Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
327 PM EDT Sat Sep 14 2019
Valid 12Z Wed Sep 18 2019 - 12Z Sun Sep 22 2019
Guidance continues to show varying ideas for an Arctic upper low
expected to start the period near the northwest corner of the
mainland at the start of the period early Wed, along with
individual systems carried along by progressive North
Pacific/Bering stream flow.
After showing considerable spread/variability through yesterday's
12Z run, guidance has come into alignment for the system likely to
be approaching the southwest mainland as of early day 4 Wed. The
ECMWF/ECMWF mean were the last solutions to join the current
solution cluster. This system should make rapid progress through
the mainland early in the forecast period.
The next system, tracking near the Aleutians mid-late week,
originates from an area of disturbed weather (98W) to the
south-southeast of Japan with some interaction with mid-latitude
dynamics occurring by Mon-Tue. To some degree as with the
preceding system, there have been some flow interaction details
that could take added time to work out given their scale. Up to
arrival of the 12Z GFS, the GFS runs were the slowest and most
southern/separated solution while the more phased 00Z ECMWF was on
the faster side. The 00Z CMC had elements of both as it resembled
the GFS into Thu but then accelerated. At the time of forecast
preparation the guidance spread suggested that a compromise
approach would be best for a single deterministic forecast--and
provide only modest adjustment from continuity. Remaining 12Z
runs including the ECMWF have bolstered support for an evolution
closer to the GFS at least into early Thu with some spread (and
timing a little faster than the GFS) developing thereafter.
For the final system of the period there is still a good signal
for its existence. Details have varied though, as over the past
day through the 06Z cycle the GFS/GEFS mean have trended
noticeably weaker while remaining solutions are still rather well
defined. The 12Z GFS has returned to a stronger feature--offering
further support for keeping the system fairly well defined. As
the system reaches the vicinity of the Alaska Peninsula by day 8
Sun a model/mean blend provides a reasonable starting point, with
the components of operational runs providing a little enhancement
to the means.
A blend/ensemble mean approach still looks best for the upper low
initially near the northwest corner of the mainland. As a whole
the average of guidance through arrival of the 12Z GFS had
adjusted somewhat westward from yesterday, though GFS runs have
been erratic with the 00Z and 12Z runs well westward of the 06Z
run. Other 12Z models show potential for even greater
retrogression.
Based on forecast preferences the forecast blend started with a
blend of the 06Z GFS and 00Z ECMWF/CMC/UKMET early in the period
and then transitioned to a model-mean blend with the means
represented by the 06Z GEFS/00Z ECMWF mean. Guidance consensus
still shows that the pattern will provide the best potential for
heaviest precipitation over the southeastern coast and Panhandle
with significant totals also extending farther north into the
southern and eastern mainland.
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