Small Craft Advisory 802, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
448 AM AKDT Wed Oct 17 2018

.ANALYSIS AND UPPER LEVELS...
A very large and broad upper level trough remains in place across
the Bering Sea and Alaska. It stretches from just east of the
Kamchatka to the Alaska Panhandle. The parent fully stacked "anchor"
low remains over the eastern Bering Sea with the weakening 987 mb
surface low center just west of the Western Capes. Weak low level
cold air advection is wrapping all the way from the western
Bering Sea, where gale force northerlies exist, into the Gulf of
Alaska. This is creating a convective low level marine boundary
layer with sufficient instability to support isolated
thunderstorms, some of which are moving inland along the immediate
Gulf Coast. Middleton Island and Kenai radar are also depicting
"streamers" (convective elements) moving north off the coast and
coastal islands, which is also indicative of the convective
environment. Thus, while inland downsloping exists, some showers
are making their way inland over Southcentral. The same general
pattern exists over Southwest Alaska as unstable onshore flow
keeps showers in place.

Finally, looking south towards 38N 168W, MIMIC TPW precipitable
water satellite imagery, as well as the global models, are picking
up on a developing low along a stationary subtropical warm front.
This basal low will become a major low as it moves into the Gulf
of Alaska Thursday and Thursday night (see the morning SPS, model
discussion, and the Southcentral discussion for more details).

&&

.MODEL DISCUSSION...
The numerical models are in excellent agreement with the general
pattern heading into the weekend. However, they remain in lesser
agreement with what is looking more likely to be a very powerful
storm force bomb cyclone Thursday into Friday over the Gulf of
Alaska. The GFS and NAM have led the way among all the models with
this storm as now all models (including the former weak outliers
ECMWF and Canadian GDPS) are depicting a rapidly developing warm
seclusion as a tropospheric PV anomaly intersects the warm front,
forming what looks to be a tropopause fold and the classic low
level bent- back front and rapid frontal fracture associated with
these types of storms. What does all this mean? It means this low
will become rather fierce over the Gulf waters and will likely
spread storm force winds across much of the Northern Gulf Coast.
Strong winds will also move into Prince William Sound and down
Cook Inlet. Track differences remain, but they are all
consolidating on a track just south of the Kenai Peninsula, then
moving slowly inland in the vicinity of PWS. Right now, the NAM
and GFS were favored, but the 00z ECMWF now supports these models
as well. The Canadian is an outlier as the 06z Canadian RDPS
depicts a more southern track, although it still rapidly bombs the
low out (35+ mb in 24 hours).

&&

.AVIATION...
PANC...Gusty southeast winds will persist through today then
slowly diminish overnight. Periodic showers will move through the
region through today, but they are not expected to be heavy.
Ceilings and visibilities should remain VFR even during these
showers, though the possibility exists for a brief drop to MVFR
conditions in a shower.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Today through Friday)...
Generally cloudy and showery weather will best describe the
weather over much of Southcentral over the next 2 days as a nearly
stationary trough remains over the eastern Bering. Cold air
wrapping around the southern/eastern edge of this low will
continue to bring weak instability to much of the Gulf and Gulf
Coast as the cold air moves over the warm waters. This has the
potential to bring thunderstorm activity over much of the Gulf
today, as well as gusty winds as heavier cells pass overhead. Gap
winds will also persist from Turnagain Arm eastward into the
Copper River Basin as a persistent pressure gradient along the
Chugach weakens through this afternoon. This will bring continued
strong winds into south/west Anchorage as well as near Palmer for
much of the day today.

The focus in the short term forecast will then shift towards a
developing North Pacific low that will rapidly intensify as it
tracks northward into the Gulf late Thu-Fri. While there are some
minor differences in how quickly this low will move through the
Gulf, confidence is very good that it will bring a broad area of
storm force winds from the North Gulf into Southern Cook Inlet
late Thu night before making landfall near Seward Fri morning.
Gap winds throughout the Gulf Coast will rapidly increase in
response, bringing gusty winds not only to the usual gap wind
areas but also into the lower elevations of Southcentral for
Friday. The second impact of this front will be for yet another
round of heavy rain to the Eastern Kenai Peninsula, including the
Seward area. The heaviest rain is likely to occur further north
towards Portage/Whittier where an additional 3 inches is possible
with strong easterly flow, while Seward will likely see more in
the neighborhood of 1.5-2" due to the less favorable flow for that
area. However since this rain is quickly coming on the heels of
recent heavy rains in the area, the potential for further stream
rises and flooding is again possible from this event.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Today through Friday)...
A vertically stacked low centered over the eastern Bering will
continue to rotate bands of showers onshore and across Southwest
Alaska today. As the low starts moving slowly south tonight and
and then stretches out into a trough Thursday, showers will
diminish and become more scattered in coverage. Thursday night, as
a strong low tracks up into the Gulf an associated easterly wave
will cross the Alaska and Aleutian Range to spread precipitation
into the Kuskokwim Valley and Greater Bristol Bay area late
Thursday Night through Friday morning. Another trough from the
same system will rotate across the mountains Friday afternoon,
spreading additional precipitation over the Kuskokwim Valley
through Friday night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Today through Friday)...
As upper level ridging builds over the Western Bering and an
upper trough over the Bering Strait digs south, the large
vertically stacked low over the eastern Bering will weaken and
elongate into a trough as it tracks slowly to the south through
Thursday. As the western upper ridge shifts east, a frontal system
associated with a Kamchatka low will swing over the western
Aleutians Thursday and into the central Aleutians Thursday night
and Friday.

&&

.LONG TERM FORECAST (Days 4 through 7)...

The forecast period commences with a strong low south of the
AKPEN with that low moving north into the eastern Bering, and the
associated warm front moving into the Gulf of Alaska. There is
decent agreement between the GFS and the ECMWF that this low will
bring another round of heavy coastal rain and more warm air into
most of Southern Alaska Saturday and Sunday.

Monday and beyond, uncertainty rises quite a bit, but it does seem
that broad cyclonic flow will draw down more cold air over the
western Bering Sea with continued warmer than average temps over
most of Southern Alaska. However, the storm track looks to move
south, bringing a break in the stormy weather.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Advisory 125.
MARINE...Gales 170 172-176 178 179 185 351 412-414.

&&

SYNOPSIS AND MODEL DISCUSSION...Ahsenmacher
SOUTHCENTRAL ALASKA...DEK
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
LONG TERM...PS