Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
740 PM EDT Tue Mar 24 2020
Valid 12Z Sat Mar 28 2020 - 12Z Wed Apr 01 2020
...Below to much below normal temperatures over the Interior and
Panhandle...
...Overview...
Multiple significant uncertainties lead to fairly low confidence
in forecast specifics. There is a short-range interaction of
shortwave energy over the Bering Sea with resulting detail
uncertainties reaching the eastern Bering through Gulf of Alaska
area by this weekend. Then individual model runs quickly diverge
for eastward progression of an upper low south of the Aleutians
and significant spread develops for how the upper ridge that
approaches the mainland from Siberia and the Bering Sea during the
weekend. Late in the period the best consensus of guidance
suggests North Pacific through Bering Sea flow will become
progressive and low in amplitude, which also leads to low
predictability.
...Guidance Evaluation/Preferences...
In order to depict the preferred pattern for this uncertain
forecast, the initial blend started with an operational model
compromise (12Z GFS/ECMWF/UKMET/CMC) day 4 Sat into day 5 Sun with
a day 6 Mon transition toward days 7-8 input from the 12Z
ECMWF/00Z ECMWF mean/12Z GEFS mean.
Through the period as a whole the dominant theme was to downplay
the GFS over the North Pacific as its upper low progression was
slower than other models--though hinted at by GEFS/CMC ensemble
means. The GFS evolution ultimately leads to low pressure
tracking over the Aleutians and into the southeastern Bering by
day 8 Wed, which not even the GEFS/CMC means show. Farther north
there is decent clustering for the upper ridge crossing Siberia
and the western-central Bering Sea during the weekend. It is
after Sun when guidance diverges significantly for the shape and
progression of the ridge, which affects surrounding flow as well.
The past couple days of model/ensemble mean runs have varied on
specifics as well, so at the moment it is hard to pinpoint
well-defined trends to latch onto. The preferred blend reflects
some elements of continuity with an upper high closing off near
the northwest corner of the mainland and then drifting into the
Arctic, allowing for a fair amount of troughing over the mainland
even if not to the extent of the deep upper low seen in the past
couple operational ECMWF runs.
This evolution of the ridge/surrounding flow and details of the
North Pacific upper low will have a role to play in the specifics
of low pressure over the northeastern Pacific/Gulf of Alaska by
Mon-Wed. It remains to be seen whether flow aloft will evolve in
such a way to bring strong surface low pressure into the Gulf by
early Tue as in the ECMWF but over recent runs there has been
decent ensemble support for a defined surface low there at that
time. Finally, confidence is obviously low for a wave forecast to
cross the Bering by next Tue-Wed but it is quite plausible for a
wave to exist in general given the progressive flow. The CMC
offers support for the last two ECMWF runs in principle and the
new 12Z ECMWF mean shows more of a hint of such a wave by late
next Wed than the previous run. In fact the new ECMWF mean
compares fairly well with the preferred blend for most large-scale
aspects of the pattern through the period.
...Weather Highlights/Threats...
Below to much below normal temperatures over a majority of the
state will start to moderate from west to east with time. The
North Slope region may see mostly above normal readings after the
weekend. The remainder of the eastern mainland as well as the
Panhandle should remain fairly well below normal into the first
part of next week. The highest precipitation totals for the
five-day period should be along the southeastern coast and
Panhandle, associated with low pressure tracking just to the south
during the weekend and another system that may track into or
develop over the Gulf of Alaska by next Tue. Terrain-enhanced
precipitation over the southwestern mainland will trend lighter
during the weekend. The Aleutians may see one or more episodes of
precipitation but with low confidence in timing/amounts.
Rausch
Hazards:
- Heavy precipitation across portions of the Alaska Panhandle,
Fri-Sat, Mar 27-Mar 28.
- High winds across portions of mainland Alaska and the Aleutians,
Fri-Sun, Mar 27-Mar 29.
- Much below normal temperatures across portions of the Alaska
Panhandle and mainland Alaska, Fri-Tue, Mar 27-Mar 31.
- Much below normal temperatures across portions of mainland
Alaska, Sat-Tue, Mar 28-Mar 31.
- High significant wave heights for coastal portions of mainland
Alaska and the Aleutians, Fri-Sat, Mar 27-Mar 28.
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