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Alaska Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1948Z Feb 15, 2018)
 
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Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
248 PM EST Thu Feb 15 2018

Valid 12Z Mon Feb 19 2018 - 12Z Fri Feb 23 2018

Latest guidance indicates a sharp and amplified upper-level ridge
extending up through the state of AK this coming weekend. ECMWF
and GEFS means suggest these 500mb heights being 2 to 2.5 standard
deviations above average. A split flow upper trough does slide
through this ridge and state on Sunday. However, this feature has
very little influence on the anomalous ridge, while most of the
upper dynamics dive southeast and pool over the northeastern
Pacific. Thus the dome of upper high pressure builds back up and
covers the majority of the state Monday through part of Tuesday.
The operational and ensemble guidance capture this scenario quite
well and added confidence in the upcoming forecast into early next
week.

By next Tuesday or Wednesday, the models begin to diverge on short
wave details and whether the upper ridge survives. The ECMWF is
very bold and aggressive in driving upper dynamics from the
Pacific and Bering Sea through the state with the upper ridge
reappearing west of AK next Friday. The last few runs of the GFS
are slower in driving a swath of upper dynamics through AK but
also depict a broad upper ridge re-appearing south or southwest of
the state. Meanwhile both means pretty much carry some resemblance
of the ridge over the state throughout with more subtle upper
dynamics/short wave passing through. WPC followed more of a mean
pattern for Wednesday through next Friday with small percentages
of the operational GFS/ECMWF sprinkled in.

The heaviest qpf over the 5 days will occur across most of the
Aleutians with pockets of heavier precipitation across west
central AK. 850mb temperatures will be slightly above normal,
mainly a .5 to 1 standard deviation above average.

Musher