Gale Warning 478, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
512 AM AKDT Tue Aug 29 2023

.ANALYSIS AND UPPER LEVELS...

Widespread areas of rainfall are continuing to push north into the
southern Mainland this morning as a deep moisture tap containing
precipitable water values as high as 1.5 inches continues to
stream north between a closed low in the Bering Sea and a strong
ridge over Northwest Canada. A weak surface low and supporting
shortwave trough have now crossed over the Alaska Peninsula from
the south into Bristol Bay, with bands of rain near the center
now stretching between Bristol Bay and the Lower Kuskokwim Valley.
Strong south to southeast flow developing aloft to the east of
this system is beginning to foster precipitation mostly forced by
upslope flow into mountainous terrain across Southcentral.
Conversely, most of the lee side of the mountains across the
western Kenai Peninsula, Matanuska Valley and Copper Basin is
mostly drying out due to downslope flow and warming/drying.

Out in the far west, a stacked and occluded low is now
approaching Nikolski from the west as it tracks along the Aleutian
Chain and as rain moves along the occluded frontal boundary into
the Dutch Harbor vicinity. A corridor of easterly winds around 30
kt north of the center continues to slowly decrease in intensity
as the low weakens. Otherwise, the rest of the Bering Sea is
relatively quiet aside from some low to midlevel stratus drifting
north of the low.

&&

.MODEL DISCUSSION...

Models are overall coming into better alignment with the key
features to watch over the next few days. There is increasingly
good agreement that southeasterly mid to upper level flow
developing across Southcentral today into Tuesday will focus most
of the additional rainfall where favored by upslope flow into the
mountains surrounding Prince William Sound and along much of the
Alaska Range. By Tuesday night and Wednesday, models are all
keying in on a negatively-tilted upper shortwave expected to lift
northeast up the Cook Inlet and Kenai Peninsula. This will likely
help focus an axis of briefly heavier rainfall following along and
ahead of the trough axis, but there is still some model
disagreement for the timing and orientation of this shortwave and
brief period of heavier rainfall moving up from the Kenai
Peninsula into Anchorage and the Matanuska Valley.

For the remnants of Damrey, models have improved significantly in
terms of spatial/temporal agreement for the system as it rapidly
deepens into a storm force low on Wednesday, then crosses into the
Bering Sea on Thursday. There is still a bit of uncertainty in
terms of where the core of strongest winds to the south of the
center will track, with any slight north or south shift in the
track of the center possibly leading to much stronger or weaker
peak winds along the eastern Aleutian Chain and southern AKPen.
There also remains good agreement that that this system will
weaken quickly on approach to Southwest late on Thursday.

&&

.AVIATION...
PANC...Southeasterly winds are trying to make it into the
terminal early this morning. The transition form a northerly
surface wind to southeast (winds aloft are southeast) has produced
some low level wind shear overnight, but tat should end as winds
go southeast. This shift in direction should also help keep
ceilings mainly VFR through the day. While ceilings could drop to
MVFR during the evening to overnight hours, it looks like they
should remain mostly VFR.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3: Today
through Thursday)...

Southcentral is looking at more rainfall due to moisture streaming
in from the south at all levels along a front that runs from well
south of the Gulf into the Interior of the state. Lifting from
this front as well as the upper level trough will be sufficient to
keep this rain around throughout the day for most of the area. As
this front lifts northward, there may be a break in rain
overnight Wednesday into Wednesday night before the next wave
moves into the region for Thursday.

As has been the case the past few days, the moisture moving into
Southcentral is very deep and 2 to 4 inches of additional
rainfall are possible for the north Gulf coast and Prince William
Sound areas as well as for the Alaska Range. This is the cause of
the Flood Watches for these areas that run into Wednesday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA/BERING SEA/ALEUTIANS (Days
1 through 2: This morning through Thursday morning)...

A pair of occluded lows, one lifting across Kodiak Island into
the Western Alaska Range, and a second one traversing into the
Eastern Aleutians, are bringing abundant moisture into the Alaska
Peninsula as well as much of Southwest Alaska with their forward
progress this morning. CIMSS MIMIC Layer shows precipitable water
values approaching one inch stretching from Unalaska into the
Lower Kuskokwim Valley. Not coincidentally, rain chances will
remain in the forecast across Southwest Alaska and much of the
Aleutians for the next couple of days. Further aiding in rain
chances today is the arrival of a shortwave trough into the
Southwest coast this morning, as the occluded low over the
Aleutians begins to break down and devolve into several smaller
disturbances. Scattered showers are forecast to spread northward
from the Western Alaska Range and into the Kuskokwim Delta through
this evening.

What remains of the occluded low is forecast to finally push into
Southwest Alaska on Wednesday, bringing with it more rain for
Southwest and scattered showers along the Aleutian Chain on its
way. All attention then shifts to the remnants of Tropical Storm
Damrey late Wednesday into Thursday. Models continue to depict the
development of a deep extratropical low lifting out of the North
Pacific and into the southern Bering by Thursday morning. The low
will be preceded by a broad front as early as Wednesday afternoon
with expected precipitation to stretch across the entire Aleutian
Chain. Furthermore, given the potential strength of the low, with
multiple models showing the low bottom out into the 960 millibar
range, an attendant threat for elevated winds will also exist. As
with the previous discussion, current thinking is that the low
will occlude before hitting the Aleutians, and subsequent winds
should begin to gradually weaken. However, sustained gales and
even storm force winds appear likely along the Aleutians as early
as Wednesday afternoon for the Western and Central Aleutians.
Further east, the highest winds are forecast to remain on the
Pacific side of the chain, though storm force gap winds through
the Eastern Aleutians will be possible, with southerly gales on
the Bering side of the Aleutians.

&&

.LONG TERM FORECAST (Days 4 through 7: Friday through Monday)...

Confidence is above average for the long term forecast, as despite
a pattern shift toward more zonal, less amplified flow, models are
in fairly good agreement through the weekend. Beginning Friday,
the extratropical remnants of Tropical Storm Damrey move inland
over Southwest Alaska, and quickly make their way into the
Interior as an upper trough over the Bering Strait works to
eject it north and east. Winds associated with the low diminish
quickly, but a large-scale pressure gradient between a North
Pacific ridge and broad low pressure over Northern Alaska will
continue to drive gusty southwesterly winds across Southwest
Alaska, as well as up Cook Inlet. Various southerly gap winds will
also be active. Westerly gap winds out of Kamishak Bay will
briefly reach gale force Friday night through the Barren Islands
as remnant troughing from ex-Damrey moves into the Gulf of Alaska.
Aside from this, the main impacts from ex-Damrey (strong winds
and coastal flooding potential) are expected to occur Thursday.
These are discussed in previous sections of the short term
forecast.

With the development of a zonal, westerly flow regime, expect
several progressive, quick hitting systems bringing rain to low
elevations, with more prolonged upslope precipitation likely to be
the main hydrologic concern. Southerly flow ends on Friday,
ending the bulk of precipitation for the Prince William Sound
area. The westerly flow that takes over will shift the focus to
the Western Alaska Range and the western slopes of the Talkeetna
Mountains, bringing more rainfall to the basins in the Susitna
Valley which are already dealing with high water levels this week.

A broad low tracks across the northern Bering Sea Friday night,
bringing a large swath of gale force winds as far south as the
Aleutian Chain through Saturday, when the low moves into mainland
Alaska. Precipitation with this low sweeps quickly east from
Southwest Alaska on Saturday into Southcentral for Sunday. This
coincides with the development of a triple point low in the
northern Gulf Sunday morning, drawing another round of westerly
gap winds, potentially as strong as storm force, through the
Barren Islands, aided by cold advection with 850 mb temperatures
dipping below 0 Celsius streaming across the Alaska/Aleutian
Ranges. Winds diminish through Monday as the low exits into the
eastern Gulf. Confidence in the long term forecast decreases by
Monday, though model guidance suggests that the next low is
likely to be approaching the region in the vicinity of the
western Bering Sea/Far East Russia.

&&