Area Forecast Discussion
Issued by NWS Anchorage, AK

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000
FXAK68 PAFC 220039
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
439 PM AKDT Fri Sep 21 2018

.ANALYSIS AND UPPER LEVELS...

The main story for today continues to be the upper level low over
the Bering. Its center is currently positioned at 60N just
west of Nunivak Island. The associated warm front is lifting along
the southwestern coast, producing low level stratus, fog, and rain.

Looking south, a ridge of high pressure is currently flattening
out over the Gulf in response to the Bering low drifting
eastward. In fact, an upper level jet streak on the south side of
the Bering low has max values near 165 knots, which is aiding to
suppress the high in the Gulf. With this current setup of a
pressure gradient between the high in the Gulf and the low in the
Bering, conditions are becoming increasingly favorable for a
Turnagain Arm wind to develop.

&&

.MODEL DISCUSSION...

Overall, models are in good synoptic agreement through the next 24
hours regarding the placement of the Bering low. By Sunday, model
solutions seem to agree on the development of a secondary surface
low spinning up over the southwestern Gulf (southwest of Kodiak
Island), but disagree on the timing and position of the low
through early next week. In the last 24 hours, models have shifted
the timing of arrival to the southern Mainland from Sunday to
Monday, so there is still quite a bit of uncertainty. At this
time, forecasters preferred the GFS/EC solution which keeps the
low further west over the NAM, which currently has a more easterly
track.

&&

.AVIATION...

An approaching front will bring high confidence rain to PANC
tonight. As the front approaches, wind off Turnagain Arm should
ramp up. The big question with the developing southeasterly winds
is how low the ceilings will get. Generally with a Turnagain wind
ceilings will stay VFR, but the front is a moist one, and the
southeasterly winds are not that deep which should allow ceilings
to lower.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

A strong vertically stacked low over the Bering will push an
occluded front across Southcentral this evening with rain
spreading southwest to northeast across the region this evening.
This will usher in a significant pattern change across the area as
weeks of record warm and dry weather will be replaced by typical
cool and wet fall weather. This will also be the first soaking
rain of the month for most non-coastal locations across
Southcentral. As the front moves across the area, a surface low
will form north of Kodiak Island tonight which will induce
pressure rises in the Prince William sound, leading to strong
winds in the Turnagain Arm, Upper Hillside, as well as the Copper
River Basin overnight. Additionally, the location of the parent
low just west of Nunivak Island will allow Turnagain Arm winds to
curve north into the western Anchorage Bowl overnight.

Precipitation should rapidly diminish later tomorrow morning into
afternoon as the front progresses east allowing drier air to
filter in behind the system. Winds will turn southerly and remain
gusty tomorrow afternoon along the Cook Inlet region as the
parent surface low remains over the west coast. Conditions should
continue to improve Sunday as a weak ridge builds in with clearing
skies, especially south of Anchorage. On Monday, another system
rapidly moves north into the Gulf near Kodiak. There are some
model differences in the track of the developing surface low but
there is good consensus that another round of rain will spread
across most of the area with potentially even stronger coastal gap
winds developing.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Tonight through Monday)

The much advertised occluded front is moving ashore this
afternoon, preceded by widespread rainfall, some of which has been
moderate at times based on radar intensities. The front will
continue to sweep across the southwestern portions of the state,
while slowing down as it weakens overnight. In it`s wake, showers
will continue through the early next week, as the parent low
migrates from Saint Matthew Island this evening, towards the Yukon
Delta over the next few days. Multiple vort lobes rotating around
the low, and other vort maxes in the southwest flow aloft, will
interact with abundant moisture in an onshore flow regime to keep
the wet pattern in place. Additionally, a low riding northeast
from Kodiak towards the Kenai Peninsula will allow for deformation
zone dynamics and a more steady rainfall to skirt the eastern
portions of the AKPEN/Alaska Range on Monday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Tonight through Monday)

An area of low pressure will track from the northern Bering Sea to
western Alaska over the next few days. It`s associated surface
front will continue crossing the Bristol Bay region/AKPEN through
this evening. Here, a steady rain will precede the front with
winds gradually dropping below small craft criteria overnight.

For the remainder of the Bering Sea and Aleutians, lobes of
vorticity with embedded vort maxes will continue to drop southeast
then east across the area, as cyclonic flow continues through the
weekend. The ascent associated with these disturbances and
abundant moisture through the low and mid levels of the atmosphere
will interact to maintain a showery pattern over the next few
days. Some of the stronger disturbances may also bring bursts of
modest cold advection aloft, enhancing precipitation coverage and
intensity, as lapse rates steepen somewhat.

As for winds, sustained gales will gradually decrease to small
craft conditions near the low pressure center over the next 24
hours, which looks to affect locations near a Saint Matthew
Island to the Pribilofs line and points eastward. Otherwise, winds
will gradually decrease below small craft criteria through
tomorrow night for most of the region.

&&

.LONG TERM FORECAST (Days 4 through 7: Monday Night through Friday)...

Monday night continues to feature a low centered in the Norton
Sound region that will be lifting northward away from the area.
Clearing looks to be gradual behind the low however, with showers
and light rain expected to continue along the area mountains
overnight. Mostly cloudy skies are expected to persist for much of
the day Tuesday, though much of southern mainland Alaska will be
dry for the day. High pressure briefly moves over the area Tuesday
afternoon and night, which will help with some clearing for
inland locations, but the forecast models disagree on how cold the
air mass gets, with the foreign models suggesting warmer
conditions than previous days.

The models have also come in worse agreement on the track and
speed of the next low approaching southern Alaska from the south
Tuesday night. The foreign models are in somewhat better agreement
with each other than the GFS. To favor consistency, the EC and
Canadian solution was chosen, indicating rain at the leading edge
of another warm air mass beginning at the coast early Wednesday
morning, and gradually spreading northward through the day. On
this point, the EC and Canadian solutions begin to diverge, with
the Canadian`s more eastward track of the center of the low along
the coast Wednesday night into Thursday suggesting a very wet
period for nearly all of Southern Alaska. Meanwhile the EC now
takes the low west of the Pribilofs, which would mean very little
rain for much of Southcentral. With such disagreement in the
models past Tuesday night, forecast confidence is low.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale 179 180 181 185.

&&

$$

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


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