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Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
436 PM akst Mon Feb 20 2017

Analysis and upper levels...
we are getting into a more progressive pattern with a upper level
trough moving eastward over the Mainland part of the state...being
driven by an amplifying upper level high that is also moving
eastward. At the surface there is a very strong low near Attu
which produced hurricane force winds over the western Bering Sea
this morning. The front associated with this low is heading
eastward across the Bering Sea and Aleutians and is the source of
the Blizzard Warning for the Pribilof Islands. The surface ridge
is over southwest Mainland Alaska in stretching into the interior
of the state. A surface low near Prince William Sound is also
keeping some snow showers in that area as well as gusty winds
through the Barren Islands. In northern Cook Inlet the atmosphere
is saturated up to around 6500 ft and then quite dry above that.
There is very weak lift ahead of the 500 mb trough which, combined
with just a little cold air advection, is producing an environment
that is squeezing every bit of moisture out of this saturated
layer and causing the persistent periods of snow. The snow is
often occurring low enough that the radar beam out of Nikiski is
above the precipitation and therefore showing nothing on the
radar.

&&

Model discussion...
models continue to do good with the synoptic pattern. The European model (ecmwf) is
hinting at a slightly faster solution for the system moving
through southwest Mainland Alaska tomorrow and southcentral Alaska
tomorrow evening and overnight but will just mean minor differences
in the forecast. Models, even the high resolution ones, are not
catching the subtle forcing bringing the snow to the upper Cook
Inlet region today. They are all similar in timing of the passage
of the 500 mb trough late this afternoon/early evening which will
bring an end to the persistent on and off snowfall.

&&

Aviation...
panc...snow showers should taper off late this afternoon or early
this evening. This will allow ceilings to remain at or above 5000
ft from that time Onward. Late tonight the clouds should
completely clear but that also adds the threat of some fog
developing late tonight and early tomorrow morning. At this time
it looks more likely there will not be widespread fog formation so
it is not in the taf, but it does remain a possibility.

&&

Short term forecast southcentral Alaska (days 1 and 2)...
light snow showers are expected to diminish by late tonight as
the upper level low over the northwest coast lifts north. A colder
dense air mass filters across southcentral for Tuesday. Strong
winds will increase through the night for Seward and Whittier as
the pressure gradient tightens along the western Gulf Coast with a
ridge settling in from the interior. The low over Prince William
Sound is forced southeast by Tuesday afternoon which allows the
pressure gradient to slacken and winds quickly taper off in
response.

The next front to impact southcentral is expected to bring
widespread snow across all of southcentral late Tuesday and
Wednesday. Snow development across Kodiak Island and the western
Kenai should start Tuesday afternoon along the surface front
which runs slightly ahead of the upper low. The upper level low
kicks into a faster pace Tuesday night as another low pushes in
behind it. The dynamic set up for this system will primarily be an
overrunning event where a warmer subtropical air mass glides over
the cold shallow air mass already in place. Precipitation chances
were generally increased across the southern Mainland and Gulf
with the system diminishing late Wednesday from west to east.

&&

Short term forecast southwest Alaska (days 1 and 2)...
current clear and cold conditions will remain the status quo
through tonight and into early Tuesday morning, as a stationary
upper trough and Arctic air mass continues to remain in place over
the area. The pattern will then begin to shift early Tuesday
morning with the approach of a frontal system from the Bering.
This system will spread snow from southwest to northeast through
the day Tuesday, along with some offshore flow and slightly mixed
air hindering surface temperatures from dropping drastically as
has been seen in previous days. Snowfall accumulation looks to be
in the 3 to 8 inch range, with isolated areas getting slightly
more than 8 inches. With the gusty winds and snowfall associated
with this frontal system, there is potential to see some blizzard
conditions mainly along the Kuskokwim Delta coast and the areas
from Dillingham north and west on Tuesday. The front exits from
west to east early Wednesday morning, but the now active pattern
will persist into the second half of the week.

&&

Short term forecast Bering Sea/Aleutians (days 1 and 2)...
the big story remains the hurricane force front tracking northward
over the western Bering Sea tonight. It looks to have peaked in
intensity today, with winds beginning to trend downward this
evening. The associated front will continue to push northeastward
over the Bering, bringing blizzard conditions to the Pribilof
Islands early this evening and overnight into Tuesday. As the
front continues to track eastward, Unalaska will get a quick shot
of snow and blowing snow, but temperatures will warm enough from
downslope to mitigate the threat by the late afternoon hours. The
actual warm air will change precipitation to rain by the late this
evening. The Alaska Peninsula is in a similar situation as the
front traverses the area Tuesday morning.

As the front exits onto the Mainland, a second strong low and
front will follow a similar pattern as the previously mentioned
system, and enter the western Bering Tuesday afternoon. The
notable difference with this system is that it will have more of
a south to north trajectory, with the warm front followed by a
west to east moving cold front. Confidence is higher that strong
winds and precipitation will remain in the forecast for this area,
as the more progressive pattern sets up shop over the
Bering/Aleutians.

&&

Long term forecast (days 3 through 7)...
a very amplified ridge setting up over much of Mainland Alaska on
Thursday will surge above normal temperatures and moisture to the
Gulf and the south Mainland through late this week. This will
initially bring above normal temperatures but otherwise quiet
weather to most areas for Thursday, but this will begin to become
more active as the remnants of a frontal band becomes
reinvigorated by a shortwave traversing the Bering Sea. This will
bring precipitation mostly in the form of rain the southwest
Mainland on Friday, which will then spread into southcentral late
Friday-Saturday. What is less certain this far out in the forecast
is determining whether surface temperatures will rise above
freezing, so the possibility of freezing rain will need to be
monitored in the coming days.

By Sunday, there is good agreement that this warm-up will once
again be short lived as the amplified ridge flattens out as it
shifts to the south. This will allow for a series of Arctic
troughs to track back into the Mainland, forcing temperatures back
below freezing and chances of precipitation to snow in most areas.

&&



Afc watches/warnings/advisories...
public...Blizzard Warning 191.
Blizzard watch 155 161.
Marine...Gale Warning 130 131 155 160 165 170>181 185.
Heavy frz spray 121 129 130 138 139 165 185.

&&

$$

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