Small Craft Advisory 338, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
355 AM AKDT Sat Apr 29 2017

.ANALYSIS AND UPPER LEVELS...
The upper level low that has been plaguing the Bering for a week
has finally weakened to an upper level trough. However, it's
eastward progression has continued, and with building upper level
ridging continuing along the Alaska/Yukon border, stronger
southeast flow has returned to Southcentral. Thus, the showers
that formed today had a steady movement towards the northwest,
which reduced the time any one location saw rainfall. On the other
hand, the showers affected almost all areas at one time or
another, so there were fewer locations that stayed completely dry.

Instability across the area increased to its highest value of the
year today on the Anchorage sounding. This helped those showers
that were around at peak heating grow higher into the atmosphere
and have more vertical real estate to produce heavier downpours
with pea-sized graupel reported for a short time in the strongest
ones. Countering that is that the mixing caused significant
drying, especially in the lower levels. So those showers that were
unable to take advantage of the full instability only produced
virga in some places. Shower coverage in Southcentral at any one
time was reduced from Thursday as a result. Outside of the
windier spots, fog has been more of a problem today in some places
such as Kenai overnight with residual moisture from the day's
rainfall activity remaining over the area.

Showers were far more extensive across Southwest Alaska yesterday
than in previous days. This was the result of the upper level
forcing from the low-turned-front approaching. Like 2 days ago in
Southcentral, yesterday the showers followed a training pattern
where repeated rounds of showers moved over the same areas, while
locales in between saw nothing or very little.

With the stacked low of the past week essentially dissipated, the
current focus across the Bering is the slowing front over the
central Bering and the approach of the parent low to the Western
Aleutians. The front has weakened to just producing small craft
wind conditions, but the cloud cover associated with both the low
and the front remains extensive.

&&

.MODEL DISCUSSION...
The movement of the front and its parent low have been well-
resolved in the models for several days now, and tonight's runs
make no exception. Thus, most any model is handling the synoptic
scale setup well. For those model differences, a blend of the NAM
and GFS was the pick tonight, with a super blend of the high-
resolution models used to resolve local wind effects. Forecast
confidence remains high. The challenges of the upcoming forecasts
remain the same as previous ones, namely determining shower
coverage and intensity, and resolving the gap winds across
Southcentral.

&&

.AVIATION...PANC remains with VFR conditions with the pressure
gradient strengthening in the afternoon. Therefore, look for the
winds along the Turnagain Arm to filter into the Anchorage Bowl
with gusty southerly flow by early afternoon hours today. Look
for the southerly winds to diminish by late evening tonight.

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A strengthen low tracking over Haida Gwaii today will send a
weaker, secondary low from the southern Gulf west over Kodiak
Island this afternoon and evening. Although this surface low will
dissipate overnight the associated upper level trough will lift
north across the Kenai Peninsula late tonight through Sunday
morning, and into Southcentral Alaska Sunday afternoon. Surface
ridging will build along the northern Gulf Coast Sunday, and
Sunday night with southeasterly gap winds increasing as front
swings from the southwest to northern Gulf.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The showery regime will continue over the southwest Mainland
through Sunday morning, with shower activity peaking during the
afternoon/evening hours especially over terrain. The area will
remain under the influence of warm, moist, south to southeasterly
flow as a warm front approaches from the Bering Sunday afternoon.
Shower activity will peak during the afternoon and evenings,
especially over terrain, before waning during the overnight hours.
As the front moves into the southwest Mainland Sunday into
Monday, rainy and windy conditions will increase along the coast
and through the gaps along the Aleutian range. However, locations
on the northwest side of the mountains will stay a bit drier as
downsloping conditions develop.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The warm occluded front continues its track through the Aleutians
as a broad low sets up across the Bering Sea. The occlusion that
is passing through the Eastern Aleutians now will make its way
toward the AKPen and Southwest Coast Saturday into early Sunday.
As it shifts east, it will continue to produce small pockets of
gale-force gusts and plenty of rain, especially on the Pacific-
facing islands. The current low pressure center west of Shemya
will yield to a developing triple-point low near the Central
Aleutians this morning. This developing low will then become the
driving-force for the weather into the weekend, producing broad
cyclonic flow across the region as it weakens in place over the
weekend.

&&

.LONG TERM FORECAST (Days 3 through 7)...
The trough centered over the Alaska Peninsula at the beginning of
next week will slowly migrate eastward towards the Gulf by the
middle of the week, with a new surface low developing over the
North Pacific and pushing into the central Gulf Tuesday night
into Wednesday. In terms of sensible weather impacts, this will
largely mean more of the same for the southern Mainland with
steadier rain focused along the Gulf coast and more sporadic
showery conditions inland as temperatures continue to run near or
slightly above seasonal normal values. Weak high pressure will
build in across the Bering Sea in the wake of the trough, yielding
fairly benign conditions. Another low looks to approach the
Western Aleutians late next week and bring a better chance of
precipitation to the chain, however confidence in this scenario
remains on the low end as model spread is still fairly large.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE WARNING...155 170 172.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JW
SOUTHCENTRAL ALASKA...JR
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP
LONG TERM...CC