Scientific Forecaster Discussion
fxak68 pafc 170238
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
538 PM akst Sat Nov 16 2019
Analysis and upper levels...
clouds can be seen on satellite rotating around an upper level
over southwest Alaska, just north of Bristol Bay. This upper level
center is bringing the snowfall to the Bristol Bay region and far
southern Kenai Peninsula. From the north Gulf Coast near through
the Susitna Valley there is an old frontal boundary which is
providing isentropic lift along it. This is the source of the
snowfall over the Anchorage and the mat-su valleys. Southwest
Alaska will continue to see colder air filter in behind the low
overnight and tomorrow.
models came into very good agreement with the 12z runs this
morning in regards to the 500 mb low that was near King Salmon and
is moving into southcentral Alaska. The subtle features
associated with this upper level low are driving the snowfall over
the Cook Inlet region today. Good model agreement persists into
Monday night when a strong and complex low moves into the
Aleutians/akpen from the southwest. This low will likely split
into at least two distinct centers, but the agreement between
models is not very good. This will bring in a large amount of
forecast uncertainty, especially in regards to both winds and
precipitation type, over southern Mainland Alaska for the middle
of the week.
snow will bring MVFR and some IFR conditions through the night at
panc. Conditions should improve early Sunday morning though some
fog and low stratus clouds may persist after the snow is done. The
low cloud ceilings are more likely than fog and ceilings may
remain under 5000 ft through Sunday afternoon.
Short term forecast southcentral Alaska (days 1 and 2)...
An upper level low centered over southwest Alaska continues to
lift inland toward the northeast. Colder air associated with this
low has moved in over southcentral, allowing for precipitation to
fall as snow across the greater Anchorage area, in addition to the
mat-su valleys. Some mixed precipitation has been reported today
down on the Kenai Peninsula. While things are off to a slower
start in the Matanuska Valley, we are expecting snow to intensify
through this evening. Winter weather advisories are currently in
effect for both the Anchorage bowl and the Matanuska Valley for
an additional 3-6 inches of snowfall by Sunday morning. As the
upper level low continues to lift toward the northeast overnight,
snow will gradually taper from south to north. By Sunday evening,
an upper level wave lifts over Prince William Sound, bringing
another round of precipitation with some additional accumulations
expected for the Susitna Valley and the Copper River basin.
Short term forecast southwest Alaska (days 1 and 2)...
a cold front draped across the Kuskokwim valley and Delta provides
a boundary for snow to develop through Sunday evening. The highest
amounts of snow are expected across the Kuskokwim valley, which
will not be significant as low level moisture is limited. A weak
trough of low pressure will remain across the southwest through
Monday, so flow will generally be out of the north. Gusty winds
increase out of the east to northeast direction along the coasts
Monday evening as the next Bering front pushes to the coast. This
front will be the next one to monitor for gusty winds and snow
Tuesday into Wednesday.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
An unsettled weather pattern is on tap through the start of the
next week. The main challenges through Tuesday surround three systems
expected to race up to the Bering from the North Pacific. These
North Pacific storms will generally shift a warmer air mass toward
the northern Bering until Tuesday...then cold air advects
southward across the western and central waters as the main low
centers moves toward Nunivak Island. The first gale to storm
force system deepens to 968 mb just before crossing to the the
southern Bering over Adak Sunday evening. The next system follows
a similar track nearing Adak mid day Monday at 965 mb. The third
system slides across the eastern Aleutians Monday night deepening
to around 960 mb. The energy from the second system gets wrapped
up into low that moves across the eastern Aleutians...allowing for
a powerfully organized system to develop on Tuesday.
Marine (days 3 through 5)...
Beginning the extended period marine forecast on Tuesday, there
will be a very large high end gale force to near storm force low
covering the entire Bering Sea and Aleutian chain, with the
associated gale force front moving into the Gulf of Alaska. There
is some uncertainty on a dangerous and compact low moving from the
Alaska Peninsula through Bristol Bay Tuesday, with some models
(the gfs) depicting a sub-960 hpa fast moving storm force low
rapidly moving northward across Bristol Bay to the Bering Strait.
For now this forecast will not include such a deep low in the
marine forecast, but mariners should remain aware of this
Eventually, the larger scale low will weaken and move inland on
Tuesday night as the gale force front continues to move east
through the Gulf of Alaska. Broad cyclonic flow will persist over
the Bering with the next potential storm force low moving into the
southern Bering/Aleutians on Wed afternoon into Thursday, moving
east into the Gulf of Alaska by Thursday afternoon. There is quite
a bit of uncertainty with this storm, but mariners should expect
high seas and unsettled weather with this storm mid to late week.
Long term forecast (days 3 through 7)...
Beginning Monday evening, a stormy pattern over western Alaska
will move into southcentral Monday night into Tuesday. A warm
front associated with a low in the eastern Bering, combined with
cold air remaining over the region, will create the conditions
necessary for a wintry mix of precipitation across southern
The first of two fronts, driven by a strong upper-level jet, will
move across Alaska bringing warm air and ample precipitation from
tapping the subtropics. The cold air will be in place for awhile
creating favorable conditions for snowfall, until the warm air
displaces the cold. This will lead to rain and possibly freezing
rain Tuesday night into Wednesday with heavier snowfall for the
higher elevations. As that front moves east, a second front looks
to move through the region Friday. While confidence is high that a
front associated with a strong low will move across southern
Alaska, uncertainty remains with the storm track of the low and
how far north or south it may end up. This track will determine if
we will see rainfall or unsettled winter weather across the
southern Mainland. Heading into Saturday night, it looks like cold
air will move into the area, as cold air from northern Alaska
wraps around the area of low pressure and makes its way south. In
summary, active weather will continue throughout the week with
impacts expected for many precipitation types: rain, snow and
freezing rain. Temperatures look to start off cold before warming
up and then ending the extended forecast period cold again.
Marine...storm 174. Gales 170-173 175 176 179 411-414.