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129
FXAK69 PAFG 191349
AFDAFG

Northern Alaska Forecast Discussion
National Weather Service Fairbanks AK
549 AM AKDT Thu Oct 19 2017

.SYNOPSIS...
Aloft...
A long wave trough over mainland Alaska will deepen through the
Fri and then persist through the weekend. A 995 DM low at H500 180
nm northwest of Barrow will move to Point Lay by 4pm Thu, to 60 nm
west of Kivalina by 4am Fri, to Nome by 4pm Fri and then over
Norton Sound Fri night and Sat. Very cold air associated with this
feature is moving over open water and causing a strong surface low
to form under this feature that will track directly under this
feature and form heavy snow showers over the open water due to
nearly dry adiabatic lapse rates with 850 mb temperatures of -12C
or colder. See the surface feature mentioned below for details.

Northern Alaska will remain cold through the weekend, with snowy
conditions along the coasts.

Surface...
A 980 mb low 180 nm northwest of Barrow will move to near Point
lay as a 980 mb low by 4pm Thu, to 60 nm west of Kivalina as a 980
mb low by 4am Fri, to near Nome as a 985 mb low by 4pm Fri then
weak slowly. A cold front stretching from this low to the Bering
Strait will move to Point Lay to the Yukon Delta by 4pm Thu, to
Kivalina to Anvik by 4am Fri, and to Kotzebue to Anvik by 4pm Fri.
Very cold air with the low and behind the cold front are causing
very unstable conditions and heavy snow showers near the low
center and along the cold front. Showers will taper off as the
front moves inland. Expect at least 2 inches of snow every 12
hours near the low center. Showers along the front taper off as
the front moves inland. There are northwest winds 20-30kt along
and following the cold front. There are north to northeast gale
force winds in the northwest quadrant of the low. Winds will
decrease in the wake of the low.

This feature is causing periods of low visibility with the snow
showers over zones 201 and 202 so winter weather advisory will
continue there today. May need to issue winter weather advisory
for zones 213 and 207 tonight.

A 992 mb low 60 nm north of Barter Island will persist through
Friday, then move to near Barrow as a 992 mb low by 4am Sat.
Snow along and north of this low will remain mainly offshore
through Fri then move into Barrow Fri night.

An occluded front stretching from Yakutat to Inuvik will persist
through tonight then dissipate Fri. This is causing light snow
along the Alcan Border and Alaska Range that will taper off
tonight. This is also causing nw winds 10g20 mph through the
Alaska Range passes and over the eastern Tanana Valley today that
will taper off tonight.

&&

.DISCUSSION...
ECMF again moves the low aloft southwest more quickly than the
GFS and NAM, and we favor the slower GFS as the low is now slowing
as it moves over open water deepens at the surface. At 06Z the
low near Point Thompson is 5 mb deeper than models initialize and
the low north of Barrow is almost 10mb deeper than any models
indicate. Due to this expect the low to remain deeper and move a
bit slower than any models indicate. Will lean towards the GFS
since it has the slowest movement of any deterministic models,
but we will move the low center a bit slower than the GFS. What
this means is that snow will last longer over the Western North
Slope than models indicate, and that NW winds in the southwest
quadrant of the low will last longer and be stronger than models
indicate. Since winds have now turned NW at Shishmaref and could
last through tonight we will issue a high surf for that area.

This also means that the deeper low near Barter Island will lead
to winds lasting longer today in zone 204 than indicated by
previous forecasts and by models.

By Fri afternoon, model locations for the low come together
better near Nome and seem reasonable.


Coastal Hazard Potential Days 3 and 4...
There are northwest winds 20-30kt blowing across the southern
Chukchi Sea causing high surf at Shishmaref today through tonight.
Winds will decrease and turn northeast at Shishmaref Friday
morning causing surf to decrease.

May see High surf at the Bering Strait and and St Lawrence Island
tonight and Fri as the strong winds just west and north of the low
move over those areas.

&&

.AFG WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory for AKZ207.

Winter Weather Advisory for AKZ201-AKZ202.

Heavy Freezing Spray Warning for PKZ230-PKZ235-PKZ240-PKZ245.

Gale Warning for PKZ225-PKZ230-PKZ235.

Small Craft Advisory for PKZ210-PKZ220-PKZ225.
&&

$$

JB OCT 17



000
FXAK68 PAFC 191239
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
439 AM AKDT Thu Oct 19 2017

.ANALYSIS AND UPPER LEVELS...
Weak high pressure and offshore flow continues to bring generally clear
skies with gap winds to much of Southcentral and the Gulf Coast
as a low in the Eastern Gulf begins to weaken as it moves inland.
The one exception to this is a weak deformation band that brought
an area of light snow to the Southern Kenai Peninsula overnight,
which should quickly dissipate this morning as the upper trough
axis moves to the east. Further west over the Eastern
Bering/Southwest coast a broad area of cold air advection in the
northerly flow is producing rain/snow showers as it cold air moves
over the warmer waters of the Bering. These are beginning to
diminish however as a gale force low begins to spread into the
Western-Central Bering. This system will continue moving eastward
through the Aleutians this week and be the focus for active
weather in the Gulf this weekend.

&&

.MODEL DISCUSSION...
The models remain in very good synoptic agreement through Friday,
with just some minor differences in the low position over the
Eastern Gulf that should have little impact in forecast
confidence. This agreement quickly begins to break down starting late
Friday as the Aleutian low begins to deepen as it moves into the
Gulf. Beginning Wednesday evening, model runs are began shifting
this further to the east into the Panhandle, however since this
was such a large departure from the previous track the forecast
was left mostly as is tonight until better model consensus is
available.

&&

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

&&

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

The high pressure in the Interior and low in the eastern Gulf are
keeping the northerly winds going today. The lack of extremely
cold air and only moderate pressure gradient for such a pattern is
keeping outflow winds on the modest side. An upper level trough
is over the Talkeetna Mountains through eastern Kenai Peninsula
early this morning and it will remain nearly stationary into the
evening and then move eastward into the Copper River Basin
overnight. This will keep some areas of clouds in the Copper River
Basin. Periods of light snow through tonight with minor
accumulations will be possible within Wrangell- St. Elias National
Park including the McCarthy area. There is also an area of clouds
that has set up along the Western Kenai Peninsula coast from
Kasilof southward. This is due to the cold advection and gusty
winds coming through the gaps in Western Cook Inlet causing some
convergence right there. Other than that, skies should be mostly
clear over the region today into tomorrow. Clouds will move into
the area in a widespread manner late Friday night or Saturday
morning. The timing of these clouds will have a direct impact on
Friday night temperatures so those are subject to be adjusted as
the timing of the clouds becomes more clear.

&&

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

A bowling ball low near Point Barrow continues to drop southward
which continues to advect colder air toward Southwest Alaska. The
atmosphere remains relatively unstable with the colder air moving
in aloft but the lack of large-scale forcing is limiting the
coverage of snow showers across the region. Other than scattered
snow showers over the next several days, a rather benign weather
pattern is in store for the region with partly cloudy skies
prevailing across the region. The advertised cool down has started
and it still looks like each of the next 3 days should see both
high and low temperatures a degree or two cooler than the previous
day.

&&

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

The area of low pressure near Shemya continues to strengthen this
morning and the satellite signature shows it`s now just
approaching the mature phase. This has allowed winds along the
frontal boundary to be just a touch stronger than originally
expected with storm force winds over parts of the far western
Bering Sea. Now that the low has matured, it will steadily weaken
over the next several days as it moves along the Aleutian Island
and ultimately into the Gulf of Alaska Saturday morning. Cold air
advection showers will build across the Bering Sea today and the
vast majority of the Bering Sea will see rain/snow showers over
the next several days with colder air filtering in aloft.

&&

.LONG TERM FORECAST (Days 3 through 7: Friday night through
Wednesday)...

An active pattern is expected through the long range forecast
through the middle of next week. The main challenge surrounds a
storm system which slides east into the gulf this weekend bringing
the potential for snow across Southcentral into early next week.

For the start of the weekend, cold air advection keeps
temperatures below normal across the southern mainland into
Saturday morning under dry conditions as low pressure tracks south
of the Alaska Peninsula to the Gulf. Weak disturbances across the
gulf will initiate rain showers along a surface low near the northeast
coast heading into Saturday morning, with a few snow showers moving
inland over the eastern Copper River. For locations along the
Bering, including the southwest coast, precipitation chances
increase through the upcoming weekend as an upper low pressure
system in the eastern Bering slowly propagates south toward
Bristol Bay. A colder air mass advecting from the northern Bering
changes precip type mainly to snow except along the Aleutians
where a mix of rain and snow prevail under moderate temperatures.

The biggest challenge of the upcoming forecast is the storm system
which tracks into the gulf on Saturday. This system is currently
expected to lift a frontal boundary to the northern waters by
Saturday afternoon ahead of a surface low. This low moves across
the central gulf Saturday night and deepens to 970 mb. This warm
core system will spread rain across the Gulf on Saturday, with
showers along the gulf coast starting initially as snow mixing
with or changing over to rain through Saturday night. The upper
level low associated with this front begins to lift north towards
Sunday morning, which brings the potential for snow to develop
across inland areas of Southcentral. Models continue to struggle
from run to run on the timing and track of this system, therefore
only small changes were made to increase precipitation chances
for the latter part of the weekend. With high uncertainty during
the mid range forecast, kept changes minimal through the middle
of next week.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 130 132 170 173-178.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...DEK
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MC
LONG TERM...KH


000
FXAK67 PAJK 192232
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
232 PM AKDT Thu Oct 19 2017

.SHORT TERM.../Thursday night through Friday night/...Rain showers
continue over much of the panhandle this afternoon with more
expected tonight, tomorrow, and Friday night. Overnight lows over
the far northern inner channels and northeast gulf coast are
forecast to be low enough that an overnight rain/snow mix is
possible during this short term forecast period, particularly for
Friday night. 1000-850 and 1000-500 thickness numbers as well as
850mb temps are all indicating this possibility as well.
Nonetheless, so low elevations expected yet. Not a lot of QPF in
the forecast for the Haines or Klondike Highways, so accumulations
there are less than an inch for any given 12-hour period.
Eaglecrest may see as much as an inch tonight and Friday night.

Satellite imagery is very clearly convective and this is borne out
in model data. Convective parameters were really quite high for
the coastal waters and the Dixon Entrance. Even so, most observed
lightning today was confined to Hecate Strait and coastal British
Columbia. As low pressure over the northern and eastern gulf
weakens, a ridge will build in first from the west and then from
the south, causing the atmosphere to begin stabilizing by Friday
afternoon. A high end gale force front, will then approach from
the west late Friday night to finish the stabilization process,
but will bring its own complications in the form of strong marine
winds and windy conditions along the southern outer coast. At
present, am expecting strong wind gusts to have begun by late
Friday night.

Used a blend of GFS and ECMWF for updates to the pressure field
through Friday night. Model divergence beyond that time was judged
too severe to make any changes to the existing forecast with any
confidence. used GFS as a foundation for winds through Friday
night. Also used GFS for PoP and QPF. Very little change to temps
with some minor adjustments to snow levels. Overall forecast
confidence is average.


.LONG TERM.../Saturday through Thursday/Model agreement is fair
for the weekend, then they differ quite a bit next week. Main
issue is timing of systems moving across the NPAC, potential for
at least 1 tropical system getting into the westerlies, and
interaction between these systems and an upper trof over W AK.
Decided to use the 12z/18 ECMWF to handle Sat, then went with
mainly WPC for Sun onward.

First main system will be a complex low moving into the gulf by
Sat. Looking like an occluded front will move N across the area
Sat but will weaken as it moves across the N half of the area.
This front will most likely bring at least gale force winds with
it over the gulf, with at least SCA level and possibly gale force
winds to mainly the more E-W inner channels. Should see some rain
with the front Sat, although as front weakens over the N, precip
may become more spotty as it encounters offshore low level flow.
The main low appears like it will move to the far N-central gulf
by late Sat night, then weaken there Sun. As this happens, looking
more showery for the area especially later Sun.

While model agreement is not good next week, they suggest at least
1 strong system will affect the area. This is when some effect
from what will be remnant of Typhoon Lan will come into play. The
model timing on how fast the typhoon moves into the westerlies
varies somewhat, with GFS/GEM faster than the EC by about 1 day.
(Of interest...the Joint Typhoon Warning Center timing is right
in the middle of the 2 extremes) It is this feature that the
GFS/GEM spins up into a powerful storm S of the Aleutians early
next week, then moves it into the gulf by late week. EC also shows
a fairly strong system but it does not have remnants of Lan in
it, and is somewhat weaker than the GFS/GEM. Still plenty of time
to monitor this situation. In between this larger system and the
one over the weekend, models suggest another low may try to move
NE into the gulf early next week but vast model differences exist
on this, so later forecasts may need to be updated to reflect this
system more than current forecast has it.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Small Craft Advisory for PKZ041>043.
Small Craft Advisory due to Seas for PKZ022-051-052.

&&

$$

Fritsch/RWT

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