Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
331 PM EDT Mon Aug 12 2019
Valid 12Z Fri Aug 16 2019 - 12Z Tue Aug 20 2019
From late week into the weekend guidance has been having
considerable difficulty in determining the specifics of the upper
ridge building from the North Pacific into the eastern Bering Sea
and closed low/shortwave energy expected to travel around the
northern/eastern side of the ridge. Each successive day has
featured various changes and spread for Arctic energy that will
likely consolidate into a compact closed low west of the northern
mainland by late this week and then for the overall shortwave's
southeastward progression as the ridge builds. Multi-day trends
have generally been westward and slower. This trend offered some
support for the 12Z/11 and 00Z/12 ECMWF runs and 00Z ECMWF mean
which were slower than other models/means. However the
significant majority of guidance faster than the ECMWF (including
the 12Z GFS) recommended a starting point between the 00Z ECMWF
and faster cluster, given the highly sensitive nature of the
evolution. The new 12Z ECMWF has jumped to the faster
cluster--with the other models generally holding serve with faster
timing than the prior two ECMWF runs.
Once the shortwave in question passes by, there is decent
agreement toward broadly cyclonic flow covering mainland
Alaska/Gulf of Alaska and northwestern Canada with some degree of
surface low pressure over the Gulf or south of the Panhandle. It
remains to be seen whether the eastern Bering ridge aloft
weakens/moves to the extent seen in some latest guidance, though
it does get pinched by the combination of northwestern North
America troughing and the broad upper low centered over the
western North Pacific. Especially given the chaos that had been
present early in the period (before arrival of the new 12Z ECMWF)
a blend provided an acceptable starting point.
Regarding the mean upper low south of the Aleutians, consensus has
been fairly stable at the large scale but there are still plenty
of embedded uncertainties with shortwaves and associated waves.
Most guidance shows multiple wave developments within/around the
overall circulation--including a stronger low forecast to evolve
toward days 7-8 Mon-Tue and possibly approach the Aleutians next
Tue. Sporadic operational runs have brought the upper low a
little farther east than what might be reasonable for such a
blocky pattern, with the ensemble means providing useful input as
confidence decreases in some individual model details.
Based on guidance available through arrival of the 12Z GFS,
today's forecast started with a compromise of half 00Z ECMWF and
the remainder 06-12Z GFS for days 4-5 Fri-Sat followed by
increasing 06Z GEFS/00Z ECMWF ensemble weight... 40 percent total
day 6 Sun and 70 percent thereafter. The early period 500mb blend
required some editing to achieve a somewhat realistic depiction.
In light of the strong clustering in the new 12Z cycle it worked
out well that the surface pattern took on more of a GFS flavor.
Late this week the flow ahead of the descending Arctic feature
should promote areas of enhanced precipitation over parts of the
mainland. With some day-to-day variability expect one or more
periods of fairly brisk winds over parts of the Aleutians and
southern/western Bering Sea. The best signal for highest five-day
total rainfall exists over the central/west-central Aleutians.
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
- Heavy precipitation across portions of mainland Alaska, Thu-Fri,
Aug 15-Aug 16.
- Heavy rain across portions of the Aleutians, Thu-Sun, Aug 15-Aug
18.
- High winds across portions of the Aleutians, Thu, Aug 15.
- High significant wave heights for coastal portions of mainland
Alaska and the Aleutians, Thu, Aug 15.
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