National Weather Service United States Department of Commerce


872
FXAK68 PAFC 250310
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
710 PM AKDT Mon Jul 24 2017

.PREV DISCUSSION... /issued 450 PM AKDT Mon Jul 24 2017/

ANALYSIS AND UPPER LEVELS...
A strong blocking ridge is centered south of the Western Aleutians
with low amplitude (nearly zonal) flow over top of this ridge
across the Bering Sea and Mainland Alaska. The main feature of
interest today is an old elongated frontal boundary which extends
from the Kuskokwim delta southwestward across the eastern Bering
Sea and down to the Eastern Aleutians. This is producing widespread
clouds and a narrow zone of rain just ahead of it. However, an
upper level wave crossing the Bering is helping to reinvigorate
the southern end of the front, resulting in increased rainfall.
Fog and stratus continue to dominate the Aleutians and much of
the Bering Sea.

A separate short-wave crossing the Alaska Interior is just
clipping the northern tier of Southcentral, with light rain over
the northern Susitna Valley and Talkeetna Mountains along with
portions of the Alaska Range. A marine layer is firmly entrenched
along much of the Gulf coast and Kodiak Island, with just hints
of some lower clouds in Cook Inlet. In between, variable clouds
extend across the rest of Southcentral

MODEL DISCUSSION...
Model guidance is in good agreement through Tuesday night.
Differences then evolve with track of closed upper low as it dives
southeastward across Southwest Alaska and into the western Gulf
Tuesday night through Wednesday. This will affect timing of rain
over the Susitna Valley-Cook Inlet-Kodiak Island corridor. For
now, will utilize a mean position of the 12z models.

AVIATION...
PANC...Ceiling will gradually lower over the next 12 to 24 hours
as weak upper wave and surface front approach from the west.
Expect to bottom out at marginal ceilings, though some patchy
IFR clouds could make there way up Cook Inlet to the terminal.

SHORT TERM FORECAST SOUTHCENTRAL ALASKA...
A weak front that is pushing through southwest Alaska towards
Southcentral Alaska will be reinforced by a shortwave tonight
helping to bring rain to Southcentral. The majority of the rain
will stay over the mountains with some light rain at lower
elevations. Then on Tuesday night a low will drop down from the
north and skirt the western edge of Southcentral Alaska. As the
low passes it will bring more widespread rain to Southcentral
Alaska Tuesday night into Wednesday. Then it will track into the
Gulf Wednesday evening.

SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A frontal boundary continues to weaken and track into the
Southwest Mainland this evening, keeping some light showery
conditions over the area this evening. Dominant onshore flow along
with a short-wave disturbance moving in aloft overnight tonight will
keep ample moisture and renewed dynamic lift over the area. This
will act to re-invigorating the precipitation over the area,
keeping it in the forecast through wednesday afternoon. Look for
heavier precipitation in upslope regions aided by orographic lift.

Areas of fog are also expected to develop and move into areas
along the southwest coast, under the persistent onshore flow, a
weak surface low developing along the Western Capes Tuesday
morning, and ample amounts of moisture at the surface. Therefore,
look for the marine layer stratus to advect into the AOR, and
areas of dense fog to form over areas surrounding the Kuskokwim
Delta and Bristol Bay coasts.

SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The western Bering and the Aleutian Chain remains under the
influence of a high pressure ridge, which is accompanied by
marine layer stratus/fog. A front is tracking across the eastern
Aleutians/Bering and Alaska Peninsula this evening through
Tuesday morning. The biggest challenge remains with how widespread
the marine layer stratus/fog will be and how it will impact the
communities along the Aleutian Chain and Bering Sea. The Western
Aleutians will likely have dense fog issues in the morning time-
frame as the high pressure builds into the Bering by Tuesday
morning. Persistent zonal/westerly flow continues to support the
marine stratus/fog to continue advecting into the central Bering
region. Another wave of precipitation will round the top of the
Bering Sea ridge over the Northwest Bering and track over the
central/eastern Bering and Pribilof Islands by Wednesday morning.

LONG TERM FORECAST (Days 3 through 7).../updated at 700 PM AKDT
Mon Jul 24 2017/

The forecast for Thursday and into the weekend continues to be a
challenging one. The upper levels across Southcentral will be
dominated by a low that will practically anchor itself in the
southern Gulf of Alaska. At the surface, a weak ridge of high
pressure will try to nose into Prince William Sound this weekend,
increasing the chances for Turnagain winds. The overall pattern is
a bit of a sloppy one. With an influx of moisture and persistent
onshore flow, coastal areas should see showers leading into the
weekend. The Anchorage area may be spared with the usual
rainshadow/downsloping - but it is still too early to hone in on
those specifics. Long range guidance is beginning to hint at
another ridge of high pressure expanding across Canada and into AK
beyond the forecast period (e.g. next work week) meaning dry and
warm weather could be on the rebound. Out west across the Bering
and Western Alaska, continued summertime stratus dominating the
forecast. The biggest weather player of note is a well agreed upon
system that will dive towards NW Alaska from Siberia Thursday,
bringing a large area of rain with minimal thunder chances. All
guidance members are in excellent agreement with this feature and
forecast confidence into the weekend is high.


AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

$$

SYNOPSIS AND MODEL DISCUSSION...SEB
SOUTHWEST ALASKA...TP
SOUTHCENTRAL ALASKA...DK
LONG TERM...SS

&&



078
FXAK69 PAFG 242218
AFDAFG

Northern Alaska Forecast Discussion
National Weather Service Fairbanks AK
218 PM AKDT Mon Jul 24 2017

.DISCUSSION...
The 12z model suite initialized well on position of the low over
the Chukchi Sea but in general initialized around 2 mb to high.
Model spread remains low in the short and mid range and but
displays considerable spread in the long term as current zonal
pattern shifts to a more blocked pattern by the end of the week.


Aloft at 500 hpa, swift zonal flow continues over Alaska with a
weak shortwave over the Chukotsk Peninsula digging south and east
over the Bering Strait Tuesday and over the lower YK Delta Coast
Wednesday. The trough strengthens into an upper level low as it
moves south and east Bristol Bay Wednesday. An upper level ridge
builds back over the central and northern Bering Sea through
Wednesday. Weak flow aloft will develop over the interior Tuesday
and continue into the end of the week. Zonal flow will continue
through the week over the North Slope.

On the surface a 996 mb low over the Beaufort Sea will continue to
move to the east as the accompanying strong front continues to
sweep east through the interior. The front is currently located
along a Eagle to Fairbanks to McGrath line and will continue to
move east into Canada this evening. Rain intensity diminishes
behind the front over the central interior to scattered rain
showers, however wetting rain is developing again with another
system moving through the Bering Strait tonight and over the
western interior tomorrow. Generally less than one half inch of
rainfall is expected with this next system however favored upslope
areas of the western Brooks Range will pick up between one half
and one inch of rain. Just enough instability forecast for the
middle and upper Tanana Valleys and Fortymile country to produce
isolated thunderstorms Tuesday afternoon and evening.

On the North Slope High Surf Advisories will continue through
this evening for surge values in the 1 to 2 foot range and 25 mph
onshore winds west of Barrow. Conditions are expected to improve
this evening as winds diminish. Along the eastern North Slope
persistent west winds combined with storm surge values in the 1 to
2 foot rage will continue to produce high surf through Tuesday
morning near Prudhoe Bay and through noon near Kaktovik.


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

&&

.FIRE WEATHER...None


&&

.HYDROLOGY...
Small Stream Flood Advisory for the western Brooks Range remains
on track. Expect another half inch to one inch over the same areas
tonight and tomorrow as next system moves out of the Chukchi Sea
and over the Seward Peninsula.

&&

.AFG WATCHES/WARNINGS/ADVISORIES...
Wind Advisory for AKZ218.

High Surf Advisory for AKZ201-AKZ202-AKZ203-AKZ204.

Small Craft Advisory for PKZ220-PKZ240.

Gale Warning for PKZ245.
&&

$$

CCC JUL 17



000
FXAK67 PAJK 242305
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
305 PM AKDT Mon Jul 24 2017

.SYNOPSIS...High pressure persists over the Gulf into Tuesday. A
weak low pressure system and its associated weather front will
impact the southern Panhandle Tuesday and Wednesday.

&&

.SHORT TERM...Widespread low status continues to plague a wide
swatch of the gulf, which advances inland and into the inner
channel waters that have entrances to the open waters. Cloudy
skies with a fair amount of sunny breaks occurred over the
northern half of the Panhandle while the south had more clouds.
The outer coastal areas from Icy Bay down to Cross Sound area kept
the clouds all day and even had light precipitation fall.
Forecasting with a heavy hand on persistence this evening before
the low starts to influence our area, starting in the south
Tuesday and progressing northward into Wednesday.

With the advance of the low, the rain chances will increase in the
southern half of the panhandle through Wednesday with the
Ketchikan-Prince of Wales are receiving the most rainfall.
Staying with only chance POPs for the northern half as we reach
Wednesday.

I feel like a broken record with the winds being 15 kt or less,
but alas that is what continues. The exception also continues to
be a late afternoon/evening enhancement in Lynn Canal and Skagway
area. Directions will be changing significantly as the low moves
up from off the coast of Haida Gwaii to off the coast of Prince of
Wales.

Overall good model agreement between the ECMWF/NAM/GFS. Used the
GFS with more NAM when dealing with the precipitation moving in.
Slight divergence with the POP with the GFS being drier, while the
NAM is wetter.

.LONG TERM.../Thursday through Sunday/...As of 1030pm Sunday...
The upper level low that we`ve been watching for the start of next
week has strengthened in the models now to the point that there
is a surface low/trough reflection...where there used to be
weak/broad high pressure. The placement of the low should allow
for lower chances for showers across the far northern panhandle,
however cannot rule out isolated/scattered showers.

After that upper level troughs look to continue to pass over the
region with southerly onshore flow, thus keeping the potential for
showers in the forecast. At the surface, expect a very weak
pressure gradient over the region develops due to broad high
pressure while low pressure settles over the south central gulf.
Have decreased wind speeds over the inner channels some to reflect
the weaker pressure pattern. Will have to watch where the new low
tracks for next weekend.

Used a blend of the GFS/ECMWF and WPC for updates to this forecast
package. Forecast confidence is average.

&&

.AVIATION...Fairly widespread clouds over Southeast Alaska with
only a few breaks. Expect to see clouds lower overnight to the
MVFR with ceilings 1500 to 2500 ft. Clouds should be lifting
through the morning. A small front/wave looks to be lifting some
rain north into southern and central by late afternoon.

&&

.AJK Watches/Warnings/Advisories...
PUBLIC...None.
MARINE...None.

&&

$$

KV/Ferrin

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