Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 170112
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
512 PM AKDT Wed May 16 2018

.ANALYSIS AND UPPER LEVELS...
Spring like weather has still not made its way to the region. A
ridge is building over the western Bering with a trough over the
eastern Bering. The 12Z sounding shows freezing levels of 2500
feet and cold air aloft from the upper level low sitting over
southwest Alaska.

A low is making its way to southern Mainland from the North
Pacific bringing in moisture. Water vapor imagery shows rain and
clouds already moving in over the southwest and Kenai Peninsula.
Light rain and southwesterly flow aloft will remain over
Southcentral as a warm front lifts along the Gulf coast. A low
west of Shemya is responsible for a front near the western
Aleutians.

&&

.MODEL DISCUSSION...
Overall the models seem to be handling the low moving from the
North Pacific pretty well, with small differences in the exact
center of the low as you move further out in time. Models also
tend to be in better agreement with placement of the low moving
over the western and central Aleutians than they have been in
previous model runs.

&&

.AVIATION...
PANC...Southeasterly winds this afternoon will diminish later
this evening. VFR conditions are likely to persist. There is a
possibility that some MVFR conditions could develop if there is
significant rainfall starting Thursday morning, but at this time
the most likely outcome will be for ceilings to fall around 5000
ft.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A weak disturbance moving up Cook Inlet this afternoon will
continue to spread light rain northward the rest of this afternoon
and into the evening hours. Low level southeasterly flow across
the Kenai and Chugach Mountains is fairly shallow and weak (as
per the 00Z Anchorage sounding), so downslope processes are not
enough to prevent some sprinkles or very light rain in the lee
of the mountains.

Right on the heels of this first trough is a more impressive
frontal system moving into the western Gulf. A strong meridional
jet streak is helping drive the front northward, while contributing
to upper level vertical motion and heavier rain than with the
aforementioned trough. Rain will spread northward to the Kenai
Peninsula tonight. Thursday will see a bit of a repeat of today
for the Cook Inlet to Susitna Valley corridor. The difference will
be stronger dynamics and more abundant moisture, so do expect
measurable rain for the population centers.

As the trough over the North Pacific becomes negatively tilted
Thursday night and Friday, heights will rise to the north, with a
ridge axis moving overhead of Southcentral. This will help push
much of the moisture westward toward Southwest Alaska, leading to
a drying and warming trend over much of Southcentral. The ridge
will move through fairly quickly as the trough then lifts
northward into the Gulf. The big question is whether there is
enough time to clear out some of the clouds before this next
system moves in. That will determine how warm temperatures get
on Friday. There is higher confidence that the Copper River Basin,
which will be more firmly underneath the ridge, will see it`s
warmest day of the year.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA...

A low in the North Pacific will replace a weak upper trough
currently over the region. This will bring in another round of
rain and showers along with increasing easterly winds. Rain will
not be heavy, but it will be widespread at times. The first front
moves in tonight, and then a series of weaker troughs will move
through thru Saturday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS...

A warm front moving into the eastern Bering Sea will bring rain
and increasing northeast winds to the Pribilof Islands and the
Eastern Aleutians. This front will fall apart as it moves west of
the Pribilof Islands Thursday evening, leaving showery conditions.
The only other feature of interest is a gale force occluded front
which will remain parked over the Western Aleutians through
Friday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Beginning Saturday, there is increasing confidence that our
overall pattern will transition to a more progressive one as the
persistent upper level ridge over Western Canada and Southeastern
Alaska will move east allowing an upper level low to reposition
itself over the Eastern Bering. This will allow several systems to
rotate into the Southern Mainland over the weekend and into next
week. Strong southeasterly onshore flow associated with these
systems will favor coastal areas for precipitation, while the
typical lee-side locations will downslope out and remain mostly
dry, though seeing continued cloudy and cooler conditions.

By mid week, a ridge will amplify over the Eastern Aleutians and
Bering and creep east. Though model solutions are widely varying
at this point, there is some hint that the ridge may displace the
upper level low far enough east to push the main area of on-shore
flow towards SE Alaska, bringing a bit of a break to Southcentral
and the SW mainland. Some model solutions however show this ridge
quickly breaking down as another strong low barrels into the the
area from the west. Either way, it appears the persistent pattern
of cooler, cloudy, and unsettled weather that has plagued the
Southern Mainland for nearly the past month will remain for the
foreseeable future.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 131 178.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...ALH
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JA
LONG TERM...KVP



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