Small Craft Advisory 498, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
454 AM AKDT Thu Jun 25 2020

.ANALYSIS AND UPPER LEVELS...
An upper level low and its corresponding elongated surface low
were present well southeast of Unalaska this morning. This low
will continue moving northeastward while weakening which will
bring Kodiak Island and the Gulf Coast region periods of rain
later today through Friday. The front associated with this low was
seen on satellite imagery pushing northward this morning. ASCAT
scatterometer data shows mainly small craft advisory level winds
associated with this front moving through the southern Gulf.
Southcentral Alaska remains under some weak upper level ridging
again this morning with weak synoptic scale forcing keeping the
weather generally quiet. Partly to mostly cloudy conditions were
present this morning.

Further west, a piece of the main upper level trough was seen
moving southward towards the Yukon/Kuskokwim Delta with the main
upper level low present further north over the Arctic Ocean. A
frontal system associated with this upper level trough remains
relatively stationary across the Yukon/Kuskokwim Delta this
morning with rain showers ongoing. This front will help to keep
the region in the unsettled pattern. The typical low stratus/fog
continues across the Bering underneath a subsidence inversion.

&&

.MODEL DISCUSSION...
Model guidance continues to be in really good agreement on the
synoptic scale features through Day 3.

&&

.AVIATION...
PANC...VFR conditions will persist throughout the TAF package.
Gusty southeasterly winds may move into the terminal again Thursday
evening.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

Weak upper level ridging is over the interior parts of Southcentral
today which is bringing a break in the showers over the inland
parts of the region. This ridging is also bringing more stable
conditions over interior Southcentral today and no thunderstorms
are expected over the region, though some afternoon and evening
showers are still possible.

An elongating surface low south of Kodiak Island is stretching a
front across the Gulf of Alaska today. This front is in the
northern Gulf this morning and is edging toward the north Gulf
coast where it will bring some more rainfall today. This front is
also accelerating the winds near the Copper River Delta and is
expected to bring gales to the marine area that runs from
Montague Island eastward tonight.

A sharp upper level trough will be approaching the Alaska Range
from the west Friday afternoon. An upper level low is attempting
to cut off at the base of this trough and the track of this new
cut-off low will determine the potential for thunderstorms over
Southcentral tomorrow. At this time, the most likely area looks
to be over the northern Susitna Valley and Copper River Basin
in the afternoon and evening. However, if this low takes a track
far enough to the south it will take the cold air aloft away from
the Susitna Valley and Copper River and therefore lessen the
thunderstorm chance in those areas. If this does happen, it could
then increase the chance over the Kenai Peninsula tomorrow
afternoon and evening as this upper level low moves through.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

An upper level trough over the northern Bering Sea will continue
to move into Southwest Alaska over the next few days. At the
surface, a cold front associated with a low just north of the
Kuskokwim Delta has brought rainfall from Bethel to Kipnuk this
morning. This front will slowly slide eastward into the Kuskokwim
Valley through this afternoon and evening as the associated low
exits the region. By Friday afternoon, most precipitation around
Southwest Alaska will diminish, before the next round of
widespread rainfall moves in on Saturday afternoon as a new front
moves into the area. Cooler temperatures will continue through the
end of the week when this colder airmass will begin to weaken
late Friday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

As a low south of the AKPEN continues into the Gulf, surface high
pressure is building over the western Bering, keeping relatively
quiet conditions and widespread stratus across the area through
Friday morning. The pattern then becomes more active with a gale
force low moving into the northern Bering, lasting through
Saturday morning. Meanwhile, another low in the North Pacific will
move towards the central Aleutians Friday afternoon, which will
also bring an area of gale force winds and widespread rainfall.
The associated front will work its way eastward over the AKPEN and
into Southwest Alaska through Saturday as the system weakens.

&&

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

Aleutians and Bering: A front swings across the Bering through
Mon. Confidence is good for Southwest small craft winds and waves
across the Bering to increase to gales in the eastern Bering on
Sat. A North Pacific low and front slides over the western and
central Aleutians for Fri and Sat. Confidence is good for
easterly gales over the Aleutians with small craft winds and
waves spreading towards the Alaska Peninsula.

Gulf of Alaska: High pressure moves across the Gulf through Sun.
Confidence is good for southerly small craft winds and waves to
spread across the Gulf ahead of a low and front diminishing Sun
along with westerly small craft winds and waves in the western
Gulf around Kodiak Island on Sun and Mon.

&&

.LONG TERM FORECAST (Days 4 through 7: Sunday through Wednesday)...

Models are coming into better agreement through the weekend, but
begin to diverge with solutions by Tuesday and Wednesday. Biggest
difference from previous model runs is the parent low near the
northern Bering Sea will not absorb the low over the Aleutians.
This will keep this low from strengthening and being a bit
colder. This will allow for the colder temperatures to remain over
northern and western Alaska but will allow the south mainlands to
remain a bit warmer than previously expected.

Sunday features a pair of low pressure systems. The parent low
over the northern Bering Sea and a weaker low centered over the
Aleutians with the upper trough extending near western Alaska.
This sets up broad southwesterly flow aloft across the Southern
Mainland. This flow taps into moisture from weakening surface
fronts moving across the Southern Mainland which will bring cloudy
conditions with rain and showers Sunday and Monday. A transitory
ridge will move across the Copper River Valley bringing brief
drier and warmer weather Sunday. By Monday, the upper low and
short wave over western Alaska Sunday will mover east of the area.
However, an upper low near the Seward Peninsula keeps generally
westerly flow across the mainland. So expect generally showery
conditions Monday. Upper ridging builds in across the southern
Mainland late Tuesday into Wednesday which will bring decreasing
shower activity. However, there may be potential for afternoon
and evening showers inland with a strengthening thermal trough.
Model differences arise Tuesday and Wednesday which lower forecast
confidence.

The Bering Sea will be generally under high pressure through
early Monday, then transition toward more active weather as a low
and front approach from the south and west Tuesday and Wednesday.
Again, confidence is low as significant model differences arise
in positioning and intensity of these systems.

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 185 119.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...MV
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH
MARINE/LONG TERM...MK/RC