Flooding in Alaska, September 21 to 27, 2017

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
454 AM AKDT Thu Sep 21 2017

.ANALYSIS AND UPPER LEVELS...

The few hours of warm, sunny and clear weather as Southcentral
passed underneath the axis of a transient upper level ridge are
coming to an end this morning. Kenai radar shows the next round of
rain spreading northward up the Kenai Peninsula into Anchorage.
The upper level ridge over Southcentral is getting quickly shunted
eastward by an invading upper level low near Norton Sound.

Precipitation is ending across the Bristol Bay region of
Southwest Alaska as well. The warm front associated with a slow-
moving broad low swirling south of the central Aleutians extends
well east of the low, and that front is the primary source of lift
for the light rainfall. As the jet is set up in a southwest to
northeasterly flow over Southwest Alaska, the rain is not
progressing much into the Kuskokwim Delta region. The low south of
the Aleutians is keeping light rainfall ongoing across much of
the chain as warm, moist air streams northward ahead of the low
and is lifted above the cooler air mass near the surface.

&&

.MODEL DISCUSSION...

The models are in excellent agreement once again with the overall
broad scale pattern, a feat considering the plethora of ways the
pattern can evolve with the upper low moving south into Southwest
Alaska interacting with the low south of the Aleutians, its
attendant warm front, and a rapidly changing jet stream pattern.
Thus, the model differences once again are limited to smaller
scale features. One such difference remains the timing of the
incoming precipitation. Thus far this morning, the radar
reflectivities with the incoming precip have been all bark and no
bite. In other words, despite indicating rain, a lot of it on the
leading edge of the precipitation shield is falling as virga.

During the day today, the NAM has been and continues to indicate
a bifurcation of the forcing across Southcentral. Some of the
energy associated with the incoming upper low goes north into the
Susitna Valley, while other energy associated with the advancing
warm front causes precipitation along the Gulf Coast. Being in
between, locations from Kenai northeastward through Anchorage and
the Matanuska Valley get relatively little, especially as
downsloping southeasterly winds kick in this afternoon. The other
models all tried to indicate the same in previous runs, but have
since backed off a bit. For consistency's sake, the NAM remains
the favorite in the short term. By Saturday however, it becomes
the outlier, racing the low and its associated energy northward
much faster than the other models, so it was eliminated as the
chosen model by that point. Confidence is high concerning the
broad pattern (cloudy and cool), but lower with smaller details
such as total precipitation amounts over the next several days.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist through the
early morning with off-and-on light rain. MVFR ceilings will be
possible for the rest of the day through into the overnight as the
next system moves through. The best chances of rain will be
through the morning with somewhat lesser chances during the late
afternoon and evening. Another round of somewhat heavier rain is
possible again Friday morning.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Rain will overspread most of Southcentral Alaska early this morning
as a warm front approaches from the south. After an initial period
of steady rain, the rain will likely become intermittent for many
areas as the warm front is shunted eastward toward Southeast
Alaska. However, a strong southerly upper level jet will then
drive upper short-waves into Southcentral along with a steady
supply of warm moist air from the sub-tropics. Deep south to
southwest flow will support periods of rain for most of the region
through Friday night. The heart of the Copper River Basin along
with Kodiak City and vicinity will stay drier than surrounding
areas due to downslope flow. Meanwhile, upslope flow will bring
heavier rain to Prince William Sound and the Susitna Valley.

Flow will begin to shift to more of a southeasterly direction
beginning Friday night, as a trough approaches from the west.
This will lead to development of a downslope hole from the
Western Kenai Peninsula to Anchorage and portions of the Mat-Su.
This will also enhance rainfall totals over Western Prince William
Sound. A Flood Watch is currently in effect for Kenai Lake and
Kenai River from Cooper Landing to Skilak Lake due to an outburst
from the Snow glacier dammed lake. The increased rainfall rates
Friday night into Saturday in the Kenai Lake-River system will
contribute to higher river levels. Will continue to monitor the
glacier lake release and forecast rainfall amounts to determine
the extent of flooding.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
A wet pattern ensues through Saturday for much of the Southwest as
a front pushes across the Alaska Peninsula and Bristol Bay this
morning then through the Kuskokwim Valley by the afternoon.
Isolated thunderstorms are possible today across the Alaska
Peninsula as an unstable environment moves in along the frontal
boundary which was evidenced early this morning south of the
eastern Aleutians. The low associated with the front moves slowly
east through Saturday with the frontal boundary stalling across
the Southwest during this time. The Kuskokwim Delta stays on the
drier side through Friday, then increasing chances for scattered
showers enters late Friday into Saturday as an upper level low
develops near the mouth of the Yukon. Patchy fog is expected along
the frontal boundary as the cool saturated air mass is conditionally
stable.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A front moving into the Aleutians this morning brings isolated
thunderstorm chances for the eastern islands and surrounding
marines through the morning hours. The associated surface low is a
slow mover as a ridge across the eastern Pacific waters blocks its
progress. This results in the frontal boundary remaining
stationary through Friday morning, then a colder air mass pushes
through the Aleutian Chain in during that afternoon. A ridge
pattern slides north to south across the western waters through
Saturday. Patchy fog will develop across much of the Bering the as
the stable ridge pattern traps moisture near the surface.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Beginning the extended period forecast Friday night, there will be
an arctic trough centered over eastern Bristol Bay with waves of
North Pacific moisture rotating northward toward the Gulf of
Alaska. This pattern will support wet weather across the region
with the heaviest rain along the North Gulf Coast. Saturday night
into Sunday morning, a stronger front will move toward the coast,
but this feature now looks much weaker than it potentially was
looking the past few days. The main impact will be increasing
winds along the coast, along with more rain across Southcentral.

By Monday and Tuesday, the upper low will begin to dig south into
the North Pacific. This change will cut off the main moisture
supply and allow for gradual drying heading through the middle of
the week.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Watch: 121 125
MARINE... Gales: 119 131 173 174 413 FIRE
WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...JPW
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH
LONG TERM...JA