Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
749 PM EDT Tue May 18 2021
Valid 12Z Sat May 22 2021 - 12Z Wed May 26 2021
...Overview...
Latest ensemble means show the pattern becoming more blocky with
time as an upper low and associated surface system drift across
the Bering Sea/Aleutians (CMC mean a little north of the
GEFS/ECens) and upper ridging builds over much of the mainland.
Multi-run trends in the means generally lean toward a stronger
depiction of both features. Ahead of the building upper ridge, a
weakening wave will track across the northeastern Pacific/southern
Gulf of Alaska during the weekend while shortwave energy farther
north could push a front across northern/central areas.
Operational models also indicate the evolution toward a more
blocky pattern but already diverge for leading details over/north
of the mainland during the weekend and then for what becomes of
the Aleutians/Bering Sea upper low.
...Guidance/Predictability Assessment...
Before the start of the forecast early day 4 Saturday, there is
already a divergence of solutions for energy that tracks just
north of Kamchatka around Wednesday-early Thursday. The result is
that the energy either feeds into the northeastern
Pacific/southern Gulf low (GFS) or tracks over/north of the
northern mainland (most other guidance). Alternatively, the 12Z
CMC tracks the upper low over the central mainland and then drops
it through the Southcentral region. The 12Z ECMWF trended toward
less northward deflection than the prior run, tracing a wider arc
than the CMC. Ensemble means have been quite weak with this
energy so far. Latest model trends balanced by guidance spread
and variability recommend adjusting to reflect somewhat more
shortwave/surface front definition but in a conservative fashion.
Farther south, guidance is better behaved for the weakening system
that tracks south of the mainland. Addition of extra upstream
energy in the GFS leads to a slightly deeper system but it is
sufficiently within the realm of plausibility to be included as
part of a favored non-CMC blend.
From the weekend into very early next week the guidance is showing
improved clustering for the Aleutians/Bering Sea system, with a
composite of latest operational runs providing a good
representation of the most likely evolution. However after early
Monday the operational models rapidly diverge for what becomes of
the initial upper low, ranging from varying degrees of elongation
and possible splitting to maintenance of a consolidated system
(12Z CMC). The 12Z ECMWF becomes the most extreme solution as it
splits into separate upper lows that track into Siberia and the
mid-latitude Pacific (ultimately building a ridge where the
ensemble means have an upper low). For the latter part of the
forecast prefer to guide the pattern toward the more stable and
similar 12Z GEFS/00Z ECMWF means with some lingering input from
the 00Z ECMWF (more compatible than the new run) and 12Z GFS, as
well as the 12Z CMC once its mainland upper low issue becomes a
minimal factor.
...Weather/Threats Highlights...
Shortwave energy and associated surface front that may cross
northern/central parts of the mainland during the weekend could
produce scattered precipitation with a drier trend progressing
from west to east. Specifics are fairly uncertain due to guidance
differences with details of the energy aloft. Meanwhile some
light to moderate precipitation associated with weakening low
pressure south of the mainland should extend into the first part
of the weekend over the southeastern coast and Panhandle. Bering
Sea/Aleutians low pressure and potentially wavy leading frontal
system will likely spread one or more organized areas of rain
across the Aleutians and into the Alaska Peninsula and nearby
areas. Precise system details that have low predictability at
this time frame will determine the magnitude and eastward extent
of rainfall over this region.
The Panhandle and parts of the far southern mainland should see
below normal highs for most of the period while the rest of the
state should see max temperatures rise from near normal values on
Saturday to above normal after the weekend as upper ridging builds
overhead. Uncertain details of shortwave energy that may cross
portions of the mainland during the weekend temper confidence in
temperatures somewhat early in the period. Expect much of the
state to see above normal morning lows Saturday-Wednesday with
perhaps just a few pockets of slightly below normal readings.
Rausch
Hazards:
No significant hazards are expected over Alaska during this
forecast period.
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