Prognostic Meteorological Discussion
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FXAK02 KWNH 221914
PMDAK

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

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

***Guidance/Uncertainty Assessment***

The medium range period begins on Thursday with a lower than
average level of model agreement that only increases in time
through next weekend and into next Monday.  There is general
agreement on the eastern Bering Sea low that is in the process of
weakening and evolving into an open trough axis.  The 00Z CMC is
differing from the model consensus as early as Thursday with much
less of an upper level ridge over eastern Siberia and the western
Bering Sea, and farther north with the Bering Sea low.  Since
these differences with the CMC escalate with time, it was not used
in the forecast process.  By Friday, the 6Z GFS is quicker with
the trough axis across the Gulf of Alaska, and slightly ahead of
the GEFS mean.  The UKMET and ECMWF are more in line with the EC
mean and previous WPC continuity in keeping the trough axis
farther to the west with a ridge over the western Aleutians.  The
GFS loses additional ensemble support by Saturday and ahead of the
model consensus.  It is also notably stronger with a second Bering
Sea low by Saturday, whilst the ECMWF and the GEFS/EC means are
much farther to the west near the Kamchatka Peninsula.  By Sunday,
the GFS and ECMWF are now out of phase over western Alaska with
the ECMWF indicating an upper level ridge and the GFS a potent
upper low.  Given that the ECMWF is more in line with the ensemble
means, it was preferred over the GFS for this portion of the
forecast period.

***Weather/Threats Highlights***

The best prospects for rain and higher elevation snow will be
across the southern third of the state on Thursday owing to
onshore flow from the weakening Bering Sea low.  For Friday into
the weekend, mostly dry weather is expected for most areas with
high pressure
governing the overall weather pattern.  Strong winds may affect
the central and western Aleutians later in the period as the
strong low over eastern Siberia begins to exert its influence over
the Bering Sea and northern Pacific.  The coldest temperatures, as
expected, should be confined to north of the Brooks Range, and
gradually moderating readings across most of the Interior.


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

D. Hamrick


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