Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
822 AM EDT Sat Oct 13 2018

Valid 00Z Sun Oct 14 2018 - 00Z Sun Oct 21 2018

...Some heavy rains expected today...

Himawari satellite imagery showed a nice southwest-northeast band
of moisture as seen in many of the water vapor channels. This is
occurring in response to a slow moving shortwave with much of the
activity focusing along the southern extent of a quasi-stationary
baroclinic zone. An anomalously moist air mass is in place,
particularly over Maui and the Big Island with moderate showers
currently falling over the latter region. The latest 12Z RAOB from
Hilo validates this with a PWAT value of 2.08 inches while October
climatology averages around 1.45 inches. The heaviest rainfall
will focus over these southern most islands through the
afternoon/evening hours before tapering off as the frontal zone
shears to the south. In its wake, surface winds should stay out of
the north in the cold advection regime before veering toward the
east-northeast by late Sunday.

Moving into next week, there are suggestions that a weak to modest
band of vorticity will cross south of the Hawaiian island chain
with the 00Z UKMET being most aggressive taking it directly over
the region. This scenario would afford an increasing threat for
rainfall early next week. Overall, the general pattern is for
Hawai`i to sit in a break between the subtropical ridges sitting
to either side. The positive height anomaly to the west near the
International Dateline should take over as the dominant feature
later in the week while amplified flow generally remains confined
to the mid-latitudes. At the surface, east-northeasterly trades
will dominate next week given the presence of surface high
pressure to the north. Wind speeds are forecast to stay in the 15
to 20 knot range with intermittent breaks given periodic weakening
of the axis of high pressure. Trade driven shower activity will be
commonplace although the guidance do not suggest appreciable
rainfall next week outside of the outlying 00Z UKMET solution.
Other global models seem to have a solid grasp on the overall
synoptic pattern. 


Rubin-Oster