Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
733 PM EDT Mon Mar 09 2020
Valid 12Z Fri Mar 13 2020 - 12Z Tue Mar 17 2020
...Overview and Guidance/Predictability Assessment...
While the occasional model run has waffled a bit in one way or
another, the general trend of model/ensemble guidance in recent
days has continued to be toward stronger ridging aloft over the
northeastern Pacific and into parts of the mainland/Bering Sea.
This ridge will promote a storm track well to the west and north
of the mainland while associated fronts may reach into the
northern half of the mainland. Guidance has been consistent in
suggesting that the leading system Fri-Sat will be much stronger
than the second that should cross the Arctic around Sun-Mon. An
ongoing uncertainty will be how moisture/energy around the
northern side of an upper low over the central Pacific--and
perhaps the upper low itself as it weakens/opens up--may interact
with larger scale flow and bring moisture into the eastern
Aleutians and western mainland.
Among guidance available through the 12Z cycle, the
ECMWF/UKMET/CMC are noticeably stronger with the upper ridge and
show greater westward extent versus the GFS/GEFS mean from early
in the forecast period. The multi-day trends toward a stronger
ridge seem to provide good support for using the 12Z
ECMWF/UKMET/CMC as the starting point of the forecast from late
week into the weekend. From late weekend into early next week
most 12Z solutions have significantly increased the degree to
which the central Pacific upper low opens up, leading to more
moisture reaching the eastern Aleutians/Alaska Peninsula as well
as some locations farther north. This change is more difficult to
gauge for now, as upstream energy dropping into the overall
central Pacific trough may play a role in addition to specifics of
the upper ridge. At the very least the preference for the ECMWF
cluster in principle would keep the shortwave/upper low energy and
moisture somewhat farther west than forecast by the GFS. Current
preference is for a conservative transition to this idea presented
in the 12Z guidance, by way of trending the forecast to about an
even weight among the 12Z ECMWF/CMC and 00Z ECMWF/ECMWF mean.
...Weather Highlights/Threats...
The initial storm tracking into/across the Arctic late this week
may bring an episode of strong winds and terrain-enhanced
precipitation to parts of the northwestern mainland but multi-day
trends for a stronger upper ridge to the south have continued to
trim away the southeastward extent of these effects.
Moisture/precipitation flowing from the North Pacific into the
Bering Sea may extend into parts of the western/northern mainland
along the fronts trailing from the first Siberia-Arctic system and
a second weaker system. Depending on evolution aloft over the
North Pacific by next Mon-Tue, precipitation may increase/spread
slowly eastward over the eastern Aleutians/Alaska Peninsula with
some of this activity extending to the north as well. Cold high
pressure will support below normal temperatures over the
southeastern corner of the mainland into the Panhandle on Fri with
some pockets of below normal readings possibly persisting over
this area thereafter. Otherwise expect above to well above normal
temperatures to prevail across the state with the greatest
anomalies most likely to be over northwestern areas.
Rausch
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 snow across portions of mainland Alaska, Thu-Fri, Mar
12-Mar 13.
- High winds across portions of mainland Alaska, Thu-Fri, Mar
12-Mar 13.
- Much above normal temperatures across portions of northern
mainland Alaska, Fri-Mon, Mar 13-Mar 16.
- Much above normal temperatures across portions of central
mainland Alaska, Sat-Mon, Mar 14-Mar 16.
- Much below normal temperatures across portions of the Alaska
Panhandle and mainland Alaska, Thu-Fri, Mar 12-Mar 13.
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