937
FXAK68 PAFC 240009
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
409 PM AKDT Fri Jul 23 2021

.ANALYSIS AND UPPER LEVELS...

Continue to watch two low pressure areas, one over the northern
Bering, the other over the western Gulf, with high pressure
remaining over the interior. In the eastern Bering, gap winds of
around 20-30 kts are occurring with the northwest winds making it
to the surface. In the easterly flow between the high pressure
area and the low over the northern Gulf, a shortwave is moving
through from the eastern interior into Southcentral. Showers and
thunderstorms are occurring with this feature, with the activity
over Southcentral limited to light rain showers.

&&

.MODEL DISCUSSION...

Models are in fairly good agreement with the synoptic features,
although there are differences with how they handle the smaller
shortwaves. The main concerns continue to be how the shortwaves
in the easterly flow are handled and what kind of an impact, if
any, will occur in the Anchorage area which appears to be on the
cusp. All in all, can expect generally cool, wet conditions with
cloudy conditions and mainly scattered showers.

&&

.AVIATION...

PANC...Predominantly VFR conditions will continue through the TAF
period, although areas of light rain with brief MVFR conditions
aren`t out of the question. Turnagain Arm winds have, at least,
briefly redeveloped, albeit lighter than yesterday. While it will
likely weaken some later this afternoon, winds are expected to
increase again this evening, then should continue through the
early overnight hours.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: This
afternoon through Monday)...

A vertically stacked low is slowly moving through the southern
Gulf and will continue to bring rain to Kodiak Island before
tapering off Saturday afternoon as it moves south of the forecast
area. An easterly wave seen especially well at 700mb is slowly
moving across the Copper River Basin and through the Talkeetnas.
This wave is bringing widespread rain to the Copper River Basin
through the Talkeetnas and will move west into the Susitna Valley
tonight. Rain is also expected to move into the MAT valley and
Anchorage sometime near or after midnight although not as much
rain is expected as farther north. This easterly wave will begin
to stall out Saturday around the Susitna Valley and south towards
the Kenai Peninsula as an upper level ridge edges in across the
northern part of the forecast area. This will keep areas of rain
going through around mid-day from the Susitna Valley and south
towards Anchorage and the Kenai Peninsula. This wave will also
keep the area slightly cooler on Saturday. By Saturday afternoon
and evening the wave will begin to weaken with rain decreasing
across the area but the clouds should hang tough most of the day.
As we head into Sunday the upper level ridge will be centered over
the area roughly from the western Alaskan Range to the Talkeetnas
this will allow the area to see breaks in the cloud cover with
spotty showers mainly over the higher elevations and slightly
warmer temperatures. On Monday continued improvement is expected as
the ridge holds steady over Southcentral with a mix of sun and
clouds with more sun in the valleys and warmer temperatures. There
will be a chance of showers on Monday mainly over the higher
terrain, but most areas will remain dry.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Tonight
through Monday)...

Quiet weather is expected for much of Southwest Alaska through the
weekend and into early next week. Expect intervals of clouds and
some sunshine basically throughout the entire forecast package.
Some impulses of upper level energy will move along the western
Alaska Range south to the Aleutian Range through the remainder of
this afternoon and into this evening. There are ongoing rain
showers occurring now for areas around King Salmon north to
Iliamna and east of Pilot Point. These instability driven showers
are expected to continue into the evening hours before
dissipating overnight. A few other weak impulses may move across
the same areas Saturday and Sunday that may cause some rain
showers to develop but this time mainly over the mountains and
higher terrain.

Otherwise, areas of fog will yet again be a threat tonight for
the same areas due to low dew point depressions and due to the
ongoing rain shower activity currently. The one thing that may go
against this for tonight is the weak northerly flow for areas such
as Bethel and Iliamna. Unlike last night with very dense fog for
Iliamna, there will be a slightly more northerly wind component
tonight that might inhibit this, therefore, the forecast for
tonight is low to average confidence. Elsewhere, King Salmon and
along the AKPen, flow will remain more variable and light,
therefore, think this is the area with better risk for the fog
overnight/Saturday morning.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3:
Tonight through Monday)...

An upper level ridge continues to remain in place over the Bering
tonight lending to the usual low stratus and areas of fog
underneath it, dense at times. A transient upper level
trough/shortwave moves into the Western Aleutians this evening and
continues traversing eastward across the Aleutian Chain through
the weekend and into Sunday evening. As a result, expect an
increase in unsettled weather with periods of rain and small craft
advisory winds with low end gale force gusts along the Chain. The
strongest winds are expected to remain on the southern side of
the Aleutians Islands over the marine waters. Timing for these
winds is Saturday afternoon through Sunday afternoon. Behind this
transient shortwave, another upper level ridge will build in from
west to east across the Bering leading to more of the usual low
stratus and areas of fog, dense at times, in the Bering and around
Shemya.

&&

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

Bering/Aleutians

A front will move through the northern Bering Sea Tuesday
afternoon. Forecast confidence remains low at this time due to
differences in solutions with respect to the intensity of this
front. Winds may exceed gale force in the northern Bering, no high
seas are expected.

Gulf of Alaska

Expect winds less than 35 kts and seas less than 20 ft for the
Day 3 through 5 period.

&&

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

Looking ahead into the extended forecast, the latest operational
model runs are slowly coming into agreement with the evolution and
progression of the synoptic scale features for the work week but
diverge in the specific details towards the end of the period. The
general pattern will be a broad longwave trough extending from
eastern Siberia into the Bering Sea with a ridge of high pressure
aloft building across the southern mainland toward the Brooks
Range by the middle of next week. An upper level wave rounding the
base of the trough coupled with moist, southwesterly flow and a
surface front will bring cool and wet conditions over the Bering
Sea/Aleutians and Southwest Alaska, with some breaks in between.
With ridging aloft over the southern mainland, expect near or
slightly above normal temperatures under mostly sunny skies and
drier conditions. Additionally, model solutions have a cutoff low
meandering over the southern Gulf while a weak shortwave
associated with the Bering trough moves south across the eastern
Aleutians into the North Pacific and phases with the Gulf low.
Thereafter, increasing uncertainty emerges as there are
differences with the exact trajectory and strength of this low as
it begins migrating toward the north Gulf coast and the ridge axis
shifts east over the AK- Yukon territory. Subsequent model runs
will be needed to iron out these discrepancies including potential
areas of precipitation and unsettled weather returning across
Southcentral by late next week.

&&


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

&&

$$

SYNOPSIS AND MODEL DISCUSSION...NR
SOUTHCENTRAL ALASKA...RC
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MV
MARINE/LONG TERM...ED/MF

000
FXAK67 PAJK 232250
AFDAJK

Southeast Alaska Forecast Discussion
National Weather Service Juneau AK
250 PM AKDT Fri Jul 23 2021

.SHORT TERM.../Friday afternoon through Sunday night/... Satellite
imagery shows the remnants of the front that moved through the
panhandle today stretched from northern Lynn Canal over toward
Yakutat, moving slowly north and weakening. Radar shows numerous
light showers still moving onshore. Rainfall amounts from this
system so far ranged from little to nothing across the northern
panhandle to 0.5-0.75 inches for the southern areas. Temps stayed
in the upper 50s to around 60 with the cloud cover and precip.
Brief bouts of gusty winds were experienced with the frontal
passage, but most areas are seeing a decreasing trend in the winds
this afternoon. Exceptions to this are Skagway, Haines, and
Yakutat where what`s left of the front is just arriving, and
around Scull Island - where winds have been blowing near small
craft strength all day.

Tonight, the surface low over the central gulf will very slowly
drift south-southeast. There are several shortwaves that will
rotate into the panhandle around the low, focusing most of the
energy (and precip) across the southern 2/3 of the panhandle. The
first of these looks to enhance rainfall tonight. Precipitation
amounts around 0.5 inches are expected for the south with lighter
amounts as you move north. Another weaker wave is expected late
Saturday night into Sunday morning, and yet another on Monday.
Models are showing a dry period across the northern 1/3 of the
panhandle during the day on Sunday, with maybe even a few breaks
in the clouds. Consequently, the warmest temperatures in the short
term will likely be on Sunday.

The forecast challenges today included dialing in the timing of
the waves and fine tuning the areas of enhanced precipitation
potential with the arrival of each wave. This will continue to be
refined with newer model runs. That said, models were in good
agreement for the short term, and confidence in the forecast is
above average.


.LONG TERM.../Sunday through Friday as of 10 PM Thursday/... The
long term period features a low in the gulf which will meander off
the SE Gulf Coast and weaken into mid next week. The low
eventually fills as ridging builds over NW canada and Southern
Alaska by the middle of next week. Ensembles are all in general
agreement with this pattern; however deterministic models diverge
on the details of the evolution of the ridging. The EC was more in
line with the ensembles keeping systems in the gulf away from the
Panhandle through the week. The GFS brings in another system from
the west faster than most guidance, so chose not to use that
solution in the long term forecast.

As far as sensible weather, showers will be diminishing Sunday as
flow weakens and backs toward the east. Most recent guidance is
now hinting at a wave to rotate northward into the Southern
Panhandle Monday through Tuesday increasing shower activity from
south to north. Heaviest and most widespread shower activity is
expected to remain across the Southern two thirds of the area
with more scattered showers making their way further north. Expect
the driest portion of the Panhandle this week to be Upper Lynn
Canal as the mountains tend to wring out much of the moisture in a
southeasterly flow pattern. The latter half of the week should
feature another drying trend as flow becomes easterly and higher
pressure builds into the region. Still kept some PoPs in the
forecast as some models were hinting at easterly wave action;
however confidence is increasing in a drier later half of next
week.

Winds through the period remain relatively light, especially
through the Inner Channels. Winds 10 kt or less expected for much
of the weekend and into next week. If more sunshine and surface
heating occur in between showers, then stronger afternoon sea
breezes may occur in Icy Strait and Lynn Canal. An increase in
winds in Northern Lynn Canal is also possible mid next week as
surface ridging builds into the Panhandle; however confidence is
still low regarding the exact pressure gradient pattern associated
with the surface ridge.

Expect temperatures to remain close to climatological norms for
this time of year with smaller diurnal fluctuations due to
increased cloud cover and showers. With 850mb temps steady around
+3-5C and mostly cloudy skies, expect highs generally in the lower
60s with lows in the low 50s. Cooler temps likely across the south
Monday and Tuesday with more widespread showers. Breaks in the
clouds during the afternoon could lead to a few warmer spots,
especially across Upper Lynn Canal.

Used a blend of GFS/EC for the mid range transitioning to mostly
WPC/EC by mid to late next week. No major changes made to the
long term forecast.

&&

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...None.

&&

$$

BFL/CM

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