Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
337 PM EDT Sun Mar 18 2018

Valid 12Z Thu Mar 22 2018 - 12Z Mon Mar 26 2018

...Blocking pattern leading to slow movement of systems across
Alaska...

The guidance showed good agreement over Alaska and upstream in the
Bering Sea through about day 5 with differences growing afterward.
Due to this, the pressures and 500 hPa heights were based on a 00z
ECMWF/06z-12z GFS/00z Canadian global/00z ECMWF ensemble mean/06z
GEFS mean blend Thursday 22 Mar and Friday 23 Mar.

Low development near the southeast coast Wed drifts southeast just
offshore parallel to the coast through Thu before weakening.  The
area of low pressure becomes ill defined and weaker afterward. The
next wave produces low pressure moving across the Gulf of AK,
which returns the front back onshore as a warm front late next
weekend into early next week.

The models/ensembles develop the initial area of low pressure wed
that drifts north into the northern Bering Sea Thu with a trailing
cold front moving slowly east across the Aleutians.  This is
followed by triple point redevelopment late Fri 23 Mar into Sat 24
Mar into the Gulf of AK, with the 12z ECMWF a few hours slower 
than other solutions.   

For next weekend, there was increasing weighting towards the 00z
ECMWF ensemble mean solution with time given model to model and
run to run differences in cyclone location/evolution.  There was
an earlier cluster of the 06z GFS/00z ECMWF/00z Canadian global in
low pressure crossing the Aleutians around 12z Sat 24 Mar.
The 12z runs of the GFS, UKMET, and 12z ECMWF moved the system
more on a west track, showing the need to mitigate uncertainty by
blending the models and means.  Confidence is still below normal
given the continuing changes, with timing differences as well
between the models and means.  The low confidence led to a blend
of multiple solutions.  

Petersen