Small Craft Advisory 425, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
456 PM AKDT Thu May 21 2020

.ANALYSIS AND UPPER LEVELS...

A long-wave upper level trough extends from the Pacific through
the State of Alaska and across the Bering Sea and Aleutian
Islands. An upper low is embedded within this trough near the
central Aleutians. A series of short-waves can be seen in water
vapor imagery rotating around this main low center, and producing
areas of rain.

An active portion of the upper level trough lies south of the
Gulf. Satellite shows increased moisture streaming northward
across the Gulf of Alaska, right ahead of a frontal system over
the southwest Gulf/Kodiak Island. Steady light rain and small
craft advisory to gale conditions are focused ahead of this front
across Kodiak and the western Gulf. Radar imagery also shows rain
extending northward to portions of the Kenai Peninsula and up Cook
Inlet to the Susitna Valley. Meanwhile, strong low level
southeasterly flow ahead of the front is leading to downslope
drying in the lee of the Kenai and Chugach Mountains. Several
shortwaves embedded within the elongated upper level trough
spreads over the Interior, Bering Sea, and Aleutians, and these
features will continue to induce moisture and clouds for those
locations.

&&

.MODEL DISCUSSION...
Model agreement is pretty good with handling of large scale
features over the next couple days. There are minor differences
with track/strength of numerous short-waves rounding the low
center along the Aleutians. Based on the massive size of the
upper level circulation around this low center, these waves will
cross much of the Alaska region and help to focus precipitation.
There are also the typical model differences with depiction of
the strength of downslope flow and whether precipitation will
fall in the lee of the mountains. This is particularly true for
the western Kenai Peninsula and Anchorage. Generally prefer the
hi-res models (NAM12, NamNest, WRF, HRRR) which more accurately
represent the actual terrain and its influence. These solutions
all hint that these areas will likely remain dry due to the
strength of downslope flow, though there certainly could be
sprinkles/very light rain as upper level short-waves pass
overhead tonight through Friday morning.

All solutions indicate a new low developing over the northeast
Pacific on Friday and tracking across the Gulf and to Southcentral
Saturday through Sunday, however there are differences in the
track of the low which will impact the strength of winds over the
northern Gulf. However, no matter which solution you go with there
is a agreement on light low level flow over Southcentral and rain
spreading onshore. For now, will use an average position of the
models for the forecast.

&&

.AVIATION...
PANC...Strong low level east to southeast flow ahead of a front
lifting northward across the Gulf will largely maintain downslope
drying at the airport, which will keep ceilings/vis solidly in the
VFR category. A series of upper level short-waves will track
overhead between about 09z and 21z, so there could be some
sprinkles and a temporary short-lived lowering of the ceiling
below the 5000 ft threshold.

Otherwise, the Turnagain Arm wind will continue to produce periods
of gusty southeast winds at the terminal. It looks like the wind
will follow a diurnal trend, strengthening each afternoon and
evening in line with the thermal trough over the Alaska interior,
then dying down overnight. The Turnagain Arm jet does look weaker
on Friday, thanks to a weakening front over the Gulf. Thus, would
expect marginally weaker winds at the terminal as well.

&&

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

A front lifting inland across Southcentral will bring unsettled
conditions through the weekend. Southeasterly winds will continue
through the evening hours bringing gusty conditions to the usual
places including Portage Valley, Turnagain Arm, Knik River Valley,
and Copper River Valley. Winds will gradually taper by Friday
morning and then ramp up again Friday afternoon before diminishing
early Saturday morning. Rain will continue along the northern
Gulf coast through most of the weekend. Midlevel southeasterly
flow is expected to keep most of the Anchorage Bowl and
northwestern Kenai Peninsula on the drier side, though increasing
amounts of moisture will allow for brief passing showers Friday
and Saturday. By Saturday evening, the midlevel southeasterly flow
shifts to become more southerly, allowing the downsloping effect
to weaken, thus increasing chances of rain are likely late
Saturday into Sunday.

&&

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

An unsettled weather pattern will continue for Southwest Alaska as
a low pressure system remains cutoff from the main flow of the jet
stream. As a result, multiple waves of upper level energy will
continue to rotate around the aforementioned low bringing a
showery pattern to the area. The Kuskokwim Mountains, Alaska
Range, and Aleutian Range will experience the most persistent
rainfall as the flow remains southeasterly with plenty of moisture
being streamed northwestward from the Pacific Ocean. This moist
air will be pushed up against the mountains and lifted (a process
called orographic lifting) which will help to squeeze even more
moisture out of the atmosphere along the eastward facing slopes.
On the west side of the mountains, downsloping will prevail which
will keep these sides of the mountains drier, though, a periodic
brief light shower or sprinkle cannot be ruled out. A more potent
upper level wave is expected to pass through the region on Sunday
which may bring a more widespread rain as this upper level
disturbance provides more lift to the atmosphere.

&&

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

A vertically stacked low will continue to meander in the Bering
leading to unsettled weather. Showery conditions will continue
throughout much of the Bering and the Aleutian Chain for most of
the forecast period. However, a potent piece of upper level energy
is expected to develop on the southeast side of the main low and
then move slowly northward and then west as it rotates around the
low near the Pribilofs. As a result, PoPs were increased
significantly to account for this piece of energy with the steady
rain anticipated to move through the Pribilofs Saturday morning.
The winds were also upped to small crafts in Bristol Bay and the
waters around where the upper level shortwave progresses.

&&

.MARINE (Days 3 through 5/Sunday through Tuesday)...

Conditions look quiet through the period and overall confidence in
the forecast is high.

&&

.LONG TERM FORECAST (Days 4 through 7/Monday through Thursday)...
Starting the extended forecast period on Monday, the pattern will
be very similar to now with a trough and closed low over the
Bering Sea and ridging over the Yukon and northern Alaska. Under
this pattern, expected continued mostly cloudy skies with areas of
light rain or showers as a plethora of short-waves rotate around
the Bering Sea low. This far out, it's hard to pinpoint the most
likely areas for rain.

The pattern will shift as we head to mid week, with the Bering
low exiting northward and two new systems coming into the picture:
one low over the northwest Pacific tracking toward the Aleutians
and a second larger and deeper low over the northeast Pacific
tracking toward the Alaska Peninsula. Model spread with the track
of these systems is fairly large, so overall forecast confidence
is low for the Wednesday through Friday time period. Considering
all of the solutions, the most likely area for active weather
(wind/rain) is the southern tier of the forecast area, along the
Aleutian Island chain, Alaska Peninsula, Kodiak Island, and the
Gulf of Alaska. Southwest and Southcentral Alaska should see some
sort of break mid week, with more sunshine and warmer
temperatures under the influence of ridging. As we head later in
the week, the system somewhere in the vicinity of the Alaska
Peninsula could start to impact these areas with clouds and rain.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE: 130 131
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...CB/SEB
SOUTHCENTRAL ALASKA...KO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MV
MARINE/LONG TERM...SEB