Small Craft Advisory 745, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
440 PM AKDT Sat Sep 29 2018

.ANALYSIS AND UPPER LEVELS...
An upper level ridge remains over the state today, which brought
patchy fog and fair skies to Southcentral and stratus for the Gulf
throughout the day. An upper level stationary low is sitting over
the southcentral Bering. This low is responsible for bringing
light rain along its frontal boundary stretching across the
eastern Bering down below the eastern Aleutians. Gale force winds
have been experienced with this front, bringing gusts above 60 mph
for Cold Bay. Gusty conditions and the potential for rain showers
remain along the central and eastern Aleutians and Bering, as
well as the AKPEN as the front continues across the region. A
second batch of moisture sits below the central Aleutians
associated with a North Pacific low that is moving northward
throughout the afternoon, ready to bring another round of rain for
the central Aleutians and Bering.

&&

.MODEL DISCUSSION...
Models are in good agreement through the short term (days 1&2).
Differences creep in with the handling of the remnants of typhoon
Trami Monday and Tuesday as it moves into the Bering Sea. Main
differences are in low center position and location of wind
maxima. Will continue to lean toward the GFS which is a good
middle ground. All said, expect a broad area of storms with
potential for hurricane force with this system.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through this
evening. IFR ceiling and visibilities are possible Sunday morning,
but were left as VCFG for now.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)
Gorgeous weather is expected to continue across almost all of
Southcentral Alaska for the next couple days. A strong ridge
building over the area will peak in intensity on Sunday. With most
areas seeing a second day in a row of full sunshine again Sunday.
That should mean record high temperatures are once again a
distinct possibility to round out the final day of the warmest and
one of the driest Septembers on record at Anchorage.

The one caveat to seeing record highs as opposed to well above
normal highs on Sunday will be the possibility of fog along the
area waterways and sheltered valleys. Most areas are a few
degrees warmer this afternoon thanks to the full sunshine and a
couple degrees of dew point drier as well. This should mean
somewhat less fog around the area Sunday morning in terms of
coverage, but it will almost certainly be around and in many of
the same areas that had fog this morning. Be aware of that
possibility if out traveling in the morning. Fog is for sure the
one and perhaps only piece of the forecast where confidence is
low.

A front near Kodiak this afternoon is slowly drifting westward,
with clearing making steady progress westward across the Gulf. It
may take until the afternoon Sunday for the clouds to finally
clear out of Kodiak given the persistent onshore flow, but
clearing and sunshine are expected there as well. On Monday, a
reinforcing front moving eastward will allow the clouds and rain
to begin advancing back to the east, with Kodiak expected to cloud
back up in the morning and rain resumes there in the afternoon.
The front will only make very slow progress, with rain not making
it into the southern Kenai Peninsula until Tuesday. Much of the
area will see steadily increasing cloudcover through the day
Tuesday as a result with the exception of the Copper River Basin.
The increasing clouds and the weakening ridging aloft will make
for steadily cooling, but still above normal afternoon highs
through midweek.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
Winds are the biggest challenge through Monday as the surface low
currently along the southeastern Bering is replaced by another
moving up on Sunday, keeping a tight baroclinic zone across the
Alaska Peninsula. Southeasterly flow will be gusty across Bristol
Bay and the Kuskokwim Delta with the highest speeds along the
coasts. Isolated to scattered showers are possible along the
coastal areas, however strong wind shear limits any cell
development. The highest rain chances are along the Alaska
Peninsula where a deformation zones remains in place through
Tuesday. Moderate to heavy rain can be expected along the
Peninsula as the next surface low moves across on Sunday through
early Monday. Rain is expected to spread inland across the
Southwest on Tuesday as a strong storm system pushes a front
across from the western Bering; however, strong southeasterly
flow and shear once again limits precipitation.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
A challenging weather pattern is expected through Tuesday as two
significant storm systems track to the Bering. The first system
organizes tonight pushing a front to the central and eastern
Aleutians. This brings gusty southeasterly winds and rain late
tonight through early Monday, then diminishes as surface low
quickly fills. Gusty northerly flow increases behind the frontal
passage with a shortwave ridge separating this system from the
next. The second system will be a powerful organized system as a
subtropical storm taps into a cold air mass over the Kamchatka
Peninsula. This system should be monitored as models continue to
show stronger winds with each new model run.

&&

.LONG TERM FORECAST (Days 3 through 7)...
(Wednesday through Saturday)
A very unsettled pattern will persist for the remainder of next
week, as the remnants of the former Super Typhoon Trami, which is
currently near Japan, move northeast towards the western Bering
Sea, then turning eastward as it merges with a trough coming off
the Kamchatka Peninsula. The numerical models all agree in this
scenario, but they differ in speed and strength as the system
bumps into a persistent ridge stretching from the North Slope
through the Kenai Peninsula, and into the Gulf. Initially, the
models are in good agreement with the pattern aloft on Wednesday,
but they quickly begin to diverge some 24 hours later, as the GFS
looks to have a faster progression with the system as the ridge
slides off to the east while weakening. The Canadian global is
considerably weaker and faster with the system, and significantly
diverges from its counterparts (GFS/ECMWF). Thus, it'll be
discounted through the period. The GFS is typically too fast with
timing. That said, the differences are close enough to keep it in
the package, and we'll use a ECMWF/GFS/WPC (Weather Prediction
Center) blend.

So here's what we can look forward too. This storm will be
unusually strong for the Bering for this time of year, as it'll be
3 to 4 standard deviations above normal. Most of the region along
and north of the western Aleutians will see hi-end sustained
storm force winds with hurricane force gusts likely, and seas of
35 to 40 feet on Wednesday. An associated occluded front will
cross into southwestern parts of the state by evening, with
widespread rainfall accompanying it, with another low developing
near Kodiak. This rainfall will also spread northeast during the
evening and into Thursday for most of Southcentral and the
Kenai/Prince William Sound region.

Meanwhile, another north Pacific low will strengthen as it
crosses the central portions of the chain, lifting north towards
Saint Matthew Island on Friday, while sending another front
through the southwest with more rainfall developing. Similar to
its predecessor, another Kodiak low will form Friday evening,
reaching Prince William Sound late Saturday, spreading rainfall
across all of Southcentral and into the Copper River Basin.
However, it's important to note that as we approach next weekend,
model differences increase somewhat, which is typical for 5 to 7
days out. Added to the mix is that everything is dependent on if
the models handle the first extra-tropical system correctly, so
forecast changes are probable for the latter half of the week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 150 155 160 165 172 174 180 181 185.

&&

SYNOPSIS AND MODEL DISCUSSION...AH/RC
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...PD