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450
FXAK69 PAFG 232222
AFDAFG

Northern Alaska Forecast Discussion
National Weather Service Fairbanks AK
222 PM AKDT Sat Sep 23 2017

.DISCUSSION...
Overall, the models are in good agreement in the short term. A
generally quiet weather pattern for the remainder of the weekend
with the biggest forecast challenge being the potential for gap
winds at times in the Alaska Range.

Aloft, at 500 mb, A 543 dam low near Demarcation Point will move
to the north and east through Monday. A 531 dam low near the
Kuskokwim Delta will move into Bristol Bay by Sunday morning. This
low will persist over Bristol Bay through Monday morning before
starting to move to the south. A 535 dam low near Sand Point will
move to near Homer by Sunday morning. This low will move north
into the Interior and weaken Sunday night. A shortwave associated
with a low in the high Arctic will move from west to east along
the North Slope Sunday night into Monday. A weak ridge looks to
build into the west coast on Monday. This ridge will be knocked
south late Tuesday by a short wave approaching from the west. This
short wave will move across the Arctic Coast Wednesday and
Wednesday night.

At the surface, a 1024 mb high 250 NM NW of Deadhorse will move to
the northeast and weaken tonight. A 1011 mb low near Mackenzie Bay
will move to 175 nm NE of Demarcation Point by 4 PM Sunday at 1001
mb and to 330 NM NE of Demarcation Point by 4 PM Monday. A front
will move across the Arctic Coast Sunday night into Monday. High
pressure will build just north of the Brooks Range on Tuesday. A
989 mb low 120 NM south of King Salmon will split into two lows
tonight. The fist low will move to the Kuskokwim Delta by 4 AM
Sunday at 993 mb and then to 121 NM N of Nelson Lagoon by 4 PM
Sunday. The second low will move east into the northern Gulf of
Alaska. High pressure will build east over the Chukotsk Peninsula
on Monday causing the pressure gradient to tighten over the Bering
Strait and northern Bering Sea.

Central and eastern Interior: Some gap winds at time this weekend
in the passes of the Alaska Range; however, at this point winds
are expected to remain sub-advisory. A front will push into the
Interior late Sunday night where it will stall out. This will
bring rainfall to much of the Interior on Monday with the best
chances for precipitation generally along a line from Circle to
Fairbanks to McGrath. Preciptiation will gradually end on Monday
night with a gradual decrease in cloud cover expected on Tuesday.
Another system will push into the Interior Wednesday night and
will stall on Thursday bringing another round of rainfall, mainly
along a line from Circle to Fairbanks to McGrath, although the
models do have some differences in the handling of this system at
at this point.

Arctic Coast and Brooks Range: Some fog and lower ceilings tonight
and Sunday. A front will bring precipitation as it moves through
on Monday. The precipitation will generally be snow, although
some rain will mix in closer to the coast. 1 to 2 inches of snow
is expected inland on Monday with less than 0.5 inches expected
closer to the coast. High pressure will build in on Tuesday and
the winds will become more southerly. This should help to clear
things up, particularly along the coast.

West Coast and Western Interior: Some rainfall this evening in the
Yukon Delta and Lower Yukon Valley. Some gusty northeasterly
winds will persist tonight along the coast, although they will be
well below headline criteria. A front will bring some rainfall to
the western Interior on Monday. With a weak ridge building in on
Tuesday expect mainly quiet weather the remainder of the week. The
models are hinting a the potential for a system moving in from the
west for next weekend; however, at this point there is still a
lot of uncertaintyin the details of this.

Coastal Hazard Potential Days 3 and 4...None.

&&

.HYDROLOGY...
No issues at this time.

&&

.AFG WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory for PKZ200-PKZ210-PKZ225.
&&

$$

SEP 17



000
FXAK68 PAFC 240003
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
403 PM AKDT Sat Sep 23 2017

.ANALYSIS AND UPPER LEVELS...
There is an upper level low over southwest Alaska and the eastern
Bering Sea with two centers. One center is near Nunivak Island,
with the other center is just southwest of Kodiak Island. The
upper low near Kodiak has an associated 989 mb surface low, and
there is another 989 mb surface low centered near King Salmon.
Plenty of moisture and showers are associated with these lows.
There is an occluded front stretching from the Kenai peninsula
southeast across the Gulf of Alaska. This is bringing a decent
amount of rain into the eastern Kenai Peninsula. High pressure is
over the western and Central Bering Sea. There is also a weak
negatively tilted high over the Copper River basin.

&&

.MODEL DISCUSSION...
The numerical models are in decent agreement for the short term
portion of the forecast (through Monday afternoon). There are some
minor differences in how they handle the low moving towards Prince
William Sound, but the agreement is better than in previous runs.
There is a split in how the surface low over southwest Alaska
moves into and across Bristol Bay Sunday into Monday. The GFS and
NAM are further west, keeping the low on the western periphery of
Bristol Bay, while the ECMWF and Canadian GEM put it into the
eastern portion of the bay. The GFS/NAM solution was preferred.
Forecast confidence is near normal today.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist with the
downslope on the lee side of the Chugach Mountains keeping
PANC out of most of the rain. While some wind shear is possible
this afternoon, it will likely be above the 2000 ft threshold for
low level wind shear being mentioned in the TAF. Any shear that
is there should dissipate by evening.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The remnants of a barrier jet remain along the northern Gulf
coastline this evening and will taper off by Sunday morning with
weak onshore flow. Meanwhile, an upper level closed low is just
east of Kodiak Island, and looks to track through the Prince
William Sound region by late Sunday afternoon. This feature is
reflected at the surface with a weak 994 mb low just offshore of
Chiniak Bay late Saturday evening, and tracks northward into the
Prince William Sound area by Sunday before dissipating by Monday.
This is a little different track for the low than was expected
yesterday as it is now farther east in Prince William Sound as
opposed to heading over the western Kenai Peninsula.
Look for the northern coastal communities to receive the majority
of the precipitation. The Anchorage Bowl and the upper portions
of the western Kenai Peninsula will have drier conditions with
downsloping occurring due to the cross-barrier flow through rest
of the weekend.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
There are two upper level lows helping to bring rain to southwest
Alaska. One is near Kodiak Island while the other is near the
Kuskokwim Delta coast. This setup has brought the heaviest rain
to the Bristol Bay area. The heaviest rains will move northwest
towards the Kuskokwim Delta area this evening as the low near
Kodiak pushes off to the northeast and the one near the Kuskokwim
Delta tracks south along the coast. As this low tracks south there
is uncertainty as to its exact track. These differences will
determine how strong winds get in Bristol Bay on Sunday. Sunday
night the low will continue its track south into the North Pacific
leaving behind light rain over southwest Alaska.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
High pressure continues to dominate the majority of the Bering
and Aleutians and will continue to do so through Monday. This
pattern will keep northerly winds with minimal rain over the
region. The exception will be the eastern Bering Sea and Alaska
Peninsula as an upper level low near the Kuskokwim Delta tracks
along the coast into the North Pacific through Monday.

&&

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

Confidence continues to increase in regards to an area of low
pressure quickly moving into the Gulf of Alaska from the North
Pacific on Tuesday. The ECMWF is the fast outlier currently but it
now looks like gale force gusts could be expected near Kodiak
Island and the northern Gulf Tuesday afternoon through Wednesday
afternoon as a front associated with the low pressure quickly
moves through the region. Rainfall with this system will upslope
along the coastal mountains but there is a bigger question for
interior locations and it really depends on the low track.
Currently it looks like inland areas like Anchorage and Palmer may
miss out on much of the precip on Wednesday.

Looking at the big picture from late next week through the
weekend, it appears the wet pattern across Southwest and
Southcentral Alaska will remain the norm. A jet streak on the
backside of an upper level low over Southwest Alaska on Tuesday
will help amplify the longwave trough across the state of Alaska.
This will keep cyclonic flow across the region with colder air
being advected south which should keep increased instability and
rain showers in the forecast for much of the state. There could
be a few breaks with some shortwave ridging across Southwest
Alaska over the weekend but zonal flow will return to the state by
late Sunday and allow for another unstable pattern to develop at
that time. Afternoon high temperatures look to remain below
normal with low temperatures across the region remaining near
normal. The majority of the Bering Sea will be under the influence
of high pressure come Tuesday with low stratus and fog. By late
Friday, a strong low will push off of Russia and into the Bering
Sea which will force the high southward into the North Pacific
and a rather wet and windy period will be observed across the
Bering next weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Warning 121 125.
MARINE...Gale 119.

&&

$$

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


000
FXAK67 PAJK 232318
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
318 PM AKDT Sat Sep 23 2017

.SYNOPSIS...A front will move in off the gulf and spread heavy
rain across portions of the panhandle through Saturday night.
Another low will cross over the southern panhandle on Sunday
night.

&&

.SHORT TERM.../Saturday evening through Sunday night/...Satellite
imagery is indicating a front located over the central gulf.
Deformation in the frontal cloud shield is also indicating the
development of what appears to be a triple point low west of the
southern outer coast. Infrared cloud top temperatures of -60C and
satellite based microwave rain rate information are all suggestive
of heavy rain in the associated front. Models all support these
things elements as well. Lightning was observed late last night
and earlier today well to the west over the central gulf, but none
has been observed this afternoon. Modeled lifted index and CAPE
not conducive for more lightning this afternoon or tonight. Still,
very cold cloud tops warrant caution. And so, have opted to
retain a slight chance of thunderstorms over the eastern gulf in
marine zone 310.

Showers already have, or will soon, transitioned to rain with no
break in precipitation. Rain will be heavy at times, particularly
over the southern half of the panhandle. 36 hour forecast storm
totals ranging from around 1/2 inch to more than 2 inches with the
heaviest amounts over the southern inner channels. Some models
are suggesting that even more precipitation is possible. As such,
this represents the most critical element of the afternoon
forecast and there will be responses from area creeks and streams.
No flooding is expected at this time, as this front will move
through fairly quickly. Look for updates from this office as this
event unfolds.

Winds associate with this front will be limited to small craft
intensity. Have small craft advisories for portions of the outer
coast as well as Cross Sound, Clarence Strait, and Northern Lynn
Canal. Winds will increase and become gusty from south to north
and from outside to inside as the front moves onshore beginning
late this evening over the southern inner channels, late tonight
over the central inner channels, and finally tomorrow morning from
the Icy Strait corridor and northward. Post frontal winds and
gusts will be less.

A few tweaks to winds to highlight gusty conditions associate with
the front`s progress north. Updated PoP and QPF using some of the
NAM, GFS and SREF to capture the details associated with the
deformation associated with the developing triple point.
Otherwise, no significant changes made to the previous forecast.
Overall forecast confidence is average.

.LONG TERM...Monday through Saturday...An upper level shortwave,
developing from a parent circulation with its center positioned
near the Aleutians, will propagate across the northeastern gulf
and lift across the panhandle Sunday night into early Monday
morning. A subtropical jet feature will assist with advecting
additional moisture with this system, but most of the
precipitation increases will primarily be confined to the more
southern portions of the panhandle. A ridge of high pressure will
then begin to build over the eastern gulf Monday. We should start
to see most locations across the panhandle dry out for the day.
Fog development may be a possibility, especially during the
morning commute on Monday, so we`ll continue to monitor this
trend for subsequent forecasts. The ridge should then begin to
weaken and retreat east before another shortwave develops and
progresses northeast across the gulf, impacting the panhandle on
Tuesday. Models continue to show additional subtropical jet setups
to assist with pushing additional moisture over the panhandle.
This type of pattern appears poised to continue through the
remainder of next week, with additional shortwaves that develop
out of the parent trough over the gulf. Due to this tropical
moisture push over the area, we expect chances of precipitation to
remain high through the week and temperatures to maybe be
slightly above average, especially in southern areas.

For adjustments to the forecast, we updated PoPs with a blend of
the ECMWF and Canadian, relying more on the EC, to continue to
show the increasing precipitation trend through the week. We also
increased winds throughout the eastern gulf waters to account for
the tighter pressure gradient that develops with the subsequent
shortwaves that progress out of the parent circulation. A lot of
uncertainty will then remain regarding the synoptic pattern after
about Day 8.

&&

.AVIATION...Phone line problems in Yakutat are preventing the ASOS
obs from being disseminated. Had to NIL the TAF there due to
deteriorating weather tonight and loss of useful webcam views by
sunset /0311Z/. Continued very low cigs/vsbys across panhandle in
our very moist air mass. Wind shear mentioned in southern
panhandle TAFs overnight tonight with vigorous short wave moving
quickly through southern areas overnight. Rain changes to showers
during the day Sun with gradual stabilization. Outlook for
Mon...possible dense fog impacting TAF sites.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...Small Craft Advisory for PKZ012-022-036-041-043-052.
Small Craft Advisory due to Seas for PKZ051.

&&

$$

Fritsch/Voveris

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