Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
317 PM EDT Sun Sep 30 2018

Valid 12Z Thu Oct 04 2018 - 12Z Mon Oct 08 2018

Typhoon Trami is in the process of becoming extratropical near
Japan, with deepening expected to occur during the short range
period as the system heads into the Bering Sea.  Most guidance
suggests that this system should achieve its lowest central
pressure around Tue-Wed, before the extended forecast starts early
Thu.  During the extended period Thu-Mon there is reasonable
agreement that the low should slowly weaken and perform some
variation of a cyclonic loop over the Bering, potentially reaching
near the western Aleutians by the weekend.  Likewise consensus
suggests a westward wobble for the corresponding upper low during
the first half of the period.  Early in the period the 12Z GFS has
adjusted eastward close to the 00Z ECMWF, favoring a compromise of
those two solutions as the best way to depict the system's detail
without the manual intervention required to refine a more
expansive multi-model blend.

To the southeast of the Bering low, models/means still signal
potential for evolution of a northern Pacific into Bering system
around days 5-6 Fri-Sat but with continuing differences in detail.
 A model/ensemble mean with the similar 12Z GFS/00Z ECMWF as the
operational model input yields only modest adjustment from
continuity--favoring a weakly defined and progressive system
though still with fairly low pressure in absolute terms.

From the weekend into early next week the latest/recent guidance
continues to show that energy within the southwest side of the
overall Bering/North Pacific upper trough will amplify but
opinions differ regarding how far southward this energy will pull
the core of the overall trough.  Thus far GEFS/ECMWF ensemble
means along with the ECMWF maintain the trough core over the
Bering while GFS runs (both operational and FV3) show greater
amplification into the Pacific.  Prefer the majority scenario as
depicted by the ensemble means/ECMWF at this time.

Over the eastern Pacific the ensemble means have trended stronger
over the past day for the upper ridge expected to build late this
week into the weekend.  A significant aspect of the forecast that
will have to be monitored toward days 7-8 Sun-Mon will be the
potential for central Pacific tropical cyclone Walaka and its
moisture to be pulled northeastward by flow between the upper
ridge and central Pacific troughing.  Only in the past day have
models started to converge in loose principle while significant
timing and track differences remain among operational model runs. 
Ensemble members are sufficiently diverse that the resulting means
lose definition after days 5-6 Fri-Sat.  A compromise between the
06Z GFS/00Z ECMWF is close to the 1500 UTC advisory from the CPHC
into late week, and then by the end of the period it seems
reasonable to lean more toward the farther left ECMWF track given
strength of the eastern Pacific upper ridge.  The 12Z GFS is
faster but plausibly even farther west.  Typically high
uncertainty in days 7-8 forecasts would favor a more conservative
depiction than the literal 00Z ECMWF.

Both the models and means have made noticeable trends toward
farther southeastward progression of the Arctic ridge/closed high
as it weakens during the latter half of the period.  The means are
not quite as pronounced with this adjustment versus some
operational runs like the 12Z GFS/00Z CMC and seem to be a
reasonable starting point to represent this developing trend while
awaiting further confirmation from future runs.

Based on the considerations for individual features, today's
forecast starts with an even blend of the 12Z GFS/00Z ECMWF early
in the period and then incorporates increasing ensemble mean
input--more 00Z ECMWF mean than 06Z GEFS due to generally better
definition in the former.  The forecast holds onto some 00Z ECMWF
weight through the end of the period while phasing out the 12Z GFS
after Sat. 

The best signal for highest 5-day rainfall totals continues to be
along the southern coast and Panhandle, due in part to a couple
triple point waves that may track just offshore.  Specifics are
very uncertain by Sun-Mon given the guidance spread for Walaka's
moisture, with solutions ranging from dry to very wet.  Mass field
preferences would lead to precipitation in the wetter half of the
full envelope.

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