Wind Chill Advisory 1, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
447 AM AKST Sun Jan 6 2019

.ANALYSIS AND UPPER LEVELS...
There is a weakening surface low north of Attu Island that is
vertically stacked and has the remnants of a decaying occlusion
wrapped into it. There is a mature low south of Unalaska and a
second mature low that is rapidly intensifying southwest of Attu
Island. In the latest ASCAT/advanced scatterometer image there is
a large area of high pressure at the surface south of the AKPEN
and Kodiak. The jet stream is south of 50N. There are two distinct
jet cores supporting the aforementioned matures cyclones. The
radars at King Salmon and Kenai are barren and both in clear air
mode. Meanwhile, the Middleton Island and Bethel radars have a few
light returns on them but nothing ominous or sinister. On the
GOES Nighttime Microphysics and the AVHRR (which are both
satellite products) fog and low clouds have been detected again
over portions of Southcentral, more specifically Anchorage,
Wasilla and Hope. The satellite has also pinged into areas of
freezing fog and low stratus in the Copper River Basin. There is a
another area of fog in Southwest Alaska over portions of the
Kuskokwim River north and east of Aniak. The coldest region again
for the southern tier of Alaska remains the Copper River Basin
where temperatures are ranging from the negative teens to negative
twenties. However, at 4 AM this morning the METAR at Talkeetna
was -21F which was a function of pure radiational cooling.

&&

.MODEL DISCUSSION...
Models are in good agreement with the major synoptic features
through Tuesday. The caveat being the decaying low in the extreme
Western Bering. But the model consensus with the train of cyclones
south of 50N in the short term forecast period looks good. Weather
models in high latitude environments in general don't handle
microscale features such as localized fog very well. Which is the
case right now for the fog and stratus in the Anchorage Metro
Area. This is the portion of the forecast is where the level of
uncertainty is bumped up a skosh.

&&

.AVIATION...
PANC...The nighttime microphysics images from GOES have detected
an area of fog/low stratus over Knik Arm, Turnagain Arm and over
portions of the Cook Inlet. The metars near the Knik Arm suggest
that the base of that batch of stratus is at 200 feet. The
stratus over Cook Inlet ranges from cigs of 600 to 1000 feet.
There has not been a lot of mixing and the forecast for the
ensuing 30 hr period still looks pretty stagnant in the boundary
layer. The best chance for VFR flight conditions at PANC will be
this afternoon but its not a guarantee. The fog/stratus situation
is actively being monitored. Amendments will be made as required.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)...
The same forecast issues from the past two days will remain again
today across Southcentral...areas of fog around Cook Inlet and
the Copper River Basin and winds through channeled terrain.
The fog and low stratus does look to mostly remain again today
and through tonight in these areas. This has led to the low
temperatures tonight being adjusted upward some as skies should
not clear out. Tonight, a short wave will move over Kodiak Island
and then progress over the Cook Inlet area for Monday and Monday
night. While this is not a strong short wave it will be enough to
increase mid and higher level clouds which may help break up the
persistent fog and stratus over Cook Inlet.

The trough along the northern Gulf of Alaska coastline has helped
increase some of the outflow winds from Seward through the Prince
William Sound area. These winds and colder temperatures have
decreased the wind chill in Thompson Pass to around 45 below zero
this morning. Since the temperatures will continue and winds
will persist near those levels through Monday morning, a wind
chill advisory has been issued for that area through Monday
morning.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 through 3/Sunday
through Tuesday)...
The first of two fronts forecasted to impact southwest Alaska is
currently coming ashore from the Bering Sea. It is producing some
light snow and windy conditions along the coast as it does so.
This is reducing visibility to a mile or so, but those conditions
are localized. Additionally, the low stratus that has been in
place is slow to move out of inland areas of the southwest. It is
clearly evident on the nighttime microphysics satellite. As the
front pushes inland, it should clear out the remaining stratus.

After this front pushes through, it will pretty much be a repeat
of the setup we have had the past couple of days. A brief break
in the weather will occur later today with the strong possibility
of stratus/fog setting up again over the area. Being that stratus
has been slow to dissipate and there is a broad deck of it throughout
western Alaska, would not be surprised to see it return after the
front moves through. These indicators show conditions are primed
for it to redevelop. The next front will then approach the coast
tomorrow night, bringing about the same weather for Tuesday.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 through 3/Sunday
through Tuesday)...

The low center with the first in succession of two lows is
pushing out of our forecast area in the Western Bering. Its
slowly decaying front is leaving the eastern Aleutians and moving
into southwest Alaska. Behind this low center, some cold core
cumulus is moving through the Bering and Aleutian Islands. It is
in association with cold air being entrained into the low.

The next low that will be moving through the Bering and Aleutians
is starting to appear in our forecast area on satellite imagery
and is entering the western Aleutian Islands. Storm force winds
with a few hurricane force gusts are still forecasted with this
low. However, as the previous discussion mentioned, these stronger
wind gusts will be brief so still steering away from issuing any
hazard products associated with them. The front still looks to
detach from the low center and lose its dynamic support as it
moves east. This means little change in the forecast for the area.

&&

.MARINE (Days 3 through 5)...

Beginning Tuesday evening, conditions are expected to improve
across the Bering Sea as a storm system tracks north out of the
area while weakening. Winds and waves are likely to subside quite
a bit through Wednesday night before the next storm system
approaches from the south by Wednesday night. The storm is
expected to be quite strong with central pressures potentially
bottoming out below 950 mb, though the system is expected to
weaken as it approaches the Western/Central Aleutians and stay
south of the island chain. This would bring upper end gale force
winds to most of the Western Bering Thursday night into Friday.
Models have been fairly consistent with this storm so far,
thus confidence is above average in the track and strength.
Though, if the low takes a more northward track and maintains its
strength, winds will likely increase into the storm force
category.

Over the Gulf of Alaska, high pressure will continue to keep the
storm track to the south and west, thus expect generally good
conditions through the week. Localized higher winds may develop
out of bays and passes along the North Gulf Coast due to northerly
outflow winds.

&&

.LONG TERM FORECAST (Days 4 through 7)...
The long term forecast begins Wednesday with high pressure over
most of Southcentral and a cold Arctic upper level low over the
northern mainland. This will keep cold and dry conditions with
offshore flow over Southcentral and most of Southwest AK through
the end of the week. A few shortwaves rotating around a low south
of the AKPEN and the Arctic low will bring some snow showers to
parts of southwest AK on Wednesday but should diminish by the end
of the day. The Arctic low is expected to shift south some
allowing even colder air to filter in over the southern mainland
on Thursday. At the same time, a strong North Pacific low will
move north towards the Aleutians, but should stay south of the
island chain as a strong polar high over Siberia will prevent the
system from pushing further north. Regardless, the system will
bring very wet and windy conditions to the Aleutians and spread
further east towards the AKPEN by the end of the week. Models are
now beginning to show the low complex spreading east into the Gulf
of Alaska by Friday as high pressure shifts east at the same
time. This would bring a pattern change to the southern mainland with
a return to warm and wet southerly flow as we head into the
weekend. There is still a fair amount of disagreement between
models as to how far east the North Pacific low gets, so a lot
could change between now and then. Best to keep track of the
forecast for updates through the week as the details are worked
out.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Wind Chill Advisory 131.
MARINE...Heavy Freezing Spray 185. Gale 414 412 170 172 173 174.
Storm 413 411 175 176 177 178 179 185.

&&

SYNOPSIS AND MODEL DISCUSSION...PJS
SOUTHCENTRAL ALASKA...EZ
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...BJB
MARINE/LONG TERM...KVP