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126
FXAK68 PAFC 221437 CCA
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
413 AM AKDT Mon Jul 22 2019

.ANALYSIS AND UPPER LEVELS...

Subsidence from an upper level ridge stretching across the
interior of the state has limited the amount of convective cloud
cover, with satellite imagery indicating only a few clusters of
cumulus clouds lingering over the higher terrain overnight.
Pockets of low stratus remain over the Southwest, but areal
coverage is trending down as the atmosphere becomes more unstable
on the back side of the ridge and the onshore flow weakens.
Satellite imagery also shows an slight increase in mid-level
clouds across the Copper River Basin overnight in association with
a sheared, weak shortwave undercutting the ridge.

The ridge is bookended by two upper-level lows, one south of the
Gulf of Alaska and a second spinning over eastern Russia. A
vigorous shortwave currently tracking north over northern British
Columbia will pinwheel around the parent Gulf low and become the
focus for the next round of convective activity across Southcentral
Alaska as it lifts to the north and west the next two days. The low
over Russia is associated with a sub gale- force frontal system
draped across the western Bering. A southwesterly flow ahead of this
system is lifting a trough north across the Pribilofs and is
beginning to displace some of the low stratus and fog that has
persisted for days across the Bering.

A vertically stacked low south of Atka is producing a small area
of easterly small-craft winds and low cloud cover across the
central Aleutians. This system, however, is quickly filling in as
a new triple-point low develops downstream well-south of Dutch
Harbor.

&&

.MODEL DISCUSSION...

Overall, models continue to agree on the bigger synoptic picture:
a northward retreating upper-level ridge, an easterly wave sliding
across the southern mainland, and a trough over the Bering moving
toward the southwest coast before lifting up to the north.
That, however, is where the agreement ends, especially concerning
the progression of the easterly wave. The biggest issue is that
guidance wants to shear the shortwave as it moves across the
Yukon, with a portion of that energy moving into the Northwest
Territories and another over southern Alaska. The system then
becomes more like a series of vorticity maxima moving across the
region Monday night through Tuesday night as opposed to one
consolidated wave, and it is the timing of these maxima that is
inconsistent between models and even between runs of a specific
model. The 00z NAM and the hi-res models are fastest overall. The
00z GFS is slower with the evolution (with the latest 06z NAM run
now in line with the GFS). The 00z Canadian and the 00z ECMWF are
the slowest. Given the fact that model consensus and consistency
is still quite poor, a blend of models with a heavier weight
toward the 00z GFS/06z NAM was used for timing and coverage of
clouds and precipitation.

&&

.AVIATION...

PANC...VFR conditions and light winds will persist.

&&

.FIRE WEATHER...

Southcentral...Warm and mainly dry conditions are expected today
with the high pressure over the area. A few mountain showers are
possible, including a slight risk of wet thunderstorms in the
Talkeetna mountains. Cooler conditions with higher humidities are
expected across the area on Tuesday as the high pressure pushes off
to the northeast and an easterly wave moves into the region. A
potential concern with this easterly wave is that in front of it,
from the western Copper River basin through the Susitna valley, it
could be a trigger for widespread convection.

Southwest...Fire weather concerns will be on the increase across
Southwest Alaska as the ridge slides east, clouds diminish, and
temperatures warm. The best chance for thunderstorm activity looks
to be Tuesday afternoon and evening as an easterly wave moves over
the interior of southwest Alaska. The greatest chance for convection
will be along the Alaska Range and the Kuskokwim Valley.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...

High pressure will remain over the area today, keeping most
locations under mainly sunny skies, light winds, and above
average temperatures. Even under the high pressure there will
still be a slight chance for showers in the mountains, and
possibly even a slight chance of some thunderstorms over the
Talkeetna Mountains.

Tuesday and Tuesday night will see a change from the dry pattern
with an inverted trough rotating around a low to the south that
will push into Southcentral Alaska from the east. This trough will
bring rain back to many areas around Southcentral Alaska with the
heaviest rain in the mountains. Although there is still some
uncertainty on the timing and strength of this trough, it looks
like the best chances for precipitation in the Cook Inlet area
will be Tuesday night. It will also bring a chance for some
thunderstorms ahead of it from the western Copper River basin
through the Susitna valley.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Monday
morning through Wednesday night)...

The upper level ridge that has been in place over southwest Alaska
slowly departs to the east through the day today. Meanwhile at the
surface, generally weak onshore flow will allow fog to persist
along the coastal areas this morning, with some low level stratus
and fog being advected inland. As the high departs the region,
several upper level waves are expected to propagate over southwest
Alaska from the east for Tuesday, inducing a pattern change. While a
rumble of thunder is possible this afternoon along the foothills of
the Alaska Range, the greater chance for convection will be Tuesday
afternoon. Isolated thunderstorms become more likely over the Middle
Kuskokwim Valley and interior areas of Bristol Bay through Tuesday
evening. By Wednesday, a front associated with a Bering low will
approach the Southwest coast, bringing precipitation and gusty
southerly winds.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3: Monday
morning through Wednesday night)...

An active pattern over the Bering continues over the next few
days. A low centered south of the Aleutian Chain will bring
gusty southerly flow (especially through the usual gaps and
passes) to the central and eastern Aleutians this morning as the
low moves east. Meanwhile, a front over the western Bering weakens
and lifts northward this afternoon. By Tuesday, an upper level
low approaches the western Bering, bringing westerly small craft
winds to the western Aleutians. This low strengthens as it moves
over the Bering for Wednesday, bringing westerly winds,
precipitation, and cloudy skies to the Bering. Several upper level
waves rotating around the low will likely allow for these
conditions to continue through the end of the week.

&&

.MARINE (Days 3 through 5/Wednesday through Friday)...

There is high confidence in active weather across the Bering Sea
and Aleutians, and Southwest Alaska coastal waters. Expect
widespread small craft winds, though there is some potential for
smaller areas of gale force winds. This will depend on the exact
strength and track of these systems, which is hard to nail down
right now. The Gulf looks quieter, though one of the storms
crossing the Bering could reach the western Gulf by Friday.

&&

.LONG TERM FORECAST (Days 3 through 7/Wednesday through Sunday)...

Weather in the extended forecast period looks very active -
perhaps a shift toward what is more typical for late summer. A
high amplitude long-wave trough over the Bering Sea, supported by
a strong polar jet, will progress eastward toward mainland Alaska
late in the week. There has been a lot of question about how far
this trough will intrude into the mainland. The latest operational
model runs and ensembles both support a more progressive solution,
perhaps bringing the trough all the way across the state next
weekend. If this were to verify, would expect cloudier, cooler,
and wetter conditions across both Southcentral and Southwest
Alaska.

As far as specific storm systems, there is decent confidence in a
low crossing the northern Bering Sea Wednesday through Thursday,
with a trailing front crossing the Aleutians Wednesday and
reaching Southwest Alaska on Thursday. Expect lots of rain and
wind with this front. After this, models quickly diverge on the
evolution of the upper trough - and thus on the track of the
surface features. However, they all indicate continuation of
progressive and dynamic upper level flow. Thus, expect unsettled
weather across the Alaska region.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...BL
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KO
MARINE/LONG TERM...SEB