Scientific Forecaster Discussion
fxak68 pafc 250026
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
426 PM akdt Wed may 24 2017
Analysis and upper levels...
a nearly stacked low is evident near Saint Matthew Island. Its
associated front is moving rapidly east across SW Alaska. Radar
imagery shows widespread rain across the north Cook Inlet region,
north Gulf Coast, and over the mat-su valleys. Rain is rapidly
changing over to showers across SW Alaska as cold air aloft moves
overhead and destabilizes the area. Strong as the broad upper low
complex continues over the northern and central Bering Sea. A
trough on the eastern spoke of complex extends across southcentral
and is exiting to the north. Gusty winds associated with the
above features were evident from the AK pen/SW Alaska to across
southcentral. Gap flows are fairly strong across Turnagain Arm,
Knik Arm, and along the Copper River.
model guidance overall is in fairly good agreement with timing of
synoptic features. Mainly performing hand edits.
panc...gusty southeast winds will continue across the Airport
complex through Thursday, but slowly diminish. VFR conditions are
expected, except just a slight chance of some MVFR in showers
Short term forecast southcentral Alaska (days 1 and 2)...
drier air working its way northeastward towards southcentral is
diminishing the coverage of the rainfall activity this afternoon.
The Turnagain Arm wind was effectively able to dry the Anchorage
bowl out today, resulting in less rainfall than originally
forecast here, though most areas with less rain got close to
This evening and tonight, precipitation will end in most areas
with the exception of favorable upslope locations along the
Talkeetna Mountains, Alaska Range, and Prince William Sound. The
drier air will also be colder, particularly aloft, making the
atmosphere become increasingly unstable. Thus, remaining
precipitation activity along the mountains will increasingly take
on a showery nature as the clouds are able to build more
vertically with time.
On Thursday, with impressively cold air continuing to move in
aloft, it will not take much daytime heating for thunderstorms to
become a possibility along most of the area mountains around
southcentral, even by late morning. The potential will also exist
for the increasing southeasterly Turnagain Arm wind converging
with the broad southerly flow up Cook Inlet for some showers to
make it into Anchorage as well. For those in the mountains,
falling snow levels won't inhibit vertical cloud growth, so
thundersnow will be possible there.
By Thursday night, while the thunderstorm threat diminishes, much
of the atmospheric instability will remain, so mountain showers
will likely continue through the overnight. For the populated
valley locations, temperatures may be the big story, as colder air
moves in. Freezing levels are expected to fall to around 1,500
feet. Nearly all locations, including Anchorage, are forecast to
see lows dip into the 30s with clearing skies. While the winds
across the area are forecast to remain strong enough overnight to
keep temperatures in most areas safely above freezing, any
locations that are sheltered and/or are elevated have the
potential of seeing frost develop Friday morning. Areas with the
greatest potential of seeing some frost include the usual cold
spots around Anchorage, the Upper Hillside, most of the passes
across southcentral, and the Copper River basin.
During the day on Friday, Mountain showers will redevelop once
again with cold air remaining in place aloft. However, without an
upper level wave or much of anything to add to the lift besides
topography, thunderstorms, if any, will be much more isolated.
Short term forecast southwest Alaska (days 1 and 2)...
the frontal system that brought widespread wetting rains
throughout the region trough this morning has moved east of the
area, allowing for unseasonably cold air to move in its wake. The
first impact from this cold air will be this evening from a deep
layer instability that will occur as a very cold upper level
trough passes overhead, which may provide enough instability to
produce a few lightning strikes over the Kuskokwim River/Delta
this evening. A chance of thunderstorms will then re-form over
the Kuskokwim valley on Thursday afternoon as as even stronger
instability develops under the upper level trough. The second
impact from this system will occur late tonight as enough cold air
reaches the low elevations along the Kuskokwim valley to produce
light snow accumulations, which may spread as far west as Bethel
for Thursday morning. If this were to occur it would be first time
since 2008 that accumulating snowfall occurred at Bethel this
late in the season.
Short term forecast Bering Sea/Aleutians (days 1 and 2)...
most of the weather through Thursday will surround the low
currently over the northern Bering that will continue to bring
unseasonably cold air into much of the Bering. This low will
bring gusty westerly winds and rain showers to much of the region
this evening. This low may bring enough cold air into the
Pribilof Islands overnight to bring snow to the region, however
with warming sst's keeping temperatures well above freezing
accumulation is not expected.
Long term forecast (days 3 through 7)...
the weekend will begin with the elongated cold upper level trough
over western and southwest Mainland Alaska. This will keep below
normal temperatures over most of the forecast area...especially in
southwest Mainland Alaska. Southcentral will be in due south flow
aloft which is favorable for precipitation. As the weekend
progresses a cut off low should develop near Kodiak off the main
upper level low allowing the main low to retreat into the Arctic.
After this the main pattern remains a bit unclear as models
continue to struggle with the synoptic pattern. Overall the
deterministic runs are all over the place with the GFS ensemble
means a little bit better. Best bet is that early next week
remains cloudy with some showers, but temperatures should be
increasing and back to near normal for much of the week. This
unsettled pattern is producing below normal confidence in the long