Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 301301
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
501 AM AKDT THU JUN 30 2016
.ANALYSIS AND UPPER LEVELS...
Upper level lows are over the western Aleutians and the extreme
southern Gulf of Alaska this morning, with a weak col over the
eastern Aleutians. The jet lies just south of the forecast area.
At the surface, lows are over Attu and near the Kuskokwim delta,
with weak high pressure over the northern Gulf. a stalled and
decaying front is draped from the northwestern Bering to the
eastern Aleutians. The thermal trough lies along the Alaska and
Kuskokwim mountain ranges.
Models are in general agreement that waves moving around both
upper lows will be the weathermakers for the next few days. The
upper low over the Gulf will slide into the panhandle on Sat into
Sun. While doing so several waves will move along its northern
flank and across Southcentral Ak at irregular intervals through
Sun. These easterly waves will bring abundant moisture to the area
south and east of the Alaska and Aleutian ranges...and will
interact with the unstable thermal trough this afternoon and
evening. Portions of the waves that actually cross the Alaska
range will also interact with the thermal trough over southwest Ak
through the weekend.
Models are also in agreement that the front across the Bering will
continue to decay, and that a wave moving around the upper low
will affect most of the Aleutian chain through the weekend.
The forecasts use a blend of models, attempting to come to a
consensus with respect to timing of the waves and onset
and duration of precipitation.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
A big rain event is still on tap for Southcentral Alaska, but
the timing has changed significantly in the past 24 hours. Heavy
rain will now arrive later than prior forecasts indicated. As
mentioned in previous discussions models really struggle with
this very dynamic and unstable pattern, though it appears the
latest solutions may all be grasping on to the same basic solution.
Water vapor satellite imagery provides a good picture of what is
on tap for the next couple days. A large low is spinning over the
Gulf with a strong trough axis lifting northward from the eastern
Gulf to British Columbia. Numerous short-waves out ahead of this
trough axis are rotating westward toward Southcentral Alaska. As
these begin to enter Southcentral today expect increasing showers
(and likely some thunderstorms). All of this will consolidate into
a large swath of heavy rain as the main trough axis arrives this
evening. This trough will elongate and weaken Friday, bringing an
end to the steady rain and leaving a much more stable air mass in
place. This will be a good environment for fog formation, so have
introduced patchy fog just about everywhere beginning Friday night.
As far as rainfall totals, there is still quite a bit of uncertainty.
The combination of the convection today transitioning to steady
rain tonight and the fact there are so many upper features
involved means rainfall totals will vary significantly. Still,
average amounts of one half inch to one inch seem reasonable for
most areas...with one to two inch totals possible where the
convection sets up. Thus, still expect rapid rises in area creeks
and streams and potential for minor urban flooding in Anchorage.
The timing of heaviest rains and rising water levels will be
overnight tonight through Friday morning for most areas. The
special weather statement which was issued yesterday provides a
good summary of these threats.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Easterly flow with embedded disturbances has become firmly
established across Southwest Alaska. Isolated to widely scattered
thunderstorms continued well into the early morning hours this
morning with marginal instability and decent wind shear (to help
maintain organization and longevity) remaining in place above the
near-surface inversion. About 150 to 200 lightning strikes were
detected by the GLD360 lightning detection network toward the
coastal areas of the Kuskokwim Delta and Bristol Bay coasts last
evening through the early morning hours this morning.
The warm, moist and unstable air mass will only become more
unstable today as another disturbance now over the Alaska Range
moves across the area today around the energetic upper level low
over the Gulf. Showers will become more widespread as the day
progresses with isolated thunderstorms. Shear will not be as
strong as Wednesday but instability will be a bit higher and will
help to keep storms somewhat organized especially toward the
coasts and along the western Alaska Range today.
Another series of shortwaves will rotate around the parent low
bringing showers and possibly embedded thunderstorms to the area
Friday into Friday night. The Friday-Friday night storm appears
to be unusually strong and deep through the atmosphere, with
locally heavy rainfall an increasing threat where an increasing
moisture-laden low-level jet streak interacts with a
strengthening surface trough from the interior Kuskokwim Valley
south into the Bristol Bay region. The NAM model depicts rainfall
amounts with this storm well over an inch, with the GFS about half
that much. Confidence on timing of heavy rainfall is still low but
the confidence is increasing that such an event will occur, as
easterly waves are notorious for their timing across the southern
half of Alaska especially during Summer.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The stationary front that sits in the Central Bering just west of
the Pribilof Islands and stretches south over the Eastern
Aleutians will continue to diminish, and then dissipate as a
disturbance approaches the Central Aleutians late tonight into
Friday with a renewed round of rain. This system will elongate and
weaken much as the current front is doing south of the Pribilofs
and in the vicinity of the Eastern Aleutians Friday night.
.LONG TERM FORECAST (Days 3 through 7)...
The extended forecast beginning this weekend will start off with
two lows, one weakening over the western Bering, and another
developing over the Gulf of Alaska on Saturday. Easterly shortwave
troughs rotating around the Gulf low will be supporting rain
across much of Southern Alaska, some possibly heavy at times. The
first of these waves will rotate southward through the Kenai
Peninsula and Cook Inlet region Friday. The second wave will
track through the Southwest Mainland while weakening Saturday,
and quickly push southward over Kodiak Island by early Sunday
morning. However, this is where confidence drops off
significantly, as the model solutions diverge greatly with regards
to the secondary wave rotating around the parent Gulf low. Either
way, expect more diurnally driven showers over parts of the
southern mainland, as the Gulf low begins to track further
eastward. The warmest temperatures will likely be west of the
Alaska Range with moderate temps across Southcentral. By early
next week, weak ridging is expected to build in behind the
departing Gulf low with continued inland showers driven by diurnal
SYNOPSIS AND MODEL DISCUSSION...DS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CC