Small Craft Advisory 437, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
501 AM AKDT Tue May 26 2020

.ANALYSIS AND UPPER LEVELS...

The easterly wave passing through the south side of the upper
level high sitting over northern Alaska has brought some
widespread rain to the Copper River Basin & the Matsu Valley.
Gulkana recorded nearly three quarters of an inch in last 24
hours and widespread quarter inch totals in the Susitna Valley
were recorded. Temperatures at Eureka dropped to freezing and
snow was recorded there and likely elsewhere above 3500 ft.
Conditions along the southern coast are quiet with scattered rain
in Valdez, Seward, & Anchorage from the southern reaches of the
previously mentioned easterly wave. Southwest Alaska is seeing a
few scattered rain showers, as the upper low in the Bering pushes
a shortwave to the outer coastline of Bristol Bay. The broad
nature of the upper level trough in the Bering is keeping the
Aleutians in a quiet pattern this morning.

&&

.MODEL DISCUSSION...

06Z model runs seem to remain consistent with a blocking high
setting up across mainland Alaska from the Yukon border back to
the Alaska Peninsula. This high will be skirted by two lows that
are advertised much differently as the end of the week nears. In
the near-term, a pattern shift is in the works for southcentral
Alaska. Uncertainty will remain though, because of the blocking
high pressure and how much influence it exerts on the upper level
lows competing to shake up the pattern. The EC is holding on to
the block, while the GFS breaks it down.

&&

.AVIATION...
PANC...Mostly VFR conditions and light winds will persist.
However, expect scattered showers to remain near the terminal
through the evening. MVFR conditions possible due to ceilings
during rain showers.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Thursday night)...
The easterly wave that brought rain to much of Southcentral
yesterday and last night will exit to western Alaska this morning,
with rain diminishing in its wake. However, another short-wave
trough has entered the Gulf and is lifting northward toward
Southcentral Alaska. This trough, which is much larger and
broader in nature, will act upon the existing moist environment
and combine with weak instability to produce showers. Low level
southerly flow will favor showers training over the same areas,
from the western Kenai to Anchorage and the Mat-Su. Showers will
diminish tonight into Wednesday as this second trough exits to
the north.

A pattern change will then lead to much warmer and drier conditions
as we head through the remainder of the week. Wednesday will be a
transition day as a short-wave ridge builds overhead and weak low
level offshore flow develops in response to a large vertically
stacked low approaching the southern Gulf of Alaska. This will
help to clear out most of the clouds and begin to warm temperatures.

On Thursday, the short-wave ridge will merge with a larger and
stronger ridge near the ALCAN border and build over Southcentral.
Strengthening offshore flow between the ridge and the low making
some progress into the Gulf will lead to rapidly warming low
level temperatures, with 850mb temperatures warming by 6 to 8
degrees Celsius from Wednesday afternoon to Thursday afternoon.
The strength of the offshore flow will largely keep sea breezes
at bay. Thus, expect high temperatures across the entire region
to reach the 60s to lower 70s.

&&

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

The persistent low that has remained in place over the eastern
Bering will finally begin to lift northward and exit the Bering
to the north this afternoon. An upper level wave has the potential
to provide enough lift to produce showers and possibly isolated
thunderstorms this afternoon, especially along the foothills of
the Alaska Range. Isolated thunderstorm development is also
possible over the Kuskokwim Mountains this afternoon and early
evening. This trend is likely to continue for Wednesday with
daytime heating allowing for showers to form, especially over the
mountains. By Thursday, a shift in the pattern arrives as an
upper level ridge over the mainland attempts to extend westward.
This will bring offshore flow and a warming and drying trend
across much of Southwest. High temperatures could approach the
mid to upper 60s on Thursday, especially across interior parts of
Southwest, which will be a noticeable departure from the cooler
temperatures seen earlier this week.

&&

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

A low centered north of the Pribilofs slowly exits the Bering to
the north this afternoon. Farther west, an inbound low spreads a
front across the western Aleutians today bringing gusty southerly
winds and rain. While the low center tracks west of the Bering,
the associated front will continue moving eastward through
Wednesday afternoon. An upper level ridge building over the
eastern Bering will prevent the front from advancing farther east,
stopping just short of the Pribilof Islands early Thursday.
However the gusty southerly winds will remain in place across much
of the central and western Bering through the end of the workweek.

&&

.MARINE (Days 3 through 5: Thursday through Saturday)...
Gulf of Alaska: A front wrapped around the north side of a low
may possibly produce gales from Thursday morning through midday
over the far southern Gulf's offshore areas. The low center will
move into the eastern Gulf through Saturday, but as it weakens,
the chance of gales will remain low. Confidence is average.

Bering Sea/Aleutians: Gales continue to be most likely Thursday
morning over the central Bering from Adak through to west of Saint
Matthew Island. The chances of those gales materializing in that
area have decreased since yesterday. Confidence quickly diminishes
as to whether a new round of gales will impact the southern
Bering and Eastern and Central Aleutians Friday into Saturday, as
model agreement on the track of a new North Pacific low that would
cause the gales is very low.

&&

.LONG TERM FORECAST (Days 4 through 7: Friday through Tuesday)...
The long-term forecast starts out with a stagnant weather pattern
in place. An upper level Rex Block of sorts starts out the long
term on Friday across mainland Alaska, with an upper level high
over the northern Yukon, with a stationary upper low over the
Gulf. A narrow area of ridging is over Southwest Alaska, with a
large trough over the far western Bering. An upper low associated
with the western Bering trough moves east to around the Central
Aleutians by Sunday. Whether or not that low gets close enough to
the still-stationary Gulf low to absorb it remains in question,
with more solutions than not suggesting that one does not absorb
it until early next week.

Either way this pattern will leave much of southern mainland
Alaska, especially Southcentral, on the wet side of things. This
is because pulses of energy circulating around the upper level
Gulf low will periodically swing east to west, or easterly waves
as we meteorologists call them, around the Gulf low and across
Southcentral. Since the waves will not be originating from
interior Canada, but rather from the Gulf, they will frequently
have lots of moisture associated with them. Thus, cool, cloudy,
and wet conditions, with potential for thunderstorms if any
prolonged clearing can occur in between waves, are expected. The
upcoming weekend may allow the Copper River Basin in particular to
end what had been a very dry May prior to last night well above
normal for total precipitation.

For Southwest Alaska, since the mountains of Southcentral will
wring out much of the moisture and the waves will be moving
towards the southwest, it's far more likely to be dry for much of
the long-term. Any stronger waves could still set off showers and
thunderstorms in the afternoons, depending on how much
instability and sunshine can develop earlier in the day. Sunday
afternoon appears to be the best chance of precipitation as of
now.

&&

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

&&

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