Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
813 AM EDT Sat May 16 2015

Valid 00Z Sun May 17 2015 - 00Z Sun May 24 2015

The 16/00Z deterministic model guidance (EC, GFS, Canadian)
offered a new solution across the north central Pacific---similar
to the one offered with the 15/00z but with a twist. Recommend a
blend of the 16/00Z ECENS and NAEFS to allow a longwave 500mb
trough to dig across the mid-latitude central Pacific on day
5---along 160W--- and slowly progress eastward---and north of the
forecast area---for days 6-7. Can't be definitive with a
particular deterministic run along 40N by day 4-5---in so much
as---it's very unclear where remnant typhoon moisture and energy
entrainment occurs along 50N latitude.   

In the north central Pacific--- two 00Z cycles, two different
ideas on how to incorporate remnant tropical southwest Pacific
typhoon energy ('Dolphin')---late in this forecast period. The
latitude is certainly a big issue for the Aleutians---but even the
day for this remnant energy to migrate east of the Dateline is in
some doubt---with the slower ECMWF and ECENS becoming nearly 10
degrees of latitude slower (@ 22/12Z) than the 16/00Z NAEFS/GEFS
along 150W-160W longitude with a long wave trough and the
character of energy embedded within it.

What has not changed however---the downstream upper-level ridge
and its major amplification along 135W from 35N to the Arctic
Circle. This 'blocking feature' is having an equally-impactful
effect on the entire east Pac and west coast Mainland forecast.

The equatorial trough and embedded mid-level disturbance near 8N
170E (mentioned yesterday) has lost some of its continuity---but
only for those who felt it to be forming another deep tropical
system and migrating westward. To its north---a well-defined
upper-level trough (in the westerlies) has impinged upon in and is
're-focusing' convection (at 16/10Z) closer to the Dateline and 8N
latitude. For the next couple of days, this upper-level trough
will be streaking middle and high clouds east-northeastward
towards Hawai'i. As was the case yesterday, this doesn't look to
be a significant sensible weather feature of the pattern. More a
curiosity at this point and periodic sky cover concern. But the
fact remains---until day 4, the flow pattern at low latitudes
remains in place and three means all hold weak upper-level
troughing above the forecast area.

Vojtesak