Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
312 PM EDT Sat Apr 14 2018

Valid 12Z Wed Apr 18 2018 - 12Z Sun Apr 22 2018

Models are in relatively good agreement with the long wave pattern
forecasting a rex blocking pattern to set up along approximately
170W this period..with a closed low moving into the Bering Sea by
Day 4, while a closed high migrates north-northwestward from the
North Slopes to 75-80N.  This general pattern will then persist at
least into next weekend. 

One of the big forecast challenges has been predicting the amount
of short wave energy that rotates southward and westward around
the base of this closed high.  The 00Z ECMWF backed way off its
solution yesterday of bodily moving a closed cyclone along the
North Slopes during the middle of the week...instead keeping the
bulk of the energy in northwestern Canada until later next week
when it moves a weaker shear axis across the Brooks Range. 
Instead today it's the Canadian solutions (particularly its
deterministic run) that favor the idea of driving significant
energy under the high across the interior.  Overall, the GFS and
ECMWF ensemble means are very closely aligned in this region and
mostly supporting the 00Z ECMWF deterministic solution, and
considering the run to run variability, will lean toward the means
over about the northern half of the state through the forecast
period. Confidence, however, is low.

Farther south, there's very good model agreement moving a strong
(mid 970s) low into the Bering Sea by Day 4 (Wednesday), with a
triple point low spinning up over the Gulf of Alaska and lifting
northward Wednesday night - Thursday.  There are some differences
with the track of this triple point low, and generally favored the
track/strength of the 00Z ECMWF and UKMET, which moves it across
the Kenai Peninsula later in the day Thursday. The 06Z GFS
appeared too disorganized with the associated short wave, while
the Canadian is pulled slightly to the east as a result of the
northern stream energy rotating under the closed Arctic Ocean
upper high. The Canadian is also too quick advancing the trailing
front into southeastern Alaska (again prefer a ECMWF/UKMET blend
here).  Beyond Day 5, the models all agree that a polar stream low
will strengthen and track south of the Aleutians.  Given the
forecast time range, the differences in the track and strength are
less than average, so have relatively higher confidence with this
feature. Overall, favored a ECMWF/GFS ensemble mean track, but
opted to decrease pressures around the low by about 4-8 mb from
Days 6-8. A quick look at both the latest 12Z ECMWF and GFS
solutions are supportive of this solution.

Klein