Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
713 AM EST Sun Feb 18 2018

Valid 00Z Mon Feb 19 2018 - 00Z Mon Feb 26 2018

A wet pattern appears set to continue across Hawai'i through the
next week. A surface boundary continues to linger across the state
early this morning, which along with relatively high Pwats
(1.25-1.50 inch range) is producing areas of deep convection with
heavy rainfall. 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 favor the development and persistence of upper-level
lows. The first such upper low is shown by all models to develop
tonight into Monday, centered just north of Hawai'i. This feature
appears 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 gradually 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 keep deep southerly/southwesterly flow in place
and will continue to supply the region with abundant moisture to
support convection, while surface flow remains east-southeasterly
due to a strong surface high well northeast of Hawai'i. Surface
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.

Both the ECMWF and the GFS suggest a brief lull in convective
activity by 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 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