Small Craft Advisory 819, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
438 PM AKDT Mon Oct 22 2018

.ANALYSIS AND UPPER LEVELS...
The vertically stacked low that brought heavy rains and strong
winds to the Gulf Sunday continues to weaken and move northwest.
The upper level low is slowly filling but is continuing to advect
cold air through the western mainland into the Gulf of Alaska. The
associated trough has also become negatively tilted in a
Northwest-Southeast direction over the eastern Bering. The trough
alignment is cutting off the moisture to the western Gulf and is
pushing much of the energy to the eastern Gulf. The jet also
supports this easterly transition as it dips well below the
Aleutian chain and then sweeps back up along Southeast Alaska. In
the mid-levels, a slight ridge is forming over the Gulf which is
assisting in the moisture cut-off process. All of these factors
are allowing for a brief reprieve in the precipitation experienced
over Southcentral over the last weekend. Winds have also
diminished throughout the Gulf and Southcentral.

The AKPEN and Kuskokwim Delta on the other hand are not immune
from the rain. Through the Aleutian chain, on the western edge of
the trough, northwesterly flow is dominating, and bringing gusty
conditions to the islands and marine areas south of Saint Paul.
Satellite imagery shows bands of stratus along the edges of the
troughs while the rest of the Bering Sea is covered in scattered
cumulus. In addition, satellite depicts the next fast moving storm
brewing in the North Pacific.

&&

.MODEL DISCUSSION...
Models are in fairly good agreement at the upper levels with the
broad negatively tilted trough over the northeast Pacific and the
north Pacific jet. At the surface, models have the same path and
intensity of the developing storm which moves across the Gulf
Tuesday. There are however are some relatively minor differences
in timing of the low. The same is true of the following low
entering the southern Gulf Thursday. Mainly will be doing hand
edits to capture local effects.

&&

.AVIATION...
PANC...VFR conditions amd light winds are expected through
Tuesday afternoon. Rain moving in by late Tuesday morning could
bring some brief MVFR conditions, but this is not likely and thus
is not in the 00z TAF.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The longwave pattern features a broad trough from the Eastern
Bering to the Gulf of Alaska. There are multiple embedded
shortwaves over the North Pacific and Gulf Of Alaska. There is
widespread difluent flow over Prince William Sound. There are a
pair of lows at the surface, one south of Kodiak and the other is
south of Middleton Island. Conditions will rapidly deteriorate
tomorrow as a strong low moves along the north Gulf coast and into
southcentral Alaska tomorrow afternoon. Both the NAM and the GFS
models show this storm but the two models are not in sync with the
timing of the track and landfall. This low may have an intensity
of 965 mb which will bring a tight pressure gradient into the
region. Expecting gales and storm force winds to materialize. In
addition to gusty winds expect another round of precipitation. The
temperatures in the boundary layer remain warm, expecting low
elevation rains and snow above 3,000 feet. The exception is the
southern Copper River basin, where enough cold air is in place
that snow is expected on Tuesday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The cloudy and showery pattern over southwest Alaska will
continue for the next few days. A series of low pressure systems
rotating through the region are responsible for this pattern. One
exception will be some brief clearing tonight over the western
Alaska/Aleutian Ranges and into the lower Kuskokwim Valley. This
will allow for some patchy fog formation and temperatures to drop
to around freezing. Otherwise, the cloudy and moist pattern will
persist.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The trough on the backside of the low in the eastern Bering is
diving south as expected. The weather associated with this trough
is currently moving through the Pribilofs. Low ceilings, rain, and
gusty northerly winds are associated with the trough movement. As
this trough moves south over the eastern Aleutians tonight, the door
will be opened for strong northerly flow to push into much of the
Aleutians. As a result, we are expecting gale force winds to develop
over the central Bering tomorrow and push over the central Aleutians
tomorrow night. On Wednesday, a trough embedded in the flow is
expected to produce gales over the western Aleutians, with strong
winds continuing for the western and central Aleutians at least
until Thursday.

&&

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

A very challenging forecast is in store for us through the
extended period, as the models remain in modestly good agreement
with the overall pattern through Saturday. Thereafter, the train
goes off the tracks as solutions diverge about as much as they
can with really no semblance of agreement. This occurs as the
persistent pattern of a deep trough over the west and multiple
systems rounding the base riding north into Southcentral, which
we've had for what seems like a few weeks now, breaks down. This
will allow some potential for cooler Arctic air to try to make
inroads into our neck of the woods. However, given the huge spread
in the numerical guidance, trying to get too specific would be
problematic at best.

For the end of this week, the various forecast solutions agree
with each other, bringing another low northwestward through the
Gulf and across the Kenai Peninsula. As stated yesterday, this low
will be in a slow weakening state, but, it'll still be in the
upper 960s to mid 970s mb range. This should still be strong
enough for some sustained gales across the northern Gulf and
periods of light to moderate rainfall, with a few pockets of heavy
rain possible on the south and east facing slopes, as orographic
lift helps maximize precipitation efficiency. The main challenge
with this storm is timing. The European (ECMWF) is the fastest of
the models, bringing precipitation into the Copper River
Delta/Valdez area Thursday morning, then spreading it westward
while reaching the Kenai/Anchorage Bowl/Mat-Su Valleys region by
late Thursday evening. Meanwhile, the GFS is about 12 hours slower
with the same precipitation pattern, and the NAM is about 18 to
24 hours slower. The timing has really not been in good agreement
over the last few model runs, so we know the rain is coming, just
when is the question. At this point we're leaning towards the GFS
solution as it's a good "middle ground" approach.

For Sunday and Monday, no changes were made to the forecast given
the pattern change and high degree of uncertainty with the model
spread. For now will just lean towards the ensembles (family of
models blended towards climatology), with just some minor tweaks
to POPs (probability of precipitation) and temperatures.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory (Snow) 141.
MARINE...Storm Warning 119 125.
 Gale Warning 120 127>129 170 172>174.

&&

SYNOPSIS AND MODEL DISCUSSION...RA/RC
SOUTHCENTRAL ALASKA...PJS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BB
LONG TERM...PD