Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 180017
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
417 PM AKDT Tue Apr 17 2018

.ANALYSIS AND UPPER LEVELS...
The storm system across the Bering is tracking slowly east with
the center approximately 200 miles southeast of the Pribilof
Islands this afternoon. The leading edge of the surface front is
diffusely stretched across the southwest mainland to the northern
gulf. Scattered to numerous rain showers continue to develop along
the boundary and also around the perimeter of the broad trough
which spans over the Bering. Gusty south to east flow increased
along the frontal boundary today with the pressure gradient
squeezed between a building ridge across the northern gulf waters
and the approaching Bering low. The exception to this is along
the Cook Inlet where a typical down inlet gradient is keeping
northerly flow along the western Kenai. The strongest winds are
reported south of the storm core with westerly gales across much
of the southern Bering.

&&

.MODEL DISCUSSION...
Models are in good agreement through the short range forecast
through the end of the week. The NAM was favored for Southcentral
and the Gulf for the higher resolution wind data. GFS was utilized
for the Southwest and Bering as it provided slightly better details
for precipitation around the slowly weakening storm system.

&&

.AVIATION...
PANC...VFR conditions persist through Wednesday. Southerly winds
are expected into this evening, becoming light northerly before
midnight. Southerly winds could again increase Wednesday
afternoon.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A front moving slowly up the coast is producing some localized
shower activity across the Kenai Peninsula, Prince William Sound,
and the eastern slopes of the Alaska Range this afternoon. Dry air
is causing any precipitation to evaporate as it tries to move
over Anchorage, but a stray sprinkle or flurry can`t be ruled out.
As the front moves into the north Gulf Coast tonight,
strengthening east to southeast winds will completely cut off any
and all precipitation chances for the Matanuska Valley, Anchorage,
and the northern Kenai Peninsula due to downsloping. This
downsloping theme will remain prevalent right through Thursday as
a secondary wave of precipitation moves into the coast on
Wednesday, but with a similar result for the leeside communities
previously mentioned.

Gusty winds through Turnagain Arm and the Knik River Valley will
continue into tonight, with perhaps some additional strengthening.
Some gusts out of the southeast are impacting South and West
Anchorage, including the airport, but as the northerly winds kick
up later tonight down the Susitna Valley in response to the
approaching front, that should cause the Turnagain Arm winds to
bend south out of town. However, the northerly winds may become
strong enough to bring the remnants of the Knik and/or Matanuska
Valley winds into parts of Anchorage as well late tonight into
Wednesday morning.

An area of low pressure is expected to form Thursday morning over
the northern Gulf, but will essentially remain stationary. Both
the low and its upper level energy will again reinvigorate the
precipitation occurring along the north Gulf Coast, particularly
around Prince William Sound. The low will eventually begin slowly
drifting northwestward into the southern Kenai Peninsula Thursday
night.

The result of these systems moving one after another into the
north Gulf Coast is a period of nearly steady, and sometimes heavy
precipitation for that area for multiple days, starting this
evening. The higher mountains should see that largely fall as
snow, but the valleys and passes will get some rain with it as
well, since the air mass moving in with these systems is a
slightly warmer one. Sunshine will be a difficult thing to come by
in most of Southcentral, except maybe right along the Alaska
Range for the next several days.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
There is an occluded front that is just off shore from Southwest
Alaska. The latest scan from the Bethel Radar (PABC) detected
isolated showers in the vicinity of Toksook Bay. Showers have also
been detected on the King Salmon radar (PAKC) by Dillingham.
Expect a spike in PWs (precipitable water) the next 18 hrs.
Additionally, the shortwave associated with this wave will become
negatively tilted tonight enhancing the dynamics over Southwest.
Widespread precipitation is anticipated for Wednesday. There was
enough diurnal heating today where most locations were able to get
into the mid and upper 40s. While the models continue to
advertise mixed precipitation type for portions of Southwest,
growing confidence that this will predominately be a rain event
for the lower elevations. On Thursday the surface low is expected
to shift southeastward crossing the AKPEN, but the cyclonic flow
aloft will continue into the weekend.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The surface low that is anchored to the occlusion is southwest of
the Pribilof Islands. This low has already maxed out and is
slowly starting to fill. The latest ASCAT (advanced scatterometer)
pass has detected gale force winds near the Pribilof Islands, the
Central Aleutians and south of the Chain. The models hold on to
these robust winds tonight and tomorrow, shifting the threat
eastward. Weak ridging will follow in its wake. Then a second
storm will spin up just east of Kamchatka and begin to push
towards Attu Island early Thursday.

&&

.LONG TERM FORECAST (Friday through Tuesday)...
The end of the week is shaping up to be an interesting weather
pattern as an upper-level low pressure system drops southward
through the interior toward Southwest Alaska, and a separate storm
located in the northern Gulf on Friday is pulled westward toward
this upper-level low. This will spread precipitation across the
major population centers around upper Cook Inlet and the Mat-Su
Valleys, and then eventually into southwest Alaska as well. There
is still quite a bit of uncertainty revolving around the moving
pieces of this storm, but there is potential for the northern
Susitna Valley and Hatcher Pass area to see accumulating snowfall,
with snowfall possibly even as far south as Anchorage. Stay tuned
to future forecast updates for additional details.

After this storm moves through, the overall weather pattern will
slip back into the pattern we are in currently, with southeast
flow across much of the Gulf (and rounds of rain along the Gulf
Coast) with cloudy and some showery conditions inland. Southwest
Alaska will also remain cloudy, with fairly wet conditions
continuing through the start of next week as several fairly weak
weather fronts impact the area.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 119 160 170 172>177 180.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...KH
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PJS
LONG TERM...AD



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