Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 271243
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
443 AM AKDT Sun May 27 2018

.ANALYSIS AND UPPER LEVELS...
A stacked low in the eastern Gulf remains nearly stationary and
continues to deliver scattered cloud cover to Southcentral this
morning. A few showers are observed on radar over the coastal
areas surrounding the northern Gulf and Prince William Sound. Over
the interior locations of southcentral dry and clearing conditions
are being observed. Further west, a large ridge is building over
the central Aleutians and Bering, and tracking eastward. Surface
observations this morning indicate small craft winds along the
eastern Aleutians and AK Pen, with gustier conditions through the
gaps. A Low near Kamchatka and associated triple point low
forming over the north Pacific (south of Shemya) are pushing a
front over the Western Aleutians and Bering this morning, bringing
gusty winds and wet conditions to the area.

With the low in the Gulf slowly tracking to the northeast and the
incoming ridge building over the AK Pen, daytime heating will be
sufficient enough to allow for convection to occur across the
Mainland, generating isolated showers and thunderstorms across the
Susitna Valley and Copper River Basin, and as far south as the
Chugach Mountains around Anchorage.

&&

.MODEL DISCUSSION...

The models have remained in great synoptic agreement with the
strength, timing, and placement of the low in the northeastern
Gulf as well as the ridge over the Bering. The NAM was the
preferred model for the morning forecast package for higher
resolution. There has been good model agreement with the thermal
trough building back over the area today. This has increased
confidence that the Susitna Valley, Copper River Basin, and areas
west of the Chugach mountains will see convective showers develop
this afternoon/evening with the possibility of a thunderstorm or
two.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The low in the northeast Gulf will remain nearly stationary
through this afternoon. This will allow the flow aloft to continue
to be weakly cyclonic and from the northeast. Some showers will
remain over the area today, and will be diurnally enhanced in the
afternoon and evening. Isolated thunderstorms are possible later
today, over the central Copper River basin through the Susitna
valley, and extending south to the western Chugach and Kenai
Mountains. The Gulf low will slide east tonight and Monday, with
a weak ridge building over the area. Some diurnal convection is
possible again on Monday, but will be more limited than today. A
few thunderstorms could pop up over the southern Talkeetnas, and
portions of the Copper River basin and the Susitna River valley.
The flow aloft will become southwest Monday night ahead of a front
approaching Kodiak Island.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
Yesterday`s trough continues to push eastward while slowly
weakening, with a transient ridge moving in from the west.
Northeast sections near the Alaska Range will still feel some
effects from the trough, as cooling aloft will produce sufficient
instability for a few wet thunderstorms to develop. However,
overall storm coverage will be less than what was seen yesterday.
The building ridge should allow for decreased cloud cover, even
near the Kuskokwim coast and through the lower portions of the
river valley. However, with clearing skies already underway some
patchy fog could lift into a stratus deck, taking until mid
afternoon before the sun starts to poke through the clouds.
Another trough will quickly follow on its heels for Monday, with
rain chances returning to the coast Monday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
A short-wave ridge of high pressure located across the central
Aleutians and Bering this morning will continue to track east,
reaching the southwest coast and AKPEN tonight. As this occurs,
another trough will move into the western portions of the area
today, with a surface low entering the western Bering overnight.
Warm advection ahead of this system may allow for areas of patchy
fog to develop.

As the low moves towards/into the western portions of the Bering
sea, a surface front will move from west to east across the
region. Winds will gradually increase to hi-end small craft
conditions for most locations, with sustained gales more likely
closer to the Aleutian chain. Here, a few storm force gusts are
also possible. As for precipitation, it`ll become fairly
widespread as the front approaches, before becoming more showery
in nature following frontal passage, at which time winds will
diminish as well.

&&

.LONG TERM FORECAST (Days 3 through 7...Tue through Sat)...
As is often the case with the extend portion of the forecast,
there is decent agreement among the models early on before slowly
(and surely) starting to diverge from one another. This leads to
fairly high confidence in the pattern to start the week, but lower
and lower confidence as we move towards the first weekend in
June. To start things off, a low over the Western Bering will
continue to weaken as it slings a front through the West Coast.
This will make for rather dreary, wet, and windy day for SW AK Tue.
Convection (mostly showers with some isolated thunderstorms) will
be likely east of the AK/Aleutian Ranges across the rest of
southern AK. The front shears apart in the lower elevations as it
tries to cross those mountain ranges into Wed. However, the more
potent upper- level trough will eject over the southern mainland.
With plenty of low-level moisture, moderate instability with a
cold pocket aloft, and decent lift, expect showers to encompass
most of the mainland. The heaviest precipitation will fall over
the southwest. This trough could also drop 850mb (5,000`)
temperatures about 5 degrees Celsius making both Wed and Thu
rather cool and wet.

However, by late Thu-early Fri, most models are now pointing to a
fairly strong noise of a ridge from the Pacific pushing into the
state. This would dramatically change the weather to a much drier
and warmer pattern. Most models hint at this feature at least
lasting 1-2 days. But the ECMWF is showing signs of closing off
the high. If this were to happen, we could move into a very
summer-like pattern for much of the early part of June. As we said
at the onset of the discussion, the farther out in time, the less
we rely on any given solution. However we have trended the
forecast slightly warmer and drier for next weekend and we will be
monitoring the progress of the ridge.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...None.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...KO/TP
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...MO



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