Area Forecast Discussion
Issued by NWS Anchorage, AK
FXAK68 PAFC 300051
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
451 PM AKDT SUN MAY 29 2016
.ANALYSIS AND UPPER LEVELS...
The high pressure system that has been entrenched over the
mainland for the past few days is beginning to retrograde to the
west, allowing a shift in the pattern for mainland Alaska. Broad
northerly flow on the eastern side of this high has brought dry
air and warm temperatures to the mainland over the past several
days. Today, this is being enhanced by an upper level jet streak
that will move southward over the southcentral region. As the day
continues, winds have already started mixing down with gusty
winds ramping up in the Susitna Valley. This should bring down
dry air from aloft as well. Although the RH values are already low
at the surface there is a significant dry layer just above 700 mb,
as seen on this morning`s sounding from Anchorage. Mixing this dry
air down should continue dry conditions around the region.
A second feature of interest this morning is a trough moving in
from the east behind the high as it continues to retrograde. This
trough is expected to pick up what little mid level moisture is
available, and combined with the upper level instability generated
by the jet streak, lead to development of thunderstorms as the
trough moves westward. Satellite shows convective activity
already this morning and a few early lightning strikes near the
Al-Can border indicate that the instability is definitely there.
Further west the pattern continues to be dominated by the upper
level high. An upper level low just south of the Aleutians will
bring precipitation to the central Aleutians today, but most of
the Bering and the west coast is currently being affected by
marine stratus, as is evident on visible satellite this morning.
As the high shifts westward, flow should be transitioning to
offshore, pushing the remaining fog in the Kuskokwim Delta out
to the Bering Sea. Once this happens, conditions will dry out
Model agreement was fairly high this morning and so confidence
in the short term forecast is high, except for the lingering fog
along the Kuskokwim coast, which was not handled well by any
model. The NAM was chosen for detail resolution in the short term.
By the Tuesday time frame, models begin to diverge on the
solution of the Aleutian low and confidence in the forecast
Winds and dry air mixing down from aloft and thunderstorms are the
two main fire weather concerns for today. As mentioned, winds are
already kicking up in the Susitna Valley, and the Kuskokwim Valley
is also seeing near Red Flag conditions this morning. The struggle
will be along the Kuskokwim coast where fog has taken more of a
stronghold than expected this morning. Winds are expected to
become more offshore with the upper level support in the
afternoon, but the question still remains as to whether that will
happen in time to reach Red Flag criteria this afternoon and
evening. Best chances for that occurring are in the eastern side
of the zone, where skies remained clear this morning.
As for the thunderstorms, conditions look good for the approaching
shortwave to kick off storms in the Talkeetnas and possibly down
along the Kenai as well. This wave will move westward into the
Kuskokwim Valley tomorrow, bringing widely scattered thunderstorms
to that region. For today, the layer of dry air close to the
surface brings a risk that a few thunderstorms in the Southcentral
region may be dry, although they will be isolated. As this trough
pushes westward, the upper level flow will change to be more
southerly, bringing more moisture in and lowering the risk for dry
thunderstorms in any location tomorrow.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
Thunderstorms are popping up around the area this evening, while
slightly cooler conditions are expected through midweek. Scattered
to numerous showers are blossoming mainly along the Chugach and
Copper River Basin this evening while isolated lightning strikes
are showing up as well. These conditions will hold through the
evening before tapering off overnight. On Monday most of the
shower activity will be confined to the Kenai Peninsula in the
morning. As the system that produced these showers exits, a
slightly cooler air mass will come in behind it and will bring
cooler but still above normal temperatures to the mainland with
no precipitation forecast.
A weakening jet aloft combined with a shortwave trough and modest
instability is forcing showers and thunderstorms across the area.
Most of this activity is scattered in nature except for the
leading edge of the wave, over the Kenai Mountains, which is
producing quite a few lighting strikes. This trough will sit over
the southern Kenai Monday before exiting. The air mass behind the
exiting disturbance is much more stable though small pockets of
instability will exist over the terrain.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
Ridging will continue to bring sunny and warm conditions to the
southwest mainland this evening, while a developing surface trough
will bring increasing offshore flow to the Kuskokwim River and
Delta. Over Bristol Bay, this trough will bring onshore flow to
most of the area, while weak instability will produce showers or
even a few cracks of thunder along the Alaska Range this evening.
As this onshore flow diminishes with clearing skies tonight,
patchy fog may develop for a few hours overnight for the low
lying areas of Bristol Bay.
An easterly wave will then push overhead on Monday, bringing
showers and a chance of thunderstorms to the Alaska Range for the
afternoon, which will then move westward into the Kuskokwim Valley
and Delta for the late afternoon and evening hours. Afterwards,
broad cyclonic flow aloft on Tuesday will maintain a chance of
showers for the Kuskokwim Delta and Bristol Bay, while sunny and
warm weather begins to return to the Kuskokwim Valley.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
The gale force low moving through the Aleutians will bring gusty
winds and rain to the region through Monday when it will begin to
dissipate over the Southern Bering. To the west of this low,
northerly flow and cold air advection will allow for persistent
fog/stratus that has developed over the western Bering/Aleutians
to begin to diminish.
.LONG TERM FORECAST (Tuesday through Saturday)...
The pattern heading into the middle of the week shifts toward
dominating low pressure across the gulf and ridging to the west.
The Bering will trend toward a drier and more stable environment
from mid week into next weekend. Meanwhile, low pressure across
the gulf advects moisture across the southern mainland through the
end of the week bringing increased cloud cover and rain chances.
After a relatively dry Wednesday, a deformation zone sets up near
the Alaska range as another low pressure system digs toward the
eastern Beaufort sea coast on Thursday. This combined with several
upper level shortwaves moving around systems will initiate convective
diurnal showers through the weekend. Models are in good agreement
on this synoptic change, however, there are differences in handling
multiple upper level disturbances from the two dominant lows. The
NAM was used to handle the transition into Wednesday and then
switched influence in the extended forecast to ECMWF along with
some of the 06Z GEFS MEAN which provided the best consistency.
MARINE...Gale Warning 155 165 170 172 173 174 175 176 177.
FIRE WEATHER...Red Flag Warning 145 153 155.
SYNOPSIS AND MODEL DISCUSSION...LF
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...DEK