Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
411 PM akdt Tuesday Mar 31 2015
Analysis and upper levels...
a long-wave trough remains over the region with a nearly zonal
subtropical jet driving systems eastward across the North Pacific
and Aleutians. The flow has become more amplified the past 12
hours with a building ridge over the western Bering out ahead of
a short-wave trough digging southeastward out of Asia. This is now
causing downstream amplification with a low/trough transiting the
Aleutians/north central Pacific. A strong jet streak and short-
wave have led to intensification of the low which has now occluded
and nearly stalled south of the eastern Aleutians. As the trough
continues to amplify this will cause the low to take a more
southerly track with the eastern Aleutians and Alaska Peninsula on
the northern periphery of the storm. The air mass over the central
Bering Sea is quite cold as is evident by the 12z sounding out of
Saint Paul...with 850 temperatures of around -14 degrees celsius. With
the building upstream ridge northerly flow is just beginning to
strengthen and will eventually lead to decent cold advection as
the low exits.
Not much has changed across southwest and southcentral Alaska as a
series of mostly very weak upper level waves track in from the
Gulf producing fairly widespread clouds and a few showers. A weak
and elongated trough axis from the eastern Bering to south of
Kodiak Island is progressing slowly northeastward toward the
southern Mainland...but there is very little vertical motion out
ahead of it. If anything...there is improving conditions right out
ahead of this as the more significant upper waves mentioned above
are pushed off to the north.
there continue to be some minor model differences with track of
low south of the eastern Aleutians as it heads into the southern
Gulf as well as with weak triple point which develops out ahead
of it. This mainly affects wind directions over the southern Gulf
which will generally be on the light side in vicinity of the main
low center. Will use an average position of the various models for
the updated forecast. Overall there is a trend toward a weaker
triple point low and farther south track of the main low.
Thus...the occluded front will make less north and west progress
resulting in less precipitation for Kodiak Island and the north Gulf
Coast. Outside of the details of the low track forecast confidence
is generally above average.
Short term forecast southcentral Alaska (tonight through Thursday night)...
After a few noticeable breaks in clouds across the region this
afternoon...clouds will begin building back across the region this
evening. Water vapor imagery depicts a weak and disorganized
short wave pushing across the western Copper River basin...that
has aided in the development of shower activity across the Kenai
Peninsula and parts of the mat-su valley during the day. The 00z
radiosonde observation from panc shows that the thermal profile has cooled (noted
mainly by 850 and 500 mb temperatures)...which indicated some
elevated instability present across southcentral Alaska. While the
temperature/dew point depression across Anchorage and the matsu valley
ranges from 10-15 degrees...once temperatures begin to cool this
evening and the column begins to saturate...scattered rain and
snow showers will be seen across the area. Greatest chances for
precipitation remain along the eastern half of the Kenai Peninsula
as a decaying low in price William sound pushes inland.
An area of low pressure skirting south of the Alaska Peninsula
will push into the western Gulf tomorrow afternoon and push a
front toward the northern Gulf Coast. Gusty winds will develop
through Portage Valley and Turnagain Arm as well as the Knik
River Valley. As winds across Turnagain Arm begin to increase on
Wednesday...it should help diminish rain chances across the
Anchorage bowl. This low will quickly push into the eastern Gulf
on Thursday and allow for precipitation chances across the Gulf
Coast to begin to diminish. Clearing skies across the interior or
southcentral will be seen Thursday and continue into the weekend.
Short term forecast southwest Alaska (days 1 and 2)...
lingering scattered rain and snow showers around the southwest
Mainland have quickly tapered off this afternoon...with some
remaining scattered showers along the Alaska Peninsula and
Aleutian Range. These showers will persist through this
evening...as a weak wave tracks over Bristol Bay while further
weakening overnight. Much of the Mainland will remain dry through
the end of the week...with the exception of parts of the Bristol
Bay zone. The next round of precipitation and gusty winds will
clip the Alaska Peninsula tomorrow morning and persist through
Thursday morning...as the North Pacific low tracks further
eastward. As the system enters the Gulf early Thursday
morning...showers will taper off and cooler temperatures will move
in over the Mainland as the ridge over the eastern Bering pushes
into the southwest coast.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
a North Pacific low south of Dutch Harbor will bring a mix of rain
and snow to the eastern Aleutians and Alaska Peninsula this
evening...along with some localized gusty winds over the area.
This system will continue an eastward track just south of the
Aleutian chain...before tracking into the Gulf of Alaska by
tomorrow afternoon. An area of cold air advection behind the
passing low will help generate some stronger gusts and snow
showers behind the system...which will be reinforced by a ridge
building into the central Bering overnight tonight. Further
west...a stronger frontal system associated with a gale force low
off the coast of the Kamchatka peninsula will bring the next round
of precipitation and gusty winds to the western Aleutians by
tomorrow afternoon. Gusty southerly winds associated with this
system will bring in some warmer temperatures from the northern
Pacific...causing the precipitation type to switch over to rain as
the warm front passes through the area...this shot of warmer air
will be reinforced by another system forming southwest of Shemya
Long term forecast (days 3 through 7)...
a frontal boundary will linger in the north Gulf and along the
coast on Thursday. Most of the steady precipitation will be along
the coast...however...increasing cyclonic flow and deep layer
moisture along with cold upper level temperatures keeps the chance
for showers along the mountains in the Cook Inlet
region...especially in the Copper River basin. This system exits
on Friday drying conditions out again. The extended forecast then
revolves around a front moving into the Bering Sea late this week.
This front is currently prognosticated to move into the Gulf in some
fashion through early next week...but guidance does not have a
good handle on the evolution of the flow pattern aloft. The best
agreement in guidance the past couple days has been keeping the
Mainland on the cool side of the jet while energy slides to the
south. This would return the area to normal or slightly above
normal temperature with the best chances for precipitation
remaining in the Gulf and along the coast.
Marine...Gale Warning 120 131 132 150 155 172 178.
Heavy freezing spray 179 180.
Synopsis and model discussion...sb
southwest Alaska/Bering Sea/Aleutians...tp