Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
608 PM EST Wed Nov 18 2020
Valid 12Z Sun Nov 22 2020 - 12Z Thu Nov 26 2020
...Strong system may approach Kodiak early Monday...
...Overview and Guidance/Predictability Assessment...
Progressive and active pattern will continue into next week,
driven in part by an energetic jet stream from Japan and across
the North Pacific. To the north, upper ridging will stretch WNW to
ESE across the Arctic Ocean into the Canadian Archipelago. 12Z
models were in good agreement in the larger scale to start the
period Sun-Tue except for the GFS. Main system of concern will
deepen just astride the eastern Aleutians Sunday and track to a
location just off Kodiak by 12Z Mon as a potentially strong (960s
mb) surface low. The GFS was notably weaker/east as a result of
less (or no) phasing which remains a possibility, but not a higher
likelihood solution given the trend and ensemble clustering. This
did shift the low much closer to the coast than 24 hrs ago along
with lower pressures. That system should move eastward to the
Panhandle but continue steadily eastward.
Next system will move through the central Bering around Mon into
Tue, but this has shown poorer continuity in the deterministic
runs. However, latest 12Z ECMWF/Canadian were close enough and
plausible that a modest blend with the ensemble means (mostly
ECMWF ensemble mean) offered a good starting point, taking the
parent low along the eastern Russia coast as the triple point
develops across the AKPen and into the Gulf. Broad cyclonic flow
west of 150W could support a trailing cold front across the
Aleutians.
...Weather/Threats Highlights...
Mon-Tue system has the potential for widespread modest rain/snow
along the eastern Aleutians and over Kodiak to the southern Kenai
along with breezy/windy conditions (40kts+). This could continue
eastward to the Panhandle but with a jog southeastward it may
focus into the southern Panhandle to British Columbia. Next system
in the Bering may have a less favorable moisture source but could
still bring some rain/snow to the island chain on southwesterly
flow ahead of the front. Too early to tell how much precipitation
may expand over Southcentral next Tue-Wed but at this point it
appears generally light to perhaps modest amounts. Temperatures
will start well above normal along northern areas thanks to upper
ridging, but will trend closer to more typical values in time.
Southeastern third of the mainland into the Panhandle will see
near to below normal temperatures.
Fracasso
Hazards:
- Heavy precipitation across portions of mainland Alaska, Sat, Nov
21.
- High winds across portions of mainland Alaska, Sat, Nov 21.
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