Scientific Forecaster Discussion
fxak68 pafc 270025
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
425 PM akdt sun Mar 26 2017
Analysis and upper levels...
there is an upper level low over Alaska extending into the western
Gulf. At the lower levels there is an inverted trough over the
western Gulf, with some moisture and showers over the Gulf and
along the Gulf Coast. There is another upper low south of the
eastern Aleutians moving slowly towards the north. There is a
fairly strong and vertically stacked 976 mb surface low associated
with that upper low. Precipitation (mostly rain) from this low
stretches across the Aleutians. An upper level high is over the
Bering Sea, with east-northeast flow at the lower levels.
the numerical models are in decent agreement through the short
term portion of the forecast (tuesday afternoon). There are some
differences in how they handle lows that move northward through
the Gulf, one tonight into Monday, and another Monday night into
Tuesday. The Canadian Gem is the fastest with both of these lows.
The NAM and European model (ecmwf) were the preferred models. Forecast confidence
is near normal.
panc...VFR conditions and light winds will persist.
Short term forecast southcentral Alaska (days 1 and 2)...
the start of the week will feature a gradual change in the pattern
as the longwave trough sitting over the Mainland retrogrades
slightly westward and allows onshore flow to become focused along
the north Gulf Coast. A surface low rotating northwestward
through the Gulf will move to the vicinity of Seward by Monday
afternoon, bringing widespread snow showers to the Gulf Coast and
initially spreading increased cloud cover over inland parts of
southcentral. Snow will likely be very convective in nature owing
to steep mid level lapse rates, which could support significant
accumulations over portions of western Prince William Sound as
well as Portage Valley and Turnagain Pass through Tuesday
morning. There is lower confidence on whether or not any snow will
make it across the coastal mountain ranges, but the combination
of a lack of initial downslope flow and a weak upper wave lifting
northward suggest at least a chance that Anchorage will see snow
showers for the first time in several weeks.
Short term forecast southwest Alaska (days 1 and 2)...
much more of the same weather expected through Wednesday evening.
A system backing into the area Monday night from the Gulf of
Alaska will bring some cloud cover over far eastern portions of
the Mainland, mainly near the Alaska Range. Some light snow is
possible in the same area, but currently a pretty low confidence
solution. Otherwise expect light northerly winds with below
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
a gale-force front nudges up to the central Aleutians this
evening. Precipitation associated with the front will likely stay
south of the chain as well, but a few local sprinkles or flurries
remain possible in the Unalaska area. Otherwise, broad but
steadily weakening northerly to easterly flow will remain over
Long term forecast (days 3 through 7)...
The only significant change to the long range was to increase the
probability of precipitation along the north Gulf Coast and Prince William Sound into the
Copper River basin Tuesday night and Wednesday. A surface low is
moving from the Gulf into Prince William Sound beginning Tuesday
supported by an upper level low. The models continue to hold the
precipitation along and east of the Kenai and Chugach Mountains.
Over the Bering Sea, Aleutians and western Alaska we will see a
big pattern change by the middle to end of this week as a large
potent low will move toward the Alaska Peninsula Thursday and
Friday. The models are in good overall agreement with this
solution through Friday. The latest model run does begin to
converge on this low moving over the southwest Mainland Saturday,
although the position and intensity of the low vary a bit. Models
are struggling to converge on a solution for the location and
intensity of a low south of the Aleutians. At this point we stuck
with nudging the current forecast toward the ensemble solution
making minor changes to the forecast.
Overall the pattern is shifting and will bring a more active
weather pattern and likely warmer temperatures to the southern
Mainland and Alaska Peninsula.
Marine...Gale Warning 155 172 174
heavy freezing spray 185