Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
235 PM EST Sat Nov 24 2018

Valid 12Z Wed Nov 28 2018 - 12Z Sun Dec 02 2018

...Large-Scale Overview...

The flow pattern remains an active one across the North Pacific
with numerous systems and interactions/phasing between multiple
streams, which introduces increasing chaos into the models from
day 6 (Fri) onward. D+8 mean 500 hPa anomalies from the ECENS and
GEFS show a mean Rex Block extending from near the North Pole into
Siberia, with mean troughing across the northwest Pacific. The
result will be a persistent source of shortwave energy exiting
east Asia and moving near the Aleutians. Potential phasing of
polar/arctic energy in this region will give at least the
potential for some systems to undergo rapid/significant
intensification, as some of the guidance indicate toward the end
of the extended period.


...Guidance/Predictability Assessment...

Model solutions have converged significantly relative to yesterday
during the early part of the extended period (Wed-Thu). A
weakening surface low should be in place across the Gulf of Alaska
on Wed with additional shortwave energy staying well south of the
region. Another significant shortwave/low pressure system should
enter the Bering Sea Wed night/Thu with a front sweeping east
across the Aleutians. Given relatively good consensus observed in
the guidance, a blend of the ECMWF/GFS along with the ECENS/NAEFS
means served as a good starting point for the forecast during days
4-5.

On Fri is when predictability begins to rapidly degrade as another
shortwave crosses the North Pacific and potentially phases with
energy farther north. Some model solutions (06Z FV3 and 00Z ECMWF)
show the potential for explosive deepening of this phased low
pressure system, depicting a surface low around 940 hPa nearing
the Aleutians Fri night. By day 7 (Sun) some solutions take this
surface low across the Aleutians and toward western mainland
Alaska, while other keep the low south of the Aleutians, moving
toward the Gulf of Alaska. Ensemble low scatter plots show an
extremely large spread with this system, and given the complex
nature of forecasting any phasing of relatively small-scale
features this far out, confidence is very low in the specifics of
any individual solution. Additional complexity arises from yet
another vigorous system following quickly behind. Given these
considerations, this forecast maintains exclusive focus on the
ensemble means from day 6 onward, which yielded a forecast
relatively close to continuity.


...Sensible Weather Highlights/Hazards...

Locally heavy precipitation will be possible on Wed as the low
weakens in the Gulf of Alaska. Persistent onshore flow in the wake
of the front will keep precipitation chances in the picture across
southern/southeastern Alaska. Precip should become more widespread
across the Aleutians by late next week in association with the
next couple low pressure systems traversing the North Pacific.
Conditions will be relatively dry in the interior, the result of
the persistent southerly flow and moisture struggling to make it
past the coastal mountain ranges. Temperatures will be near to
slightly above average for most of the state, as the active North
Pacific flow regime will inhibit any significant southward
incursions of arctic air across Alaska.


Ryan

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