FXAK68 PAFC 181257
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
457 AM AKDT Mon Oct 18 2021
.ANALYSIS AND UPPER LEVELS...
A broad mid to upper trough wrapped to a diffused upper low near
Kodiak brings steady light to moderate rain over Kodiak Island and
the northern Gulf, including Western Prince William Sound. A
barrier jet meandering over the northern Gulf has generated strong
easterly winds with high end gales near Middleton Island and
mainly south of Cordova and Valdez overnight. Meanwhile, a vast
upper ridge runs across the Bering to Southwest Alaska then turns
east through Southcentral to beyond Wrangell Mountains. Multiple
shortwaves over the Alaska Peninsula and across the Gulf enhances
the potential for increased rain, clouds and gusty winds in those
areas this morning.
Models handle the synoptic systems over the Bering Sea much
better than the revolving and emerging lows over the Gulf and
Southcentral days ahead. Two separate lows over the Bering will
bring active weather across the Aleutians and Southwest Coast, and
models are really straight forward until Thursday afternoon when
the solutions begin to diverge. ECMWF takes the Northwest Pacific
low slightly closer to the north of the eastern Aleutians while
GFS positions its low right over Pribilof Islands which is about
150 nm difference in position. At the Gulf, developing lows from
the North Pacific are absorbed toward a robust frontal system in
the eastern Gulf leading up to a 270 nm variation in the placement
of the lows among NAM, GFS and ECMWF by Wednesday night. Clearly,
lows associated with the front could be a bit difficult to follow
as some could either stay on track or detour from the northern
Gulf in the coming days. For this reason, the forecast confidence
is low beyond Wednesday.
PANC...VFR conditions will persist, with gentle northerly winds
continuing through today and Tue morning.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The main weather feature in the area today is the very strong
front in the Gulf that is wrapped around an elongated low with a
center near Kodiak and another about 300 miles due east of the
first low. There will be multiple lows that develop and roll along
the front the next few days as the front will remain rather
stationary from the eastern Gulf to near Prince William Sound.
Kodiak remains under the strong easterly winds and persistent
rainfall. Winds should start to slowly diminish this afternoon,
but it will remain windy through tonight around the Kodiak area.
This pattern is also pushing rain toward the north Gulf coast, but
the way the front is bent around the lows, it is not pushing
inland. Therefore the rain will be a bit muted even along the
north Gulf coast today until one of those lows along the front
that was mentioned earlier is able to push inland a little bit.
This looks to be most likely to happen Tuesday night as an upper
level wave moves northward through the Gulf and into Southcentral.
This is moving in response to the main upper level trough digging
southeastward across the eastern Bering Sea at that time.
The biggest uncertainty is where this low will move. Models are
basically moving it across the Kenai Peninsula from southeast to
northwest. However, whether it track over Seward and across the
northern part of the Peninsula or crosses just the southern tip of
the Peninsula will have a bit of impacts as to where there will be
the most precipitation and how the wind field behaves. At this
point, going with the more northerly solution, but this will need
to be watched closely until it happens, or at least tips it`s
hand as to which route it will take.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2: Today through
A transient ridge moves over the Southwest Monday, bringing
clearer skies to start the workweek. Temperatures this morning
will start in the 20s before climbing into the 30s throughout the
Unsettled weather returns as a low moves across the Bering
Sea, pushing its front into Southwest Alaska tonight. Snow will
fall initially, then daytime heating and continued warm air
advection will transition the precipitation into a rain/snow mix
or rain through Tuesday afternoon. Further south, at locations
like Dillingham and King Salmon, warm air aloft will keep
precipitation as rain even as temperatures Tuesday morning hover
near freezing. As temperatures decrease into the evening,
precipitation may transition back into a mix or snow before
slowly tapering off. Overall, precipitation is expected to be
light with minimal snow accumulations.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2: Today
A ridge continues to move across the Aleutian Chain, bringing
light winds and drier conditions. Over the Eastern Aleutians and
AKPEN, lingering rain and gusty gap winds will diminish through
Monday morning as the ridge moves overhead.
Behind the ridge, wind and precipitation chances increase as
fronts track across the Aleutians from a low in the northern
Bering Sea. A warm front moves across the Western Aleutians
beginning Monday morning, bringing gale force winds and light
rain. Rain remains likely with this front as it tracks across the
Central and Eastern Aleutians, though wind speeds diminish as the
front weakens. A cold front follows on the heels of the warm
front, bringing a chance for showers across much of the Aleutians
from Monday evening through Tuesday evening. Snow or rain/snow
showers are possible over the Pribilofs on Tuesday with the cold
front. However, rain showers are expected for the remainder of the
Aleutians as heat from the warmer ocean waters modifies the air
.MARINE (Days 3 through 5/Wednesday through Friday)...
Forecast confidence remains above average on Wednesday as guidance
shows similar solutions with the placement and intensity of
synoptic scale features. However, forecast confidence
significantly decreases on Thursday as the next low pressure
system enters the western Bering/Aleutians as guidance shows
significant differences with respect to the placement and
intensity of this low. One common theme is gale force winds are
likely in the southern Bering Sea and along the Aleutian Chain
Thursday and Friday.
Gulf of Alaska
Forecast confidence remains above average for Wednesday as
guidance shows similar solutions with respect to the placement and
intensity of synoptic scale features. Gale force winds across the
northern Gulf will ease on Wednesday with the weakening of a
barrier jet. Confidence is high that winds will remain below gale
force on Thursday as a transient ridge of high pressure moves
across the Gulf. Confidence decreases during the forecast period
on Friday as the next low moves into the vicinity.
.LONG TERM FORECAST (Days 4 through 7/Thursday through Sunday)...
A cloudy, cool and wet pattern is expected for Southcentral
during the extended period. A positively tilted upper trough will
extend from the northern Bering Sea through the central Gulf of
Alaska. This pattern will allow for moisture to advect from the
Gulf into Southcentral as the trough-axis will remain southwest of
the Kenai Peninsula and thus result in southeasterly onshore
flow. Lower confidence exists with respect to precipitation type.
Snow chances increase as elevation increases.
Southwest will also remain cloudy and cool during the extended
period. Showers and intermittent precipitation is likely, though
steady precipitation is less likely due to the southeasterly flow
downsloping on the Alaska Range.
The Bering/Aleutians will remain in a cloudy and cool pattern as
broad upper troughing will continue to extend across the area. We
are monitoring the potential for a large low pressure system
moving from the North Pacific into the Bering Sea on
Thursday/Friday. Forecast confidence remains low with respect to
the placement and intensity of the low center. However, there are
more similarities between the GFS/GEFS and the ECMWF/EPS with
respect to the placement and intensity of this low, meaning for
now a GFS/ECMWF ensemble blend is the preferred solution.
MARINE...Gale Warning: 119, 120, 130-132, 136, 138, 150, 155,
173-178, 185, 351, 411-413.
SYNOPSIS AND MODEL DISCUSSION...CB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KC/CJ