Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
753 PM EDT Sun Aug 15 2021
Valid 12Z Thu Aug 19 2021 - 12Z Mon Aug 23 2021
...Overview...
Today's guidance continues to show broad cyclonic flow aloft from
the Arctic into the North Pacific while a ridge over the eastern
Pacific gradually weakens somewhat, with the overall pattern
leading to a period of unsettled weather and rather cool daytime
highs across much of the state. There is still a fair amount of
uncertainty in some specifics for individual shortwaves within the
various steams and how the separate streams may interact.
...Guidance/Predictability Evaluation...
Overall the most agreeable feature remains the upper shortwave and
associated low pressure expected to track into the mainland from
the west during the latter half of the week. There is still some
spread, especially with the latest GFS runs deeper than most other
guidance for the shortwave and deeper/northward for surface low
pressure. The 12Z GFS was also at the deep edge of the 12Z GEFS
member envelope for the surface system. Yesterday's continuity
favoring moderate amplitude of the upper trough has held up well
with today's consensus.
Predictability and confidence are lower with other aspects of the
forecast, albeit with a little more definition for features of
interest. Over the past day the signal has increased somewhat for
a northern Pacific trough aloft to support a surface wave that
could track anywhere from well south of Mainland Alaska (06Z
GFS/12Z ECMWF) to the mainland itself (00Z ECMWF) with 12Z/18Z GFS
runs between these extremes and the CMC becoming slower than other
solutions as it has tended to do in recent runs. Latest ensemble
means are starting to hint at this wave and suggest a surface
track that may reach near the Alaska Peninsula and continue into
the Gulf. A slightly more defined version of an ensemble mean
average would provide a reasonable adjustment at this time given
how much uncertainty still exists. An upstream feature could drop
into the Bering Sea late in the period but with minimal confidence
for specifics thus far. Across the Arctic, there is reasonable
agreement that one upper low will depart northeast of the mainland
to start the period with a trailing compact upper low tracking
north of Siberia and possibly reaching close to the northern
mainland. The 12Z CMC becomes quite fast versus other guidance
for the trailing upper low/trough while the 12Z ECMWF is fastest
among non-CMC solutions. Ensemble means generally favor an
intermediate timing.
A 12Z operational model blend provided a good intermediate
starting point early in the period with the main focus being the
system tracking into the mainland from the Bering Sea.
Transitioning this blend toward an average among the 12Z/00Z
ECMWF, 12Z/06Z GFS, and 12Z UKMET (plus starting to include a
small 12Z GEFS/00Z ECens weight) by mid-period early Saturday
yielded the desired solution for the wave that may track near the
Alaska Peninsula while also maintaining somewhat more detail than
the means elsewhere. At that time the CMC was removed due to
increasingly poor comparisons to other guidance in multiple areas.
The rest the period increased ensemble mean input while
maintaining minority components of aforementioned GFS/ECMWF runs.
...Weather/Threats Highlights...
The Bering Sea system tracking into the mainland during the latter
half of the week along with a leading wavy front will spread
organized precipitation over much of the central and southern
mainland. Some pockets of enhanced activity are possible. There
is still some uncertainty over the northward extent of moisture.
The most likely scenario keeps the North Slope fairly dry.
Precipitation should exit eastern areas by the weekend.
Confidence remains low for details of the North Pacific wave whose
track will determine the northward extent of moisture and
precipitation intensity from late week through the weekend.
Current preferences would bring some precipitation into the
eastern Aleutians and Alaska Peninsula then eastward along the
southern coast and into the Panhandle while some lighter activity
could reach a little northward. The full range of possibilities
extends from a more suppressed/drier solution to a more northward
extent of meaningful moisture. Meanwhile expect some
precipitation to accompany a front reaching the northwestern
mainland next weekend as the anchoring system tracks over the
Arctic. The unsettled pattern will keep much of the state below
normal for high temperatures through the period. Lows should be
more moderate with best potential for consistently above normal
readings over some locations in the southern mainland.
Rausch
Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can also be
found at:
https://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
https://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html