Scientific Forecaster Discussion
fxak68 pafc 150129
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
429 PM akst Sat Dec 14 2019
Analysis and upper levels...
a weak upper short wave is lifting north across the southcentral
this afternoon. In response showers are evident across much of the
Gulf, Kenai Peninsula, and the Cook to Susitna Valley corridor.
Though strong southeast cross-barrier flow minimized precipitation
potential to the Lee (west) of the Kenai and Chugach Mountains. A
stacked low pressure center was over the eastern Aleutians with
an occluded front extending from just south of the Pribilof
Islands to over Kodiak Island. Rain/snow associated with this
front as evident across the pribilofs, southwest Mainland, to rain
over Kodiak Island. Widespread rain and showers were also
observed over the eastern Aleutians and Alaska Peninsula. Strong and
gusty cyclonic flow, associated with the large low center over the
eastern Aleutians, was evident by brisk northerly winds to its
west and strong easterly winds stretching across the Gulf of
Alaska. Tight east to west pressure gradients produced strong
easterly winds along Turnagain Arm.
models from a synoptic perspective are in fair agreement. There are
some positional differences in the surface low near Kodiak/Alaska pen
tonight and Sunday. However, the general sensible weather should
be fairly close.
panc...VFR conditions will persist. Winds across the Airport
complex will become north by early evening. This, combined with
southeasterly winds aloft, will bring low level wind shear through Sunday.
Short term forecast southcentral Alaska (days 1 and 2)...
a trough producing widespread showers across Prince William Sound
will continue to lift north overnight as it slowly weakens. A
front associated with a low pressure system over the central
Aleutians will track northeast from the western Gulf and approach
the northern Gulf and southcentral coast Sunday. Rain along the
front will continue to provide wet conditions for Kodiak city
tonight through the weekend. This area of precipitation will
advance with the front, spreading over the southcentral coast
Sunday and Monday. This front will also produce winds up to storm
force and high seas from the Barren Islands to the Copper River
Delta beginning Sunday. The high winds and rough seas are expected
to persist until Monday morning.
Despite this wet pattern, the Anchorage bowl and western
Kenai Peninsula are not receiving much, if any, precipitation.
This is due to downsloping from rain falling on the windward side
of the chugach/Kenai Mountains which makes it difficult for
precipitation to make it to the leeward side. These dry conditions
look to remain through Monday night. Winds look to remain
southeasterly, which will keep Turnagain Arm wind gusts around 30
to 50 mph through Sunday before slowly tapering off. Patchy fog in
the Copper River basin is the greatest forecast challenge, as
temperatures look to stay cold ahead of the front. Whether fog
occurs or not will be a matter of the sky clearing overnight. If
it clears, temperatures will be able to drop closer to the
dewpoint. This, along with little to no wind, are ingredients
needed for fog development. If clouds linger and temperatures stay
a bit warmer across the basin, widespread fog may not have the
opportunity to develop.
Short term forecast southwest Alaska (days 1 and 2)...
a low pressure system 150 miles southeast of St Paul island this
afternoon will be responsible for a round of active weather for
southwest Alaska on Sunday. Gusty winds will impact the area, with
Bristol Bay and coastal areas seeing the strongest winds through
Sunday evening. While precipitation will be primarily rain across
southwest Alaska, areas of the Kuskokwim valley and Kuskokwim
Delta will see snow. As winds return to the northeast, southwest
Alaska will see some mixed rain and snow showers on Monday as
colder air comes back in.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
a low pressure system 150 miles southeast of St Paul island will
be responsible for strong winds and widespread rain and snow
showers across the Alaska Peninsula and eastern Aleutians through
Sunday night. As the system weakens Monday, rain showers will
remain through Monday night. Another strong low pressure system
will enter the western Aleutians Sunday evening, with another
round of strong winds and precipitation.
Marine (days 3 through 5)...
the storm track during this period will be from the Aleutian
Islands eastward to the Gulf of Alaska. High end small craft
advisories to gale warnings are expected as this low pressure
system moves through. Additionally, expect light to heavy freezing
spray from Hooper Bay to Nunivak Island.
Long term forecast (days 3 through 7)...
the global models are in decent agreement with the longwave
pattern for North Pacific and Bering through Friday December
20th. At the onset of this forecast period there are two lows,
one near Kodiak Island and a Kamchatka low. There is a ridge axis
between the two systems. There are some minor differences with
the timing of two different surface lows but the forecast
intensity is only a few millibars off, and the forecast track is
very similar. Overall, the run to run continuity with these two
lows has been decent. Expect these systems to move from west to
east with the first storm tracking into the Gulf of Alaska and
later towards Southeast Alaska. All the while, the second storm will head
into the Bering with the frontal boundary pushing across the
Aleutian chain. For southcentral Alaska, temperatures will be near
or slightly above normal. Again, for the Anchorage Metro area the
chances of snow in this forecast period range from zero to slight
Marine...Storm Warning 119 120 130 131.
Gale Warning 125 132 136>136 141 155 165 170>179.