Small Craft Advisory 716, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
540 AM AKDT Sat Sep 28 2019

.ANALYSIS AND UPPER LEVELS...
Ridging continues over eastern Alaska with the ridge axis near
the Canadian border. This setup allows for south to southwesterly
flow over much of the southern mainland and Alaska Peninsula.
Layer precipitable water satellite imagery shows a long fetch of
moisture in the southerly flow heading towards the Alaska
Peninsula and southwest Alaska. Out west there is a low over the
western Bering with its front moving through southwest Alaska
bringing showers to the area.

&&

.MODEL DISCUSSION...
The models are in good agreement until Monday when they struggle
with the position of a low entering the Gulf. The differences are
how far inland rain would push with the NAM keeping the majority
along the coast while the GFS pushes rain farther inland. A
middle ground solution was preferred with keeping the heaviest
amounts along the coastal mountains.

&&

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

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Sat and
Sun)...
A wet and windy weekend is set to unfold for nearly all of
Southcentral. The frontal system responsible for this is moving
into the Barren Islands this morning and will spread rain and
winds across the area through the day. Winds will continue to
increase through the day, especially for the southeast to
northwest oriented gaps of the Turnagain and Knik Arms, and the
Copper River Valley. Winds will also get gusty along the Anchorage
Hillside but, for the most part, they will not make it into most
of the Anchorage Bowl as the down-Inlet gradient remains too
strong. There could be a brief period as the surface low tracks
north through Bristol Bay where the weakening front will help to
mix some of these winds through town later Sun morning into early
Sun afternoon. The winds will be breezy out of the southeast, but
should be short-lived and not nearly as strong as the Turnagain
Arm or the Hillside.

Rain will spread up the Kenai Peninsula today and over the Cook
Inlet Region tonight. As previously mentioned, the moisture
content with this system is quite impressive. The system is just
starting to impinge upon Kodiak. Their upper air sounding this
morning showed a precipitable water value of 1.05". The sounding
also showed a serious push of warm air as the freezing levels
also spiked up over 10,000'. This rain will move into the rest of
Southcentral in 2 phases. The first will be the initial warm
front. Southeast winds across the mountains will produce some
downsloping for inland areas with this feature. Thus, we expect
QPF amounts to be on the lower side for locations that are in the
lee of the terrain. The second phase will be with a weak surface
low that develops over the Southern Cook Inlet on Sun morning.
This feature will slide through the area creating deep southwest
flow. It will bring widespread rain to all areas with 0.25" to
0.40" expected for the Anchorage Bowl. With the associated surge
of warm air, snow levels will sky rocket above most of the
mountain tops. As some of the recent mountain snow starts to melt
and rain falls moderate to heavy at times, we will closely
monitor river levels to see how they respond

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
East to southeasterly winds will increase today ahead of an
approaching frontal system with the strongest winds through the
gaps and passes in the Alaska and Aleutian range. As the low moves
north, strong southerly flow will push onshore during the late
afternoon and evening. Widespread rain associated with the front
will reach the coast by midday and quickly spread inland through
the afternoon and evening becoming moderate to heavy at times.
Storm total precipitation amounts will average around an inch with
2 inches or more possible in higher elevation locations where the
terrain orientation favors extra lifting from the strong
southerly flow associated with the front.

The frontal system will move east across the Alaska Range Sunday
morning with cooler, showery southwesterly flow pushing in behind
it for Sunday and Sunday night as an upper level trough moves up
to the Southwest Alaska coast. The upper level trough will swing
inland on Monday with the bulk of the energy associated with it
shifting around the south side of the trough and moving into the
western Gulf. The general flow will turn northwesterly on Monday
and then strengthen rapidly Monday night as upper level ridging
builds into the eastern Bering and the low in the western Gulf
spins up.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
A frontal wave developing south of the eastern Aleutians will
strengthen as it tracks to the north-northeast today to cross the
western Alaska Peninsula near Cold Bay early this afternoon. The
surface low will continue to deepen as it tracks across the western
periphery of Bristol Bay this afternoon to make landfall near Cape
Newenham this evening. With the low in rapid spin up phase of
development and expected to continue to deepen as it moves inland
east to southeasterly Gales ahead of the low with a southerly push
of Gales behind the low supported by pressure rises to its south look
likely.

Farther to the west, broad upper level troughing over the western
and Central Bering with several smaller embedded shortwaves will
shift gradually to the east through the weekend. The upper level
trough will reach the eastern Bering Sunday night and Monday with
stronger northwesterly flow developing across the Alaska Peninsula
Monday night as the upper trough continues east into the Gulf.
Upper level ridging will build over the western Aleutians and
western Bering Sunday night, move to the central Bering on Monday
and eastern Bering Monday night.

&&

.MARINE (Days 3 through 5/Monday through Wednesday)...
All signs point toward a strengthening low in the Gulf on Monday
through Tuesday, though it's hard to say where in the Gulf it
will be located. Based on the expected strength and size of the
storm, widespread gale force winds appear likely, with some
potential for areas of storm force winds. Winds will come down
on Wednesday as the storm exits to the eastern Gulf. The Gulf
storm could also produce gale force winds along the Alaska
Peninsula. Otherwise, don't see any impactful storms across the
Bering Sea/Aleutians through the period.

&&

.LONG TERM FORECAST (Days 4 through 7/Tuesday through Thursday)...
A deep low will be positioned in the Gulf of Alaska Tuesday,
though confidence in it's location is low. There is pretty high
probability of strong winds and moderate to heavy rain along the
Gulf coast and Kodiak Island. The low position will affect whether
the rest of Southcentral Alaska see anything out of this. In any
case, the low will exit eastward on Wednesday, with drier
conditions moving in.

Upper level flow will become quite progressive behind the initial
Gulf low, with a trough driving eastward out of Russia Tuesday
and Wednesday and reaching the Gulf Thursday and Friday. Models
disagree on timing and amplitude of this trough and display poor
run-to-run consistency so overall forecast confidence is low. In
general, expect a "weakish" low and frontal system to cross the
Bering Sea Tuesday and Wednesday, with rain spreading into
Southwest Alaska late Wednesday. All of this will then shift to
the Gulf and Southcentral Thursday or Friday. There is agreement
that the trough will amplify in the Gulf as a ridge builds
upstream in the Bering Sea ahead of recurving tropical systems
over the west Pacific. Thus, would expect good potential for
another round of heavy rain and strong winds along the Gulf coast.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE WARNING 119,130,131,150,155,160.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...DK
SOUTHCENTRAL ALASKA...MSO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JER
MARINE/LONG TERM...SEB