Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
318 PM EDT Sat Mar 17 2018

Valid 12Z Wed Mar 21 2018 - 12Z Sun Mar 25 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 GFS/00z-12z Canadian global/00z ECMWF ensemble mean
blend Wednesday 21 Mar and Thursday 22 Mar 22.
Low development near the southeast coast Tue then drifts southeast
just offshore parallel to the coast through Wed.  The GFS
continues a few hours faster than the GEFS Mean and operational
ECMWF and Canadian global, so the gfs was given less weighting in
these areas.

A trough leaves eastern AK while ridging drifts east from the
Bering Sea into Western AK later Wed. The model clusters separate
at this point, with the 06z-12z GFS/GEFS members moving the ridge
faster east than the 12z ECMWF/ECMWF ensemble members and mean.

The models show low pressure moving east across the Bering Sea in
the middle of next week with the 12z ECMWF exemplifying the
pattern uncertainty by moving the low north out of the north
Pacific into the eastern Bering 12z Fri 23 Mar. 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 and several
mb deeper 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.   

Petersen