Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
245 PM EDT Mon Mar 19 2018

Valid 12Z Fri Mar 23 2018 - 12Z Tue Mar 27 2018

The initial large-scale pattern is well agreed upon with a
full-latitude trough sweeping through the western Aleutians while
a mean ridge sits across mainland Alaska toward the polar regions.
Meanwhile, broad negative height anomaly sits over the North Pole
with extension down into the Yukon Territory. How this all evolves
becomes a big question mark, particularly with regard to how the
mean ridge breaks down over the weekend. The GFS and ECMWF differ
in the evolution of this ridge erosion as well as the
re-establishment of its return. The 00Z ECMWF supports a compact
upper low which will peel off from mean trough near the Yukon
Territory which would undercut the ridge and form a rex block.
Meanwhile, the past few runs of the GFS suggest maintaining more
of an omega block signature as downstream energy does not show any
retrogression back toward mainland Alaska. During this evolution,
multiple additional interactions with surrounding features takes
place which leads to a very chaotic looking synoptic-scale pattern.

Underneath this blocking regime, a chain of shortwaves appear
primed to sweep across the Aleutians into the Gulf of Alaska
during much of the forecast. This will maintain unsettled
conditions across the entire southern extent of the state. A pair
of cold fronts pushing across the northeastern Pacific should each
be foci for organized precipitation, particularly the one on
Friday given better low-level convergence fields. Otherwise,
locations to the north should be under the influence of surface
ridging which would keep conditions cool and dry for the most
part. There is reasonable confidence in the timing of the first
system as models have begun to exhibit better clustering. Farther
upstream, while details with the wave placement are reasonable on
Saturday, timing and placement issues prevail thereafter as the
system approaches the western Aleutians. While general troughing
should continue into early next week, model spread becomes much
too great to offer a whole lot of detail in the forecast.

The preference was heavily based on ensemble means after Day
5/Saturday given the complexities discussed above. Kept 30 percent
of an operational influence, 12Z GFS and 00Z ECMWF specifically,
in the forecast early on before resorting to a complete ensemble
mean based approach. From this weekend onward, took a three way
combination of the 00Z ECMWF ensemble mean with the 06Z GEFS/00Z
NAEFS ensemble means. After coordination with some of the local
forecast offices, felt it was prudent to maintain a heavier dose
of the 00Z ECMWF ensemble mean given the preference to form a rex
block setup.


Rubin-Oster