Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
712 AM EST Wed Nov 19 2014
Valid 00Z Thu Nov 20 2014 - 00Z Thu Nov 27 2014
For Hawai'i, the 19/00Z guidance maintained good continuity with
the migration of a mid-latitude upper-level trough and attendant
cold front across Hawai'i---24/12Z to 26/12Z.
The 19/00Z deterministic ECMWF, GFS-Parallel and Canadian attempt
to close off a 500mb low just north of the state around the
25/00Z-25/12Z, then migrate the system slowly northeastward during
the latter portion of the medium range forecast period---days 6-7.
The 19/00Z and 19/06Z GFS are more progressive with the front and
delay the formation of the cutoff feature until it is downstream
of the Islands. The presence of a 500mb cutoff---or lack
thereof---will likely make for a challenging precipitation
forecast between Monday and Tuesday.
At this point, the deterministic guidance is unsure when--and
actually where--this cutoff will develop---with its formation,
depth and forward momentum directly tied to an Aleutian Low's
progression across the Gulf of Alaska. Using a blend of the 19/00Z
ECENS/GEFS will preserve and maintain height falls and support the
timing of the surface front (with upper-level support) from Sunday
into Tuesday afternoon. In the coming days, the forecast may then
require some alterations---with the possible cutoff generating
some additional 'shear-line' clouds and showers in the wake of the
surface frontal passage.
But at this point in time---and 6-7 days out---the depth and
overall size of the cutoff is far from clear---with respect to the
sensible weather impacts it may generate upon Hawai'i.