Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
702 AM EST Mon Feb 19 2018

Valid 00Z Tue Feb 20 2018 - 00Z Tue Feb 27 2018

A wet pattern appears set to continue across Hawai'i through much
of the week. A surface boundary continues to linger across the
state early this morning. An upper low that developed over the
past 24 hours essentially over Hawai'i has, for the moment, pushed
the deepest moisture plume just east of the islands. The large
scale pattern across the central Pacific is not expected to change
appreciably over the next week. Well to the north of Hawai'i, a
blocking ridge is expected to weaken slightly, but will remain
prevalent. South of the ridge, the pattern will continue to favor
the development and persistence of upper-level lows. Models show
the aforementioned upper low across Hawai'i as relatively
transient, with most guidance showing it dissipating by mid-week
as another, deeper upper low develops farther to the west. Models
show a general consensus that the surface boundary over the state
this morning will continue to drift westward and weaken as the
initial upper low weakens. As this second upper low develops by
late in the week, a ridge axis is forecast to develop just east of
Hawai'i. The combination of these to features will bring a
resumption of deep southerly flow, and will supply the region with
abundant moisture to support convection once again. Meanwhile,
surface flow remains east-southeasterly due to a strong surface
high well northeast of Hawai'i. Low-level ESE flow should
gradually strengthen through the week and into next weekend as the
aforementioned surface high drifts southward, strengthening the
pressure gradient across the Central Pacific. Looking out to the
beginning of next week, a number of model solutions show the upper
low west of Hawai'i starting to break down as a larger upper
trough amplifies farther to the west, bringing the upper low under
the influence of gradually increasing westerlies. Models show
varying solutions as to how quickly this occurs, and confidence in
the specifics at this time range is low.

Both the ECMWF and the GFS suggest the brief lull in convective
activity continuing into Tue as the initial upper low weakens and
the surface boundary drifts west of the state. But a resumption in
the potential for convection and heavy rainfall is shown from Thu
onward as the upper low takes hold farther west, and deeper
moisture once again begins to stream across Hawai'i east of the
low.

Ryan