Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 211304
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
504 AM AKDT Sat Apr 21 2018

.ANALYSIS AND UPPER LEVELS...
The major players in our upcoming pattern change are coming into
place this morning. First comes the retrograding upper-level
trough digging in from the Arctic. It is currently settling into
the Bering Sea. Second is a complex upper-level low spinning in
the northern Pacific. The jet stream is feeding into the Pacific
system with an embedded shortwave trough pulling poleward. Further
to the east over the Gulf of Alaska, a weaker trough sits,
complete with a pocket of -32C air aloft. The cold air aloft and
seasonable surface temperatures promote instability through the
column and scattered to numerous showers across the northern Gulf,
and a lesser amount inland due to moisture blocking and downslope
flow.

&&

.MODEL DISCUSSION...
Overall, good synoptic agreement. The discrepancies come from the
phasing of the Pacific shortwave into the Bering upper low. All in
all we`re only talking about a 50 mile difference west to east
across the Alaska Peninsula, but it could be the difference
between seeing accumulating snow or only rain near that boundary.

&&

.AVIATION...
PANC...VFR conditions will persist. Gusty Southeast winds will
develop again this afternoon and into the evening. Winds should
turn Northeast and lighten significantly by midnight.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Saturday
and Sunday)...
The weather will be slow to change today as the upper level low
digs westward and southcentral Alaska remains in moist
southeasterly flow. Numerous rain showers are evident over the
northern Gulf on the Middleton Island radar with some making it
across the Kenai and Chugach mountains. There will be a brief
respite overnight before the next front moves into the area from
the south. It will bring Gale force winds and rain to Kodiak
tonight and then make it to the north Gulf coast by early Sunday
morning. Warmer air is associated with this system so the freezing
level will increase. The gusty gap winds through Turnagain Arm
will continue and should move over west Anchorage this afternoon
and evening. After midnight, the approaching front will cause the
winds to turn down the Inlet and toward the incoming low.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Sat and Sun)...
The well-advertised Arctic trough is pushing through the area
this morning. Light snow will continue to fall from Aniak through
the YK-Delta coast through the morning. However, accumulations are
expected to be quite limited as surface temperatures are near or
above freezing. The trough will push into the Eastern Bering and
begin to pivot towards becoming negatively tilted today. This will
allow it to pull up more moisture from another low south of the
Aleutians. With some weak afternoon instability, expect another
round of showers to develop over interior Bristol Bay and spread
W-NW.

By Sun, the energy turns into a deformation band with most of the
precipitation just off the west coast. So Sun will actually be a
relatively dry day across most of the area. By late Sun night,
this deformation band gets pushed back to the east bringing
another round of steady rain inland.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Sat and
Sun)...
The area will start to show some signs of a more active pattern
over the next few days. First, one area of low pressure will spin
northward into the Alaska Peninsula today. This will bring some
small-craft winds and moderate rain to the peninsula by this
evening. On the backside of this feature (stretching from near
Cold Bay up through the Pribilof Islands), the airmass should be
cold enough to support steady snow. Snow accumulations look to be
less than an inch for population centers at this time as surface
temperatures are still quite mild. However, should the cold air
shift to the east a bit and snow continue a bit longer, parts of
the western Alaska Peninsula could see some more snow.

By Sun morning, the next low pressure system moves towards the
Western Aleutians. It will swing an gale-force front into the area
with a hefty amount of rain associated with it. This front
stretches out across the entire chain as a warm occlusion reaching
the Alaska Peninsula by Mon morning. The parent low will linger
over the Western Bering. This will keep nearly the entire domain
under broad cyclonic flow with primarily easterly winds north of
the chain with gusty westerly winds on the Pacific side.

&&

.LONG TERM FORECAST (Days 3 through 7...Mon through Fri)... A
progressive pattern will remain steadfast over the region through
the end of next week, yielding widespread precipitation and gusty
conditions across much of the Bering/Aleutians and southern
Mainland. Models are showing a series of lows and associated
fronts tracking through the Bering and north Pacific throughout
the period, with the fronts eventually pushing into the Gulf
region as the low centers approach the southwest coast. The areas
west of the Chugach and Alaska/Aleutian ranges are still expected
to see more of a downsloped-regime under southeasterly winds, with
some small breaks in the wind periodically which will allow some
precipitation to spill over. However, by Thursday when the flow
changes more to the SW over the Cook Inlet region, we should see
more precipitation making it into the area. Along the Gulf coast,
steadier precipitation is expected with each frontal passage,
along with gusty winds along the northern Gulf and through favored
channeled terrain.

Models are all now starting to show agreement on a fairly strong
low moving from south to north over the Eastern Bering Tue-Wed of
next week. There is still some disagreement on the exact track and
depth of the system, however, they all paint a fairly similar
synoptic picture. The low should be quite broad and be somewhere
in the low 970 mb range, so this looks much more like a Fall-type
system than late April. Some of the typical impacts would be
strong winds and the possibility of high-surf and coastal
flooding. We will be monitoring this system closely as it develops
over the next few days.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 119 120 130 131 132 138 139 178.

&&
$$

SYNOPSIS AND MODEL DISCUSSION...
SOUTHCENTRAL ALASKA...
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...
LONG TERM...



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