Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
706 PM EDT Thu Aug 19 2021
Valid 12Z Mon Aug 23 2021 - 12Z Fri Aug 27 2021
...Heavy rainfall possible across portions of southern Alaska and
the Panhandle...
...Overview and Guidance Evaluation...
An amplified pattern will take shape over Alaska during the
extended range period. An initial closed low will slide along the
Southern Coast Monday-Wednesday before the main trough shifts into
Western North America late week. This should send a frontal system
and plenty of moisture across the Gulf into the Panhandle region
during the first half of next week. The next system moves behind
this will move into south of the Aleutians by next Wednesday and
reach the Gulf by Friday. To the north, a compact closed upper low
will slide just north of the state early week while another
shortwave sends a low pressure system into the northern Mainland
region later in the week.
The latest 12z cycle of models and ensembles show good enough
agreement early in the week to warrant a general model compromise.
After Tuesday, the uncertainty remains high in the details,
especially regarding the late week system moving south of the
Aleutians into the Gulf. There are both timing and strength
differences in the models with this system. The ensemble means are
in better agreement on the overall synoptic pattern, but as usual,
don't maintain the details and tend to wash out systems. As such
the WPC progs for days 6-8 quickly trend towards the ensemble
means, with small contributions of the deterministic GFS and ECMWF
just to help maintain some of that system definition.
...Weather/Threats Highlights...
The amplified pattern expected next week should maintain wet and
unsettled conditions across much of Alaska. Southerly flow ahead
of the first cold front moving across the Gulf early week will
continue to direct significant moisture into southern Alaska
fueling moderate to heavy rainfall from parts of the southern
Coast to Panhandle the first half of next week. General troughing
ahead of the next system moving into the Gulf by next Thursday
will maintain the heavy rain threat through much of the week for
parts of southeast Alaska and especially the Panhandle. Some of
the highest elevations across southern Alaska may also see
measurable snowfall. A couple of shortwaves interacting with an
arctic frontal boundary should allow lighter precipitation to fall
across parts of northern Alaska as well early in the week. A
stronger low pressure system should enter northwest Alaska next
Thursday bringing increased chances for organized rainfall next
Thursday into Friday. Moderate to heavy precipitation is possible.
The pattern will continue to support below normal high
temperatures over much of the state through next week, with the
highest anomalies likely across the North Slope region. The
overnight lows should be more moderate with the best potential for
above normal lows over southern and eastern parts of Alaska.
Santorelli
Hazards:
- Heavy rain across portions of the Alaska Panhandle and mainland
Alaska, Mon-Tue, Aug 23-Aug 24.
- Heavy rain across portions of mainland Alaska, Sun-Mon, Aug
22-Aug 23.
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