Gale Warning 406, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
425 PM AKDT Sun Sep 8 2019

.ANALYSIS AND UPPER LEVELS...
The upper level disturbance that passed over the region yesterday
giving us the widespread rainfall, has stalled over the far
western Copper Basin (based on satellite imagery) as it bumps into
a ridge centered over the Yukon. Further west, a long wave trough
stretches from a low over the northern Bering southward through
the central Aleutians. Also associated with this is another weaker
low passing by to the south of Shemya.

&&

.MODEL DISCUSSION...
The models have been too aggressive with the disturbance bumping
into the ridge as it currently sits over the far western Copper
Basin versus the central portions of the valley as earlier
thought. As a disturbance rides north in the southerly flow, the
ridge over the Yukon builds westward, causing the vorticity ribbon
to retrograde further west. With much drier air at 700 mb and
above compared to yesterday, as the incoming disturbance interacts
with a moist southerly low level flow, the latest short range/hi-
res models are not surprisingly painting a much bigger coverage
of precipitation overnight for the Mat-Su Valleys south through
the Kenai Peninsula.

Then, as another weakening front pushes into southcentral from the
west, rainfall coverage is also projected to be higher for Monday
as additional lift and moisture aloft interacts with the
disturbance mentioned above, with additional vort maxes rotate
into southwest Alaska in the cyclonic flow around the Bering low.
This helps foster increased precipitation coverage seen in the
model data.

&&

.AVIATION...
PANC...Light winds will prevail through the forecast period. Some
MVFR conditions are possible on Monday morning with some showers.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
The wave of energy that pushed east yesterday is now retrograding
back to the west, unable to advance due to a stubborn ridge
parked over the Copper River Basin and Yukon. As this feature
drifts back to the west, it will run into another wave associated
with a larger trough over the eastern Bering and Southwest Alaska
that is pushing east over the Alaska Range. The result is an
increase low-level convergence and the development of a swath of
light rain from Whittier and Portage, across the Anchorage Bowl,
up to Talkeetna beginning later this evening and lingering through
midday Monday. Although rainfall is likely to be a steady light
rain (maybe beginning as a few sprinkles), QPF amounts are not
expected to be near the previous event, maybe around 0.10" to
0.25".

As this first system lifts northeast on Monday, the broad trough
from the Bering will push across the AK Range. This should make
for a cool and cloudy day across all but the Copper River Basin
with another shot of rain beginning late Monday for the Kenai
Peninsula and areas north.

Showers linger into Tuesday as this second wave lifts north across
the interior. Another system approaches from the Gulf, with a
surface low and weak front moving east of Kodiak City overnight
Monday, lifting north to the Kenai Peninsula and Prince William
Sound on Tuesday. This system is then reinforced by a deeper low
moving across the Gulf pushing more moisture into the western and
northern Gulf, keeping showers across Prince William Sound
through midweek.

Given the lingering low-level moisture from the previous rain
event coupled with more moisture feeding in from the Gulf, areas
of fog are also possible along the Kenai Peninsula and Cook Inlet
overnight tonight into early Monday and again Monday night into
early Tuesday as the waves progress across Southcentral.

&&

.FIRE WEATHER...
With the Copper River Basin still under the influence of high
pressure, this area remains the primary area for fire weather
concerns through the next couple of days. Areas east of Eureka
will remain generally dry through Monday with above normal high
temperatures in the lower to mid 60s and minimum RH values of 40
to 45%. By Tuesday, clouds and scattered showers will begin to
move over the Basin. Elsewhere, clouds and periods of light rain
are expected tonight through Monday with the rain tapering off and
transitioning to showers by Tuesday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
Scattered showers will continue over much of southwest Alaska
through Tuesday morning under weak low pressure. Gusty winds are
possible over the Kuskokwim Delta through this evening. Partial
clearing is likely Tuesday under a weak ridge. A strong front will
approach from the west on Wednesday morning, brining heavy rain
and stronger southwesterly winds over coastal areas through the
day. The prolonged westerly winds across the Bering south of the
associated low could likewise potentially lead to storm surge
and/or coastal erosion issues. It is too early to determine
specific locations, but the overall large-scale pattern supports
these potential impacts.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
Scattered showers will continue across the Aleutians through
Monday, south of a weak low. Benign conditions are expected across
the central and eastern Aleutians under a weak ridge. A strong
front will track across the Bering on Wednesday, bringing strong
southerly winds and moderate to heavy rain moving west to east
over the Aleutians.

&&

.MARINE (Days 3 through 5/Wednesday through Friday)...
Confidence is high that a Gale force low will track eastward
across the northern Bering Sea Wednesday through Friday. This will
lead to widespread gale force westerlies across the southern
Bering Sea and much of the coastal waters along the Aleutians and
all the way to Southwest Alaska. There is a chance for a small
area of storm force winds along the west coast of Alaska Wednesday
and possibly just south of the low center over the Bering Sea
offshore, but these winds would be brief and over a fairly small
area. The long fetch of westerly winds across the southern Bering
Sea will effectively build up the seas through this time period,
reaching a range of around 15 to 20 ft.

The western Gulf and nearby coastal waters will be quiet until
the front out ahead of the Bering low arrives on Thursday. The
front will be in a weakening state as it arrives and small craft
advisory winds are likely. However, there is a possibility a new
low may form along the front and strengthen winds. Thus, there is
some potential for gale force winds Thursday into Friday.

&&

.LONG TERM FORECAST (Days 4 through 7/Thursday through Sunday)...
The beginning of the extended period will start out wet and windy
as we deal with the remnants of multiple typhoons. Starting
Thursday, the extra-tropical low which formed from Typhoon
Lingling will be sitting over the northern Bering Sea, with a
strong north-south oriented front pushing onshore across Southwest
Alaska. By this time, moisture from a second Typhoon (Faxai) will
be feeding into the front side of the trough and headed to
Southwest Alaska. Thus, expect a ton of available moisture in the
atmosphere. When you add dynamic lift to the equation, expect
widespread heavy rain.

The heavy rain will spread from Southwest Alaska on Thursday to
Southcentral Alaska and the Gulf Thursday night and Friday. There
will be strong southeasterly flow ahead of the front, which
oftentimes produces downslope drying on the west side of mountain
ranges. However, in this case flow will quickly veer to south to
southwest as you head up in the atmosphere. When you factor in the
high amounts of moisture and intensity of the rainfall, downslope
flow will be a minor player with this storm and would expect all
of Southwest and Southcentral Alaska will see at least moderate
rainfall out of this. While it has been wet of late, we remain
well below normal for rainfall this time of year. Thus, do not
expect flooding will be a big threat.

The threat storm surge induced erosion and flooding along the
Southwest Alaska coast will be something we'll continue to watch
Thursday through Friday. The Bering low will track eastward to
Southwest Alaska with multiple days of a long fetch of westerly
winds across the southern Bering Sea and into Southwest Alaska -
all very favorable for producing storm surge.

By Saturday, conditions will become more showery over the
Southern mainland as the remnants of the Bering Low and upper
trough settle overhead. Then expect a drying trend as we head
through the weekend and this all begins to exit eastward.

Beyond this initial system, models diverge significantly and
forecast confidence is low. With upper level flow likely to remain
progressive, would expect additional another storm to move into
the Bering Sea by next weekend. All indications are the west
Pacific tropics will remain active, as is common this time of
year. So the threat of recurving tropical systems affecting Alaska
will likely continue.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale Warning 178.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...PD
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AP
MARINE/LONG TERM...SB