Gale Warning 602, NWS Anchorage, AK

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

.ANALYSIS AND UPPER LEVELS...

A vertically stacked low continues to weaken and fill this
morning as it spins just off the coast of Cape Newenham. Colder
air wrapping around the center of circulation has advanced from
the central Bering through the eastern Aleutians and AKPEN and
into the western Gulf. This is evident by both the areal coverage
of open-cell stratocumulus depicted by satellite and the
combination of radar imagery and surface observations indicating
showery precipitation falling from Cold Bay to King Salmon. In the
western Gulf, a band of clouds and rain is moving south to north
toward Kodiak Island as a weak trough extends from the base of
the main low.

Farther east, southeasterly flow out ahead of the trough, coupled
with steep lapse rates due to colder air aloft, has allowed
showers to persist across the Southcentral coast overnight. These
showers, however, will become less numerous through the morning
hours as a mid-level ridge amplifies and moves toward the coast
from the southeast.

&&

.MODEL DISCUSSION...

Models have come into better agreement with the development and
evolution of a surface low that looks to impact Southcentral on
Tuesday. However, there remains a bit of uncertainty regarding the
track and intensity of this feature. Both the NAM and GFS have a
well-organized low around 970mb moving into western Prince William
Sound by 00z Wednesday. The EC and Canadian models had a slightly
more southerly track and were slower. Given the consistency of
the GFS and that the NAM is now coming into agreement with the
GFS, a blend of these two models was used for the forecast. A
similar method (this time weighted more toward the NAM) was used
to handle the second low moving through the eastern Gulf on
Thursday through day three of the forecast. However, there is
still quite a bit of uncertainty with this system regarding
overall track and strength as it enters the northern Gulf.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through the
current TAF package.

&&

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

The remnants of the front associated with the strong low over the
area from yesterday are in the western Gulf of Alaska and
Southcentral Alaska this morning. This front will move westward
today and rain showers will taper off as it does bringing in some
clearing skies for overnight. Areas around Cook Inlet that are
more prone to fog will likely not see any as the northerly
gradient will persist and keep the atmosphere mixed well enough to
keep it from forming.

Conditions will rapidly change tomorrow as a strong low will
develop in the eastern Gulf overnight tonight and move rapidly
along the north Gulf coast and into southcentral Alaska Tuesday
afternoon. This low will be fairly compact but still quite
strong. The compact nature of it will actually minimize the strong
winds down Cook Inlet and around Kodiak Island so wind forecasts
were lowered in those areas with the morning package. However,
the northern side of this low will be where the pressure gradient
will be very tight and therefore brought storm-force winds to
parts of the north Gulf Coast marine waters and into Prince
William Sound.

The actual track of this low, which will rapidly weaken as it
moves on shore, will dictate whether or not the gale-force winds
make it into Passage Canal. With a low of this nature, Whittier
can get gale or storm-force winds for a few hours right after the
low passes west of them. However, it is borderline whether the
low will make it far enough west to get strong pressure rises into
Passage Canal. Therefore, went gale though the range of possible
winds are from small craft to storm-force tomorrow.

What is not as sensitive to the track of the low is that
regardless of its path most of Southcentral Alaska should see
precipitation tomorrow and/or tomorrow night as it moves over the
region.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...

A persistent cloudy pattern with occasional showers continues
through Wednesday under a broad upper level trough. A surface low
centered just off the southwest coast this morning pulls further
away later this morning as the next front pushes west across
Bristol Bay and the Kuskokwim Valley. This front rapidly
diminishes by late afternoon before it reaches the coast. Patchy
fog may develop along the western Alaska and Aleutian Ranges early
Tuesday morning as a weak temperature inversion sets up just off
the surface. Another front moves across the region from the gulf
late Tuesday into Wednesday and also weakens as it near the
southwest coast.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...

A significant shortwave dives along the backside of the low
spinning over the eastern Bering today bringing gusty northerly
flow, in the small craft range for marine waters, along with rain
showers. This energy closes off a new parent low over the western
Alaska Peninsula early Tuesday which allows a colder air mass to
advect across the Bering from Kamchatka. Gusty northerly flow is
expected to increase to gales across much of the Bering for
Wednesday into Thursday. The forecast should be monitored for
winds and waves as the polar jet noses toward the Central
Aleutians on Wednesday with models still differing slightly on
the path of the strongest winds.

&&

.LONG TERM FORECAST (Days 4 through 7: Wednesday Night through
Sunday)...

There have been a few changes to the long-term forecast regarding
the low expected to impact the Gulf on Thursday. The model trends
now are suggesting a slower forward speed of the low across the
Gulf. Thus, while the fastest models had the low making landfall
Thursday evening in yesterday's forecast, now the fastest ones
have it moving in on Friday morning. The GFS remains the slow
outlier, not moving the center inland into the Kenai Peninsula
until late Friday night. Secondly, the models are now showing a
steady weakening trend in the low. Thus, its strongest point as
far as lowest central pressure goes is when it enters the Gulf
Wednesday night. The steady weakening should keep winds down
somewhat, especially considering the slowing forward speed of the
low. However, since the low will be starting off in the mid-960s
when it enters the Gulf, it will still take some time for the
winds to appreciably diminish, and as such widespread gales are
still expected over a large area of real estate over the Gulf
waters. Since the center of the low is tracking on more of a
southeast to northwest direction, that should diminish the barrier
jet strength along the north Gulf Coast Thursday into Thursday
night.

The downside to these changes, primarily with the slowing speed of
the low, is that will allow for a more prolonged period of rain
associated with the low along the North Gulf Coast, particularly
along the coastal Kenai Peninsula. The rain will begin sometime
late Wednesday night/early Thursday morning, and continue into
Saturday morning. Depending on the track and intensity of a front
that is not well agreed upon in the models, its moving through
Saturday may cause the rain to resume Saturday afternoon and
continue into Saturday night. The good news once again is the
weakening low should prevent anyone from getting any heavy rain,
but rather a light to moderate rain. It remains unchanged that the
heaviest rain along the Kenai Peninsula into Seward will be after
the low center passes south of Seward on Friday morning. After
that point, the winds will turn out of the south and draw Gulf
moisture into the Kenai Mountains and Seward for the rest of the
day.

As regards the colder air across the Bering, that aspect of the
forecast remains on track early in the period. The core of the
coldest air will be over the western Bering Wednesday night. It
will move over the Pribilofs and the central Bering Friday
morning. The Canadian model stops the eastward advance there.
However, both the GFS and EC continue it into the Kuskokwim Delta
by Saturday morning. Those two models begin to diverge greatly
after that, with the GFS taking the core of the cold air mass well
north after that, leaving the rest of the cold air to warm up pretty
quickly, while the EC leaves the cold air in place over much of
Southwest Alaska through Sunday. Even with this most aggressive
solution, the EC still does not bring much, if any, of that cold
air into Southcentral. Stay tuned.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...None.
MARINE...Gale Warning 120 127 128 129 351 352.
Storm Warning 119 125.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...JW