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FXAK02 KWNH 261848
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Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
247 PM EDT Thu Apr 26 2018

Valid 12Z Mon Apr 30 2018 - 12Z Fri May 04 2018

Models and means show a transition to a more amplified regime
compared to the progressive pattern in place during the short
range time frame, as a northeastern Pacific ridge builds over
eastern Mainland Alaska and western Canada and an eastern
Siberia/Bering Sea trough extends into the northern Pacific.
Depending on the exact orientation of the trough, the downstream
ridge axis may begin to tilt back into the mainland late in the
period.  Higher latitude flow appears to remain fairly blocky at
least for the first half of the week with an upper low meandering
in the vicinity of the Chukchi Sea and modest ridging over the
Arctic.

As the aforementioned pattern develops, the first half of the
period (early Mon into early Wed) generally reflects a refinement
of details/strength compared to yesterday`s forecast.  A blend
among the 12z GFS/00z ECMWF and to a lesser degree 00z GFS/00z CMC
helps to mitigate what guidance spread still exists for individual
features while keeping systems more defined than the means.  A
progressive Bering system should reach the northwestern mainland
as of 12z Mon while modest waviness reaches the Gulf of Alaska.
The next in the series of Bering systems is most likely to reach
the southwestern mainland/Bristol Bay by 12z Wed.  Between early
Tue and early Wed the models/ensemble members diverge considerably
for latitude of best defined low pressure but are more agreeable
for timing, with the favored blend providing an intermediate
solution.

By the latter half of the week the primary focus will be on
another system tracking out of the northwestern Pacific.  Over
recent days there have been decent signals for its existence but
considerable model/ensemble spread for track and timing.  Ensemble
means and some operational runs have been favoring a track into
the Bering in recent days but the last three ECMWF runs through
00z/26 have been closer to the Aleutians with the 00z run actually
slightly south of the Aleutians.  GFS runs are trending to this
southern track as well, and in fact drop south of the ECMWF after
next Thu.  The 00z ECMWF mean holds onto a southern Bering track
but has a notable southeastward trend versus prior runs.  Given
the spread/trending in guidance thus far, prefer a conservative
approach by way of adjusting the forecast more toward the ensemble
means (more 00z ECMWF mean relative to 00z NAEFS) by days 7-8
Thu-Fri but still keeping a minority of operational input.  This
provides some account for recent trends while lowering the risk of
having to make a significant reversal.

Ahead of this system during Wed-Fri, northern Pacific troughing
aloft along 150-160W may interact with energy/moisture from
low-mid latitudes of the east-central Pacific.  This aspect of the
forecast will require monitoring as model/ensemble guidance is
suggesting potential for significant precip totals over parts of
Alaska`s southern coast and panhandle.


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

Rausch

$$





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