Scientific Forecaster Discussion
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
409 PM akst Friday Nov 27 2015
Analysis and upper levels...
the main upper low is anchored over the northwest Bering Sea.
Another upper low is now over the Alaska Peninsula and moving
north then northwest...eventually rotating into the main upper
center over the next couple of days. The main surface low is
underneath the Alaska Peninsula upper low with a weak surface low
in Bristol Bay and another south of Kodiak Island. The associated
front has pushed in along the north Gulf Coast overnight and the
moist warm southerly flow persists into the Mainland. Troughing
across the interior north of the Alaska Range coupled with the
pressure rises behind the front is driving the strong winds over
southcentral Alaska today. This is resulting in the typical cross
barrier flow precipitation shadow across the Chugach Mountains and
upslope areas of precipitation along the Alaska Range. Colder air
is wrapping around the west and south quadrants of the Alaska
Peninsula low as evident in showery weather seen in satellite
the models remain in very good synoptic agreement through the
weekend. The next system that will race along the Aleutians to the
Alaska Peninsula by Monday morning is handled well by all models and
agreement is good between the models. This allows US to continue
to use the higher resolution models for the digital forecast
preparation to better capture the details of the strong gap wind
and the areas of precipitation.
Short term forecast southcentral Alaska (days 1 and 2)...
the pattern of the next couple is revealing itself this afternoon
as the last major front/shortwave of the longwave trough passes
overhead into the interior. The coldest air in the upper trough
will be overhead for the next couple days. When combined with the
warmer than average low level temperatures this will lead to a
convective showery pattern. There is a lack of defined upper level
features to focus any release of instability. Thus...most of the
steady showers will be confined to the north Gulf Coast in upslope
flow over the terrain. Some of these showers will carry over into
the Cook Inlet region but coverage should not be as much.
Precipitation type should remain mostly rain in these showers but
a heavier burst of precipitation could result in a changeover to
snow. Wind will taper off pretty quickly this evening which will
allow temperatures to fall off from moderate values in the middle 40s
across the region.
Short term forecast southwest Alaska (days 1 and 2)...
the broad upper low over the Bering will bring multiple waves
across the southwest through the weekend keeping chances for snow
and rain. Temperatures will gradually becoming cooler through the
weekend as the surface low near the southwest coast lifts to the
north allowing a colder air mass to filter in along westerly flow.
The next front moves into the the Alaska Peninsula early Sunday
bringing warmer temperatures and gusty gale winds across the
marines for the end of the weekend.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
gusty northwesterly flow begins to taper off toward Saturday
morning as low pressure weakens across the eastern Bering. The
next low will quickly move into the western Bering late tonight
and then track along the islands...reaching the eastern Aleutians
by Sunday. This system will bring a warmer air mass and gusty gale
winds as it moves through the chain during the weekend. The front
will make it to the Pribilof Islands late Saturday and then stall
as the surface low approaches Dutch Harbor on Sunday.
Long term forecast (days 3 through 7)...
beginning the extended period on Sunday evening...a broad upper
trough over the North Pacific and Bering Sea will be making slow
eastward progress...with a departing and weakening upper trough
over eastern southcentral Alaska. The associated Bering front will
move into the Gulf of Alaska Sunday night and Monday...then merge
with a preexisting cutoff low near haida gwaii. The front will
reach gale force is it tracks into the Gulf...but then weaken as
it merges with the aforementioned haida gwaii low as it tracks
toward the northern Gulf. This rather common semi-permanent
Bering low regime will favor showery weather across coastal
locations in the Gulf...with the bulk of cold air remaining over
the Bering Sea...which in itself tends to enhance the North
Pacific polar jet stream. Southcentral will remain under the
influence of weaker lows rotating around the trough into the
Gulf...with east to southeast low level flow favoring a rather
dramatic downslope precipitation pattern across inland locales.
The first large Bering low will move east and weaken by late
week...only to be replaced again by another large Bering low as
the North Pacific jet stream makes slow eastward progress heading
into the weekend...and as the Stout northeast Pacific Ridge (the
current source for the very warm pattern in place now) deamplifies
and weakens. What this means...impact wise...is there will not be
any significant cold air outbreaks or significant inland
storms...but it will result in a moderation of the very warm
temperatures closer to normal by the weekend. Forecast confidence
is extremely high with the general pattern and inland
impacts...and moderately high regarding weak lows rotating around
the larger trough as they trek into the Gulf of Alaska. These
differences however are not all that significant until late week.
Marine...Gale Warning 155 170 171 172 173 174 175 176 177 178.
Synopsis and model discussion...surface observation
southwest Alaska/Bering Sea/Aleutians...kh