Small Craft Advisory 740, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
448 PM AKDT Thu Sep 27 2018

.ANALYSIS AND UPPER LEVELS...
A very amplified and broad upper level pattern is beginning to
dominate the weather over the state. The first feature of note is
a broad ridge of high pressure over the Northeast Pacific,
allowing for deep southerly flow that is surging moisture from the
Hawaiian Islands northward towards Bristol Bay. This is occurring
as a nearly stationary low spins over the Western Bering, leaving
much of the mainland under the influence of the southerly flow.
This is bringing rain to much of the Gulf Coast into the Kenai
Peninsula and Bristol Bay, which is beginning to spread northward
into the mainland and eastern Bering. Under this rainfall, a
trough in the western Gulf brought gale force winds from the
north Gulf into Shelikof Strait, along with fairly strong winds
along Western Turnagain Arm today. These winds are however all
coming down as the trough weakens.

&&

.MODEL DISCUSSION...
The biggest difference in the models has been the trend to a much
further west and slower progression of the rainfall over the South
Mainland. As a result large changes were necessary to the rainfall
forecast over southwest Alaska for the forecast updates today
with good confidence. There is now great agreement that a second
round of rain will push into Southcentral late tonight, requiring
large changes to the rain forecast there as well.

&&

.AVIATION...
PANC...Rain and generally VFR conditions will continue into
Thursday morning before diminishing.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
The stalled front along the coast begins to lift slowly north
late tonight spreading light rain across the southern mainland.
Gusty winds taper down and rain diminishes Friday afternoon with
the front pulling northward. However, gusty winds and higher rain
chances linger across the western gulf through Saturday as this
area remains the influence from a trough to the west. A ridge
builds across the eastern gulf through the central mainland late
Friday through Saturday, bringing a blue bird day to start the
weekend.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
(Tonight through Sunday)
A wet 24 hours is in store for the region, as today's numerical
model runs have shifted the main axis of the tropical fetch
further westward (Cook Inlet yesterday versus towards the
AKPEN/Bristol Bay today). This is also confirmed when looking at
the Pacific Mercator view of the CIMSS MIMIC water vapor imagery.
As such, POPs and QPF were raised significantly. The rainfall has
already spread to locations south of a Dillingham to Port
Alsworth line, and this will continue to lift north on Friday, as
an area of low pressure and an associated warm front lifts north
across the area.

Then for your Saturday, a ridge of high pressure aloft will build
northwest from the Gulf across most of southwest Alaska.
Meanwhile, multiple waves of low pressure and associated warm
fronts will lift north across the central Bering. The resulting
pattern will allow for a brief warming trend as temperatures and
heights aloft rise, aided by a southeast down-sloping wind which
will really boost afternoon temperatures. This will be most
prevalent in the lee (northwest side given the wind direction) of
the mountains, with most locations here climbing into the lower
to mid 60s. This pattern remains in place for Sunday, with heights
lowering some as the high recenters to near Fairbanks. Still,
above average temperatures in the lower 60s in the lee of the
mountains are likely, with upper 50s to near 60 likely for the
other locations.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
(Tonight through Sunday)
A wet and unsettled period will continue through the weekend, as
a rather complex scenario of storm systems interact. Starting with
the western half of the Bering and Aleutians, not much has
changed from yesterday, as we'll have an area of low pressure
wobbling across the western portions of the Bering, with a
stalled occluded front located from southwest of buoy 35 to just
west of Adak. This front may edge eastward some, as the initial
western low meanders southeast towards Adak. The first of many
surface lows will eject north to northwest across the central
parts of the chain near Dutch Harbor, before turning northwest
near 60N and then gradually turning southeast. In some ways this
is like a broad Fujiwhara effect. In any case, gale force winds
will be common along the frond and on the back side of each area
of low pressure, as disturbances aloft dive into the trough,
giving some enhancement to the downward mixing potential near the
lows and a tighter pressure gradient near the front. Seas will
remain elevated most to the time, with waves increasing to near 16
feet as each disturbance crosses the area.

For the eastern half of the Bering and Aleutians/AKPEN, Multiple
surface lows will track north northwest from the Pacific,
generally crossing near Dutch Harbor, before making a gradual turn
towards the west one they get near/west of the Pribilof Islands.
Each wave of energy will bring an associated warm front northward,
with periods of moderate to heavy rainfall and sustained gale
force winds accompanying each storm/front. Waves will be in the 9
to 13 feet range north of the peninsula/island chain, with higher
waves of 13 to 17 feet south of the land areas due to a longer
fetch length.

&&

.LONG TERM FORECAST (Days 4 through 7)...
Beginning the extended period forecast Sunday night, there will
still be a very impressive 575 dm 500 hpa blocking high (4 sigma)
over the state of Alaska with a large negative tilt trough over
the Bering Sea. A secondary low will be in place south of the
Alaska Peninsula. It does now seem that the blocking high will
remain strong over Alaska through Monday night and may even hold
through Tuesday, keeping most of southern Alaska dry into
Wednesday before precipitation begins to move north along the Gulf
Coast. This trend means precipitation threats will be a day or so
later than previously thought.

Attention quickly turns to the western Bering Sea where the
remnants of Super Typhoon Trami will begin to exert an influence
on our waters. Unfortunately, much uncertainty remains at this
juncture, even though the low is expected to move into western
Bering Monday night and Tuesday (less than 5 days out). The global
numerical models range from a weak and southerly track barely
clipping the Aleutians (12z ECMWF) to a powerful 948 hpa storm
force low over the entirety of the western Bering Sea and Western
and Central Aleutians (18z GFS). The 12z Canadian NH splits the
difference between the outlier models in both track and strength,
so that solution will be favored for now which will likely hint at
potential gales for the far SW Aleutians. It should be noted, for
marine purposes, that this storm has the potential to be very
strong, but much uncertainty remains at this juncture so a more
conservative approach will be taken, although not as conservative
as the weak ECMWF solution.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 131 138 178 180 181.

&&

SYNOPSIS AND MODEL DISCUSSION...DEK
SOUTHCENTRAL ALASKA...KH
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...JA