Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 290036
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
436 PM AKDT TUE JUN 28 2016
.ANALYSIS AND UPPER LEVELS...
The most notable feature on the surface map this afternoon
is the slowly weakening area of low pressure that has been
wobbling in the south Gulf. To the north of this feature
is a fairly prominent area of high pressure that stretches from
the PAC NW along the coast and into the interior. To the east of
the Kamchatka peninsula is a weakening surface low with a front
stretching to the southeast and laying just west of Dutch. The
primary upper level jet helping to support these lows is sliding
south of 50N at this time. A few showers are seen on radar this
afternoon across the mountains, but they are beginning to weaken.
On the Kenai Peninsula, a seabreeze was noted on radar returns
and looked to be the primary forcing mechanism for showers forming
near the mountains due east of Kenai. Other showers are working
from east to west north of Homer and south of Ninilchik. Most of
these showers will rain themselves out over the course of the next
few hours. A dryer flow will be starting overnight helping to dry
the column out some across Southcentral.
The overall synoptic picture seems to be well handled by all
models in the entire weather suite this afternoon. All models
have a good placement of both the upper level low over the
Gulf of Alaska as well as the low across the western Aleutian
chain. Differences do not start to show until after 48 hours.
Those differences are rather minor in the grand scheme of things.
Through the late part of the week all guidance weakens the low
over the gulf and slowly shifting to the northeast. The ridge
separating the Gulf and Bering Lows will erode some but pinch off
across the interior. Models really begin to diverge come this
weekend when trying to pin down details on the evolution and
degradation of the both upper level lows. At the surface the story
is very similar to the upper levels. The biggest overall
differences don`t appear until late in the week. That said...
there are some timing and spatial differences being worked on with
h50 shortwave timing and trending. All guidance was lock in step
with the weak wave moving across our area this morning and
providing some showers across valley. The second wave is being
handled a little faster by the NAM and bringing showers into the
area by Weds night. One thing all guidance does capture is the
wave dropping across the Copper River Basin and across the
Anchorage Bowl bringing in a good swath of rain. A third quickshot
(almost as a one - two punch) will be right on the heels of the
previous wiggle in the flow bringing more to the area 12 to 18
hours later. The next shortwave beyond the first grouping will
slide from east to west across the same area into Fri afternoon.
The remains of a weak jet streak will slowly shift north -
helping to give a little upper level support to the late week
SSS.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The weak 1013 mb low remains in the southern Gulf, and has a stack upper
level low spinning shortwaves around this feature through midweek.
These waves will track from east to west across the Gulf, and
affect the southern Kenai southward to the Kodiak Island region.
Therefore, the aforementioned regions will receive majority of the
showery precipitation with these synoptic features through Wednesday.
There is a weak ridge over the interior that is resulting in
clearing of skies, and should prevent any thunderstorm activity
this evening. By Wednesday afternoon/evening look for isolated
thunderstorms along the Alaska Range and the Talkeetna Mountains.
The thermal trough will drop down from the north, and
thunderstorms will likely develop through the Tanana Valley.
Combine this with the easterly/southeasterly storm motion, look
for some thunderstorms to cross the Alaska Range into our AOR.
Meanwhile, beginning late tonight through Wednesday and beyond
the forecast becomes extremely challenging over Southcentral
Alaska. The upper level low opens into a trough and swings
northward across southeast Alaska. This will send a steady series
of "easterly waves" across British Columbia, and the southern
Yukon into Southcentral. These usually produce 6 hours or more of
steady rain along with widespread clouds, and cooler temperatures.
Models traditionally struggle with the track and intensity of
these waves, often not gaining a good handle on them until 6 to 12
hours out. The first of these upper level waves will enter the
Copper River Basin late tonight then track westward across
interior Southcentral during the day Wednesday. Used a fairly
broad-brush approach to precipitation forecast with this do to
varying representations in guidance and overall low confidence in
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
The synoptic pattern that will last into Friday features an upper
level low over the Gulf that will slowly track east and high
pressure to the north. This will leave Southwest Alaska in an
unstable easterly flow pattern. The combination of afternoon
heating and easterly waves rotating around the low will kick off
showers over the area as the waves pass and during times of peak
heating. Even with the weaker waves moving through there should be
enough instability for thunderstorm development through Thursday.
Thursday also looks to have the strongest shortwave which will
lead to more rain on Thursday than the previous two days.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The front that has pushed into the Central Bering is now stalling
and will stay in approximately the same place through Thursday
before dissipating. Rain and winds will be associated with this
front, but will also gradually diminish as the front fades.
Throughout much of the Bering and Aleutians abundant low level
moisture has allowed for areas of fog to develop. These areas of
fog look to linger into Wednesday but will diminish later in the
week as the pattern becomes more active.
.LONG TERM FORECAST (Days 3 through 7)...
The extended forecast beginning late week will two large lows, one
over the western Bering and another over the Northern Gulf of
Alaska. Easterly shortwave troughs rotating around the Gulf low
will be supporting rain across all of southern Alaska, some
possibly heavy at times. The first of these will be rotating into
southwest Alaska by Thursday night, with the possibility of
another moving into Southcentral Friday and Friday night.
However, this is where confidence drops significantly as the model
solutions diverge greatly with secondary waves rotating around
this parent low. Either way, by the weekend, expect more showery
conditions inland as the Gulf low begins to move east. The
warmest temperatures will likely be west of the Alaska Range with
moderate temps across Southcentral. By Monday, weak ridging will
build in behind the departing Gulf low with continued inland
showers driven by diurnal heating. The Bering low will move east
into Southern Alaska by Tuesday, with more clouds and rain chances
coming with it.
SYNOPSIS AND MODEL DISCUSSION...SS
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DK