FXAK68 PAFC 050037
Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
437 PM AKDT Sat Apr 4 2020
.ANALYSIS AND UPPER LEVELS...
There are two defining weather features for our forecast area
today. This first is a high pressure system in the North Pacific
whose ridge is pushing north into the eastern AKPEN. The other
feature is a low centered just southwest of St. Lawrence Island
and moving slowly northeast. The ridge of high pressure is just
strong enough to force most of the energy north of our forecast
area, but southwest AK is not escaping this system. In the upper
levels, all of this is supported by a longwave trough digging
south through AK and pushing high pressure south into the North
Pacific. A band of energy is trapped between these two features
and is moving through southwest AK. This setup is also creating
on-shore flow into southwest Alaska and enhanced wind speeds for
both southwest AK and the eastern Aleutians.
The ridge of high pressure is also keeping southcentral AK out of
most of the weather. However as with SW AK, it is not pushing
north far enough to keep weather entirely out. Therefore, cloud
cover and small pockets of instability producing snow showers have
been able to exist in Southcentral AK today.
Models continue their run of good agreement in the short term.
This is keeping forecast confidence high. One area of difficulty
is in southwest Alaska, where they are having trouble picking up
on precipitation coming off the Bering Sea. The forecast is being
manually adjusted to account. Otherwise, models are performing well.
PANC...Some low level cloud cover will linger into the overnight
hours, but VFR conditions are expected to persist. Winds will be
picking up from the south overnight and be gusting 25-30 kts early
tomorrow morning. Winds should subside midday tomorrow.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3;
Tonight through Tuesday)...
The upper-level long wave pattern will begin to transition tonight
as the ridge situated over the Gulf moves east and gives way to
an upper-level low tracking from the northern Bering to interior
Alaska. As it does, a moist, southwesterly flow will develop out
ahead of an upper-level wave and associated cold front that will
enter the western periphery of Southcentral Sunday. Gusty
southwesterly winds will develop overnight through early Sunday
from Cook Inlet north to the Susitna Valley in advance of the
front. In addition to gusty winds, areas of snow will develop
along the front, with accumulating snow likely for areas north of
Talkeetna (Broad Pass), Hatcher Pass, and the higher elevations
around Prince William Sound. Enough warmer air will still be
around on Sunday for precipitation to be a mix of rain/snow for
lower elevations and areas along the coast. The front will
continue east through Sunday with gusty west-southwesterly winds
persisting across Cook Inlet, Kachemak Bay, and the Barren
Islands. Strong westerly winds will also develop across Prince
William Sound with winds gusting through favored bays and passes
and rain changing back to snow as the front pushes through.
Although winds are expected to diminish on Monday as the initial
trough exits the region, a second, trailing shortwave should
provide enough instability for a few lingering snow showers over
the higher terrain.
Conditions trend clearer, drier, and cooler Monday afternoon
through Tuesday as the upper-level trough axis slides east of the
area and a weak ridge attempts to build over the region. The next
frontal system then moves into the Gulf late Tuesday with the
potential for precipitation and strong winds for Kodiak Island and
the immediate Southcentral coast through Wednesday.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Tonight
Over the next 12 hours, there will be a significant pattern shift
across Southwest Alaska. The relatively warm conditions will
rapidly end as a powerful cold front sweeps across the area from
west to east. Ahead of the front, temperatures are just above
freezing in most locations across the Kuskokwim Delta. Conversely,
highs got well into the 50s today south of King Salmon. Once the
front moves through, temperatures will stay in the 20s across the
Kuskokwim Delta with low to mid 30s expected further east.
Another effect behind the front will be a significant increase in
the winds along with snow bands setting up in the very unstable
atmosphere. This will begin tonight, and persist through the day
on Sunday. Winds gusting as high as 65 mph are expected late
tonight into Sunday morning. This is the result of a strong and
powerful low moving into Norton Sound, with the wraparound behind
the front accelerating as the low deepens. The combination of the
strong winds and the snow bands that develop will lead to rapidly
deteriorating conditions in portions of the Kuskokwim Delta,
particularly north of Kipnuk. The snow bands will be localized.
Thus, outside of them, conditions will remain very windy, with any
previously fallen snow being blown around, but otherwise expect
mostly cloudy conditions. Inside the bands, expect near whiteout
conditions at times. Since the bands will be moving around, do not
expect long periods of whiteout conditions, but persistent bands
could cause visibilities to drop to a half mile or less. Thus, a
Winter Weather Advisory has been issued from late tonight until
Sunday evening. The worst conditions are expected at the coast,
but the bands will be able to move well inland, so areas such as
Bethel will also see periods of reduced visibilities in strong,
gusty winds through the day Sunday.
The winds will begin diminishing Sunday night as a small area of
low pressure moves near the Pribilofs. There is still significant
uncertainty as to the eventual track of the low, but its
relatively close proximity to the coast should keep most of the
area mainly cloudy Monday and potentially into Tuesday, depending
on how long it persists. Otherwise, expect well-below-normal
temperatures to persist through the day Tuesday, with highs in the
20s and 30s and lows in the single digits and teens.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3: Tonight
A strong cold front near the Pribilofs this afternoon will rapidly
move into Southwest Alaska this evening. Behind the front strong
northwesterly flow of much colder air is producing widespread snow
showers, which will persist over nearly the entire Bering Sea
through Sunday. A low moving off the Kamchatka Peninsula moves
into the western Bering Sunday afternoon, and strengthens as it
moves over the Pribilofs Monday afternoon. There is significant
uncertainty where it moves from there, but generally a slow
eastward drift towards Southwest Alaska is expected. A large area
of high pressure moves over the western Aleutians Monday, then
gradually shifts east across the central Aleutians on Tuesday. A
weak low moves over the Northern Bering Tuesday afternoon as
a separate front moves into the western Aleutians on Tuesday.
.MARINE (Days 3 through 5/Tuesday through Thursday)...
High pressure centered southeast of Adak Tuesday afternoon and low
pressure to the northwest of Savoonga will create a tight
pressure gradient across the northern Bering, where some sustained
gales are possible. Further south and west, a front will approach
the western Aleutians ahead of an incoming area of low pressure.
The center of high pressure then moves to Dutch Harbor on
Wednesday and into western Alaska for Thursday. This allows the
front to trek eastward into the central Aleutians and Bering, with
sustained gales possibly developing.
Gulf of Alaska:
A strengthening area of low pressure approaches the Gulf Tuesday evening,
with a sustained gale force front preceding its arrival. As this
low deepens further across the northern Gulf Wednesday morning,
high-end sustained gales, with storm force gusts, are anticipated
to develop. This low then moves ashore between Cordova and Yakutat
Wednesday evening, with sustained gales continuing near the
northern Gulf Coast. An offshore flow pattern develops on Thursday.
.LONG TERM FORECAST (Days 3 through 7/Wednesday through Saturday)...
Guidance is in fairly good agreement leading to high confidence on
the long term upper-level pattern in the middle to later parts of
next week. There will be an upper level trough over the mainland
and a modest upper level ridge over the Bering. This will lead to
cold temperatures over the mainland of Alaska with warmer
temperatures over the Bering and Aleutian Chain. The upper level
ridge over the Bering will slowly work towards the mainland
Thursday leading to a warming trend. Beyond Thursday night is when
the models begin to diverge on the upper level pattern and the
placement of features. While the trend is there on both models for
a warming trend, the degree to how strong this warming trend will
be is still uncertain. This is leading to only low to moderate
confidence in the forecast for temperatures Friday into Saturday
of next week. The placement of features in the upper level flow is
in question as one model has the upper level ridge further west
and more amplified while another is less amplified.
PUBLIC...Winter Weather Advisory 155.
MARINE...Gale 119 120 130 131 139 141 179 180 181 185
Heavy Freezing Spray 180 185
SYNOPSIS AND MODEL DISCUSSION...BB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW