Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
830 AM EDT Wed Sep 26 2018

Valid 00Z Thu Sep 27 2018 - 00Z Thu Oct 04 2018

Currently, the tail end of a baroclinic zone and associated
pre-frontal trough were noted just to the north of Kauai fostering
the development of northeastward progressing showers. As the
frontal zone begins to move toward Kauai and Oahu, some heavy
rainfall will be possible the next day or so as precipitable water
values spike into the 2 to 2.25 inch range. Relative to September
climatology, this is easily 1 to 2 standard deviations above
average. Difficult to pinpoint where the heaviest amounts would
occur, but most of the guidance suggests the potential for 1 inch
6-hour totals, particularly on Thursday as the slow moving
boundary sits over the region.

Eventually conditions will improve by Friday as the boundary
shears out with a surface high building over the subtropical
Pacific. Synoptic-scale pressure gradients should tighten with an
area of disturbed weather tracking south of Hawai`i. The Central
Pacific Hurricane Center is currently advertising a 50 percent
chance for tropical cyclone development during the next 5 days.
While impacts would stay well south of the island chain, enhanced
trades are expected with sustained winds in the 15 to 20 knot
range. Into early next week, a mid-latitude longwave trough
crossing the International Dateline will lift up this wave with a
position well west of Hawai`i. If the cyclone remains strong
enough as suggested by the 06Z GFS, some residual enhanced wind
field could continue into next week. This is in stark contrast
from the 00Z ECMWF which does not draw the system northward.
Ultimately, from Friday onward, conditions should be much drier
except for some scattered trade driven shower activity. By Day
7/October 3, the next cold front will be approaching the northern
islands but any precipitation would arrive shortly after. 


Rubin-Oster