Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
559 PM EST Thu Dec 17 2020
Valid 12Z Mon Dec 21 2020 - 12Z Fri Dec 25 2020
...Guidance and Uncertainty Assessment...
Upper troughing across the North Slope into the Bering Sea will
consolidate near the western Aleutians and ease southeastward by
late next week. To the east, upper ridging will build from the
Gulf Monday into eastern areas/Panhandle and into northwestern
Canada in response to the troughing upstream. The models and
ensembles generally show this progression but vary on the details
for much of the period. Lead system will meander its parent low
near the western Aleutians as the occlusion pushes into the AKPen.
12Z guidance was close enough (minus the Canadian) such that a
blended solution sufficed as they each differed on timing/track of
the parent low and other areas of low pressure around it, perhaps
along the front itself. Without any clear trends, could not weight
one solution much more than the other early in the period. That
system will move into the Gulf and perhaps dissipate as it
encounters upper ridging, opening the door for another system from
the central North Pacific. That system should lift northeastward
toward the southern Gulf per the ensemble consensus. No use in
trying to favor one model over the other (GFS was quicker, ECMWF
slower and closer to the ensemble means) given the pattern and
lead time.
...Weather/Hazards Highlights...
Areas along the southern coast and to the Panhandle could have
moderate to perhaps locally heavy precipitation early in the week
as the frontal system (and developing triple point low) move
across the Gulf. Several inches of rain/melted snow are likely
over the multi-day period, focused over the southeastern Kenai
peninsula. Lighter showers are likely over the Aleutians and
southwestern areas of the mainland under the upper low.
Temperatures are forecast to be much below normal over the
central/eastern Interior will trend much milder (about 10-20
degrees above normal) for the middle and end of the week as
southerly flow overspreads the area.
Fracasso
Hazards:
- Heavy precipitation across the Alaska Peninsula into southern
Alaska, Mon-Wed, Dec 21-Dec 23.
- Heavy precipitation across portions of the Alaska Panhandle into
southern Alaska, Wed, Dec 23.
- High winds across the Alaska Peninsula into southern Alaska,
Mon-Tue, Dec 21-Dec 22.
- Much below normal temperatures across much of the lower
elevations of eastern Alaska, Sun-Tue, Dec 20-Dec 22.
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
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