Small Craft Advisory 221, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
441 PM AKDT Sun Mar 17 2019

.ANALYSIS AND UPPER LEVELS...
Satellite imagery shows an impressive front pushing into the north
Gulf coast this afternoon associated with a gale-force low
lifting northward towards Kodiak Island. The Middleton Island
radar continues to show rain falling along coastal areas from this
feature, which has already brought high QPFs to the region,
including more than a half inch of rain to Kodiak and Cordova and
over an inch to Seward in the past 24 hours. Further inland, the
Anchorage Bowl remains dry due to the downsloping effect, there
are some gusty wind conditions along the Turnagain Arm. The
deformation band over the southwest mainland continues to bring
some snow stretching from the AKPEN to the Kuskokwim Valley. A
weak front over the western Aleutians is progressing eastward
associated with a vertically stacked low situated positioned south
of Shemya.

&&

.MODEL DISCUSSION...
The numerical models are in rather good agreement through Tuesday
afternoon. There are some minor differences in the exact location
of a small surface low in the northern Bering on Monday. There are
also some minor differences in the location of a low and the
timing of its associated front on Tuesday in the Gulf. These have
only minor impacts on the forecast. Thus the forecast confidence
is higher than normal for the afternoon package.

&&

.AVIATION...
PANC...VFR conditions are expected through Monday afternoon. Gusty
southerly winds are expected late this evening, diminishing late
tonight.

&&
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Tonight
through Tuesday)...

An area of low pressure currently approaching the south side of
Kodiak Island will be the primary weather maker for Southcentral
for the next 36 hours or so. The low is tracking north, and is
expected to move into southern Cook Inlet Monday morning. Then it
will move into the Susitna Valley and dissipate Monday evening.
Along this track, onshore southeasterly winds causing low-
elevation rain and mountain snow will continue through low
dissipation. The dry slot of the occluding low has passed
Anchorage, allowing for a few breaks in the cloud cover and some
sunshine this afternoon. Clouds will refill the skies tonight as
the low approaches.

When the low moves to near Afognak Island tonight, one last round
of heavier rain and snow is expected along the eastern Kenai
Peninsula, with 6 to 12 inches of snow expected in the Kenai
Mountains around Turnagain Pass. The southeasterly winds will
continue to rain-shadow the Anchorage area through Monday, which,
along with the Turnagain Arm and Knik Valley winds, will send
temperatures to around record high territory (upper 40s) Monday
afternoon in both Anchorage and the Matanuska Valley. The Copper
River Valley wind is also expected to continue through Chitina and
Gulkana, allowing temperatures to approach 50 in both of those
locations due to the downsloping winds. The center of the surface
low will pass Anchorage Monday afternoon. Behind the low, the
winds will turn southwesterly and up-Inlet. This will cause the
Turnagain Arm winds to rapidly weaken Monday evening, which in
addition to the approach of an upper level wave and associated
additional moisture, will likely cause brief periods of light rain
and snow Monday evening in Anchorage and the Matanuska Valley, as
the precipitation cools the air column. No significant
accumulations are expected due to the light intensity and
temperatures staying at or above freezing.

The skies will clear Tuesday morning behind the wave. Then a
lingering front over the central Gulf is expected to begin
retrograding westward as another North Pacific low tracks south of
Kodiak Island. The front may potentially bring additional chances
of rain and snow to Southcentral Tuesday night.

&&

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

As weak low and trough push north across southwest Alaska in
conjunction with an upper level low pushing north through the
Bering Strait the resulting upper level deformation zone and
accompanying lift will enhance the precipitation tonight across
the Nushagak Hills to the Sleetmute and Lime Village areas. Low
level warmer air at the 850 mb level coupled with weak cross
mountain flow and down slope warming will hold temperatures
around the freezing mark overnight resulting in a mix of
precipitation, especially along and near the Alaska Range.
Another weak upper level low will push through western Alaska
Monday night while at the surface weak high pressure is building
over the southwest mainland. This will usher in colder low level
air, although little if any moisture will be available for
significant snowfall. Clearing skies and radiational cooling
tonight and Monday night will increase the chance for fog,
especially over the Kuskokwim Delta and Bristol Bay region tonight
and Monday night.

&&

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

Northwest flow will move into the eastern Bering Sea as a ridge of
high pressure builds into the central Bering Sea in advance of the
next incoming low. This low will trek along just south of the
Aleutians through Monday night then stall south of Unalaska
through Tuesday. Behind this low a ridge will build over the
western Aleutians and western Bering Sea. Another low will then
move into the western Bering Sea Tuesday serving to nudge the
ridge of high pressure east as it breaks down.

&&

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

...Bering Sea/Aleutians...

There is increasing confidence of northeasterly gales developing
over the eastern half of the Bering on Wednesday through Friday as
a series of lows track south to north over SW Alaska and the
AKPEN. Right now, there is more uncertainty regarding the exact
location and strength of the winds on Wednesday/early Thursday
due to model disagreement of the surface low moving over the
AKPEN. Model guidance shows better agreement with a stronger low
moving towards the AKPEN on Friday, thus more confidence that
there will be northeasterly gales developing from the Pribilofs
east. Looking towards the end of Friday, there is decent
agreement that a storm force low will push across the western
Bering/Aleutians.

...Gulf of Alaska...

Wind and wave heights are expected to subside through the day on
Wednesday as the region will be in between systems. There is
increasing confidence that a gale force low will enter the Gulf by
Thursday with easterly gale force winds spreading across most of
the region on Thursday and Friday. Wave heights in excess of
20-25 ft are also likely by Friday with increasing long fetch
winds from the east and eventually south/southeast. Storm force
winds could also develop near the North Gulf coast Thursday and
Friday during this time associated with an easterly barrier jet.

&&

.LONG TERM FORECAST (Days 3 through 7, Wednesday through Sunday)...

Not much change is expected in the longwave pattern this week
with persistent and strong southerly flow over the Gulf supporting
a barrage of lows over the southern mainland. These lows will
continue to tap into subtropical moisture leading to very heavy
precipitation over the coast. Models show total liquid
precipitation amounts over the coastal mountains along Prince
William Sound approaching 10 inches through the end of the week.
The southerly flow will also pump in warm air with snow levels
hoovering between 1500 and 2500 ft. Strong cross barrier flow
(from the east) will continue to promote downsloping for locations
on the west side of the mountains, including the western Kenai,
Anchorage, and the Mat-Su. Weather in these locations will be
cloudy but mostly dry with temperatures well above normal, climbing
into the 40s each day.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...
 Small Craft:119 120 125 128 130 131 132 138 139 155 160 170 172
             173 175 176 177 178
        Gale:150 174
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...AH/BL
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...SA
MARINE/LONG TERM...KVP