Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 170104
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
504 PM AKDT Thu Aug 16 2018

.ANALYSIS AND UPPER LEVELS...
There is an upper level low over the Alaska Peninsula that is
slowly moving towards the southeast. There is a weaker upper low
centered over the eastern Copper River basin, that is moving
northeast and dissipating. Lingering moisture from these two lows
over southern Alaska is continuing to bring some showers to the
area. There is an elongated upper low over the western and central
Aleutians, with a fairly moist front draped across those areas.
The associated surface low is centered to the south of the front
over the northern Pacific. High pressure at the surface and aloft
is over the western and central Bering Sea.

&&

.MODEL DISCUSSION...
The numerical models are in good agreement through Saturday
afternoon. There are some minor differences in how they handle the
moisture and precipitation, lowering the confidence a bit.
Therefore the forecast confidence is just a little above normal.

&&

.AVIATION...
PANC...VFR conditions will persist. Gusty southerly winds this
evening will diminish late tonight.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Tonight through Sunday)...
Showers will continue to develop over the next 24 hours, mainly
on the windward sides of the mountains through this evening. This
is occurring as weak lobes of vorticity rotate around a low near
Middleton Island, while combining with diurnal heating and
orographic lift. Heading into your weekend, a ridge of high
pressure will build across Southcentral for your Friday and
Saturday, helping to decrease shower coverage some while confining
them to mainly the mountains once again. A fairly potent front
will begin to cross the region from southwest to northeast on
Sunday, bringing increasing winds and rainfall potential with it.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Tonight through Sunday)...
A widespread cumulus field has developed across the Lower
Kuskokwim Valley and Bristol Bay this afternoon. Many of these
cumulus are producing showers with light to briefly moderate
rainfall rates. As these are being fueled mostly due to daytime
heating, the loss of it this evening will lead to a rapid
diminishing of rainfall coverage. Friday will be the "calm before
the storm" period for Southwest Alaska, with a decent weather day
on tap. A few isolated afternoon showers are possible again Friday
afternoon, particularly on the east side of the Kuskokwim
Mountains, but they will be much fewer and further between than
the activity going on presently.

Then on Friday night, the leading bands of rain will move into the
Alaska Peninsula region. In addition, the winds will ramp up
significantly, especially through the gaps in the Alaska Range
through Lake Iliamna. Compared to areas further south along the
Alaska Peninsula, Southwest Alaska will get a glancing blow from
this storm, as the center turns westward well south of the Alaska
Peninsula by Saturday morning. Thus, most of the associated
rainfall on Saturday will be south of King Salmon, and along the
eastern slopes of the Kuskokwim Mountains. There should be
significant downsloping and gusty winds through the gaps, which
will leave the immediate lee of the Alaska Range relatively dry,
and most of the Lower Kuskokwim Valley and Kuskokwim Delta will
remain dry throughout the event, as the rain is not expected to
encroach that far north during the day.

By Saturday night however, the occluded front wrapping around the
north side of the low will advance northward enough to get nearly
all of the Lower Kuskokwim Valley and Kuskokwim Delta into at
least some on-and-off rain, which will continue through the day
Sunday for much of Southwest Alaska. As the front causing the
precipitation weakens, the precipitation should taper through the
day Sunday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Tonight through
Sunday)...
A front over the western and central Aleutians tonight will
consolidate into a strong low south of the Alaska Peninsula Friday
afternoon and evening. This will lead to widespread gale force
winds around the Alaska Peninsula and eastern Aleutians from
Friday night through much of the day on Saturday. Gusts in this
area could get up to 65 mph at times. A strong occluded front
associated with the low on Saturday will weaken by Sunday morning
as it moves west of Dutch Harbor, through the Pribilofs, and into
the Kuskokwim Delta. The front will rapidly weaken through the day
Sunday and dissipate by Sunday evening. A separate front will
move into the far western Aleutians on Saturday, and slowly
progress eastward into the central Bering and Aleutians through
the day on Sunday. This front will bring small craft conditions to
the western Bering and Aleutians into Sunday. Between the strong
low south of the Alaska Peninsula, and the front in the western
Bering, a weakening area of high pressure through the central
Bering may lead to some localized fog concerns with otherwise
light winds.

&&

.LONG TERM FORECAST (Sunday night through Thursday)...
The long term, beginning Sunday night, starts with ridging over
the eastern Gulf that extends over the southern Mainland and
ridging over the western Aleutians and Bering. In between the two
ridges is a low centered south of the Alaska Peninsula with a
front pushing in over southwest Alaska. The eastern ridge is
holding strong which will stall the front over the southern
Mainland bringing rain to the area into mid next week. The
western ridge will be breaking down which will allow for another
system to move into the western Bering early next week.

Mid next week the models diverge on what happens with the low in
the Bering. The GFS keeps the low over the western Bering which
would have little impact on the southern Mainland. However, the
EC has the low moving into the eastern Bering which would bring
widespread rain to the southern Mainland. Because of these
differences the ensembles were preferred.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 132 138 150 155 160 165 170-172 174.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...PD
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
LONG TERM...DK



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