Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 PM EDT Wed Apr 04 2018

Valid 12Z Sun Apr 08 2018 - 12Z Thu Apr 12 2018

Today's forecast reflects refinements/adjustments for some
features in the forecast but maintains reasonable continuity for
the overall pattern evolution.  There is still reasonable
consensus for a primary storm track within a zone from the
northern Pacific/Aleutians/southern Bering Sea into the northeast
Pacific, with dynamic support for each system coming from
moderately progressive flow emerging from Asia.  Meanwhile an
Arctic ridge will drift a bit westward with time and eventually
allow for some degree of cyclonic flow or elongated weakness. 
Compact energy/possible closed low should linger over the
north-central mainland during at least the first half of the
period, and an elongated ridge to the south--representing an
extension of a western Canada mean ridge-- will likely weaken with
time.

Within the North Pacific corridor, the overall array of guidance
for the system near the Aleutians around Mon has not changed
significantly from 24 hours ago.  The 00z ECMWF continues to be
somewhat on the southeastern side of the full envelope while other
solutions vary in some details but overall provide decent
clustering.  A model blend with minority weight of the 00z ECMWF
maintains reasonable continuity and has support from the average
of recent ensemble means.  This system should catch up to leading
low pressure over the eastern Pacific by Tue-Wed uncertainty over
interaction specifics tempering confidence in surface details to
the southeast of the Alaska Peninsula in that time frame.

Guidance spread looks a little broader than yesterday for the next
Aleutians system days 7-8 Wed-Thu.  The 00z ECMWF/ECMWF mean have
become more ill-defined than other models/means in spite of a
common signal for meaningful upper trough energy crossing the
northern Pacific.  On the other hand confidence is not yet
sufficiently high to go too strongly toward the deep solutions of
the GFS/GEFS/CMC.  At this time prefer to maintain a forecast
similar to continuity by way of a model/mean blend.

Farther north, the Arctic ridge aloft holds onto somewhat greater
strength in today's guidance consensus.  The 06z-12z GFS/06z GEFS
mean show a farther eastward position and/or extent of the ridge
versus most other solutions while the 00z GFS compares more
favorably to the majority cluster.  Even with the different ridge
evolution in the 12z GFS (keeping the initial compact northern
mainland upper low in place longer) it does eventually develop
cyclonic flow to end up closer to consensus by the end of the
period.  Meanwhile consensus shows somewhat lower heights versus
yesterday within the late period cyclonic flow/weakness aloft--a
nod to some previous operational model runs.  By the end of the
period a significant question mark will be the southwestward
extent of mainland energy and degree of interaction with North
Pacific flow.

Based on combined considerations for individual features, days 4-5
Sun-Mon emphasizes operational model guidance (00z
GFS/ECMWF/CMC/UKMET) with modest ensemble input while the rest of
the forecast trends toward a more even blend of the GFS/ECMWF/CMC
with 06z GEFS/00z ECMWF means.

Rausch