Scientific Forecaster Discussion
fxak68 pafc 230118
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
418 PM akst Fri Nov 22 2019
Analysis and upper levels...
broad cyclonic flow with embedded waves continues across much of
the Mainland and Gulf of Alaska. Unstable conditions with the cold
upper trough is producing widespread rain showers with isolated
to scattered thunderstorms across the Gulf and coasts. Mixed
showers were evident across southcentral. With cooler air over the
southwest Mainland showers were predominately snow though there
were a few rain showers over Bristol Bay associated with a wedge
of warmer air. The presence of widespread open cell cumulus clouds
and showers across the Bering Sea is indicative of cold advection
over the relatively warmer water surface. Also, gusty winds have
been reported in showers along the Aleutians.
models overall are in good agreement with the synoptic features
in the short term. They still struggle some with the finer
details, such as, the location of shower activity.
panc...an upper trough moving north tonight will bring periods of
MVFR ceilings and/or visibilities with showers through Saturday
Short term forecast southcentral Alaska (days 1 and 2)...
a surface low making its way up Cook Inlet this afternoon will
continue to produce areas of showers along western Kenai
Peninsula, Anchorage bowl, and mat-su valleys through Saturday.
Showers will also continue across Prince William Sound along a
surface trough. Colder air is wrapping around the aforementioned
low, so as it and its associated upper-level trough lift north
overnight, rain showers will mix with and changeover to snow
showers. The southwesterly flow both at the surface and aloft will
keep the more persistent showers over the mat-su valleys where
light snow accumulations are possible. For Anchorage and portions
of the Kenai Peninsula, the majority of the precipitation will be
exiting the region by the time the coldest of air works in. Thus,
little to no snow accumulation is expected, although an inch or so
along the Anchorage hillside is not out of the question.
Just as the shower activity begins to diminish for all but the
higher terrain and passes late Saturday, a second upper-level
wave will move across southcentral. This will usher in even
colder air and help enhance snow shower activity across the
chugach and Talkeetna Mountains as well as portions of the Susitna
Valley. This feature will also signal the start of a significant
cooling trend that will be further enhanced by the exit of the
main upper-level low late Sunday and the development of northerly
flow. By the end of the weekend, the main weather focus will be
the downward trend in temperatures and the development of gusty
outflow winds across coastal southcentral.
Short term forecast southwest Alaska (days 1 through 3)...
light snow showers are still lingering in southwest Alaska on the
backside of the last low pressure system that pushed through the
area a couple days ago. A chance of precipitation will remain
until the weather clears briefly on Sunday. Expect a drop in
temperatures with this clearing. On Monday, a fast moving frontal
system will push directly into southwest Alaska. It will bring
gusty winds, precipitation, and warming temperatures. With it
being a few days away, we are still honing the forecast for this
system. Right now, it looks like blowing snow and heavy snow ahead
of warming temperatures is likely, then a quick warm up as the
system pushes inland. There is still too much uncertainty to know
if blizzard conditions will be possible. With that, parameters
like precipitation type are still coming together that will make
or break the forecast, so stay tuned to forecast updates as we
will know more in the next day or so.
Short term forecast Bering Sea/Aleutians (days 1 through 3)...
the Bering Sea starts out quiet with northerly flow behind a low
pressure to the east over Mainland Alaska. As northerly flow does
this time of year, snow showers are embedded in this cold air
push. This will all change Sunday into Monday with a change in the
weather pattern. First on Sunday a shortwave ridge will move
through Bering. Following quickly behind it will be the first of
two low pressure systems that will switch winds to southwesterly
direction. The first low will move through Sunday and bring gale
force winds with it for much of the Bering. A second, more
powerful low will move into the Bering Monday. This low will have
widespread gales with a swath of storm force winds in the central
Bering. This low will continue to push east through the Bering
Marine (days 3 through 5)...
areas of west to northwesterly gales will are likely in the
western Gulf and through the Barren Islands Sunday night and
Monday with localized strong outflow winds near terrain gaps along
the northern Gulf Coast. Offshore flow will diminish Monday night
through Tuesday with wind remaining at Small Craft Advisory
levels or less in the Gulf and adjacent coastal waters through
Wednesday afternoon. A very strong front pushing across the
Bering will bring widespread gale to storm force south to
southwesterly winds Sunday night through Tuesday. Overall, winds
will diminish Tuesday night through Wednesday as ridging builds
into the western Bering with gales limited to the waters near the
eastern Aleutians and Alaska Peninsula where a developing North
Pacific frontal wave will approach midweek.
Long term forecast (days 3 through 7)...
on Monday a cold upper level trough will be in place over
southcentral Alaska with strong offshore flow persisting through
the day and then diminishing Monday night as upper level ridging
builds in from the west and the trough exits to the east. A front
in the eastern Bering will reach the Kuskokwim Delta coast midday
Monday and push inland over southwest Alaska through Monday
evening. A second stronger and rather large frontal system
stretching across the Bering will catch up over the first and
spread additional heavier precipitation across southwest Alaska
Monday night through Tuesday.
While there are some potential concerns for freezing rain to be
briefly mixed in with the snow expected from the first front,
strong southwesterly flow aloft accompanying the warm advection
with the stronger second front should change precipitation near
the southwest Alaska coast over to rain fairly quickly with snow
or a possible wintery mix lingering longer further inland. For
southcentral Alaska, remnants of the first frontal system will
push up the western side of the upper ridge on Tuesday bringing
good chances for snow with it to the Susitna Valley, Talkeetna
Mountains and Anchorage area Tuesday afternoon through Wednesday.
A North Pacific frontal wave interacting with the trailing
southern end of the cold front associated with the big Bering low
will spin up a new low south of the eastern Aleutians Tuesday
night and Wednesday. The associated warm front will reach the
Alaska Peninsula and Kodiak Island Wednesday morning and then push
over the greater Bristol Bay area, western Gulf and Kenai
Peninsula Wednesday afternoon and evening. The front will continue
to the northeast, bringing warmer and moister air into
southcentral Alaska Wednesday night creating significant forecast
challenges regarding how far north and inland the rain vs snow
line will push during a time period with heavy precipitation rates
over much of the area.
A colder upper level trough will push into southwest Alaska
Wednesday night and Thursday, continuing east into Thursday night
and Friday. The strong cold air advection that it brings as well
as its interaction with all the warm, moist air entrained in the
frontal system ahead of it will bring the potential for a
significant snow event to southcentral for the end of the week.
Marine...Gale Warning 150 165 185.