Areal Flood Advisory 4, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
450 AM AKDT Mon May 3 2021

.ANALYSIS AND UPPER LEVELS...

The upper-level longwave pattern is dominated by a trough
extending from the Gulf of Alaska west to the central Bering and
Aleutians. Downstream, an upper-level ridge is positioned from the
Canadian Rockies north and west to the Al-Can border. Embedded
within the longwave trough are a duo of upper-level lows, the
first of which is centered over the southern Gulf. The second
is currently over the Pribilofs and continues to move southeast
toward the eastern Aleutians.

The upper-level pattern lends to a complex synoptic set up at the
surface. The first upper-level low is vertically stacked and is
associated with a 996 mb low over the southern Gulf. This feature
is extending a front northward. Broad easterly flow ahead of and
along the front is pushing moisture across the northern Gulf, with
rain falling over Kodiak Island. Showers are also present around
Prince William Sound this morning, the result of a weaker
shortwave lifting north and west ahead of the main upper-level
low.

Mixed rain and snow is also falling this morning along a frontal
boundary extending from the AKPEN north over the eastern Bering.
This boundary is separating a more seasonal airmass with 850mb
temperatures around -5C from a wintry airmass over the central
Bering with 850 mb temperatures dropping to -12C. Thus,
precipitation falling east of this front is falling as rain. West
of the front, brisk northwesterly winds and snow showers are
occurring from Saint Paul and Unalaska to Cold Bay. A surface low
along the front south of Saint Paul is strengthening this morning
as the upper-level low drops south. This is helping to enhance the
winds and show shower activity across the western AKPEN.

A third surface low over the central Bering is also beginning its
trek southward this morning. This feature has good upper-level
support, aided by the 500 mb low moving south and a north-south
oriented 140 kt jet streak currently over the central Aleutians.

&&

.MODEL DISCUSSION...

Models remain in good synoptic agreement leading to above average
forecast confidence through the short term. The challenges again
are with local effects such as the strength of the Turnagain Arm
wind and the Knik Valley wind as diurnal heating across the
interior battles with increasing cloud cover and a developing
down inlet flow given the synoptic pattern. There is also good
agreement on the development of snow showers and gusty
northwesterly winds over the eastern Aleutians this morning.
However, there is still some uncertainty as to the intensity of
the shower activity, especially given that the intensity of
yesterday's snow shower activity over the western AKPEN out
performed model solutions.

&&

.AVIATION...
PANC...VFR conditions will persist. The Turnagain Arm wind is
expected to bend back over the terminal by late afternoon or early
evening. Gusts are not expected to be overly strong, peaking
around 20kts. The winds should relax again after midnight.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Wednesday night)...

A low will weaken over the central Gulf today, remaining just out
of reach of a trough south of the Alaska Peninsula lifting
northward toward Kodiak and the southwestern Gulf of Alaska.
Previous forecasts were based on the idea that this low would be
pulled westward toward Kodiak Island as the upper trough
approached. As a result, rainfall on Kodiak Island will initially
be quite light. However, expect a new surface low to form near
Kodiak Island in response to the upper level short-wave and
rainfall rates will increase this afternoon and persist through
tonight. The airmass accompanying the trough is quite cold for
May, with 1000-850 thicknesses as low as 1285 to 1290 meters. As
this airmass reaches Kodiak Island late tonight through Tuesday
morning, expect some snow in the mountains and perhaps even a mix
of rain and snow down to sea level. Surface temperatures at sea
level will remain above freezing, so do not expect any snow
accumulation. The low will exit southward on Tuesday, with
precipitation tapering off through the day.

For Southcentral, the remnants of the trough currently over the
Gulf along with a weak surface front will push onshore along the
north Gulf coast tonight through Tuesday, bringing areas of rain
(and mountain snow) to the eastern Kenai Peninsula and Prince
William Sound areas. Some of this moisture will spread inland in
the form of showers on Tuesday, aided by cold air advection aloft
and the resultant increased instability. While precipitation will
not be widespread, it is looking more favorable for showers to
track across Anchorage and the Mat-Su.

The entire region will dry out Tuesday night through Wednesday and
remain dry through Thursday. Very weak cyclonic flow aloft will
persist, so some clouds will likely remain in the region. Still,
would expect at least some breaks of sun each day along with a
gradual warm-up in temperatures.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1-2: Monday morning
through Wednesday morning)...

A low over the central Bering continues to move southward. This
will keep warm north to northeasterly flow in place across
Southwest today. A rain/snow mix over the Alaska Peninsula this
morning will eventually change to all rain as warm air works back
in. As the low tracks toward the southeast and moves into the
western Gulf, cloud cover will gradually increase across the area
from east to west on Tuesday. Cold air wrapping back in around the
low will keep showery conditions in place across the Lower
Kuskokwim Valley and interior Bristol Bay region through early
Wednesday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1-2: Monday
morning through Wednesday morning)...

A low centered over the Pribilofs is moving southward toward the
central Aleutians, spreading abundant cold air across the central
Bering. Gusty northwesterly winds will continue for the central
Bering today with an intense shot of widespread snow showers along
the central and eastern Aleutians. As the low continues to move
southward, precipitation will taper by this afternoon. Farther
west, a gale force front arrives over the western Aleutians
bringing gusty southerly winds and rain. The front will gradually
weaken as it reaches the central Bering by Tuesday afternoon.
Broad low pressure begins to move in across the Bering for
Wednesday morning.

&&

.MARINE (Days 3 through 5: Wednesday through Friday)...

Bering: A North Pacific low moves along the Aleutians through
Friday. Its front pushes ahead of the low over the Bering into
Southwest Alaska by Friday. West of the low, high end small craft
winds with areas of gale force winds diminish over the Bering
Thursday. Small craft winds ahead of the front expected with areas
of gale force winds developing near channeled terrain over the
Eastern Aleutians and Alaska Peninsula, diminish Friday.

Gulf of Alaska: Expect winds less than 35 kts and seas less than
20 ft for the Day 3 through 5 period.

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

A low centered over the western Aleutians and its associated front
will bring rain generally west of Cold Bay on Thursday. With
relatively higher pressure to the east, gusty southeasterly winds
are also likely over Unalaska. Model agreement is poor on exactly
where the low will track thereafter and how broad the overall
circulation will be. In any case precipitation looks to continue
over the central/eastern Aleutians into the weekend with drier,
northerly flow behind it.

Farther east, quiet conditions are expected on Thursday with only
some light showers possible near higher terrain in the afternoon.
A front will move into the Gulf on Friday, potentially bringing
steady rain to Kodiak and extending westward into the Bristol Bay
area. The rest of the Mainland looks to remain dry. The front will
merge with a weakening low in the eastern Gulf Friday night and
continue to track northward. Model agreement is poor through the
weekend, but the pattern would indicate precipitation along the
coast and near higher terrain, with Anchorage and the Matanuska
Valley remaining mostly dry in downsloped easterly flow.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Advisory 145.
MARINE...Gale Warning 132 150 170 172-174 177 178 352 411-413.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...MK/AP