Small Craft Advisory 273, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
358 PM AKDT Tue Mar 24 2020

.ANALYSIS AND UPPER LEVELS...
The low pressure system that was in the North Pacific yesterday
morning crossed the Aleutian Chain this morning, bringing the
warm front up through the Bering Sea and onshore along the West
Coast. Warm over-running with temperatures hovering right at
freezing brought freezing rain to the Kuskokwim Delta this
morning, but surface temperatures have warmed up since then and
the precipitation has switched over to rain. In Bristol Bay,
temperatures at the surface have warmed more rapidly, so the
switch over to rain was very rapid. East of the Alaska Range the
upper level ridge continues to bring northerly flow and drying
conditions. A broad cirrus shield covers Southcentral and parts of
the Copper River Basin but otherwise weather is quiet.

&&

.MODEL DISCUSSION...
Models show some slight differences tomorrow morning, with NAM
and Canadian bringing more precipitation southward over the Copper
River Basin and Talkeetna Mountains. This solution was preferred
over the slightly drier GFS. After that time period the models are
in good agreement through the short term.

&&

.AVIATION...
PANC...MVFR ceilings are expected tomorrow as a trough moves
southward and brings rain and snow to the area. Visibility could
vary depending on how much snow mixes in but since this is
expected during the day, the precip is expected to be more rain
than snow so visibility should remain fairly high.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 through 3)...
Clouds will thicken through the evening and overnight hours in
advance of an upper-level wave that will move across the region
tomorrow. Areas of light precipitation (a mix of rain and snow)
will develop first across the Susitna Valley and Talkeetna
Mountains early Wednesday morning, spreading south and east
through the day. The best chance for any accumulating snow will be
over the higher terrain as well as the northern half of the
Susitna Valley and Copper River Basin. Farther south, enough
warmer air will yield a transition from snow to a mix or all rain
for the Matanuska Valley south to Anchorage and the northern
Kenai Peninsula. Along the coast, any precipitation should fall
entirely as rain.

As the system moves over the Gulf tomorrow night and a weak ridge
builds in behind it, the clearing skies and weak wind flow could
allow for the development of fog from Cook Inlet north across the
Mat-Su Valleys overnight Wednesday through early Thursday morning.
Northerly outflow winds will also be on the increase Thursday as
the gradient between the high over the interior and low over the
northern Gulf tightens. These winds will subside by late Friday as
the flow turns southerly ahead of the next frontal system moving
into the Gulf.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3)...
A front moving into Southwest Alaska continues to spread rain
across the Kuskokwim Valley this afternoon. The front will be
reinforced by a Bering low tonight, which itself moves into
Southwest Alaska by Wednesday morning. This pattern will keep
clouds and warm air in place across Southwest Alaska. Thus, not
expecting any cooling in the temperatures tonight. In fact, with
steady onshore southerly flow, temperatures may even creep upward
with time tonight. Either way, the freezing rain threat is over in
all except the most sheltered areas of the Lower Kuskokwim
Valley, as the rest of Southwest Alaska will be seeing all rain
through the day Wednesday.

Cooler air and clearing skies will follow behind the front from
interior Alaska Wednesday night and through the day on Thursday.
The cold air looks to reach the Alaska Range and the Nushagak
Hills Thursday. The next low from the Bering will put the cool-
down on hold starting late Thursday, but will really overspread
the area Thursday night into Friday. This should mean most of
Southwest Alaska picks up several inches of snow. With broad
westerly flow, it will be the west facing slopes of the mountains
that gets most of the snowfall.

Despite some differences in the timing of the exit of this feature
Friday night, all of the models show a much colder air mass having
no problem extending south right through all of Southwest Alaska
Friday night through Saturday. Gusty winds and clearing skies will
accompany the falling temperatures.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3)...
A broad low swirling across the western Bering this afternoon
will open up into a surface trough tonight, then move into
Southwest Alaska on Wednesday. A second low tracking well west of
Attu will bring a front across the western Aleutians Wednesday
night, which moves into the northern Bering on Friday. It will
strengthen into a low Thursday night near St. Matthew Island,
which in turn will move into Southwest Alaska on Friday. Strong
high pressure moves into the Bering behind the low on Friday
night. This will force a polar air mass across the eastern Bering
with strong winds expected Friday night into Saturday.

&&

.MARINE (Days 3 through 5)...
Gale force southerly winds are expected across the western
Bering/Aleutians on Thursday. Gale force northwesterly gap winds
are also expected in the western Gulf on Thursday. Forecast
confidence for Thursday remains high. Gale force west-
southwesterly winds are expected across the central/eastern Bering
and Pribilof Islands Friday morning. Only moderate confidence
exists on Friday and Saturday for funneling westerly winds through
Unimak Pass. High confidence exists for gale force winds in the
western Gulf on Saturday with low confidence in storm force gap
winds along the AKPEN. There is also the potential for 20-25ft
long period northwesterly seas in the western Gulf on Saturday,
though confidence is low.

&&

.LONG TERM FORECAST (Days 3 through 7)...
Strong confidence exists in upper level ridging persisting across
the Alaskan mainland on Thursday. The upper level ridging will be
strongest across the southwestern portion of the mainland and
thus temperatures are expected to be warmer than normal. However,
this upper level ridging will be of short duration as an upper
trough will move into the western mainland Friday afternoon ahead
of a surface cold front. As a result, temperatures across the
southern Alaskan mainland will fall as a much colder airmass from
Siberia moves into the region. Forecast confidence decreases
into the weekend as significant differences arise between
guidance. The only area of congruence between guidance is the
jetstream remaining south of the Alaskan mainland which will allow
for colder temperatures. Confidence remains very low Sunday
onward as there is a significant amount of disagreement between
guidance with respect to the place of upper level synoptic
features which leads to a strong disagreement in the placement of
surface features.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...LF
SOUTHCENTRAL ALASKA...TM
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JW
MARINE/LONG TERM...ED