High Wind in Alaska, November 29, 2017

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
419 AM AKST Wed Nov 29 2017

.ANALYSIS AND UPPER LEVELS...
A powerful low remains over the eastern Bering Sea this morning as
a potent North Pacific disturbance has retrograded northwest
across the Alaska Peninsula and absorbed the previous low to
become the dominant feature, presently located approximately 90
miles northwest of Cold Bay. Gusty northwest winds are present on
the back side of the system, impacting portions of the Aleutian
chain west of Dutch Harbor although gusts have remained below
warning level as of early this morning. Farther east, a front
associated with the Bering low is quickly pushing inland along the
Gulf coast, bringing numerous rain showers and gusty winds to
many coastal locations. Inland portions of Southcentral AK remain
mostly dry as a result of persistent downslope flow in the lee of
the coastal mountain ranges. Finally, satellite imagery reveals a
very unstable and convective air mass over the North Pacific
behind the front, which should be a harbinger of a continued
showery pattern along the Gulf coast in the coming days.

&&

.MODEL DISCUSSION...
Models have a good handle on the synoptic pattern through late in
the week as the Bering low tracks eastward into the Gulf and then
eventually into the northeast Pacific. However, they continue to
struggle to resolve the numerous waves tracking northward through
the Gulf in the next 24-36 hours, introducing challenges to the
forecasts for both winds and precipitation in the short term.
Primarily utilized Hi-Res NAM guidance for the morning package in
an attempt to more accurately depict these features.

&&

.AVIATION...
PANC...Light snow showers will spread into the terminal after 18z
this morning as a weakening front continues to push inland along
the Gulf coast. This will lower cigs below 5000 ft, with a brief
window of MVFR conditions possible prior to 00z this afternoon.
Conditions will improve once again thereafter as precipitation
tapers off.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Low level flow along the Gulf coast, Prince William Sound, and the
Kenai Peninsula up to Anchorage will rapidly weaken early this
morning as a front pushes onshore and rapidly dissipates. The
airmass is quite unstable behind this front as indicated by the
low level lapse rates in the 12Z Kodiak sounding: 7.1 degrees C
from 0 to 3 km and 8.5 degrees C from 3 to 6 km above ground
level. Cold cloud tops are also very evident in IR Satellite
Imagery. The combination of the weakening low level flow, unstable
air, and a plethora of upper level waves rounding the Bering Sea
low and tracking northward across the Gulf will allow some showers
to progress inland today. The flow will remain out of the
southeast, so expect light snow accumulation of less than one inch
for most areas, especially for Anchorage and the Matanuska
Valley. Locally higher amounts are likely in the mountains and
across the western Susitna Valley.

The bulk of showers will remain along the Gulf coast and they
will be heavy at times. The southerly flow today is particularly
favorable for showers to train up Resurrection Bay into Seward.
The big forecast challenge is precipitation type. Seward is just
above freezing this morning, but southerly winds behind the front
will likely cause a spike in temperatures. Meanwhile, cooler air
will move in aloft which is more supportive of snow. It could be a
battle between rain and snow, with heavier showers producing snow
and lighter showers producing rain. Forecast mainly rain along
the immediate Gulf coast this morning, transitioning to rain and
snow or all snow this afternoon and evening. Snowfall will be wet
along the coast and accumulation will be highly variable.

Showers will persist, mainly along the coast, through Thursday.
The Bering low will then exit eastward across the Gulf bringing
a drying (and cooling) trend to Southcentral and the Gulf late
Thursday through Thursday night.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...

Our much-talked about low continues to slowly retrograde (move
westward) early this morning, just to the northwest of Dutch
Harbor. Vort maxes rotating around the periphery of the low will
bring bursts of weak ascent, aiding in the showery precipitation.
However, the best dynamics are removed west of the region, thus,
only light amounts of rain or snow will accumulate. This low will
continue to wobble in the same general vicinity through today,
before finally starting to slide off to the southeast. As this
occurs, easterly surface winds will gradually become northeasterly,
with precipitation diminishing. A shortwave ridge building across
the region will bring drier weather for late Thursday through
Friday morning, before another front brings widespread
precipitation back to the area.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...

A strong area of low pressure continues to trek slowly westward
off of the central Aleutians between Nikolski and Dutch Harbor
early this morning. This low is forecast to meander in place
through the day before sliding off into the Gulf of Alaska on
Thursday. Meanwhile, a transient short-wave ridge currently
edging into the western Aleutians will continue its eastward
journey through the next 36 hours, on the heels of the departing
low. Although no warning level winds have been observed as of yet,
there is still some potential for this to occur through mid
morning as the pressure gradient increases. The disturbance
rotating around the area of low pressure are the remnants of
yesterdays system of similar strength, that got absorbed into the
main system. It will dive southeast towards Nikolski through mid
morning.

Infra-red imagery has shown some cloud top cooling ahead of this
feature, indicating a locally enhanced cold core still exists.
This combined with the increasing pressure gradient may be just
enough to produce a few hurricane force gusts, but the window on
this potential is closing fast. The latest ASCAT pass also showed
winds were only around 45 knots, adding to less confidence of this
occurring. However, we left the warning intact to account for
increased momentum transfer potential with the incoming
disturbance.

Otherwise, as the ridge progresses from west to east over the next
couple of days, another area of low pressure will enter the
western Bering Friday morning. This low will send a fast-moving
cold front eastward during the next 48 hours. The associated cold
front will enter western portions of the chain tonight, reaching
the AKPEN by Friday morning. Widespread precipitation will
developing along the front, with more showery type precipitation
closer to the incoming low.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The long term forecast beginning with Thursday night starts with a
low over the western Bering that will push its front over
southwest Alaska on Friday and then Southcentral Alaska Saturday.
After that the forecast gets messy. Models are having a difficult
time handling a North Pacific low that will track toward the
Alaska Peninsula Sunday.

Some models are bringing it east toward various locations around
the Gulf while the majority bring it into the eastern Bering. The
timing of this low has also been inconsistent as well as its
strength. Some models have it dropping into the 950 mb range just
before it reaches the Alaska Peninsula while others are higher.
Uncertainty with this system is high.

By late Monday into Tuesday there are large differences with the
operational model runs leading toward an ensemble mean approach
for much of next week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None
MARINE...Storm Warning 172 173 174 175
		 Gale Warning 155 170 176 177 178 185 351 352 411 412 413

&&

SYNOPSIS AND MODEL DISCUSSION...CB
AVIATION...CB
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...DWK