Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
808 PM EDT Wed Jul 28 2021
Valid 12Z Mon Aug 02 2021 - 12Z Fri Aug 06 2021
...Heavy rainfall possible for parts of western and northern
Alaska...
...Overview and Guidance/Predictability Evaluation...
The latest guidance continues to favor a strong amplification of
an upper trough with embedded closed low located between Siberia
and the Aleutians while upper ridging builds downstream over
portions of the mainland and into northwestern Canada. With ample
moisture advecting into the Mainland within this flow pattern,
this will yield favorable conditions for enhanced rainfall -
possibly with significant amounts over the West and North. This
setup will also help maintain an area of low pressure over the
eastern Gulf of Alaska near the Panhandle/southern Mainland
coastline; which may also keep periods of cloud cover and light
rain over the area.
Unfortunately, the degree of spread on both large and small-scale
details have not improved significantly with the last couple of
model cycles. However, the ECWMF did have downward trend with the
heights for midweek over central Alaska while the GFS has not been
consistent. The ensemble means are closer to the ECWMF with the
overall evolution. Confidence with this forecast continues to be
lower for much of the extended period. The preferred blend for
this forecast used roughly and 80/20 weighting of the 12Z
ECWMF/GFS/CMC/UKMET to the ensemble means initially and into the
middle period then shifting more toward the ECWMF late in the
period.
...Weather/Threats Highlights...
One or more periods of potentially heavy rainfall will be possible
across the western and northern portions of the Mainland.
Although the spread in the guidance lows the confidence a bit on
when and where, the best time frame appears to be around Sunday
into Monday- this is when areas of enhanced moisture will likely
be in proximity of the favored higher terrain of Seward Peninsula
and Brooks Range. Some of the highest totals may occur across
these locations and additional rain may linger beyond Monday. A
leading shortwave and surface system may produce one period of
focused precipitation over the northwest early in the weekend
while the combination of a stalled front and possibly some western
Pacific tropical moisture included within southerly flow ahead of
a Bering Sea storm could provide the added enhancement thereafter.
Some of the rainfall totals may be close to climatological
extremes. Mostly light to scattered rain is expected at times for
the Panhandle ahead of the lingering northeastern Pacific upper
low.
For much of the eastern half of the Mainland and points into the
northwestern Canada will have warming temperature as the upper
ridge builds during this period. Daily temperatures across the
northern Coast will likely be higher than seasonal average for
late July into early August; while near or below normal daily
maximums are observed elsewhere across the state. Southern
portions of the Panhandle could see pockets of warmer readings.
Overall, the daily minimums are expected to be above normal, with
only a few localized areas below normal.
Campbell
Hazards:
- Heavy rain across portions of mainland Alaska, Sat-Sun, Jul
31-Aug 1.
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