000
FXAK68 PAFC 201211
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
411 AM AKDT Sat Jul 20 2019

.ANALYSIS AND UPPER LEVELS...

The upper-level pattern remains much the same as previous days,
with much of southern Alaska situated underneath a col or area of
weak flow between two upper-level lows positioned over the Gulf
and Beaufort Sea and two ridges positioned over the Bering and
Yukon. A weak shortwave is attempting to pierce the region of the
col from the northwest but is weakening and shearing apart as it
moves over the Alaska Range. This feature is expected to weaken
further and become absorbed into the flow around the main low over
the Gulf.

At the surface, a weak surface low well south of Seward is
advecting a more stable, marine airmass along with low stratus
and areas of rain from Cordova and Prince William Sound south and
west across the eastern Kenai Peninsula and Kodiak Island.
Farther inland, an unstable airmass, slightly moderated by a
cooler, southerly flow aloft, produced another round of isolated
showers and thunderstorms across the Copper River Basin as well as
over portions of the Kenai and Talkeetna Mountains yesterday.
These areas of convection, however, diminished quickly following
sunset. More organized convection did linger through the overnight
hours over the Alaska Range, aided by the upper-level support
from the aforementioned shortwave.

Elsewhere, low clouds and onshore flow continues across southwest
Alaska as conditions remain stable underneath the ridge. Gusty
northwesterly winds across the Pacific coast of the AKPEN are
slowly winding down as the pressure gradient between the ridge and
trough relaxes. Farther west, widespread stratus and areas of fog
linger as the ridge extends across the Bering and Aleutians.

&&

.MODEL DISCUSSION...

Models remain in good agreement with the overall synoptic
picture. The biggest challenges in the short term continue to be
the extent of low cloud cover across Southcentral and the
potential development of a marine layer advecting up Cook Inlet
this morning. Most guidance shows low-level moisture advection in
association with up-inlet flow lending credence for the
development of marine stratus; however, the evolution of this
remains to be seen as there is an absence of this anywhere in the
Cook Inlet this morning. The challenges in regard to the extent of
cloud cover and possible marine layer development will also have
a cascading impact on not only temperature but smoke transport as
well. Therefore, confidence in these forecast parameters is low.

Beyond the weekend, guidance is signaling the return of
widespread shower and thunderstorm activity south and east of the
Alaska Range as the airmass destabilizes and easterly shortwaves
propagate across Southcentral Alaska. There is average confidence
in this solution given overall model agreement. Differences are
with the timing of these shortwaves moving across Southcentral.

&&

.AVIATION...

PANC...General VFR conditions will prevail with southerly winds
continuing through the morning hours and ceilings remaining at or
above 5,000 ft. The challenge to the forecast remains the extent
to which smoke and low cloud cover reaches the airport complex as
a result of the enhanced up-inlet flow, mainly during the morning
hours. The Turnagain wind and scattered cloud cover overnight has
resulted in a weaker surface inversion as compared to previous
nights, so the thinking remains that any smoke advected over the
terminal will not be thick enough to reduce visibility below VFR.
Still, there is a slight possibility that smoke may reduce to 6SM
at times this morning once the Turnagain winds decrease.

The previous forecast mentioned the potential for a marine layer
to move up Cook Inlet this morning. This also looks less likely
as satellite imagery shows a lack of marine stratus. Therefore,
any indication of low clouds has been kept out of the 12Z TAF
package.

&&

.FIRE WEATHER...

Southcentral...Fairly cloudy conditions are expected over the
area through tonight along with some isolated to scattered
showers due to a low pressure system in the Gulf. This will result
in cooler daytime temperatures and higher humidities. Skies will
begin clearing on Sunday as the low moves south and high pressure
moves in. The atmosphere will be fairly stable, with only the
Copper River Basin having a slight chance of thunderstorms today.

Southwest...Fire weather concerns remain minimal over southwest
Alaska through the weekend as the ridge maintains its presence
over the region with continued cloud cover, weak onshore flow and
cooler temperatures.

&&

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

A low pressure system in the Gulf will push moisture into
Southcentral today. This will result in cloudier and cooler
conditions along with some isolated to scattered showers. The low
will push off to the south tonight, allowing high pressure to
build into the area. Showers will come to an end and skies will
slowly clear on Sunday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3; Saturday
morning through Tuesday morning)...

A weak upper level ridge over the eastern Bering has begun to
slowly drift eastward this morning. Meanwhile at the surface, the
ridge is centered over the central Aleutians, allowing for
onshore flow across the southwest coast. This, combined with
plentiful moisture, will allow for low level clouds and fog along
the coast through the morning hours today. Light showers are
possible late Saturday overnight into Sunday, though any chance of
precipitation will favor the coastal areas. As the ridge travels
eastward through Tuesday, chances of showers diminish, though low
level fog and stratus will continue to be the forecast challenge.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3;
Saturday morning through Tuesday morning)...

Northwesterly flow over the eastern Aleutians and Alaska Peninsula
will subside this morning as the ridge of high pressure in place
over the eastern Bering slowly drifts eastward. The next two
features of note include a North Pacific low and a Siberian low.
First, the North Pacific low approaches the central Aleutians and
lifts a front over Atka Sunday morning before moving eastward
Monday. While the low center remains south of the Aleutian Chain,
precipitation will spread from west to east, reaching Unalaska by
Monday evening. Secondly, an upper level trough currently over
Siberia supports the strengthening of a surface low through Monday
morning. While the low center will not enter the Bering, the
front associated with the low is expected to approach the western
Bering by Saturday evening. This will bring a broad swath of
southwesterly small- craft winds to the far western Bering waters.
By Sunday morning, winds increase to gale-force as the front
lifts into the northwestern Bering Sunday afternoon. Winds
diminish by Monday as the front weakens. Low level stratus and
areas of fog across the Bering are likely to persist under this
pattern through Tuesday.

&&

.MARINE (Days 3 through 5; Monday through Wednesday)...

Sub gales are expected to persist through Wednesday over the Gulf
of Alaska waters. A front moving into the western Bering Sea
Monday may result in near gale force wind west of 180.

&&

.LONG TERM FORECAST (Days 4 through 7; Tuesday through Friday)...

Long range models are generally in good agreement through
Wednesday then begin to diverge. The overall general concept of
low pressure in the north central Bering Sea with a trough over
the Bering and moist onshore flow into western Alaska is
consistent between the models. The Bering Sea and Aleutians will
see a continuation of lows tracking across the region.

The mid to long range for the Gulf and southern mainland looks to
be under generally weak low pressure over the Gulf with weak
ridging over the southern mainland. The models are in good
agreement with the upper level high moving into the Yukon
Territory with an upper level low over the Gulf of Alaska. This
pattern is indicative of a return to more seasonal temperatures
and precipitation over the southern mainland for the middle to
later part of next week.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE... Gale warning 185 411
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...SA

542
FXAK67 PAJK 201441
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
641 AM AKDT Sat Jul 20 2019

.SHORT TERM.../Through Sunday night/...A weak ridge is situated
over Southeast Alaska and a broad area of low pressure with
multiple low level circulations spans the Gulf of Alaska. Aloft, a
large upper level low is located over the Gulf. The Polar Jet
remains well north while the Sub-Tropical Jet is well to the
south. A minor enhancement of easterly upper level winds exists
around the northern periphery of the upper low and easterly wave
clouds are evident in satellite imagery. As with yesterday,
convective conditions exist aloft, while solidly stratiform
conditions exist near the surface with trapped low level moisture.
Low ceilings and light winds are expected once again today.
Exception may be Lynn Canal, where a southerly wind between 10 and
20 kts continues. Periods of light rain and/or drizzle will occur
over most of the panhandle today, with drier, but continued
cloudy conditions overnight. Sunday should be a little better with
light precipitation mainly over the southern zones and along the
coast mountains up to about Tracy Arm.

Biggest change to the short term forecast was to go with a `high
PoP, low QPF` depiction in grids to reflect another day with
periods of light rain and drizzle. Changes to high temps were
made using bias corrected National Blend and GFS. No significant
changes to overnight lows. Overall forecast confidence is average.

LONG TERM.../ Monday to Saturday as of 9 pm Friday / Aloft the
general pattern is fairly similar but not progressive and not
supporting of strong features to spread across the area. An upper
level low west-southwest of Haida Gwaii roughly near 50N/140W will
weaken and drift closer to Haida Gwaii through the week. Models
diverge late week on the track of the low and how fast it will
move out.

Surface pressure pattern over the gulf and northwest Canada shows
very little in way of any pressure gradient, with weaker pressure
inland, which has been supported by the thermal heating leading
to lower pressures. By the end of the week, models try to show
the upper low in the gulf pushing off into Canada as weak ridging
builds over the northeast Gulf and Panhandle. Confidence remains
low through the end of next week as there is large model spread
in timing and placement of synoptic features.

Kept pops in the 20 to 40 percent range through the week using a
blend of the GFS/SREF for pops on Monday. No significant weather
systems are anticipated through next week. Easterly waves moving
in from the east around the upper low in the Gulf can increase
pops for certain areas next week, however at this time there is
low confidence in timing and placement of these waves. Daytime
highs are still expected to slowly climb for much of the panhandle
into the 70s for Monday and Tuesday. Slightly above average
temperatures expected for the rest of the week. Lows will not get
that cool with lows from the upper 40s to lower 50s.

&&

.AVIATION...Light rain or drizzle is adding to the trapped
moisture for most of the Panhandle. Driest areas are N Lynn Canal
and southern terminals - PAKW and PAKT, Although there are some
low clouds there, too. A few areas of LIFR so far. Not looking
for a lot of improvement later today.

&&

.MARINE...Earlier, N Lynn Canal kept going at 20 KT, but is now
laying down. Considering that it may ramp up again this afternoon.
If you are working on the outside the new model runs are predicting
the surface ridge building over the Gulf again by Sunday and
Sunday Night. Look for westerlies with not to much wind but 4 to 5
FT seas with a short period.

&&

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

&&

$$

Fritsch/JCC/Bezenek

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