Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
320 PM EDT Wed Jul 31 2019
Valid 12Z Sun Aug 04 2019 - 12Z Thu Aug 08 2019
Guidance has continued to vary with the forecast of the strong
system tracking across/near the extreme northern mainland during
the weekend (late short range/early extended time frame). An
episode of heavy precipitation and strong winds may accompany this
system. After a significant faster trend through the 12Z/30
cycle, the majority cluster adjusted back somewhat to varying
degrees of compromise. On a favorable note the new 12Z runs seem
to exhibit less dramatic adjustments versus yesterday. There is
still some spread with the 12Z ECMWF a bit on the slower side of
the current envelope. The 06Z GFS was a northwestern extreme.
The 00Z ECMWF and 12Z GFS/00Z CMC provided the best ensemble
mean-supported starting point for this system, though questionable
CMC particulars with approaching Arctic energy required phasing
out its solution fairly quickly after the start of the forecast.
For the overall Sun-Thu period the ensemble means agree fairly
well that upper ridging should persist to the south of the
mainland while a ridge also builds over the eastern Bering
Sea/extreme western mainland, in response to troughing/possible
embedded upper low reaching the western Bering. Aside from
varying run-to-run details the ECMWF/CMC runs have generally been
the most consistent in signaling this western Bering trough/upper
low. The 12Z GFS is closest to the means and ECMWF/CMC in
principle versus the 00Z GFS that was quite flat with
Bering/mainland flow and 06Z GFS that was quite weak over the
western Bering but very strong with the downstream ridge. The 12Z
GFS is also strong with the ridge but reasonable comparison to the
mean pattern merits minority inclusion in the forecast blend.
Ahead of the western Bering trough upper low there is still a
reasonable signal that a weak wave will track northward over the
Bering. The front extending south from the wave should serve as a
focus for rainfall over parts of the eastern Aleutians from the
weekend into the first half of next week.
Based on the above considerations the day 4 Sun forecast started
with a blend of half 00Z ECMWF and the remainder 12Z GFS/00Z CMC.
After phasing out the CMC the remainder of the period was based on
lingering input from the 00Z ECMWF/12Z GFS along with the 00Z
ECMWF/NAEFS means and 06Z GEFS mean.
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 rain across portions of mainland Alaska, Sat, Aug 3.
- High winds across portions of mainland Alaska, Sat, Aug 3.
- High winds across portions of mainland Alaska, Sun, Aug 4.
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