Area Forecast Discussion
Issued by NWS Anchorage, AK

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060
FXAK68 PAFC 121312
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
512 AM AKDT Mon Apr 12 2021

.ANALYSIS AND UPPER LEVELS...

A longwave upper-level trough is over the Bering Sea with a
stationary area of low pressure just west of the western Alaska
coast. Over much of the mainland persists an upper-level ridge of
high pressure. This ridge is propagating eastward, with the trough
moving in behind it and towards Southcentral.

These features are responsible for precipitation across the
Kuskokwim Delta, the Alaska Peninsula, the north Gulf and Prince
William Sound and the Kenai Peninsula and Susitna Valley. Besides
the Kuskokwim Delta (largely snow) and the Aleutians (largely
rain), precipitation types have been mixed as temperatures teeter
back and forth across freezing throughout the day/night. Winds are
gusty across the Alaska Peninsula and the north Gulf coast due to
a surface front from the aforementioned upper-level trough and
low.

&&

.MODEL DISCUSSION...

Models remain in good agreement especially for today. There are
some discrepancies with a low moving north across the Gulf for
Tuesday, however, models are in much better agreement than they
were yesterday.

&&

.AVIATION...
PANC...Wind shear concerns continue through the morning. After
this, winds are expected to be calm before becoming southeasterly,
this evening, with up to 25 kt gusts. Ceilings and visibility
will remain VFR through the TAF period.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3/Today
through Wednesday night)...

A strong front lifting northward across the northern Gulf will
dissipate along the north Gulf coast and eastern Prince William
Sound this afternoon. An upper level short-wave trough helping
to drive the front northeastward this morning, will weaken as it
exits quickly northward. Showers will linger behind the trough,
primarily along the south sides of mountain ranges due to broad
southerly upslope flow. Another upper level wave will cross
Southcentral late tonight through Tuesday morning, enhancing
showers. The precipitation type forecast is extremely challenging.
The airmass over Southcentral has warmed, with rain falling along
the Gulf coast. For much of the region the airmass is still cold
enough to support snow. However, expect surface temperatures to
warm into the upper 30s to lower 40s for most areas, so it will
be right on the threshold of rain and snow for many areas. In any
case, don`t expect significant precipitation anywhere.

The next storm system is coming into focus, with a deep low
tracking from the Pacific into the western Gulf Tuesday afternoon
and then onshore across the Kenai Peninsula on Wednesday. This
storm looks impressive, with a jet streak and strong short-wave
driving toward Prince William Sound Tuesday night. As a result,
expect widespread heavy precipitation along the coast.
Precipitation will spread inland ahead of the low, but likely
won`t be long-lived for most places as strong downslope winds
quickly develop across the coastal mountains. As the low heads
inland, strong pressure rises will contribute to what could be
some very strong gap winds. Stay tuned, as this will depend
heavily on exactly what track the low takes as it heads inland.
Warmer air will accompany the system and combine with winds to
further warm temperatures across the region. Thus, expect rain
will be the predominant precipitation type along the coast, with
rain and snow inland and heavy snow in the coastal mountains.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3: Today
through Wednesday)...

A frontal boundary associated with a vertically stacked low over
the central Bering has weakened and moved eastward over the Alaska
mainland. In the wake of the low, moist, onshore flow and gusty
winds will keep snow showers ongoing over parts of the Y-K Delta
and surrounding high elevations, with a wintry mix of precipitation
over the AKPEN and Bristol Bay through Tuesday. Meanwhile, a
storm system is expected to move northward from the Pacific by
Tuesday afternoon with another round of strong east-northeasterly
winds and a snow/rain mix over the middle Kuskokwim Valley,
southern AKPEN, and all snow over the Y-K Delta through Wednesday
night.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Day 1 through 3: Today
through Wednesday)...

Northwesterly cold air advection coupled with wrap around
moisture on the backside of a Bering low will keep snow showers
ongoing over much of the Bering, including the Pribilofs, and
Aleutians through Tuesday. Meanwhile, a shortwave trough and
associated low is expected to move southeastward from the
Kamchatka Peninsula and bring widespread gale force winds and a
wintry mix of precipitation between the western and central
Aleutians by late Monday afternoon. As the low exits, expect winds
to remain strong, especially areas prone to funneling with
northwesterly gap winds in the central and eastern Aleutians
through Wednesday. A ridge of high pressure will begin expanding
over the Bering by Thursday and bring fair, mild weather over much
of the area through the weekend.

&&

.MARINE (Days 3 through 5)...

Bering Sea/Aleutians:
Forecast confidence remains above average for Wednesday and
Thursday as broad high pressure ridging extends across the central
Bering and Aleutians. Winds will remain under gale force during
this time, except for areas prone to funneling northwesterly gap
winds south of the Alaska Peninsula. Forecast confidence becomes
low on Friday with a potential low pressure system developing east
of the Kamchatka Peninsula. This low may bring gale force winds to
the western Aleutians on Friday.

Gulf of Alaska:
A low pressure system moving northeastward across the western Gulf
will bring widespread gale force southerly winds to the northern
Gulf on Wednesday. High seas of 20-25 ft are expected in the
northern Gulf on Wednesday as there is an unlimited fetch for
these seas to develop in congruence to the southerly gale force
winds. These seas will remain long in period. Conditions will ease
on Thursday. Despite differences in solutions with respect to the
extent the winds will ease, there is confidence that winds will
ease to below gale force. Weak transient high pressure will move
over the Gulf on Friday with winds remaining below gale force.

&&

.LONG TERM FORECAST (Days 3 through 7)...

A Rex Block pattern is expected to dominate the area Thursday
through Sunday as an upper level low will be over the North
Pacific with upper level ridging over the Bering Sea and Alaska
mainland. This Rex Blocking pattern may remain over the area
during the entirety of the extended period. For sensible weather,
this means that lingering precipitation across Southcentral Alaska
will taper off on Thursday with skies becoming more sun than
clouds and a warming trend with respect to temperatures. Ensembles
clearly show this warming trend as there is a consistent solution
with 850 mb temperatures becoming anomalously warm towards the
end of the extended forecast period. The weather pattern will then
remain quiet through the rest of the extended period. Forecast
confidence remains above average.


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...Gale 119 125 175 176 177 178 185 412.
 Heavy Freezing Spray 185.
FIRE WEATHER...NONE.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...CRK
SOUTHCENTRAL ALASKA...SB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MF
MARINE/LONG TERM...ED



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