Scientific Forecaster Discussion
fxak68 pafc 200106
Southcentral and southwest Alaska forecast discussion
National Weather Service Anchorage Alaska
506 PM akdt Wed Jun 19 2019
Analysis and upper levels...
A weak ridge of high pressure extends from the Gulf of Alaska
north into the southern Mainland. A weak surface and upper level
low is over the Kuskokwim Delta as another weak low is southwest
of this in the Unalaska area. A weak area of high pressure is over
the central Aleutians into the central Bering Sea as the remnants
of a front extends from the western Bering Sea across the Adak
Over the Copper River basin to the Talkeetna Mountains we are
seeing the instability (cape) increase in the wake of the
departing easterly wave. Enough moisture remains over the basin
to allow for convective development.
The models are in good synoptic agreement. However, differences
arise in the details, especially with regard to where the
instability will be greatest this evening and Thursday. The GFS
holds the convective threat more north than the NAM in the Copper
River basin, though the idea is the Copper River to the Talkeetna
Mountains is the main area of concern.
Panc...VFR conditions and light winds will persist.
For the Swan Lake fire/western Kenai Peninsula...as weak high
pressure builds over the southern Mainland, warmer and drier
conditions will develop over the next couple days. The sea level
pressure pattern will remain quite slack, so winds will generally
remain light. On Friday, a front will lift northward across the
Gulf, but confidence is increasing that this front will remain
south of the Kenai Peninsula. The only impact will be a few clouds
and perhaps a slight uptick in low level winds.
In the outlook for the weekend and early next week all signs
indicate hot and dry conditions, with temperatures likely pushing
80 degrees or higher on the hottest days. There will be a chance
of thunderstorms each day, though it's a bit too early to say
exactly where they will set up and what the storm motion will be.
Short term forecast southcentral Alaska (days 1 through 3:
tonight through saturday)...
The weather pattern the next several days across southcentral
looks to be a mainly quiet one as far as large-scale systems GOES
as ridging building over interior Alaska works to keep any
larger-scale lows well to the south over the Gulf. However, there
will be a number of smaller scale weather features to note that
will have an impact on their respective local areas.
First off is the threat of showers and isolated thunderstorms.
Showers have been making their way southward off the Alaska Range
into the Copper River basin this afternoon. This will be a common
theme the next several afternoons, though that focus will shift
westward to include the Talkeetna Mountains and Susitna Valley
Ridging building across much of southcentral, especially the
Copper River basin, will lead to a slow warming trend in the
temperatures. The warmer temperatures Thursday and Friday may
help to lower relative humidity levels, especially in areas away
from any marine cloud cover and away from any thunderstorms.
The lack of any strong weather features should help to keep winds
light and variable for much of the area, including the interior
Kenai Peninsula. This should help to keep wildfire smoke generally
contained close to the fire. Sea breezes will be the primary wind
concern, especially for small craft through Kachemak Bay,
Resurrection Bay, and Port Valdez. The warmer temperatures across
the interior should keep those sea breezes redeveloping each
afternoon in those areas, potentially reaching up to 20 kt at
Finally, the one synoptic feature of note will be a low tracking
across the far southern Gulf Thursday night into Friday before
weakening substantially. Along its northern periphery along the
Gulf Coast, winds to Small Craft Advisory levels (25 kt) are
possible. The associated northeast winds will also impact Shelikof
Strait with similar winds and seas up to 10 ft. The interior
ridging should block the worst conditions from affecting any
areas away from the immediate Gulf Coast.
A mix of clouds and sun are expected for southcentral, with the
cloud cover largely being associated with afternoon showers and
storms along the mountains, the remnant cloud cover from those
showers, or clouds associated with the Gulf low.
Short term forecast southwest Alaska (days 1 through 3:tonight
A vertically stacked low over Bethel will slowly lift north of
the region with a surface trough trailing back over the upper
Kuskokwim Delta. These features have allowed an isolated
thunderstorm to develop near Kipnuk this afternoon, and we can't
rule out a few more developing given the cooler air aloft acting
with daytime heating and orographic that will at times lift the
precipitation high enough to allow for charge separation
(lightning). Otherwise, the lingering surface trough will allow
a few showers to develop for Thursday and perhaps into Friday
for this location.
Otherwise, the main items of interest will be a North Pacific low
approaching Kodiak, which will be close enough to bring some
showers to portions of the Alaska Peninsula for Thursday. On
Friday, a thermal trough building westward will bring a chance of
showers and thunderstorms to locations north of the Nushagak hills
and east of the Kilbuck Mountains as afternoon highs increase a
few degrees. The Kodiak low will also maintain a chance of
showers for portions of the Alaska Peninsula.
Short term forecast Bering Sea/Aleutians (days 1 through 3:
tonight through saturday)...
"big bubble, no trouble" sums up the current weather pattern for
most of the Bering and Aleutians, with the main concern being some
locally dense fog for the Pribilof Islands that will like develop
again for your Thursday morning. Otherwise, the main item of
interest will be an approaching area of low pressure and its
surface front, which should start to impact the western Bering
and Aleutians on Friday with increasing winds and periods of rain.
Marine (days 3 through 5)...
Beginning the longer term marine forecast on Saturday, the storm
track will be shifting west over the western Bering Sea as a gale
force low moves in Saturday, then lifts north and weakens through
Monday. While the strongest winds will remain over the western
Bering/Aleutians, there will be longer fetch southerly winds which
will bring some higher seas as far east as the Pribilof Islands.
Dutch Harbor may see a brief period of gusty southerly winds as
well. Otherwise, the eastern Bering Sea will remain quiet with sea
stratus/fog and generally lighter winds.
A departing low will be replaced by light west winds over the Gulf
of Alaska Saturday through Monday, with high confidence that no
storms or strong fronts will be anywhere near the Gulf of
Alaska/north Gulf Coast.
Long term forecast (days 4 through 7)...
There is no change in the overall thinking of the extended period
forecasting beginning Saturday evening as an interior ridge builds
as a Gulf low departs to the southeast. The Yukon interior ridge
will build west as offshore flow across southcentral results in
further warming through the weekend. Uncertainty still exists from
Tuesday through the remainder of the week as the global numerical
guidance continues to struggle with low amplitude disturbances
passing through the upper ridge. However, the signal in the noise
is with warm interior temps inland, this will be enough to promote
the potential for inland thunderstorms, mainly across southcentral
where the greatest instability will reside. The timing of these
low amplitude shortwave troughs is very uncertain at this juncture,
but any of these waves could spark off rather widespread thunderstorms
across parts of southcentral (favoring interior and the mountains)
Farther west, a series of lows will be tracking across the Bering
Sea and sending clouds/more stable air into southwest Alaska as
early as Monday. Much uncertainty develops Wednesday as all the
global models (ecmwf/GFS/Canadian and their respective ensembles)
depict an impressive array of highly disparate solutions from high
amplitude ridging/warming over the interior with fronts passing
into the southern Mainland. These disparities arise from potential
northern stream influences from the Arctic to storms moving into
the Bering Sea from the Pacific. For now, expect warm temps (above
climo across the southern mainland) to last through Wednesday
before uncertainty grows. However, with that said, there is still
a strong signal that the warm period will persist through the end
of the week in most of the guidance (ensembles and statistical mos).