Small Craft Advisory 451, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
500 AM AKDT Sat May 30 2020

.ANALYSIS AND UPPER LEVELS...

This morning's upper level layout is anchored by 2 distinct upper
level lows. The first of which, is the low straddling the southern
Gulf and Northeast Pacific. This well stacked feature has been a
mainstay for the past 48 hours and continues to produce the
easterly flow pattern that is responsible for the afternoon
showers and convection that we've seen the past couple days.
Immediately to its west over the western AKPEN is an area of high
pressure that marks the divide for our second upper low out in the
Northwest Pacific. This system possesses more baroclinicity and
continues to gain support from a jet streak moving along 44N that
is helping to deepen the troughing. It is also spinning up a
secondary wave over Dutch Harbor, which has seen 35 kt winds and
some rain this morning.

&&

.MODEL DISCUSSION...

Synoptic scale features are aligning between models for the first
48 hours and warrant minimal concern. Continuing to address the
details in showers and convection will demand the most attention.
Monday's model outputs seem to see a bit of separation. This
appears to be a result of the Aleutian low tracking into the Gulf
and lifting a front into the north Gulf coast. The same general
easterly flow pattern remains, but shortwave disturbances are showing
slightly different solutions.

&&

.AVIATION...

PANC...VFR conditions and light winds will start the day. Light
rain will develop midday and gusty southeasterly winds late
afternoon at terminal.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Monday night)...
Today will be the beginning of a transition to cooler and wetter
weather across Southcentral. An "easterly wave" - the name given
to an upper level wave moving in from Canada - will track across
the southeastern Copper River Basin, Prince William Sound, and the
Kenai Peninsula today, bringing steady rain. The thermal trough
will be pushed northward toward the Alaska Range. Instability
combined with some influence from the easterly wave will lead to
showers and isolated thunderstorms from the northern Copper River
Basin across the Talkeetna Mountains to the Susitna Valley.
Anchorage and the Matanuska Valley look like they will remain
right on the edge between the area of steady rain to the south
and showers to the north, making for a difficult forecast. There
may be some short periods of light rain, but generally expect
drier conditions for these areas.

Additional weak upper level waves will propagate westward across
interior Southcentral tonight through Sunday. The closed low over
the Gulf will then open up into a trough and lift into the Yukon.
At this point, waves moving through the trough will take on more
of a northeast to southwest motion across the Copper River Basin.
Therefore, expect the Copper River Basin and nearby areas to see
the bulk of rain Sunday through Monday. There is a chance of rain
or showers elsewhere in Southcentral, but forcing looks weak, so
it doesn't look like a washout by any means. The thermal trough
will continue to be pushed north and west, with a much cooler air
mass settling in over Southcentral. Thus, expect below normal
temperatures Sunday and Monday, with highs topping out in the 50s
for most areas.

Meanwhile, a low tracking south of the Aleutians will send a
front into the southern Gulf Sunday night through Monday. The
front will be elongating as it arrives, so northward progression
will be slow. The latest model guidance indicates that an upper
level short-wave will induce development of a low over the central
Gulf. Confidence is low, but generally expect stronger winds and
rain across the Gulf and Kodiak Island in response to the front
and low.

&&

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

High pressure continues over southwestern Alaska with a thermal
trough rounding the Alaska Range to the Kuskokwim Delta. This
trough is forecast to be the strongest this afternoon, bringing
chances for wet thunderstorms to the middle Kuskokwim Valley,
Kuskokwim Delta and west of the Alaska Range. After this, the
trough will move northward pushing chances for thunderstorms to
the northernmost parts of our area of responsibility for Sunday
afternoon. On Monday, expect increased chances for precipitation
across the southwest, although, timing and location of this
rainfall remains a forecast challenge.

&&

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

A front associated with a low to our northwest is currently
stationed from the northwestern Bering, across the Pribilofs and
into the eastern Aleutians. This feature will bring increased
winds and chances for rain to these areas as it continues to
propagate eastward. This front should weaken this evening with
winds and rainfall tapering off. Water vapor imagery shows an
increase in moisture, ahead of a low in the North Pacific, making
its way into the western Aleutians. This low will move across the
Chain, bringing gale force winds and more rainfall to the central
and eastern Aleutians on Sunday and Monday as it looks to remain
nearly stationary during this time. Late Monday it should resume
its track east towards the Alaska Peninsula.

&&

.MARINE (Days 3 through 5)...
Gulf Of Alaska: A new low will move into the GOA Monday and
Tuesday but the surface low will hover south of Kodiak and slowly
push east as time elapses weakening along the way. Moisture will
get wrapped around the back side of the low which will keep the
chance for precipitation in the region.

Bering: Monday afternoon there will be a surface low south of the
AKPEN which will bring northerly winds to much of the Bering. The
pressure gradient forces at this time do not look impressive for
the Bering, so relative mild weather is expected for this forecast
period.

PJS

&&

.LONG TERM FORECAST (Days 3 through 7)...
The NCEP RMOP (National Centers for Environmental
Prediction/Relative Measure of Predictability) pings into the
level of uncertainty in the Bering and the low in the GOA during
Days 3-7. However, the RMOP confidently scores the high pressure
in the Central and North Pacific. Long story short, there will be
multiple lows forming in the Bering although the consensus on the
low tracks is not perfect and the Gulf Of Alaska will also be wet
and unsettled for this forecast period. The significance of the
high in the North Pacific is that lows will follow the path of
least resistance and the placement and intensity of the dome of
high pressure gives us insight to where the corridor is for these
two lows to potentially move through. Looking at the
deterministic model guidance, at the onset of the forecast period,
the longwave pattern features broad troughing from the Yukon
Territory to Kamchatka. There are numerous shortwaves embedded in
the pattern with a closed low south of Cold Bay. The closed upper
level low moves east-southeast and a new closed low develops over
the Western Bering later in the period. To reiterate, overall the
pattern looks unsettled and wet. There is also an upper level high
north of the the Brooks Range but it won't impact the sensible
weather for the southern tier of the state.

&&

PJS

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 155 165.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...LTB
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CK
MARINE/LONG TERM...PJS