Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
633 PM EDT Mon Jun 22 2020
Valid 12Z Fri Jun 26 2020 - 12Z Tue Jun 30 2020
...Overview and Guidance/Predictability Assessment...
Once again the latest ensemble means provide a fairly stable and
coherent picture of the large scale pattern evolution through the
period while some operational runs develop significant
differences. Thus today's forecast maintains an approach that
starts with an operational model consensus early followed by
increasing emphasis on the ensemble means (reaching 70 percent by
days 7-8 Mon-Tue) along with remaining inclusion of operational
guidance that is closest to the means.
Operational models continue to be quite varied among each other
and in some cases consecutive runs for the Arctic upper low and
surrounding flow. Some GFS runs including the 12Z version have
been trying to split the upper low in a curious fashion, with the
12Z run actually tracking the main low across the northern
mainland late in the period. CMC runs have also been straying
from the ensemble mean cluster (which keeps the upper low
northwest of the mainland) with the 12Z run dropping it into the
northern Bering Sea before tracking toward the southwestern
mainland. The 12Z ECMWF briefly brings the center close to the
northwest mainland but an average of the last two ECMWF runs is
closer to the means. Preference is to stay near the ensemble
means until they provide a strong trend toward an alternate
scenario. Already by day 5 Sat the latest ECMWF runs diverge on
details of leading energy shearing eastward/northeastward across
the mainland--also arguing for use of both runs as part of the
forecast blend.
Among latest guidance the notable difference for the system that
tracks into the central Pacific/Aleutians during the weekend is
that the 12Z GFS is weaker than other guidance with the upper
trough/low. In fact by day 5 Sat the GEFS mean is closer to
consensus than the operational run so the forecast used the GEFS
mean in place of the GFS after Fri. Questionable northern stream
evolution in the CMC led to its removal from the forecast after
Sat. The UKMET compared well to consensus to the end of its run
early Sun. Faster trends that were starting to emerge yesterday
have generally followed through, but the 12Z ECMWF has adjusted
somewhat slower than the previous run. The ensemble means and an
average of the two ECMWF runs provide a good starting point for
this system which should rapidly weaken once it passes the
Aleutians.
Recent continuity has been good for the system expected to reach
the Gulf of Alaska by the start of the period early Fri. What
scatter there has been in the guidance over the past couple days
has generally been gravitating toward the ongoing forecast.
...Weather/Threats Highlights...
Panhandle precipitation associated the system reaching the Gulf by
Fri will quickly taper off late this week. The mainland will see
some scattered convection/rainfall, with increasing uncertainty in
specifics depending on the details of the Arctic upper low and
possible shortwaves embedded in the surrounding flow. In the
early part of the period the best focus should come from shortwave
energy near the western coast of the mainland on Fri and then
progressing/shearing eastward or northeastward. Assuming the most
likely scenario for the system tracking into the Aleutians by this
weekend and quickly weakening thereafter--including minimal
phasing with flow to the north--expect an area of precipitation to
spread into the Aleutians/Alaska Peninsula and then to some degree
across parts of the mainland. Some local enhancement may be
possible between the Aleutians and southern coast of the mainland
but with low confidence at this time. Continue to anticipate
below normal highs over much of the state during the period.
Coolest anomalies should be over the west and south as well as the
Panhandle late this week into the weekend. The southwest may
moderate a bit by early next week. The northern coast and
northeast corner of the mainland should be closest to normal or
even slightly above normal. Low temperatures will likely be more
moderate and best potential for above normal lows will be along
the northern coast/over the northeast as well as in some pockets
over the south, especially after the weekend.
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: No significant hazards are expected over Alaska during
this forecast period.
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