National Weather Service United States Department of Commerce


089
FXAK68 PAFC 210112
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
512 PM AKDT Fri Sep 20 2019

.ANALYSIS AND UPPER LEVELS...
This afternoons CIMSS Layer precipitable water satellite loops
shows a strong tropical feed of moisture into a low pressure
system that`s currently located in the northern Gulf. This storm
is also located in the base of a long wave trough that extends
from western Alaska into our neck of the woods.

Further west, a zonal flow has developed across the Bering Sea
and Aleutians, which an area of low pressure crossing northern
portions of the Bering, and another low poised to enter the
western Aleutians.

&&

.MODEL DISCUSSION...
The numerical guidance remains in good agreement through the next
48 hours, before starting to diverge for the start of next week.
In general, they all show another storm moving into the Gulf of
Alaska. However, the European (ECMWF) is slightly faster than the
remainder of the models. Earlier runs also had more discontinuity
in strength, track, and timing. The new datasets from the 12z run
have come into better agreement, but, there are still some
important differences.

In general, they (the models) agree on the track and strength of
a fairly strong storm as it crosses the Aleutians and into the
AKPEN. After this point is where things begin to diverge, as they
disagree on when a new low forms along the triple point, along
with its strength and track. This affects the location of where
the heaviest rainfall and stronger winds will be located. Overall,
the active weather pattern will continue.

&&

.AVIATION...
PANC...Rain will persist through about 18Z Saturday. While
visibility may be briefly reduced in heavier bands of rain, expect
VFR ceiling/vis to prevail for most of the time. Drier air will
then begin to filter in after 18Z Saturday, bringing an end to
the rain and allowing ceilings to rise.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Tonight
through Sunday)...
The wave train of lows with breaks in between continues for
Southcentral through this weekend. The low that caused all of
today`s rain is moving toward eastern Prince William Sound. The
center of the low is expected to loop back to the west tonight,
then will drift to the southeast through the day Saturday. As the
low retrogrades west across Prince William Sound tonight, light
rain with periods of moderate rain is expected to continue for
Anchorage, the Matanuska Valley, and points east across the Gulf
Coast and the Copper River Basin. Anchorage will be on the western
edge of the rain, so any slight eastward deviation in the track
of the low could result in Anchorage staying much drier, and as
such forecast confidence in the duration and where the heaviest
rains will set up tonight is low. It appears likely, however, that
the rain will remain nearly stationary with somewhat heavier rain
over East Anchorage, the Hillside, and Eagle River. This is
expected to help keep the river levels high, and the Urban and
Small Stream Flood Advisory for Anchorage continues as all that
water flows down the streams.

On Saturday, as the low turns back to the east, the rain will
diminish as the low weakens and will shift eastward with time. The
very slow movement of the low will make this a gradual process.
The further west you go, the more likely you`ll see some sunshine
later in the day Saturday. Mostly clear skies will move across the
Kenai Peninsula, Anchorage, and the Susitna Valley by Saturday
evening, with lingering clouds in the Matanuska Valley, Copper
River Basin, and points east. The mostly clear skies will be
associated with a cooler air mass, the coolest of the season so
far. Thus, those aforementioned areas where the skies clear will
be threatened with near freezing temperatures away from the water,
including the interior Kenai Peninsula, east Anchorage, and the
Susitna Valley. Temperatures were nudged upward a few degrees for
the Matanuska Valley and especially the Copper River Basin where
clouds are expected to remain for at least some portion of
Saturday night. By the predawn hours Sunday morning, even the
Matanuska Valley skies should be clearing.

A mix of sun and clouds is expected for the day Sunday for much of
Southcentral, with more clouds near the coast. The next strong low
to enter the Gulf will move in Sunday afternoon, with rain breaking
out over Kodiak Island by mid-afternoon, along with strong easterly
winds. The rain will move up the coast into Sunday evening and
through the overnight Sunday night. Currently it appears as though
the usual rain shadowing areas including the Western Kenai Peninsula,
Anchorage, and the Matanuska Valley will be largely rain shadowed
Sunday night with much less rain for those areas.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3; Tonight
through Monday)...
Scattered showers and clouds from King Salmon north along the
Alaska Range will diminish overnight as a trough to the east
weakens and a ridge builds over the region. The clearing
conditions and weakening northerly winds will again allow to
overnight lows to drop near the freezing mark. There is also the
potential for patchy fog overnight tonight into early tomorrow
morning, especially from Bristol Bay north to the middle and upper
Kuskokwim Valley. Clouds will be on the increase over the Y-K
Delta beginning Saturday as the next frontal system advances east
toward the coast. Widespread rain showers will move over the coast
by late morning, moving slowly inland through the day as the
front and its associated surface low weakens and lifts northeast
to Norton Sound. A deformation zone developing between this
feature and a deepening low over the central Aleutians will allow
precipitation to persist across Bristol Bay Saturday night into
Sunday. More widespread precipitation and gusty southeasterly
winds are then expected across Bristol Bay and the AKPEN late
Sunday as the low tracking just south of the Aleutian Chain pushes
an occluded front toward the southwest coast. The rain lingers
into Monday with the gusty winds turning to the northeast as the
low passes just south of the AKPEN and continues its trek east
into the western Gulf.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3;
Tonight through Monday)...
Westerly gales and widespread ares of rain showers will develop
overnight across the Northern Bering this evening as a low
tracking east deepens. Areas of gale-force winds and waves above
10 feet will continue through Saturday from Saint Matthew Island
and the Pribilofs east to Kuskokwim Bay before diminishing
Saturday night as the low weakens and lifts into Norton Sound.

Farther south, a ridge stretching across the eastern half of the
Aleutian Chain will continue to slide east as a developing north
Pacific low approaches the western half of the chain. By Saturday
night, this system reaches the central ALeutians with gales
developing along its occluded front. The surface low continues to
strengthen Sunday as its front sweeps toward Bristol Bay and the
AKPEN. widespread gales and areas of moderate to heavy rain are
likely along the front with northerly storm-force winds developing
along the backside of the low across the central Aleutians. This
area of storm-force winds moves east from Dutch Harbor to south of
Cold Bay by Sunday night as the low moves just south of the
AKPEN. The northwesterly winds the weaken to gales and eventually
small-craft winds on Monday as the low tracks into the western
Gulf.

&&

.MARINE (Days 3 through 5)...
An occluded storm force low will continue moving eastward into
the Gulf Monday. Broad north/northwest flow at sub-gale magnitude
will prevail over the Bering Sea, though gales are likely to
persist south of the AKPEN through Monday night, as the backside
of the projected low will be over the AKPEN/western Gulf. In the
Gulf of Alaska, there continues to be uncertainty regarding the
progression of the low Monday. The last several runs of the GFS
have spawned strong secondary lows in the Gulf. Though we`re not
buying into these mesoscale features just yet, there is moderate
confidence in gales somewhere in the Gulf Monday...but uncertainty
remains with location and duration. The low will exit to the east
Tuesday into Wednesday, and winds over the Gulf will diminish,
though stronger gap winds will likely persist a bit longer over
the Barren Islands. Meanwhile, a strong low will approach the
Bering. This will likely bring gales, if not storm force winds to
the western Aleutians/Bering by Tuesday, spreading to the central
Bering Wednesday.

&&

.LONG TERM FORECAST (Days 3 through 7)...
A relatively progressive pattern will continue as a low tracks
eastward into the Gulf Monday, bringing widespread precipitation
to the Gulf/North Gulf Coast Monday. Meanwhile, broad northwest
flow will persist over the Bering, bringing showers as cold air
passes over warmer waters. Brief upper ridging will develop over
the Bering in the Tuesday to Wednesday time frame, and as the Gulf
low exits to the east, conditions will trend drier. That said, a
jet streak along the eastern edge of the ridge will maintain
cloudy conditions and scattered showers over the southern mainland
Wednesday.

Looking farther west, a strong low will approach the western
Bering/Aleutians roughly in the Wednesday to Thursday timeframe.
This low will originate from Tropical Storm Tapah, which is
currently 300 miles east of Taiwan. As per usual, models are
struggling with the evolution of this low. Strong winds and a good
shot of rain are likely over the western and central Aleutians
Thursday into Friday. The wet pattern looks to extend into
Southcentral next weekend, but it`s too early to have much
confidence in details.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Urban and Small Stream Flood Advisory 101.
MARINE...Gale Warning 130 131 132 138 173 174 176 180 185.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...PD
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TM
MARINE/LONG TERM...MM



000
FXAK69 PAFG 202326
AFDAFG

Northern Alaska Forecast Discussion
National Weather Service Fairbanks AK
326 PM AKDT Fri Sep 20 2019

.SYNOPSIS...
Still expect significant rain across the Eastern Interior tonight
and Sat as a low in the Gulf of Alaska moves north over the
Eastern Interior. A cold front will sweep east across the Interior
on Sat. Snow level across the Eastern Interior will drop from
5000 ft today, to 3500 feet about midnight tonight, and to between
2000 to 1500 feet Sat and to the surface sat night, although
little if any precip will occur Sat night. Since the ground will
be warm do not expect snow to stick below 2000 feet, but still
expect 1-2 inches of snow above 2000 feet over the Eastern
Interior on Sat, with 4-6 inches in the Alaska Range above 3000
feet, and higher amounts above 4000 feet. Could see 4 inches of
snow in the Central Brooks Range tonight and Sat as well.

Aloft...
The long wave pattern consists of a trough over the Bering Sea and
Mainland Alaska with a ridge over Western Canada. It looks like
this will hold in place through Mon. This will keep temperatures
in Northern Alaska near or below normal. A series of short wave
troughs will rotate NE across northern Alaska over the next
several days bringing periods of rain, with snow at elevations.

A low aloft just west of Point Hope will persist into Sun and
possibly into Mon. Weather associated with this will be discussed
in the surface section below.

A short wave trough over the Central Interior will move over the
NE Interior tonight, and over the Ern North Slope on Sat. This
will cause a frontal wave along the Arctic Front to move NE
tonight and Mon. This will cause and surface trough to develop
over the North Slope on Sat and persist into Sun.

A second short wave trough now along the West Coast of Alaska will
move over the Western Interior tonight, to Wainwright to Denali by
4pm Sun, to Barrow to Big Delta by 4am Sun, and to Barrow to Eagle
by 4pm Sun. This will cause snow and rain when it encounters
moisture in the Central and Eastern Interior and North Slope Sat
and Sun. Precip will end along the trough, with clouds clearing
west of the trough.

Surface...
A vertically stacked 995 mb low just west of Point Hope will
weaken slowly through Sat night. This is causing SW winds 10-15
kt across the southern Chukchi Sea through Sat night along with
scattered rain showers.

A low pressure trough will develop along the Arctic Coast of
Alaska Sat, then move just north of the Coast on Sun, then weaken
in place through Mon. This will keep stratus and fog along the
Arctic Coast. through Sat night. Winds will turn SW most areas
Sat night as the trough moves offshore and then stratus and fog
will diminish.

A 973 mb low in the Northern Gulf of Alaska will weaken slowly
through Sat then moves east on Sat night. An occluded front
associated with this low that lies from Seward to Fairbanks to
Eagle to Yakutat will remain stationary tonight with a 987 mb low
developing near Eagle tonight. The occluded front will move to
Seward to Fairbanks to a 990 mb low near Arctic Village by 4pm
Sat, with the front and low dissipating Sat night. Rain has spread
north of the Eastern Interior along this front, and rain will
expand to cover most of the Eastern Interior tonight as the
surface low develops near Eagle and rain will continue into Sat.
Expect 0.25 to 0.50 inch of rain over the Eastern Interior with
this feature.

An Arctic front that lies from Demarcation Point to Ruby to King
Salmon will move to Barter Island to Bettles to Denali by 4am Sat,
to Arctic Village to Northway by 4pm Sat, to Arctic Village to
Burwash Landing bt 4am Sun, then weakening Sun. Snow level across
the Eastern Interior will drop from 5000 ft today, to 3500 feet
about midnight tonight along the front, and then fall to between
2000 to 1500 feet Sat west of the front and to the surface Sat
night, although little if any precip will occur Sat night. Since
the ground will be warm do not expect snow to stick below 2000
feet, but still expect 1-2 inches of snow above 2000 feet over the
Eastern Interior on Sat, with 4-6 inches in the Alaska Range
above 3000 feet, and higher amounts above 4000 feet. Could see 4
inches of snow in the Central Brooks Range tonight and Sat as
well. Will see temperatures fall during the day Sat behind the
Arctic front across the Eastern Interior, and also see West winds
15 mph gusting to 25 mph on Sat.

A 994 mb low 200 NM west of St Matthew Island will move to the
Yukon Delta as a 992 mb low by 3pm Sun, and to near Nome as a 992
mb low by 4pm Sun, and over the southern Chukchi Sea as 994 mb low
by Sun. Expect winds 15-25 kt around this low along with a broad
areas of rain around the low. The rain will move into the Yukon
Delta Sat and spread north to the Seward Pen Sat night and Sun,
and into the southern Chukchi Sea Sun night and Mon.

A low moving over the AKPEN Sun will move into the Gulf of Alaska
Mon. This will do little more than push slightly warmer and drier
air north over the Interior Sun night and Mon.

&&

.DISCUSSION...
Models initialize well aloft and show similar pattern through 4pm
Mon. After Mon start to see significant differences between
models on the strength of features as remains of Tropical Storm
Tapah goes extra-tropical Tue/Wed in the NW Pacific and Wrn
Bering.

At 850 mb the NAM and Canadian models both push the cold air east
more quickly on Sat than the GFS and ECMF models. Models come back
into closer agreement at 850 mb on Sun and Mon. Since the surface
low in the Gulf of AK at 15Z is deeper than any models indicate,
and since the NAM has a deeper feature moving north into the Ern
Interior tonight and Sat, it makes sense to use the stronger winds
and stronger and faster low level cold advection shown by the NAM.
This will be a big impact on the timing of the cold advection and
lowering of the snow level over the Interior on Sat.

On precip, the NAM does not depict the areal coverage well in the
Eastern Interior this morning and differs with areal coverage from
the GFS and ECMF. At this point will use a blend of the GFS and
ECMF for areal coverage of precip and for amounts, but may reduce
precip Interior Sat PM to reflect the faster frontal movement
shown by the NAM..


At the surface at 15Z, models verify well most features, but area
several MB too weak on low just west of Point Hope and other low
just north of Inuvik, and also several MB too weak on strong low
in the Nrn Gulf of AK. Models show similar patterns through Mon,
but NAM indicates stronger gradient over the Eastern Interior on
Sat. Since the low in the Gulf is deeper than models indicate, we
favor the stronger winds shown by the NAM as this low may be
remain deeper than models indicate as it moves north over the Ern
Interior. Since models are too weak on the Arctic Coast lows, will
bump up winds 3-5 kt near those low centers from what models show,
especially over the southern Chukshi Sea. Will also use the NAM
winds as they better depict the stronger gradient Sat associated
with s slightly stronger low moving into the Ern Interior than
models indicate.

Bottom line is we will use the NAM for wind fields and
temperatures due to the stronger than forecast low moving into the
Ern INterior Sat which will induce a faster cold frontal movement
to the east on Sat as the NAM depicts. Will bump up winds from
NAM in the Chukchi Sea 3-5 kt through tonight to account for the
stronger than forecast low, and bump up gusts on Sat in the Ern
Interior to indicate the stronger low in the Ern Interior on Sat
as well. We will use a blend of the GFS and ECMF for precip areal
coverage on amounts, but lower pops and amounts in the Ern
Interior Sat PM to indicate the faster cold frontal movement.

Coastal Hazard Potential Days 3 and 4...
None through Tue. Will need to watch potential strong storm
development in the middle of next week that could bring high surf
to much of the West Coast Thu and Fri next week.

FIRE WEATHER...
Wet and cool through Sat, with RH above 40% through the weekend.

&&

.HYDROLOGY...
Expect 0.25 to 0.50 inch of rain across most of the Eastern
Interior today through Saturday, with up to 1.00 inch in the
Alaska Range. Freezing levels are now 5000 ft but will drop to
3500 ft about midnight tonight and to 1500-2000 ft on Sat. This
means that most of the precip in the Alaska Range will fall as
snow after tonight, so that will lessen the runoff rate there.
Expect rivers to rise slightly across the Eastern Interior
through the weekend, and to remain steady in other areas. No
Flooding issues expected.

&&

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

$$

JB SEP 19


000
FXAK67 PAJK 202246
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
246 PM AKDT Fri Sep 20 2019

.SHORT TERM...MIMIC indicated precipitable water traces a long
tentacle of subtropical moisture north-northeastward across the
North Pacific squarely into Southeast Alaska. Precipitable water
and IVT values for the Yakutat 12Z sounding are not record worthy,
but still are very near during one of our wettest times of year.
With the previous systems priming us, and a few before that, we
have reached the point of concern with minor flooding for the
Taiya River near Skagway, Montana Creek in Juneau, and the Indian
River in Sitka. Rainfall amounts of 1 to 2 more inches are
expected tonight with isolated amounts up to 3. Increasing
cyclonic curvature over the northern Panhandle at 500 mb also led
us earlier to increase winds through the Juneau area for gusts to
60 mph and for much of the rest of the Panhandle of 40 to 50 mph.
Winds will slowly come down at Juneau this evening but remain
strong over Haines, Skagway, and Ketchikan through early Saturday
morning.

Rainfall will turn more showery by Saturday as more unstable air
migrates in. Weak ridging will occur across our CWA through most
of the weekend, but continued southwesterly flow in the low levels
will allow for showers to continue during this time period.
Showers may become more scattered in nature, but QPF amounts are
still relatively high, in general to account for short-duration
more intense rainfall. The next big system arrives Sunday night
into Monday. This will bring another round of very strong winds
and heavy rainfall for much of the Panhandle.

No changes were made to the forecast so as to free staff to
monitor and react to the current dynamic system.

.LONG TERM.../Sunday through Friday as of 10 pm Thursday/
The weather will be most active during the first half of the long
range period, then a quieter pattern is becoming more likely
during the latter part. A strong occluded front will move NE into
the area Sun night into Mon. Looking like a significant low will
form and move NNE into the gulf by Mon morning, then curve back
to the W and merge with the original parent low that will be
moving E into the gulf during Mon. This combined low should move
across the gulf and through SE AK Tue. After that, there may be a
couple of weaker systems that might move in toward midweek, then
looking like late week into the weekend will see upper ridging
developing over much of the gulf and SE AK. Used mainly the 12z
GFS to handle things through 00z Tue, then WPC/NBM after that. Did
use 21z SREF to help with POPS through late Sun night.

One of the main model issues is how far E the developing low will
track before it curves back and merges with the parent low. 12z GFS
represented a middle ground compromise, taking low into the
central gulf, with the 12z Canadian further W and the 12z ECMWF
further E. The timing of the occluded front moving onshore was
slowed some. Overall, system looks fairly wet and windy, but with
the main upper level jet sinking S fairly early in the period, a
prolonged heavy rain event is less likely with the front. May get
some heavier precip Mon night into Tue as the combined low moves E
and onshore flow increases.

Precip chances look like they will decrease for the latter half
of the long term as upper ridging takes over across the area.
Still some important amplitude and positional differences to work
out with the placement of the ridge axis, so cannot completely
remove mention of precip further out yet.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...Strong Wind until 7 PM AKDT this evening for AKZ021-023-027.
High Wind Warning until 7 PM AKDT this evening for AKZ025.
Areal Flood Advisory until 10 PM AKDT this evening for AKZ025.
Areal Flood Advisory until 9 PM AKDT this evening for AKZ023.
Strong Wind until 7 PM AKDT this evening for AKZ017-020.
Strong Wind through late tonight for AKZ028.
Strong Wind through Saturday morning for AKZ019.
Areal Flood Advisory until 1 PM AKDT Saturday for AKZ018.
Strong Wind from 1 AM AKDT Saturday through Saturday morning for
AKZ018.
MARINE...Gale Warning for PKZ013-031>036.
Small Craft Advisory for PKZ011-012-021-022-041>043-051>053.

&&

$$

JWA/RWT

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