National Weather Service United States Department of Commerce


000
FXAK68 PAFC 250005
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
405 PM AKDT Sun Sep 24 2017

.ANALYSIS AND UPPER LEVELS...
There is an upper level low centered just south of Cape Newenham
this afternoon. An associated surface low is located just to the
east of the upper low center. A trough extends east-northeast from
the upper low into Southcentral Alaska. Showers associated with
the low extend from the Canadian border westward to the Pribilofs.
High pressure is over the Western Bering Sea, but this is quite a
"dirty" ridge with plenty of cloud cover and even some showers
across the Bering and Aleutians.

&&

.MODEL DISCUSSION...
The numerical models are in good agreement through Monday evening.
After that, they begin to show some differences with exactly how
the low pushing in from the south develops. The ECMWF is quicker
with the eastern side of the front (off the coast of the
Panhandle), with the Canadian Gem a little slower and the GFS and
NAM the slowest. The models are also a little different in how
they handle the main surface low south of Kodiak Island on
Tuesday. This creates some uncertainty in the forecast, and
confidence is thus a bit below normal today.

&&

.AVIATION...
PANC...VFR conditions and light winds will likely persist. There
is a risk of some fog tonight. The chances of this developing
depends on cloud cover. If there is some clearing, then fog could
develop.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The remnants of a low over the Gulf and Prince William Sound will
continue to usher moisture into southcentral Alaska which will be
most visible in the form of cloud coverage and shallow pockets of
fog. As time elapses, the cold pool of air aloft will continue to
modify and move inland resulting in a drying trend over the
Anchorage Bowl and the Mat-Su area. All the while, enhanced
values of precipitable water will continue to advect into the
Cordova and Valdez region. The robust ridge that is off the coast
of California will continue to amplify with the ridge axis nosing
its way deeper into the Gulf Of Alaska on Monday. This should
bring stable conditions as far north as Prince William Sound and
the Kenai Peninsula. Reduced pops over the Gulf this forecast
package for Monday. Additionally with weak ridging influencing the
region fog may develop in some areas. By Tuesday the ridge axis
will shift eastward with southerly flow setting up over the Gulf
of Alaska.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Mon and Tue)...
One low will pull away from Bristol Bay, allowing for a brief
period of drying, before another moves in to take its place. The
first low is currently drifting from north to south just outside
of Bristol Bay. This will allow some wrap around rain to continue
to fall Monday along portions of the SW Coast and in a band from
the Alaska Range to the YK-Delta Coast. Late Mon into early Tue, a
drying trend will ensue as some induced ridging builds aloft. The
second low will then creep up from the North Pacific towards
Kodiak Island on Tue. Rain will start once again over the Alaska
Peninsula around daybreak. The more widespread impact will be the
increasingly strong northeast winds. Gusts could reach 20 to 30
mph along the Bristol Bay Coastline by afternoon, with even higher
winds expected in Shelikof Strait. This dry, offshore flow will
also help to keep the rain in check and should actually allow for
most inland places to stay dry on Tue.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Mon and
Tue)...
Northerly flow will be the rule across the Bering. This will keep
gusty winds and the potential for showers in the forecast,
especially along the north side of the chain. There is limited
cold air near the Bering Strait at this time, so not expecting
anything overly strong with near neutral temperature advection. A
ridge with an associated closed high pressure center will drift
towards the Western Aleutians on Mon. This will bring some warmer
temperatures to Shemya and Attu, and could lead to foggy conditions
developing once again.

&&

.LONG TERM FORECAST (Days 3 through 7)...

Models have come into better agreement with an area of low
pressure moving across Kodiak Island on Wednesday. The recent
forecast reflects the increase in model agreement with gale force
winds likely across the northwestern Gulf of Alaska and Shelikof
Strait Tuesday night and Wednesday. In addition, it looks like a
piece of the subtropical jet will bring an influx of moisture to
allow for some decent rainfall over Kodiak Island and the Gulf
coast. Interior locations will remain showery as the low doesn`t
take a favorable track for significant downsloping conditions.

Otherwise, the forecast hasn`t deviated much from yesterday in
thinking that the upper level trough digging out across Alaska
late this week will quickly be shunted eastward by a transitory
shortwave ridge. Behind this ridge will be yet another upper level
trough but this time it`s the Arctic jet stream that is
strengthening rather than any subtropical influence. With colder
air expected to move over the Bering Sea and much of Alaska, a
cooler and showery period of weather is expected through the
upcoming weekend.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Warning 121 125.
MARINE...None.

&&
$$

SYNOPSIS AND MODEL DISCUSSION...BL
SOUTHCENTRAL ALASKA...PS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MO
LONG TERM...MC


982
FXAK69 PAFG 242245
AFDAFG

Northern Alaska Forecast Discussion
National Weather Service Fairbanks AK
245 PM AKDT Sun Sep 24 2017

.DISCUSSION...
The models are in overall good agreement out through the middle
of the week. The main weather concern today will be the potential
for precipitation Monday in for the Interior as well as the North
Slope and Brooks Range.

Aloft, at 500 mb, a 531 dam low will persist over Bristol Bay
before starting to to move south Monday night. A shortwave
associated with this low will sit over the southeastern Interior
tonight into Monday. A trough associated with a low in the high
Arctic will move from west to east across northern Alaska tomorrow
into Tuesday. While the models agree on the timing of this trough
moving across the state, they differ on how cold the air is aloft
The NAM has temperates a few degrees colder at 850 mb than the GFS
or the Ecmwf, particularly in the northern portions of the
Interior on Monday night. This will in turn have some impact on
snow levels Monday night in the northern Interior. An upper level
ridge will build over the Chukotsk Peninsula on Monday and over
the West coast of Alaska on Tuesday. A shortwave moving eastward
through the Chukchi Sea will help to knock this ridge southward
Tuesday night. The aforementioned shortwave will move across the
Arctic Coast on Wednesday. An upper level ridge will build along
the Canadian Border Wednesday into Thursday.

At the surface, a 998 mb low near Bristol Bay will persist
through Monday, gradually weakening. A cold front will move across
the Arctic Coast of Alaska Monday into Tuesday. An area of high
pressure will build east just north of the Brooks Range Tuesday
behind the front. This area of high pressure will push east on
Tuesday night as a low pushes across the Chukchi Sea well north of
the coast. High pressure will build over the Chukotsk Peninsula
tonight, this will strengthen the pressure gradient over the
Bering Strait and northern Bering Sea for tonight and tomorrow.

Central and Eastern Interior: A front will move in to the Interior
this evening and stall. The front is expected to stall along a
line from Circle to Fairbanks to McGrath. Precipitation is
expected along this front from this evening through early Tuesday
morning with the heaviest precipitation expected early in the day
on Monday. An upper level trough associated with a low in the
high Arctic will spread colder air aloft tomorrow and tomorrow
night. This will cause snow levels to drop between now and Tuesday
morning. Snow levels this evening across much of the Interior will
be around 5000 feet and by Tuesday morning the snow level drop to
1000 to 2500 feet (higher from Fairbanks south). Current thinking
is that the precipitation type will mainly be rain tonight and
Monday (with the main exception being above pass level in the
Alaska Range). On Monday night in areas above 2000 feet the
precipitation type will change to mix of rain and snow and by
early Tuesday morning to all snow. fortunately the precipitation
will be tapering off by early Tuesday morning. Up to three inches
of snowfall is possible along the Steese Highway at Eagle Summit
Monday night. Conditions should clear up during the day on
Tuesday. Another round of precipitation will move from southeast
to northwest through the Interior Wednesday night into Thursday.

West coast and Western Interior: Some precipitation is expected
along a stalled front on Monday mainly for areas south of the
Yukon River. High pressure should keep things relatively quiet
for Tuesday and Wednesday. There is some potential that a storm
system will affect the west coast over the weekend; however, at
this point, there is still a lot of uncertainty about the details
of this given the differences between the models.

Arctic Coast and Brooks Range: A weather front will pass along the
Arctic. As this front passes through it will bring snow. Generally
1 to 2 inches of snow is expected as the front moves through in
areas away from the coast. Lesser amounts are expected closer to
the coast where rain is expected to mix in with the snow.

Coastal Hazard Potential Days 3 and 4...None. The models are
hinting at the potential for a storm affecting the West Coast next
weekend; however, at this point, there is still a lot of
uncertainty about the details of this given the differences
between the models.

&&

.HYDROLOGY...
No issues anticipated at this time.

&&

.AFG WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory for PKZ200-PKZ210-PKZ220.
&&

$$

SEP 17



000
FXAK67 PAJK 242308
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
308 PM AKDT Sun Sep 24 2017

.SHORT TERM...Tonight through Tuesday...As of 1500 this afternoon,
current WV satellite imagery shows a shortwave lifting across the
northern gulf and northern portions of the SE Alaskan panhandle,
with a ridge of high pressure currently positioned over western
Canada. This shortwave developed from a parent trough that is currently
positioned over the Aleutian Islands of mainland Alaska. In
addition, WV imagery continues to show a very moist subtropical
jet advecting moisture with the flow between the ridge and the
deeper trough over the western gulf, which will impact our weather
for the early half of the work week. Current precipitable water
analysis even shows anomalously high PW values with this stream of
moisture.

As this shortwave continues to exit the area tonight, we should expect
PoPs to diminish during the overnight hours. The ridge will then
begin to build back in over the eastern gulf early tomorrow and
possibly help in decreasing cloud cover, mainly across northern
portions of the panhandle. This should aid with fog development
overnight tonight into tomorrow morning due to relatively stable
conditions and a very moist ground. The ridge will be weak and
should quickly propagate east before an additional shortwave
begins to lift out of the southern gulf and across our area during
the Tuesday timeframe. As mentioned previously, a subtropical jet
is assisting with bringing additional moisture to the area, so
this new system on Tuesday will contain subtropical
characteristics. This means that additional precipitation, a much
higher snow level, and slightly above average temperatures with
not a large diurnal spread can be expected for most of the area.

Generally, adjustments to the forecast were made using a
combination of the ECMWF and the GFS, as there was generally good
agreement between the two.

.LONG TERM...Big concern in the longer range is for a likely
atmospheric river event around mid to late week. Models are still
sorting out details on this but all indications are a significant
rain event is in store for at least part of the area. GFS and
ECMWF ensembles show precip water values 2-3+ standard deviations
above average with maximum on Wed and Thu. Ended up using 18z GFS
to start long range for Tue, then went with mainly WPC for Tue
night onward.

With details such as where main frontal band will hang up while
some low pressure waves move N along it, confidence in where
heaviest rain will occur is no more than average, but it appears
somewhere over the NW half of the area will be the wettest during
midweek. Models hint that far SE may even dry out for a bit as
waves moving along the front push precip band back to the N
around Wed night and Thu. Would expect significant rises on rivers
and streams where heaviest precip occurs, but too early to tell if
any flooding may occur. Will also need to watch out for potential
mudslides with ground fairly saturated from rain events occurring
before the midweek period.

For latter part of the long range, models differ on how fast main
trof moves out of the area, which will affect how quickly precip
diminishes. Keeping near climo POPs for much of the area for next
weekend attm.

&&

.AVIATION...Phone problems continue at Yakutat. Conds low enough
to warrant NIL TAF. Very low cigs/vis many TAF sites. Transition
to drier and more stable atmosphere except at very low levels/sfc.
Very difficult forecast problem Mon morning about whether low
stratus will be several hundred feet aloft for good visibility or
surface based resulting in dense fog. TAFs try to convey a little
of both.


&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Small Craft Advisory for PKZ012.
Small Craft Advisory due to Seas for PKZ042-051>053.

&&

$$

Voveris/RWT

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