Small Craft Advisory 411, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
434 PM AKDT Sun May 21 2017

.ANALYSIS AND UPPER LEVELS...
A high amplitude pattern is in place with a trough over the
northeast Pacific and downstream ridge along the west coast of
North America. Deep southerly flow has produced an atmospheric
river which is pointed right at southeast Alaska. Meanwhile, the
combination of an upper level low over the western Gulf, a short-
wave moving onshore along the northeast Gulf coast, and a surface
low approaching Prince William Sound is leading to widespread
rain across Southcentral. The available moisture and upward
vertical motion is not quite as impressive as over Southeast
Alaska, so the rain is generally light, but steady. Rain intensity
will increase along the north Gulf coast this evening as the low
reaches Prince William Sound and southerly upslope flow sets in.
There will also be a deformation band on the back side of the
surface low, so there should be a band of heavier rain across the
western Kenai and Cook Inlet.

Meanwhile, weather is much quieter over Southwest Alaska with a
mix of sun and clouds and a few showers. Similar to yesterday, the
combination of day-time warming and a short-wave on the western
side of the upper level low will lead to increased instability in
the afternoon/evening hours. Thus, expect scattered showers and
isolated thunderstorms, especially over inland areas.

Out west, a 140kt+ upper level jet continues to drive a trough and
surface cold front eastward across the Bering Sea and toward the
Southwest Alaska coast. Fairly cool air behind the cold front is
leading to instability off the surface. Thus, winds across the
region are quite gusty in nature. It's a fairly cold rain along
the front with temperatures in the upper 30s to lower 40s.

&&

.MODEL DISCUSSION...
Numerical forecast guidance is in very good agreement with the
significant features over the next few days and forecast
confidence is generally high. The primary forecast challenge
is capturing all of the local detail in a progressive pattern.
Specifically, there will be a lot of fairly strong and gusty
winds over Southcentral and coastal waters over the next few
days. A combination of pattern recognition and hi-res models
will really help hone in on where and how strong these winds
will be.

&&

.AVIATION...
PANC...The primary forecast challenge is determining how low
ceilings drop tonight. There is some weak low level cross-mountain
flow, but it slowly shifts around to south by 06z, so marginal
ceilings are likely. Ceilings will lift during the day Monday as
precip tapers off, but instability will lead to strengthening
southerly winds, both up Cook Inlet and out of Turnagain Arm.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
It's looking like a very cool and wet pattern over the next few
days, with a bit of a break in the steady rain Monday night
through Tuesday night. The upper low over the western Gulf will
open up into a trough and lift northward across Southcentral
tonight, driving the surface low onshore across Prince William
Sound. This will cause low level flow to shift around to southerly
which will enhance rainfall intensity along the north Gulf
coast/Prince William Sound as well as the northern Susitna
Valley/Copper Basin. Rain will begin to gradually taper off across
Southcentral on Monday, lingering longest in the upslope areas
mentioned. In addition to the widespread rain, also expect fairly
widespread gusty southerly winds to develop overnight tonight as
the surface low heads inland and higher pressure builds in behind
it. Low level instability will also lead to stronger winds mixing
down to the surface, so expect strengthening winds during the day
Monday as temperatures begin to warm up a bit.

The upper trough and surface front crossing the Bering Sea will
reach Kodiak Island Monday morning, but will be in the process
of elongating and weakening. Thus, after bringing some rain to
Kodiak Island don't expect more than some showers as the upper
trough crosses Southcentral Monday night through Tuesday, mainly
over inland areas.

More significant rainfall is on the way for mid week as a trough
digs over the north central Pacific and downstream ridge builds
over the Gulf. This will set up another atmospheric river, this
time pointed at the southern mainland of Alaska. Expect rainfall
to move into Southcentral Tuesday night into Wednesday. Strong
low level southerly flow will re-develop resulting in periods
of moderate to heavy rain for much of the region. The one big
exception will be the heart of the Copper River Basin which
will remain dry due to the southerly downslope flow. The surface
pressure gradient will also support gap winds in the usual places
(Turnagain Arm/Knik River Valley/Copper River).

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Showers will continue to develop through the early evening hours
across Southwest Alaska as an upper level trough situated along
the west coast provides just enough upper level support to support
widespread showers. In addition, clearing skies across the Lower
Kuskokwim Valley will allow for some destabilization east of Aniak
and west of Sparrevohn which will likely create isolated
thunderstorms during the later afternoon/evening hours. Today
should be the last day for thunderstorm potential as a rather
impressive pattern change is on the way. Cloud cover will engulf
Southwest Alaska on Monday ahead of an amplifying trough over the
Bering Sea which will help bring period of moderate and
potentially heavy rainfall to the region Tuesday night through
Wednesday. Rainfall amounts around half of an inch will be likely
for most locations across Southwest Alaska with upwards to an inch
likely along upslope and coastal locations.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A frontal boundary will continue to push across the eastern
Bering Sea this evening before dissipating along the west coast of
Alaska. Periods of light to moderate rain will be seen along the
Alaska Peninsula through early Monday morning. Gusty winds along
the back side of the front will be seen across the eastern
Aleutians and Alaska Peninsula with winds gusting upward to 30 to
35 mph at times. The next system to impact the Bering Sea has
already begun to take shape just to the east of Kamchatka. First
off, the upper level trough over the Bering Sea is expected to
amplify as a result of an upper level low pushing off of
Kamchatka. This will result in a surface low tracking across the
western Bering Sea on Monday which will help push a frontal
boundary into the central Bering Sea by Monday night. A second low
pressure system will form just south of Adak early Tuesday
morning which will tap into some very moisture rich air which will
bring periods of moderate to heavy rainfall to much of the
Aleutians on Tuesday before quickly tracking eastward.

&&

.LONG TERM FORECAST (Days 4 through 7)...
An amplifying trough digging through the Bering sea and Aleutian
chain will continue to shift eastward Wednesday morning, with a
blocking ridge building over the Gulf of Alaska slowing the
eastward progression of the trough. This will keep a steady supply
of wet and windy conditions over much of the southern mainland
through the end of the week. Models continue to come into better
agreement with bringing this unseasonably moist airmass (based on
forecasted precipitable water values) to mainland Alaska.

One of the biggest struggles in the extended forecast will be the
timing of when high pressure builds back into the gulf and if/when
showers over the southern Mainland tapper off. Latest model runs
suggest this wet pattern will begin to dissipate during the
Memorial Day weekend, as models remain firm on bringing the ridge
of high pressure over the Northern gulf. However, there still
remains some uncertainty as these models are showing a chance of
some showers making it over the ridge and into the cook inlet
region Sunday. If this ridge builds into the area stronger than
currently progged, there remains some potential to bring warmer
and drier conditions to Southcentral Alaska over the weekend and
into Monday. Given the current model discrepancies, forecast
confidence remains low in a completely warm and dry scenario.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 174 176.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SB
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MC
LONG TERM...PEPE