drop in the strong wind regime 
through the day on Sunday. Over the central and western Bering Sea, 
expect more variable and rapidly changing conditions ranging from 
storm to gale force winds to sub small craft conditions as the low 
centers will be pushing north along the dateline. 

In the Gulf of Alaska the main area of strong wind (gale and storm 
force gusts) will be along the immediate coast and Prince William 
Sound Friday. These outflow winds will be tapering off Friday night. 
Following this, high pressure over the Gulf of Alaska will result in 
generally benign conditions Saturday and Sunday. 

&&

.LONG TERM FORECAST (Days 3 through 7)...
The most active region for the long term will be over the Bering Sea 
as we continue to see storms moving north along the date line Friday 
and Saturday. Then another large strong storm will move along south 
of the Aleutians as frontal low then tracks north and west into the 
central Bering Sea by Monday. Meanwhile, the Gulf of Alaska remains 
under the influence of a large high through Wednesday. As for the 
sensible weather, we are still expecting a strong warm push over the 
Alaska Peninsula and mainland beginning Saturday through the early 
part of next week. 850 mb (~5000 ft) temperatures are expected to 
push above 0 C by Monday over the mainland, likely resulting in 
temperatures running about 10 to 20 degrees above normal over the 
southwest Alaska Saturday then around 10 to 15 degrees above normal 
over Southcentral Alaska. This unseasonably warm airmass will 
persist over the southern mainland through at least the end of the 
week, with ensemble means hinting at the pattern holding strong even 
further out. 

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory: 131.
MARINE...Gales: 119 120 121 125 127 128 132 137 170 173-177 179 185
                351 352 411-414.
         Storms: 130 131 178.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...RJA
SOUTHCENTRAL ALASKA...BC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO/DK
MARINE/LONG TERM...SA/KVP