Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
607 PM EDT Tue Sep 29 2020
Valid 12Z Sat Oct 03 2020 - 12Z Wed Oct 07 2020
...Heavy rain possible Sunday for portions of coastal Southcentral
to the Coast Ranges...
...Overview and Guidance/Predictability Assessment...
Upper low is forecast to slowly move eastward from around
southwestern Alaska/Bristol Bay this weekend to the Gulf by the
middle of next week. Several embedded disturbances will rotate
through/around the upper low, deforming it as additional
shortwaves drop in from the northwest. The 12Z guidance offered a
reasonable starting point for the forecast, transitioning quickly
toward the 12Z GFS/ECMWF and their ensemble means for the
remainder of the period.
After passage of a robust system in the short range, next system
will be another well-defined, but probably not as deep, area of
low pressure that will likely pass just south of Kodiak
toward/into Prince William Sound on Sunday. 12Z Canadian/UKMET
drifted away from the tightly-paired 12Z GFS/ECMWF (ensemble means
just west) by Mon and their inputs were phased out. Front will
lift into NW Canada by early Mon. Next system from the
northwestern Pacific will enter the far western Bering around next
Tue. By then, the upper pattern will start to flatten, allowing
zonal flow to increase. ECMWF ensembles were still much quicker
than even the 12Z ECMWF/GFS, though the Canadian was just about as
fast (though much different in northern stream flow). Maintained a
relatively steady pace on par with the forecast yesterday, near a
12Z ECMWF/GFS middle ground. Uncertainty in how the downstream
trough/upper low exit (quicker or slower) will dictate how quickly
the pattern can flatten.
...Weather/Threats Highlights...
Low pressure will spread modest to locally heavy rainfall over
southeastern-facing areas of Kodiak and the Kenai Peninsula toward
Prince William Sound eastward to Yakutat Sunday, gradually
diminishing as showers on Monday. Several inches of rain are
possible along with breezy conditions. Less rainfall is forecast
over the mainland after Monday, but will increase over the western
and then central Aleutians next Tue/Wed.
Temperatures are forecast to be near to well above normal over
most of the mainland in southerly flow. Near to below normal
temperatures are forecast for the Aleutians under the upper low
early in the period and over the Panhandle.
Fracasso
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
Hazards:
- Heavy precipitation across portions of southern Alaska to the
Alaska Panhandle, Sun-Mon, Oct 4-Oct 5.
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