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Alaska Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1932Z Feb 15, 2019)
 
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Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
232 PM EST Fri Feb 15 2019

Valid 12Z Tue Feb 19 2019 - 12Z Sat Feb 23 2019

From Tue into Thu expect progressive flow that will continue to
support Bering Sea/North Pacific systems that should track into
Mainland Alaska while separate waves track across the northern
Gulf of Alaska.  These systems will provide a focus for locally
enhanced precipitation and winds.  Late next week into the weekend
there is good agreement in the guidance that a higher
amplitude/longer wavelength regime will develop, with troughing
over Siberia/western Pacific and a ridge building from the
east-central Pacific into the mainland and Arctic.  This
late-period pattern will tend to favor low pressure over the
western Bering Sea and a potentially wavy front extending into the
Aleutians.

Already at the start of the forecast early day 4 Tue there is a
discrepancy over the details of late short-range system.  GFS/FV3
GFS/ECMWF runs and a decent number of GEFS/ECMWF ensemble members
show the primary low tracking to near Norton Sound by 12Z Tue
while a triple point wave tracks over the Gulf of Alaska in
contrast to the 00Z CMC/UKMET that track the primary low well
southward.  The GFS/ECMWF cluster is much closer to established
continuity and represents the general tendency for
southwest-to-northeast track of lows in this general pattern, so
would prefer to lean in that direction at this time.  The one
concession to the guidance spread is to make the system weaker
than the literal GFS/ECMWF solutions.

A fair amount of spread persists for the next system, a
combination of low pressure tracking into the western Bering and a
wave tracking northeastward from the central Pacific through the
Aleutians.  In contrast to the first system (for which the CMC
mean followed the southern operational CMC track), all three
ensemble systems agree on a fairly northward track for strongest
low pressure.  The 00Z ECMWF/CMC agree with the means in principle
though like yesterday the means merge the two features while
operational runs generally place more emphasis on the central
Pacific wave.  GFS runs have tended to be on the southern edge of
the spread but the 12Z GFS has nudged a bit northward (albeit not
yet close to the means).  The 06Z FV3 GFS is somewhat northward of
the operational 12Z run.  There has been a noticeable consensus
trend toward a stronger shortwave ridge aloft downstream, which
seems to favor the ensemble mean/ECMWF/CMC scenario--and is close
to continuity aside from slightly slower timing.  The
corresponding Gulf wave has less sensitivity to the specifics
farther northwest.

The ensemble means maintain better than average
agreement/continuity for low pressure reaching the extreme western
Bering Sea by the latter half of the period and leading frontal
system stretching across the Bering/Aleutians.  One of the greater
question marks by Fri-Sat will be the possible existence of one or
more frontal waves.  At this time model/ensemble spread is
sufficiently great to favor the means which do not yet depict a
well-defined wave.

Across the Arctic most models/means show some degree of
progressive flow aloft before upper ridging builds into and beyond
the mainland late in the period.  The ECMWF remains one of the
deeper solutions for its high latitude upper low/trough.  The 06Z
GFS/FV3 GFS trended in that direction but the 12Z GFS backed off,
while the ECMWF mean still shows a trough weaker than the
operational run.  At the very least what trends exist from
yesterday to today may support somewhat more definition of the
mean surface front indicated over the North Slope region.

Combining preferences for systems/time frames, the manual forecast
started with the most common ideas of the 12Z GFS/06Z FV3 GFS and
00Z ECMWF/CMC on day 4 Thu.  Then the forecast introduced 06Z
GEFS/00Z ECMWF ensemble means while trimming away 12Z GFS input,
followed by further ensemble mean input such that day 8 consisted
of all means.  The ECMWF mean has been leading the GEFS mean in
trends toward a stronger shortwave ridge reaching the mainland by
day 5 Wed and stronger trough/ridge pattern for days 7-8 Fri-Sat
so the blend gave the ECMWF mean somewhat more weight relative to
the GEFS mean.

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