Small Craft Advisory 444, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
457 PM AKDT Thu May 28 2020

.ANALYSIS AND UPPER LEVELS...

Upper level high pressure is over the northern part of the state
with an axis stretching to the Kuskokwim Delta coast. A closed
upper level low is just south of the Gulf of Alaska. This
vertically stacked low is the one that has been in that general
vicinity the past few days. This pattern is producing easterly
flow aloft over Southcentral. A weak easterly wave is in the
western Copper River Basin early this afternoon and headed
westward in this flow. This could be the focus for some afternoon
showers or isolated thunderstorms.

Light northerly flow aloft is in place over Southwest Alaska
where the ridge is suppressing convection. Another low is in the
western Bering Sea bringing rain into that region.

&&

.MODEL DISCUSSION...

Synoptic agreement between the models remains good into early next
week. The issue remains dealing with easterly waves (upper level
disturbances moving from east-to-west) over Southcentral and
Southwest mainland Alaska. While these easterly waves are
notoriously difficult for models to handle, the 12Z and 18Z GFS
and NAM model runs are showing better agreement in the timing of
these waves over the next two days which is increasing forecast
confidence.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist.

&&

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

A low moving into the Gulf combined with high pressure over the
Arctic is leading to increased northerly flow over region. As
such, warmer temperatures and drier conditions are currently
observed. A Red Flag Warning is currently in effect for the
Susitna Valley. Weak instability may lead to scattered showers and
isolated thunderstorms this afternoon/evening along the Alaska
Range. Due to the upper level flow pattern, any storms will move
east to west. Similar conditions are expected for Friday.
Relative humidity values will be slightly higher over the Susitna
Valley and thus fire concerns will be lower. However, as the
aforementioned upper low moves farther north, more potent
shortwaves/vorticity maximums will propagate over the Copper River
Basin and the Mat-Su Valley. This will lead to scattered showers
and thunderstorms over these regions. Cloud cover will increasing
Friday night, with a surface low developing over the Yukon
bringing more widespread rain into Southcentral, including
Anchorage by Saturday. While the timing/position of precipitation
is still somewhat in question, the wetter pattern is expected to
continue through the weekend.

&&

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

The upper level ridge continues to build over the Interior today
and tomorrow. This should continue the pattern of offshore flow
and warming temperatures for the next couple of days. On Saturday
an upper level trough will move through the Kuskokwim Valley and
inland Bristol Bay. This will bring extra moisture and
instability, so thunderstorms are forecast to begin over the
mountains and then extend through the lowlands. This is a change
from yesterday's forecast when the more widespread thunderstorms
looked like they would occur Friday.

&&

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

The front that was over the Western Aleutians yesterday is slowly
progressing eastward and is expected to be over the Eastern
Aleutians by Saturday. At this point a new low pressure center
will develop and move into the Central Aleutians. Winds are
expected to increase ahead of this low and gale force winds are
expected just south of the Aleutian Chain. After the first front
weakens on Friday, most of the precipitation will remain in the
southern Bering Sea, with conditions improving for the Pribilof
Islands.

&&

.MARINE (Days 3 through 5)...

Gulf Of Alaska: A new low will move into the GOA Monday. There is
a chance this new low could bring a barrier jet capable of gale
force winds to the north Gulf coast late Monday into Monday night.
Expect gusty winds at Hinchinbrook Entrance. There is some model
divergence on Days 4 and 5 which adds an element of uncertainty
for this region past Day 3. But the model solutions generally
point to an active pattern but the tracks of the lows after Day 3
are very different. Stay tuned for updates.

Bering and North Pacific: Monday morning there will be a potent
low near Unalaska, and the models are generally suggesting that
the surface low will be between 979 mb to 982 mb. There is a
tight pressure gradient north and south of the chain, so expect
gusty winds and enhanced seas. This low slowly moves eastward just
south of the Aleutians and is at 994 mb to 997 mb by Tuesday
night. Again there are some timing issues that become more
pronounced as the low moves from the Day 3 to Day 5 time period.
Stay tuned for more details.

&&

.LONG TERM FORECAST (Days 3 through 7)...

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 the
largest cluster over Adak/Unalaska to the Pribilof Islands. The
closed upper level low moves south of the chain by 12z Tuesday.
By 18z Wednesday a closed arctic low develops and moves toward St
Matthew Island. Overall, the pattern looks unsettled and wet.
There is a small upper level high between the Brooks an the Alaska
Range. Places like Talkeetna have the best chance for dry weather,
but that is not a slam dunk especially since we are in the time of
year where afternoon and evening convection develops routinely as
a function of day time heating. The instability parameters for
Southwest AK also merit close monitoring since there will be
additional lift from the shortwaves passing over the area.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALES: 173-176, 413.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...EZ
SOUTHCENTRAL ALASKA...AP
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...LF
MARINE/LONG TERM...EZ