156
FXAK68 PAFC 171334
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
534 AM AKDT Wed Jul 17 2024

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

Discussion:

The large complex low continues to spin over west-central Alaska as
it meanders inland before progged to start dropping south by late
tonight into Thursday morning. While this low has been the main
driver of the unsettled weather pattern we have seen over the past
week, a change in the pattern is looking more certain by this
weekend. For this morning, a shortwave trough rounding the southeast
periphery of the parent low brought a few showers to the eastern
Kenai overnight and is now lifting towards the Talkeetna Mountains
and southwestern Copper Basin. As such, may see a few rain showers
across the Susitna Valley and Copper Basin through mid-morning
before the wave moves too far east. However, the Susitna Valley will
not have too much of a break before the upper low begins to drop
south with increasing chances of more widespread rain developing
across the Susitna Valley this afternoon and spreading southward
with time.

Models have struggled the past few days on how to handle this upper
low, but have finally come into good agreement with taking the
southern track down towards the southern Cook Inlet by Thursday
afternoon. A low moving across the central Aleutians this morning
will phase with the trough from the western Alaska low as it moves
into the southwestern Gulf. This low will then become the dominant
low with our southward diving low tracking over Kodiak Island before
rounding the the southern half of the Gulf low as a shortwave
trough. Models are also agreeing that a band of rain will develop
before transitioning to more of a broad deformation band across the
western Kenai up through the Mat-Su Valley as the low moves towards
the Cook Inlet. Highest precipitation amounts look most favorable
for the northwest quadrant of the Kenai, Anchorage Bowl, and into
the Mat-Su Valley where a quarter to a half inch of rainfall is
possible through Thursday night. However, even a slight deviation in
the track of the low will shift the area of heaviest rainfall.

Looking into the weekend... The new Gulf low will set up residency
placing much of inland Southcentral in a warmer, and somewhat drier
pattern. The warmer temperatures filtering in from western Canada
will begin to increase instability over the Copper Basin. With low
spinning over the Gulf, easterly waves will also move through
setting the stage for the return of more convective type weather
pattern for the Copper Basin.

- PP

&&


.SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days
1 through 3: Today through Friday)...

Conditions continue to look fairly benign in the short-term, with
weather trending warmer/drier in Southwest Alaska versus
windier/wetter across the Bering Sea and Aleutian Islands as we
head towards the end of the week. Some uncertainty persists with
the timing of rainfall and thunderstorm chances, but forecast
confidence generally remains high.

Diving into the details...the current weather pattern features a
longwave trough over Mainland Alaska and the Eastern Bering Sea,
while a longwave ridge extends across the remainder of the Bering
Sea and Aleutian Islands. Shortwave troughs embedded within the
longwave pattern will bring rain across Southwest Alaska through
today, with rain gradually tapering off from west to east. With a
robust upper low moving across the Western Alaska Range, there
remains a slight chance for isolated thunderstorms in the
afternoon and evening. While the energy associated with this upper
low could invigorate convection, thermal instability looks
relatively poor, especially with lingering cloud cover from the
this morning`s rain. As such, thunderstorm chances remain quite
marginal. By Thursday, the approaching ridge of high pressure will
lead to gradually warming temperatures and a gradual suppression
of rain. Daytime highs climb into the 60s and low 70s by Friday,
with higher temperatures likely over the weekend.

For the Bering Sea and Aleutian Islands, the building ridge will
likely lead to widespread fog and low stratus today as moisture
becomes trapped near the surface. Fog is inherently difficult to
forecast and we forecasters tend to have a low bias (i.e. we
under-do fog coverage and severity). Therefore, be aware that fog
may become more widespread and thicker across the Bering Sea and
Aleutian Islands than is currently reflected in the forecast. By
tomorrow, an occluding low moves in from the west, leading to
windier and wetter conditions. Forecast confidence is high that
winds will remain small craft speed or lower. The front gradually
pushes eastward through the remainder of the workweek, reaching
the Pribilof Islands by Friday night.

-KC

&&


.LONG TERM FORECAST (Days 4 through 7: Saturday through Tuesday)...

Much of the long-term period will be characterized by an omega
pattern. A large upper-level low will be present in the Bering Sea
with another large upper-level low in the North Pacific in close
proximity to the Gulf. A high pressure ridge will be between
these two features. This type of pattern will keep most of the
Bering Sea, Aleutian Chain, and southern Alaska Peninsula active
as numerous shortwaves rotate around the Bering low. The ridge,
initially over Southwest Alaska Saturday and the first half of
Sunday looks to shift eastward towards Southcentral Alaska while
it weakens. As the ridge shifts eastward and weakens, this will
open the door for a frontal system to move towards Southwest
Sunday night. There are still questions on the strength of the
front and whether or not it holds together enough to deliver
precipitation to the interior locations. The coast has the best
chance at seeing rain from this system. Conditions look to remain
showery and unsettled over Southwest for Tuesday as additional
shortwaves rotate over the area from the Bering.

As for Southcentral Alaska, the North Pacific/Gulf low will help
to drag in easterly waves from Yukon Canada over the area. This is
a good pattern for interior locations of the Copper River Basin
and the Susitna Valley to receive precipitation. With riding
nudging westward from the Yukon, temperatures also look to warm
over Southcentral relative to what is being observed in the short-
term period. Depending on day-time heating and instability,
potential thunderstorm development is not out of the question
across Southcentral each afternoon Saturday through Tuesday.
However, confidence is extremely low with where exactly each
mesoscale easterly wave will track.

&&

.AVIATION...

PANC...MVFR/VFR conditions are expected to continue through at
least 03Z with the potential for IFR ceilings overnight Wed.
A few light rain showers are possible this morning with more
widespread rain setting up by 03-06Z.

&&


$$

429
FXAK67 PAJK 171347
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
547 AM AKDT Wed Jul 17 2024

.SHORT TERM.../through Wednesday night/...The next surge of
moderate to heavy rain has begun. Still expecting upwards of 2 to
3 inches of rain over 24 hours for areas along Icy Strait
(including Gustavus, Hoonah, and Juneau), and south to near
Petersburg. Much lighter amounts, around an inch or less,
elsewhere.

After the center of low pressure moves out of the area, SE AK will
transition to the backside of the low which will push the rain
east and southeast. As this happens, rain rates will decrease,
cloud cover will decrease, and wind speeds will decrease.

.LONG TERM.../Thursday through Tuesday/...
Overview: A relatively quite weather period is likely for SE AK. A
number of breaks are in the mid to long range, not exactly dry but
not the intense rainfall and stronger winds we have been seeing.

Mid level low with trough extended into the AK Gulf moves eastward
then drops to the south into the weekend. As part of a blocking
pattern, to the west a ridge then another low, this mid level
pattern will shift in position somewhat by likely not change until
late next week. For Thursday remnants of the Wednesday front will be
over the southern region keeping rain going. Points north of
Frederick Sound not have higher chances of rain breaks that day.
With the 500 mb low in place flow pattern will move a low extending
from a deeper system over the far SW Gulf northward into the SE Gulf
by Friday. For now winds around the low expected to be in the 20 kt
range with lighter winds to the north. Light to moderate rainfall
amounts move in with the system with most models keeping precip over
the south with diminishing rates as the low moves northward. GFS
tracks furthest north with highest winds and pop. By late Saturday
this low will have dissipated, but the main low remains in place
into next week and thus can keep pushing precip bands over the
panhandle. The southerly flow and weather breaks will let day time
highs pick back up into the upper 60s to low 70s. With the run to
run inconsistencies seen that past few days not fully jumping on any
one solution so overall keeping with SE AK climatology: mostly
cloudy chance of rain.

&&

.AVIATION...Conditions deteriorate through the morning hours on
Wednesday as another system arrives. Anticipate widespread IFR
and LIFR conditions arriving, along with LLWS for some locations.
Gusty surface winds are expected. Conditions will begin to improve
during the afternoon and evening hours, especially across the
Northern panhandle, as the wave departs.

&&

.MARINE...Main marine concerns are centered around the quick
moving low that will move through the panhandle today. Early this
morning, wind speeds ahead of the low are around 5 to 15 knots out
of a general northerly direction. But as the low moves north,
these winds will quickly shift to the south and pick up speeds to
around 15 to 25 knots.

The timing of that switch will be closely watched but for now,
expecting marine zones south of Icy Strait to see the switch this
morning. Areas along and north of Icy Strait look to see the
switch late this morning to early afternoon.

After the low clears out, wind speeds will decrease, to around 5
to 10 knots, this evening into overnight from south to north. The
lighter wind speeds will last into Thursday.

&&

.HYDROLOGY...The forecasted rain amounts of 1.5 to 3 inches for
the area today and the saturated soils will cause rivers and
streams to rise rapidly. There is good ensemble agreement on this
heavy rain event with 50 to 70% chance the area will see the
forecasted rain amounts and these values are very anomalous for
this time of year. With that said there are flood advisories out
for the Chilkat River area, Taiya River area and the Mendenhall
valley area for minor flooding through Thursday morning. Flooding
is expected to develop through Wednesday afternoon and persist
into the evening time frame. A flood watch is still in affect for
the full Juneau borough as other non gauge rivers/streams that may
flow out of their banks. For other areas outside of the flood
advisories, there is a special weather statement out for
significant river rises but not flood is expected for those areas.
Some of the large rivers may stay above flood stage longer due to
the lag in draining all the rainfall runoff and melted ice.



&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Watch through late tonight for AKZ325.
MARINE...Small Craft Advisory for PKZ011>013-021-022-031>036-641-642-661-
662.

&&

$$

SHORT TERM...GJS
LONG TERM....PRB
AVIATION...GFS
MARINE...GJS
HYDROLOGY...GJS

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