Scientific Forecaster Discussion
fxak68 pafc 181259
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
459 am akdt Wed Sep 18 2019
Analysis and upper levels...
Low pressure is moving through southcentral AK, with a shortwave
trough currently moving quickly up Cook Inlet as of 5am this
morning. The back edge of this short wave is well defined on
radar, with a clearly distinguished line where the rain
transitions into showers. On the broad scale the upper low over
southcentral Alaska is moving northeast and will be merging with an
area of low pressure sitting over interior Alaska. Meanwhile, the next
low pressure system is working its way east just south of the
Aleutian Islands. This is a rapidly moving system with strong
zonal flow that is driven by the jetstream. Beyond that, another
low is already lining up in the pattern just south of the
Models continue to handle this fall pattern well, at least in the
short term. As a result, little adjustment to the forecast was
needed overnight. This also gives US high confidence in the
forecast for the incoming weather system.
Panc...VFR with intermittent mfvr conditions will be present
throughout the day, due to cloud cover and rain. Windy conditions
will also continue, with wind shear present over the airfield
through early morning.
Low pressure moving through southcentral Alaska will continue to
blanket most of the region with wetting rains. The Copper River
basin continues to remain the exception. That region will see
rain, just not enough enough to be considered a wetting rain.
Short term forecast southcentral Alaska (days 1 and 2)...
The front has moved rapidly up Cook Inlet early this morning.
This will allow a short break in the rain before the widespread
showers associated with the cold air around the upper level low
move into the region. This will transition the steady rain to the
more on- and-off showers though the rainfall in these showers may
be heavy at times. There is an outside chance for a thunderstorm
over the Cook Inlet area today as the cold air aloft moves into
the area and this pattern is a common one to produce a thunderstorm
in September. However, it does look unlikely today as there will
probably not be quite enough instability for thunder though it cannot
be completely ruled out. Winds have been quite strong through
Turnagain Arm overnight. As the front moves north of the area the
winds should move into the Anchorage bowl. While the winds will not
be quite as strong in Turnagain Arm after the front moves through,
that will be the time it gets noticeably windy in Anchorage.
The next system will move into the Gulf of Alaska Thursday night
and bring widespread rain back into the region at that time. The
arrival of the front and rain was sped up a little on this
Short term forecast southwest Alaska (days 1 through 3; today
Showers are currently present across all of southwest Alaska under
a low. These will gradually become more scattered through the day
as the low weakens farther inland. A weak upper level ridge will
move into the area overnight tonight, leading to skies briefly
clearing into Thursday morning. As such, patchy fog could develop
especially given all the recent precipitation. Partly sunny skies
will persist through the evening. A trough will descend into the
region from the north overnight Thursday into Friday, bringing
cooler temperatures (lows near freezing in the Kuskokwim delta),
while a surface low will approach the Bristol Bay coast. Scattered
showers will persist through the day with possible snow across
higher elevations of the Alaska Range.
Short term forecast Bering Sea/Aleutians (days 1 through 3;
today through friday)...
Showers and gusty westerly winds are continuing across much of the
Aleutians south of a low. Winds will gradually diminish through
the day and showers will become more scattered as the low weakens
over the Mainland. Another low will quickly track south of the
Aleutians today through Thursday. Winds will remain generally
light, however scattered showers will persist. A ridge will
gradually build in from the west on Friday, bringing settled
weather from west to east into this weekend, however the usual
fog/low stratus is likely.
Marine (days 3 through 5)...
Beginning the extended period marine forecast on Friday, a strong
gale force low will be in the Gulf of Alaska with high pressure
over the Bering Sea. The gale force low will remain in place over
the Gulf, but cold air advection will be in full force through the
Barren Islands with possible storm force winds by Saturday.
Thereafter, much uncertainty develops as a low will move from the
north pac toward the Aleutians and Gulf of Alaska. The most likely
scenario is the low brushes the Aleutians and deepens as it moves
toward the Alaska Peninsula and Kodiak Island, likely reaching
gale force by Sat night or early Sunday. A triple point low will
form over the Gulf of Alaska Sun night and Monday, and may be a
high end gale force to low end storm force low. Prepare for high
winds and seas by late weekend over the Gulf.
Long term forecast (days 3 through 7)...
Beginning the extended period forecast on Thursday evening, there
is be a broad longwave trough stretching from the North Slope to
the North Pacific. A shortwave trough at the base of the trough
will be tracking from south of the Alaska Peninsula eastward, with
a developing low along a warm front at the surface. The low will
move into the Gulf of Alaska and move into Prince William Sound on
Thu night and will move inland through Saturday. Pieces of Arctic
energy will also dive down, so colder air will be filtering in,
which will likely deposit some high elevation snow across parts of
the Alaska Range and Talkeetna Mountains/Chugach Mountains (likely
above 5000 feet). However, the bigger story will just be another
round of rain, heaviest across the Gulf Coast and parts of the
Copper Basin, although rain will likely impact all of southcentral
Friday and Saturday before ending Sat night.
Yet another low will skirt the Aleutians and move into the Gulf of
Alaska on Sunday and Monday, bringing more rain to the coast and
chances of rain inland (the exact track of the low remains
somewhat uncertain, as well as if it is a closed low or open wave,
which impacts precipitation distribution inland). Long story
short, the pattern will be much more active than the first half of
Marine...gale warning: 131 132 150 155 170 171 172