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fxak68 pafc 180109 

Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
509 PM akdt Tue Sep 17 2019

Analysis and upper levels...
there is a large upper low over the Bering Sea this afternoon. The
leading warm front extends from Bristol Bay eastward through
Kodiak Island into the western Gulf. The cold front is situated
from Bristol Bay southward across the Alaska Peninsula and then
well south into the Pacific. A triple point low is just forming
over western Bristol Bay. Considerable precipitation is moving
into southern Alaska with these features. Showery precipitation is
occurring over much of the Bering Sea and Aleutians, except over
the southwest Bering and western Aleutians where there is a weak
ridge of high pressure.


Model discussion...
the numerical models are in fairly good agreement for the short
term portion of the forecast (through Thursday afternoon). The NAM
does become an out-lier on Thursday, not really developing a
triple point low south of the Alaska Peninsula on Thursday that
the other models seem to have a good handle on. Due to the good
model agreement, the forecast confidence is a little higher than
normal for this time of year.


panc...VFR conditions and light winds will persist until this
evening. Rain will move up the Cook Inlet this evening and
conditions will drop to MVFR around midnight. Some wind shear is
expected as well, until fairly strong southerly winds kick in
Wednesday morning.


Fire weather...
wetting rains are expected tonight and Wednesday over most of
southcentral. The exception is in the valleys of the Copper River
basin, where precipitation is expected to be fairly light. For
the Cook Inlet area, storm total amounts are expected to range
from about 0.5" to 1.5".


Short term forecast southcentral Alaska (days 1 and 2: tonight
through thursday)...

A very wet next 24 hours are in store across southcentral, as the
area makes big strides in its quest to recover from the
summertime drought. A low over the Pribilof Islands contains
abundant moisture with it. As it moves north and east, the rain
will overspread southcentral tonight. Much of radar activity so
far is virga due to a very dry air mass as evidenced on the 00z
Anchorage sounding. With the low expected to track into the
Kuskokwim Delta tonight and into interior Alaska on Wednesday, the
broad atmospheric flow will be out of the southwest. This is the
ideal wind direction for southcentral to get rains. Thus, despite
a period of strong winds along the hillside and Turnagain Arm this
evening and tonight, the downsloping factor should be minimal. Any
dry air should be overwhelmed quickly as this system is a very
efficient precipitation maker. Rainfall amounts through Wednesday
are expected to be around an inch for much of southcentral. The
notable exception will be the Copper River Basin, One of the few
places were mountains block a southwesterly flow. Rainfall amounts
of only one to two tenths of an inch are expected out there
through Wednesday.

The steadiest rain and most of the storm total rainfall will be
overnight, with a brief break in the rain possible around mid-
morning Wednesday. By around lunchtime Wed, however, cooler air
moving in aloft will greatly increase the instability. This along
with additional upper level forcing will make for widespread
shower activity through Wednesday afternoon, even including the
typically downsloped interior regions. Thus, more shower activity
will add to the rainfall totals. The shower activity will
gradually abate Wednesday evening, especially in the lower
elevations, while showers continue in the mountains.

It is now the time of year where snow levels will become more of a
concern with each precipitation event. Fortunately, with this
incoming storm, that will not be an issue for most. With warm air
moving in with the rain, snow levels should remain at or above
6,000 feet, falling to near 5,000 feet Wednesday afternoon. Thus,
some light snow amounts are possible near mountain-top level in
the talkeetnas/Hatcher Pass.

Scattered shower activity is expected for much of Thursday, mostly
over the mountains. The approach of the next system very late in
the day Thursday and Thursday evening will bring the next round of
precipitation to southcentral. More on that in the long-term
discussion below.


Short term forecast southwest Alaska (days 1 through 3; Tue night
through friday)...

An occluded front situated over the eastern Bering will continue
to track east overnight, spreading rain further inland. Rain will
transition to showers overnight and linger into Wednesday as the
associated surface low and upper-level trough both push onshore
keeping conditions unsettled with a moist, westerly flow. These
showers will diminish from west to east late Wednesday as the
trough moves over southcentral Alaska and a ridge builds. The
ridge will allow some clearing overnight Wednesday into early
Thursday, which could introduce some early morning patchy fog
across interior Bristol Bay and the Kuskokwim valley. This ridge
quickly departs on Thursday as an area of low pressure tracks east
along the north coast of the akpen, bringing another round of
rain into Bristol Bay. As this low weakens and departs the region
on Friday, a northwesterly flow aloft will establish a cooling
trend for the end of the week. A series of shortwaves embedded in
the flow will allow for scattered showers to persist along the
coast and the upslope side of higher terrain.


Short term forecast Bering Sea/Aleutians (days 1 through 3;
Tue night through friday)...

Areas of showers and gale-force winds will continue from the
central Aleutians to the akpen through the first half of Wednesday
as a low over the pribilofs this afternoon tracks inland over the
Kuskokwim Delta. Showers will also develop tomorrow over the
central Bering as a shortwave trough digs southeast from the
russian coast. Farther south and west, a Kamchatka low will
advance east tonight, pushing a front along the western Aleutians
by early Wednesday morning. Rain and brief gales will accompany
the front; however, winds will weaken to small-craft levels as
the front moves farther east and the low itself tracks south of
the chain. By Thursday, the southeasterly track of the low will
move much of the rain and stronger winds south of the chain. There
will, however, be some gusty northerly winds across the western
Aleutians in the wake of this system. These northerly winds shift
over the eastern half of the chain by late Thursday into Friday as
a low spins up near Kodiak Island. Out west, a ridge building
over the western Bering on Thursday will keep conditions rather
tranquil until the next front approaches the western Aleutians
late Friday.


Marine (days 3 through 5)...

Beginning the extended period marine forecast on Friday, a strong
gale force low will be in the Gulf of Alaska with high pressure
over the Bering Sea. The gale force low will remain in place over
the Gulf, but cold air advection will be in full force through the
Barren Islands with possible storm force winds by Saturday.
Thereafter, much uncertainty develops as a low will move from the
north pac toward the Aleutians and Gulf of Alaska. The most likely
scenario is the low brushes the Aleutians and deepens as it moves
toward the Alaska Peninsula and Kodiak Island, likely reaching
gale force by Sat night or early Sunday. A triple point low will
form over the Gulf of Alaska Sun night and Monday, and may be a
high end gale-force to low end storm-force low. Prepare for high
winds and seas by late weekend over the Gulf.


Long term forecast (days 3 through 7)...

Beginning the extended period forecast on Thursday evening, there
is a broad longwave trough stretching from the North Slope to the
North Pacific. A shortwave trough at the base of the trough will
be tracking from south of the Alaska Peninsula eastward, with a
developing low along a warm front at the surface. The low will
move into the Gulf of Alaska and move into Prince William Sound on
Thu night and will move inland through Saturday. Pieces of Arctic
energy will also dive down, so colder air will be filtering in,
which will likely deposit some high elevation snow across parts of
the Alaska Range and Talkeetna Mountains/Chugach Mountains
(likely above 5000 feet). However, the bigger story will just be
another round of rain, heaviest across the Gulf Coast and parts of
the Copper Basin, although rain will likely impact all of
southcentral Friday and Saturday before ending Sat night.

Yet another low will skirt the Aleutians and move into the Gulf of
Alaska on Sunday and Monday, bringing more rain to the coast and
chances of rain inland (the exact track of the low remains
somewhat uncertain, as well as if it is a closed low or open wave,
which impacts precipitation distribution inland). Long story
short, the pattern will be much more active than the first half of


Afc watches/warnings/advisories...
Marine...Gale Warning 150 155 170 171-176 181 185.
Fire weather...none.



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