Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
336 PM EDT Sun Aug 11 2019
Valid 12Z Thu Aug 15 2019 - 12Z Mon Aug 19 2019
Today's guidance continues to depict an increasingly
amplified/blocky pattern but with meaningful spread for exact
placement/orientation of the large scale features not to mention
embedded shortwaves. For the strong North Pacific ridge building
into the eastern Bering Sea, guidance appears to have stabilized
for the day 4 Thu forecast but trends beyond that time extend the
multi-day tendency toward a stronger ridge. The most prominent
model discrepancy involves the 00Z ECMWF straying east of
established consensus for the ridge and downstream amplifying
trough (the latter forecast by consensus to extend from
northwestern Canada into the northeastern Pacific). Thus the
first consideration is to phase out 00Z ECMWF input for the latter
half of the forecast period. Teleconnections relative to the
ridge's positive height anomaly center suggest that the
northeastern Pacific trough may end up being closer to the 00Z
ECMWF mean than somewhat farther west GFS/GEFS mean runs.
The greatest embedded uncertainty involves an upper
trough/possible embedded low forecast to be near the northwest
corner of the mainland as of early day 4 Thu. Individual model
runs have varied considerably over the timing and track/amplitude
as the trough/low drop into the favored larger-scale mean trough
and likely support surface low pressure between the Gulf of Alaska
and just south of the Alaska Panhandle. Recent 12-hourly runs
have oscillated on timing and of particular note latest UKMET runs
are on the faster side of the envelope after being on the slower
side early yesterday/Sat. Also GFS runs had been mildly fast but
the new 12Z GFS adjusted back to the 00Z ECMWF/CMC. The new 12Z
CMC supports the 12Z GFS/GEFS and 00Z ECMWF mean cluster in
principle. However the 12Z ECMWF has completely changed its
evolution, and while its scenario is too different from remaining
guidance to recommend for a deterministic forecast, it highlights
the continued lack of confidence.
By late in the period the details of Arctic flow that may
approach/reach the northern mainland become another question mark.
The 12Z GFS favorably adjusted to a more conservative solution
relative to the 06Z and especially 00Z runs. Thus far ensemble
means are inconclusive beyond the theme of northwesterly flow.
Meanwhile there is decent consensus on a deep layer low over the
northwestern Pacific but with underlying uncertainty over the
specifics, some of which rely on the evolution of the eastern
Bering ridge aloft. Waviness along the associated front may brush
the western Aleutians. A blend approach with steadily increasing
reliance on the means will provide a reasonable starting point.
Forecast considerations led to starting with primary emphasis on
the 12Z GFS/00Z ECMWF-CMC for days 4-5 Thu-Fri followed by fairly
rapid removal of the 00Z ECMWF and increase of ensemble mean
guidance (more ECMWF ensembles versus the 06Z GEFS). By day 8 Mon
the forecast started with a 70/30 blend of the ECMWF mean/GEFS
mean. The resulting forecast reflects the stronger ridge trends
after Thu but otherwise maintains reasonable continuity for the
overall pattern evolution.
Based on the most likely forecast scenario, moisture ahead of the
shortwave dropping through the mainland may produce areas of
locally significant precipitation Thu-Fri. Guidance is consistent
in showing the potential for a period of fairly strong winds over
parts of the central-western Aleutians with one or more bands of
enhanced rainfall as well.
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
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