Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 240032
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
432 PM AKDT TUE AUG 23 2016
.ANALYSIS AND UPPER LEVELS...
The upper air pattern over the area continues to amplify today. A
digging trough is sharpening with a trough axis stretching from western
Alaska through the Bering Sea, Aleutians, and into the northern
Pacific Ocean. The main low circulation is south of the central
Aleutians. Wrapping around the base of the trough is a potent
upper level jet. This jet is supporting strong deep southerly flow
and subtropical moisture advection into southwestern and
southcentral Alaska. This is evidenced through precipitable water
values on morning soundings anywhere between 110% and 130% of
normal throughout Southcentral Alaska and the southwest Mainland.
A weak disturbance tapping into the deep saturation this morning
brought more heavy rainfall to Southcentral Alaska. However, as
the upper-level support has weakened, rain is tapering off into
lighter showers this afternoon.
With the heavy rainfall that has been observed over the last few
days, flooding continues to be a concern along rivers in the
Susitna Valley and the north Gulf Coast. With rainfall expected
to continue through the middle of the week, flooding will remain a
concern and rivers will continue to be closely monitored.
Models are in good synoptic agreement through the short term with
the main differences lying in the details. A warm front
approaching the northern Gulf Coast from the south will bring the
next big wave of rain Wednesday night. Models are in good general
agreement with the front, however there are slight differences in
the location and timing of upper disturbances embedded within the
onshore flow. This is all resulting in high confidence in the wet
pattern continuing through Thursday with some smaller
uncertainties on the exact placement and timing of the heaviest
rain. Models are then in good agreement with a transition to a
drier pattern starting towards the end of the work week.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The wet pattern in place for several days continues through the
first half of Thursday. Water vapor satellite depicts an
organizing wave moving north toward the Alaska Peninsula, with an
upper level warm front signature approaching the southern
coastline. This feature will be the leading edge of our next
significant moisture push later tonight into Wednesday. If there
is a silver lining with this system, it is that stability will be higher
than with the Monday-Monday night system, thus rainfall amounts
should not be quite as robust with this coming system. Heaviest
rainfall amounts will favor south and east-facing slopes tonight
into Wednesday as southeasterly lower level flow will increase in
unison with the southwesterly upper flow. This means Western
Prince William Sound and western Susitna Valley areas will see the
heaviest rainfall amounts, with Anchorage and Palmer areas seeing
much lesser amounts within the downslope hole. As the low level
flow becomes southwesterly in tandem with the upper flow late
Wednesday into early Thursday, rainfall will begin to favor south-
and west- facing slopes, and more widespread steady rainfall can
be expected over much of the region, including Anchorage.
By Thursday, a surface front finally begins to move east through
the Copper River Basin and central Gulf, with the potential for a
significant dry slot moving north and east through the Cook Inlet
region by midday ahead of the main upper level low. Along with
bringing some clearing skies and sunshine, this will help
contribute to increased instability over much of Southcentral
Alaska, with the potential for scattered to numerous showers and
isolated thunderstorms to develop. 0-6 kilometer shear values
look to be rather decent at 25 to 40 knots, and if instability
trends upwards, it is possible for isolated thunderstorms to be
fast-moving and somewhat organized, so this bears some watching.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Southwest Alaska remains in a moist, unstable and showery air mass
in between two troughs this afternoon with the potential for a
few isolated thunderstorms through the evening hours. The next
developing frontal wave will reach the Alaska Peninsula tonight
and Southwest Alaska Wednesday morning. Showers will change over
to steadier and widespread rain as the front lifts north with
breezy east winds increasing through gaps in the Aleutian Range.
Rain will change back over to showers Wednesday night then taper
off to a more scattered coverage through Thursday morning as the
front moves further off to the east. Showers will pick again
somewhat Thursday afternoon as solar heating supports increased
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
A warm front out near the western Aleutians will push gradually
east tonight and Wednesday to weaken and dissipate as it moves
into the central Aleutians Wednesday night and Thursday. The
associated cooler front following behind will reach the western
Aleutians late Wednesday night, then swing east to the central
Aleutians through Thursday afternoon. Upper level ridging over the
western Bering will move into the central Bering through Wednesday
night and into the eastern Bering Thursday.
.LONG TERM FORECAST (Days 3 through 7)...
The advertised pattern change is still on schedule but just a
touch quicker than expected yesterday. By Thursday evening high
pressure will begin to build across the Alaska mainland which will
help begin the pattern change as offshore flow develops. This flow
will help dry out the lower levels of the atmosphere and decrease
rain chances Friday through the beginning of next week. The upper
level pattern is also changing but the upper level ridge will
remain anchored over Southwest Alaska with a broad upper level low
over the Alaska/Canada border. This complicates the forecast a bit
as it will likely keep a few showers near higher terrain in the
Copper River Basin and possibly Susitna Valley. However, much of
Southwest and Southcentral Alaska will easily see some of the best
weather this August has yet to offer. Confidence in the nice
weather decreases on Monday as models are struggling on how to
resolve an upper low of the Bering Strait. This feature, although
well outside of our forecast area, will impact the amplitude of
the building ridge over the western half of Alaska. Should this
low end up splitting it`s energy with a low in the Arctic and
another low near the Aleutians then the Alaska mainland could be
in for an extended nice period of weather. On the other hand, if
this feature gets cut off, then it will limit the extent of the
upper level ridge and likely bring parts of Alaska back into a
wetter period by the middle of next week.
PUBLIC...Flood Advisory 125.
SYNOPSIS AND MODEL DISCUSSION...SA
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR