Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
347 PM EDT Sat Mar 23 2019
Valid 12Z Wed Mar 27 2019 - 12Z Sun Mar 31 2019
As was the case yesterday there are some common themes in the
forecast of large scale pattern evolution. These include a
southward or southeastward drift of an upper high likely centered
over the southeast corner of the mainland early Wed followed by
another ridge building over and/or northwest from the southwestern
mainland by next weekend, and a far western Pacific upper trough
(anchored by a low near Kamchatka) that should elongate eastward
just south of the Aleutians next weekend. However operational
models and individual ensembles go astray with some aspects of the
forecast fairly early in the period with differences persisting or
increasing with time. The resulting low confidence in any
specific forecast solution requires today's forecast to include
more ensemble mean guidance than usual early in the period (about
half) and exclusively means by next weekend.
One major point of contention continues to be with details of
shortwave energy initially over the Bering Sea/Aleutians on day 4
Wed and any upstream energy. The combined fast/strong depiction
of the upstream shortwave in 00-06Z GFS runs (reaching the Bering
on Thu) still seems to be overdone. The GEFS mean has trended
somewhat to the GFS idea early but then dampens the feature more
by Fri. The 12Z GFS has also trended weaker to add support for
toning down the earlier GFS scenario. On the other hand slow
upstream progression in the ECMWF/CMC and ECMWF mean lead to a
slow moving trough near the western coast of the mainland (in the
former two) or moving across the mainland (the latter). The ECMWF
mean idea of having a trough crossing the mainland is rather new
compared to earlier runs and not very well supported by other
guidance or multi-day means which have been showing a very strong
core of positive height anomalies centered near the southern coast
of the mainland. There are yet other solutions including the 00Z
UKMET/12Z GFS which use some of the Bering/Aleutians energy to
form a compact upper low near the Alaska Peninsula--another very
low confidence scenario.
By next weekend the ECMWF and to a lesser degree ECMWF mean/CMC
introduce another major difference, pulling the building western
ridge far enough northwestward to allow height falls/surface low
pressure and moisture to extend into the northeastern Pacific--in
contrast to other solutions that show stronger ridging over/near
the mainland and keep the northeastern Pacific pattern more
suppressed. At the very least the ECMWF mean is not as extreme as
latest operational ECMWF runs but it still represents a meaningful
change from previously established consensus. The corresponding
CMC mean is much closer to the latest GEFS means.
Also during next weekend, the ensemble means are somewhat more
agreeable among each other with the overall evolution of upper
troughing and surface low pressure stretching eastward just south
of the Aleutians. Not surprisingly the operational models vary
considerably for specifics across the western 2/3 of the North
Pacific.
Forecast considerations and the desire to reflect as much
continuity as possible in light of a very uncertain forecast led
to blending the 00-06Z GFS/06Z GEFS and 00Z ECMWF/ECMWF mean early
in the period followed by a transition to 2/3 total weight of the
06Z GEFS/00Z NAEFS means versus the 00Z ECMWF mean. Adjustments
to the initial blend led to decreasing precipitation probabilities
next weekend along the southern coast of the mainland to reflect
better the GEFS/NAEFS emphasis. Typically low bias of ensemble
mean wind speeds led to some manual increase of wind but balanced
against the high uncertainty of speed/direction at any particular
location and valid time.
Rausch
Bullet points from the WPC Days 3-7 Hazards chart can be found 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