High Wind and Heavy Rain in Alaska, October 26 to 28, 2019

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
521 AM AKDT Sat Oct 26 2019

.ANALYSIS AND UPPER LEVELS...
A 974 mb low continues to spin southeast of Atka, helping to push
a tropical surge northward. At the same time, a low lifting north
from Hawaii is entering the western Gulf, with a secondary and
more pronounced tropical feed. To its east, a blocking area of
high pressure was positioned across the eastern Gulf, helping to
set the stage for a deep and prolonged tropical fetch, extending
down towards Hawaii.

These affects are already being seen as the latest Goes West
satellite cloud top temperatures pushing -65 degrees Celsius
across southwest Alaska, which based off of the King Salmon
sounding would indicate cloud tops approaching or exceeding 35,000
feet...which is pretty remarkable for this time of year.
Additionally, this morning's sounding from Saint Paul indicated
precipitable water values were over 200 percent above normal. To
further illustrate this, recent rainfall measurements from Dutch
Harbor indicated between three and a half to six inches of
rainfall has accumulated over the past 24 to 36 hours.

&&

.MODEL DISCUSSION...
The models remain in excellent agreement through the period for
the most part. The overnight runs have just a slight difference
from earlier runs, as the GFS and ECMWF continue to show a wave of
low pressure developing along the front as it rides north from
the AKPEN to the Kuskokwim Delta this morning and afternoon. This
has a downstream affect on frontal timing as it progresses into
Southcentral, and on a more localized level, if some of the
Turnagain winds push into the Anchorage Bowl. Otherwise, previous
versions align with the current forecasts.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through the
morning hours. However, this will change during the afternoon as
a front approaches. Pattern recognition indicates the potential
for rotor waves to develop, with wind shear conditions really
beginning to increase after 21z. There remains some question as
to how far past the terminal the southeast winds make it, as they
could become northerly from time to time.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Sunday night)...
Wet and windy weather is on the way for much of the region as an
atmospheric river currently over the eastern Bering Sea and
Southwest Alaska shifts eastward into the western Gulf and
Southcentral Alaska. The overall forecast for the next couple days
is on track, though model guidance has shifted with timing of the
many surface and upper level features. Changes to the forecast
this morning focused on these changes in timing. Most notably,
the onset of precipitation will be delayed. The Winter Weather
Advisory for snow in the northern Susitna Valley (north of
Talkeetna) remains on track, with snow starting by late this
morning and becoming heavier tonight. Did back off total snow
accumulation just a tad, as the airmass steadily warms tonight
into Sunday. Meanwhile, heavy rain in the Seward area won't
really get going until early Sunday. Thus, have pushed the Flood
Watch further out in time to better reflect the most likely time
period for flooding.

While the timing of rain has changed, the expected storm total
amounts are on track. There will be a ton of low level winds and
cross-barrier flow with this system. However, with flow out of the
south aloft and anomalously high amounts of precipitable water
being pumped northward into Southcentral, expect the usual
downsloped areas (western Kenai Peninsula, Anchorage, Matanuska
Valley) to see periods of rain Saturday night through Sunday
night. Thus, have increased the probability of rain for these
areas.

There is potential for gusty winds across most of the Anchorage
area today. Low level southeast winds will increase through the
day as a warm front moves across the western Gulf and pressure
gradients steadily tighten. However, based on forecast soundings
the layer of southeast winds will be fairly shallow, with winds
above mountain top out of the south to south-southwest. This shift
in wind direction is favorable for formation of rotors, which
could mix stronger winds down toward the surface. In addition, as
the surface warms there should be a weak unstable layer off the
surface, with potential for a weak inversion aloft as downslope
warming ensues. Lastly, most of the hi-res model guidance shows
southeasterly winds spreading across the Anchorage Bowl and Glenn
Highway north of Anchorage this afternoon. Have strengthened the
winds in the latest forecast for all of Anchorage. Do not expect
damaging winds, just widespread gusty winds up to 60 mph or so on
the hillside and 25 to 40 mph in town. Down Inlet flow will
strengthen late this afternoon through evening which will bring an
end to these winds, with light north to northwest winds moving
back into most of town.

Have also pushed back the timing of the High Wind Watch for
Turnagain Arm and the upper hillside of Anchorage until Sunday
afternoon through Monday morning. With everything slower to
progress eastward, the frontal systems and associated frontal
waves just appear too far west to produce warning level winds.
The down inlet pressure gradients will also limit the strength of
winds along the upper hillside tonight through Sunday. The best
potential for high winds looks to be Sunday afternoon/evening as
the cold front approaches from the Bering while a surface wave
rides up the front and northward across western Alaska. There is
also some potential Monday morning as the front then pushes
through Anchorage and down Inlet flow finally weakens. From a
pattern perspective, this looks like a borderline situation and
perfect scenario to stick with a High Wind Watch.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1-3: Sat-Mon)...
Wet and windy conditions start off the weekend across Southwest
Alaska as a front continues to move inland, which will persist
through Sunday as the associated front becomes nearly stationary.
Rain moving inland early today will spread across the region
through this afternoon and evening. Strong winds start off along
the Southwest coast this morning and make their way inland this
afternoon across Bristol Bay and along the Aleutian Range. Interior
Bristol Bay will see strong winds with gusts of 65 to 70 mph
through the weekend. The strongest winds and associated gusty
conditions should start to weaken by Sunday evening. By Monday,
the heaviest precipitation will decrease, but a showery pattern
will continue for much of the Southwest.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1-2: Sat-Sun)...
The strong low, remnants of former-typhoon Neoguri, continues to
be positioned south of Adak this morning, with southeasterly gale
force winds across the AKPEN and eastern Aleutians. These winds
will slightly increase through the afternoon hours, bringing storm
force winds into Bristol Bay, which will weaken through the
evening hours.

Meanwhile, storm force winds persists across the western Bering
and Aleutians as cold air advection continues to be pulled in on
the backside of this low. The storm force winds will diminish to
gales through the afternoon as they slowly track into the central
Bering and Aleutians through Sunday.

&&

.MARINE (Days 3 through 5: Monday through Wednesday)...
Bering Sea/Aleutians: A narrow area of gale force northwesterly
winds on Monday will continue across the central Bering from Adak
northward. As the low weakens, the gales in that area should
diminish by Monday evening. Isolated gales are also possible off
the coast of Cape Newenham in southeasterly winds Monday morning,
which will diminish by the afternoon. No gales are expected Monday
evening through Tuesday night. On Wednesday, a new gale force
North Pacific low will move into the eastern Bering and Aleutians.
Gales will be possible during the day south of Dutch Harbor,
Bristol Bay, and are most likely south and east of the low over
the waters south of the Alaska Peninsula.

Gulf of Alaska: On Monday, a barrier jet will produce gales along
the North Gulf Coast and Prince William Sound through the day. The
winds will rapidly diminish Monday evening. No gales are expected
anywhere in the Gulf through Wednesday morning. Gales will be
possible in the Kodiak Island area waters Wednesday afternoon and
evening.

&&

.LONG TERM FORECAST (Days 3 through 7: Monday through Friday)...
A large low over the Bering will be drawing copious Pacific
moisture into Southern Alaska to start off the long-term on
Monday. An atmospheric river aimed at Southcentral will be
producing heavy rain from Seward eastward through Cordova Monday
morning. This feature is expected to accelerate eastward through
the day, so this will be the tail end of the heavy rain in Seward.
More on that in the hydrology section below.

Less extreme rainfall amounts are expected on Monday through
Prince William Sound since the atmospheric river continues moving
east. Nevertheless most of Prince William Sound, including Valdez
and Cordova, should get between 1 and 2 inches of rain Monday
afternoon and evening. Widespread shower activity will follow in
the wake of the atmospheric river across much of southern Mainland
Alaska as the southerly onshore windflow continues. Thus, very
little in the way of sunshine is expected. In addition, strong
winds through Turnagain Arm and the Anchorage Hillside are likely
going into Monday, with only a gradual weakening to the winds
expected through the day.

The parent low over the Bering weakens as it moves into Southwest
Alaska Tuesday evening. However, this will be in advance of a new
North Pacific low that will approach the Alaska Peninsula and
Kodiak Island on Wednesday. There is still significant
uncertainty, with the GFS and Canadian models in decent agreement,
while the EC is significantly slower with the low's movement.
Going with the GFS/Canadian agreement, steady rains move back into
much of Southern mainland Alaska on Wednesday. The low slows to a
crawl either over Bristol Bay or Cook Inlet, which will keep wet
weather ongoing in Southcentral through Thursday, with dryer
weather moving in from west to east on Friday. This may eventually
allow for a return to seasonably cold conditions on Friday,
assuming this solution in the models pans out, along with much
drier weather as well for the weekend.

&&

.HYDROLOGY...
An atmospheric river moving across the coast of Southern Alaska
the next few days will cause several hydrologic issues across the
area. First up will be the Chignik area, where as much as 6 inches
of rain in 36 hours is expected starting after midnight tonight
and continuing into Sunday morning. Since this will be a large
amount of rain for that area in such a short time, flooding and
mudslide concerns exist, but the exact nature of the impacts
remains to be seen. A Special Weather Statement has been issued
highlighting this.

The atmospheric river is expected to next impact the southern
Kenai Peninsula, especially the Seward area, from Sunday into
Monday. This will largely be the result of a slowing of the
eastward movement of that feature in between fronts. Between 3 and
5 inches of rain are expected for that area over a 24 hour period
from about noon Sunday through noon Monday. This is expected to
cause flooding concerns at the airport, at the Lowell Creek
diversion tunnel, and perhaps elsewhere in the area. A Flood Watch
has been issued for the area.

Other areas of concern include the Anchor River area north of
Homer, where a prolonged period of heavy rain may lead to flooding
concerns. The primary forecast question is how prevalent will the
downslope drying be in that area. There is significant
disagreement as to this question, so it too will need to be
monitored. Another concern is the rivers and streams that drain
the Talkeetnas into the eastern Susitna Valley. Rapidly rising
freezing levels may lead to rain falling on top of some freshly
fallen snow, which is also on top of the couple feet of snow
already on the ground in the higher elevations. This could cause
some flooding concerns in that area in addition to increasing the
avalanche threat.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 145. High Wind Watch 101.
         Flood Watch 125.
MARINE...Storm Warning 130 131 160 177 185.
	     Gale Warning 119 120 132 136 137 138 139 141
                      150 155 165 175 176 178 180 181.

FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...PD
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...AH
MARINE/LONG TERM...JW