Gale Warning 434, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion...CORRECTED
National Weather Service Anchorage AK
851 AM AKDT Mon Sep 9 2019

.ANALYSIS AND UPPER LEVELS...

In the upper levels, an upper level high is currently over the
Yukon. An upper level trough is over the eastern Bering. Shortwave
ridging associated with the high is building westward over the
Copper River Basin. Meanwhile an upper level disturbance and
negatively tilted trough is moving northeastward with time over
Southwest Alaska. The approach of these two features towards each
other has lead to rain developing across Anchorage and the Mat-Su
tonight as deformation banding formed. For the area in advance of
the Southwest Alaska trough, periods of rain have developed over
much of the Kuskokwim Delta, becoming more scattered over Bristol Bay.

Temperatures across most of the area are generally in the upper
40s to mid 50s most everywhere across mainland Southern Alaska.
The Copper River Basin is a notable exception, where clear skies
are allowing temperatures to fall into the 30s. Patchy fog and
lower ceilings are also present, as winds largely remain light.
Breezy conditions are impacting Bristol Bay, with winds of 15 to
25 mph occurring around that area.

&&

.MODEL DISCUSSION...

The models remain in very good agreement through the short-term.
All are handling the development of the rain over Southwest
Alaska, the deformation band and associated rain over
Southcentral, and have the general weather pattern well.
Disagreements among the models in the long-term arise by Thursday,
mostly in regard to the track and strength of the Bering low. It
is important to remember the origins of the low, which by Thursday
will be a conglomeration of the moisture from two former typhoons,
Lingling and Faxai. The models have historically been very poor
at handling the transition of typhoons into extratropical lows. So
taking the moisture and dynamics of an already strong
extratropical low that was once a typhoon and infuse the moisture
from a second, it becomes very understandable that there can be
large model discrepancies. With that said, there are areas of
general agreement, such as the low stalling over the northern
Bering, a strong leading front, and good timing on the associated
rainfall, even if storm track and amount of rainfall are still
points of disagreement. Thus, forecast confidence is at least
average, even if the details still need to be worked out.

&&

.AVIATION...
PANC... Rain shower activity ongoing across Anchorage early this
morning is causing MVFR ceilings, with brief periods to IFR
levels. By late morning, the shower activity should diminish,
leaving cloud cover behind for the rest of the day. More shower
activity will move in by late afternoon with the approach of
another slow moving wave. More MVFR to IFR ceilings are possible
again overnight tonight along with continued showers and patchy
fog.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2...Mon and
Tue)...
The week will start much like the weekend ended...mostly cloudy
with periods of rain. A weak deformation zone is stretching out
from the Turnagain Arm up through the Susitna Valley. Along this
boundary, the radar has been active all night with on and off
periods of light rain. This will continue through the morning
hours before finally shifting off to the east and dissipating
this afternoon. The Copper River Basin continues to remain the dry
spot across Southern AK. Some fog is starting to creep up the
Copper River Valley and should increase through the early morning
hours. It will burn off by this afternoon allowing for plenty of
sunshine out there once again today.

A broad upper-level trough will move from SW AK over the Alaska
Range today into Tue. While the low clouds and steady rain should
abate later today, they will be quickly replaced by widespread
rain showers. Upper-level energy will start to consolidate this
evening around the Cook Inlet. This coupled with weak south to
southwest flow will lead to likely PoPs once again as we head
overnight into Tue. The winds across Turnagain Arm will also
respond to this energy starting to becoming gusty from the
southeast. As the trough slides to the east, most of the area
will remain in a cloudy and showery pattern. The exception will be
the Copper River Basin. The trough does not quite make enough
eastward progress to bring widespread rain or showers to the
basin. However, cloud cover should increase helping to keep
temperatures a bit cooler until a stronger system starts to move
north in the Gulf of AK late Tue into Wed. While models are still
struggling in how to handle this system, it does look to push some
steady rain and gusty east to southeast winds at least into the
North Gulf Coast by late Tue/Wed.

&&

.FIRE WEATHER...The general wet pattern with good wetting rain
chances and high afternoon humidities will persist to start the
week. The only place featuring any fire weather concerns really is
the Copper River Basin. High temperatures out there will continue
to hover in the mid to upper 60s with afternoon RH values around
40%. There will be some breezy winds out of the Copper River Basin
this afternoon. In general though, the synoptic pattern is
shifting to much more like Fall with cooler and wetter conditions.

&&

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

Scattered showers continue this morning over the Kuskokwim Delta,
lower Kuskokwim Valley and Bristol Bay areas as a shortwave
rotates northward around a weakening low in the northern Bering.
Showers will diminish in coverage somewhat this afternoon, but not
clear up completely as cold air aloft will still support some
instability. Brief ridging will develop over the Bering Sea Tuesday,
thus expect quieter conditions over the Southwest mainland, and
cloud cover should diminish a skosh. A strong front with a deep
subtropical fetch will arrive Wednesday, bringing heavy
precipitation and strong southwest winds particularly along the west
coast. The prolonged southwesterly winds across the Bering south of
the associated low may bring about storm surge and/or coastal
erosion issues. The impacts will be dependent on the overall track
of the low which is uncertain at this time... this time frame will
continue to be monitored and updated in the next few forecast
packages.

&&

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

Scattered showers over the Bering Sea are diminishing as a ridge
builds in from the western Bering. Generally benign conditions are
expected today over the Bering and Aleutians. Tuesday, a gale
force low will approach the western Bering, with its leading front
making it into the central Bering by mid-day. Gales will spread to
wide swaths of the Bering and parts of the Aleutians by Wednesday,
as the low progresses eastward. Heavy precipitation and strong
southerly winds are likely, given the southern extent of the front
and subtropical moisture draw. Along the low, high wind waves are
likely, on top of heavy swell...wave heights could exceed 20 ft in
the heaviest areas.

&&

.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 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: 178 185 411 412.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...MO
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MM
MARINE/LONG TERM...SB