Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
325 PM EDT Sun May 06 2018

Valid 12Z Thu May 10 2018 - 12Z Mon May 14 2018

...Guidance/Predictability Assessment and Preferences...

00 UTC ECMWF ensembles cluster better with the 12 UTC GFS than the
ECMWF days 4-6 for a major Gulf of Alaska low with recent ECMWF
runs seemingly overdoing height falls into a lead/warm amplified
ridge over the mainland. The 12 UTC ECMWF run has trended away
from this a bit. Meanwhile, the 00 UTC GEFS ensembles cluster best
with WPC continuity and the southward system shift over the Gulf
also seems favorable considering high flow amplitude.

Accordingly, WPC Alaskan medium range guidance was primarily
derived from a blend of the 12 UTC GFS and 00 UTC GEFS/ECMWF
ensemble means days 4-6 and then mainly these ensemble means days
7/8 amid growing forecast spread. Manual forecast modifications
were then applied to the WPC blended solution to depict deep low
pressure offshore consistent with dynamic support aloft. In this
fashion, WPC continuity was well maintained overall.


...Weather/Threat Highlights...

Warm ridging will be over the Alaskan interior/North Slope this
week as an upper low weakens over the southern Bering Sea. Leading
shortwave trough energy will meanwhile lift over the eastern Gulf
of Alaska and gradually weakens into the lead ridge position,
along with associated surface system and precipitation/weather
focus.

There is substantial later week into next weekend potential for
potent low genesis upstream into the Gulf of Alaska with
consolidation of closed trough energies. WPC progs show a deep
surface low pressure system with potential for high winds/waves
and wrapping heavy precipitation slowly tracking from the south of
the Aleutians/Alaskan Peninsula to the Gulf of Alaska. This would
increasingly threaten coastal communities and maritime interests. 

WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can be found at:

http://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
http://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html

Schichtel