The Weather Prediction Center

College Park, MD


Alaska Extended Forecast Discussion




Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
253 PM EDT Sun Jul 22 2018

Valid 12Z Thu Jul 26 2018 - 12Z Mon Jul 30 2018

Aside from subtle adjustments most guidance looks remarkably
similar to yesterday's consensus.  Multi-day means valid near the
end of the extended forecast time frame continue to be very
agreeable across much of the Northern Hemisphere, suggesting above
average confidence in the large scale pattern.

The majority cluster shows eastern Asia energy capturing a leading
upper low ejecting from southwest of Kamchatka in the short range
period.  The overall trough should then amplify into the Bering
Sea/Aleutians and likely interact with the leading (well-behaved
in recent forecasts) upper low just south-southwest of the Alaska
Peninsula as of early day 4 Thu.  Specifics of this interaction
will likely remain an unanswered question for a while.  By the
weekend there is reasonable agreement that energy aloft over the
Bering should form a closed low with a corresponding moderately
strong surface low, with this system lingering into early next
week.  Over the past day the GEFS/ECMWF means have converged quite
well after some strength differences yesterday.  The past three
ECMWF runs have been a little more consistent than the GFS with
the strength/track of the system mid-late period but as a whole
latest models/means are well within typical guidance error for
forecasts 6-8 days out in time.  The primary adjustment from
yesterday is that the system seems to take a little longer to
evolve.  Moisture focus will transition from the southern coast
ahead of the system just off the Alaska Peninsula at the start of
the period toward a persistent flow into the extreme eastern
Aleutians and Peninsula late in the forecast in response to the
Bering system.

As for other isolated solutions that appear to have low
probability but require monitoring:  The 00Z CMC strays from
consensus fairly early in the period by digging upstream energy
well westward while keeping the leading upper low stranded over
the North Pacific.  Note that the CMC mean conforms to the
majority scenario.  Another potential complication could come from
Tropical Storm 14W in the western Pacific.  Thus far the 12Z GFS
and 06Z parallel (FV3) GFS are the only solutions that are so fast
with northward progression that the feature tracks into the
southern Bering ahead of or coinciding with the upper trough that
amplifies over the Bering.  Consensus/JTWC forecasts keep this
system sufficiently slow/westward that it does not get
incorporated into the Bering Sea evolution.

To the north and east there is reasonable continuity with a strong
and progressive short range/early extended Arctic storm leaving a
trailing front draped over northern Mainland Alaska as of early
Thu.  Eastward movement of a trailing ridge aloft will help to
push this boundary northeastward as a warm front with time. 
06Z/12Z GFS runs are still somewhat faster than remaining models
with the Arctic storm but at least slower than the 00Z GFS. 
Consensus persists in merging the high latitude ridge with the
initial ridge extending from the Panhandle/western Canada into the
southeastern mainland, leading to a broad ridge extending from the
northeastern Pacific/western Canada through the eastern/northern
mainland late in the period.

Today's forecast started a blend of the 00Z ECMWF/06Z GFS/00Z
UKMET (in order from higher to lower weight) for day 4 Thu into
day 5 Fri.  As was the case yesterday clustering has continued to
be sufficiently good to allow for 70 percent model input days 6-7
Sat-Sun and 50 percent on day 8 Mon.  Ensemble input came from the
06Z GEFS/00Z ECMWF means, while departure of the Arctic system
permitted introduction of the 00Z GFS as part of the overall blend
mid-late period.

Rausch


WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can be found at:

http://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html





Last Updated: 253 PM EDT Sun Jul 22 2018