Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
818 AM EDT Wed May 01 2019

Valid 00Z Thu May 02 2019 - 00Z Thu May 09 2019

Light to moderate trades should persist across Hawaii through Thu
night, with isolated showers mainly in favored windward locations.
Model consensus has significantly improved with respect to a
shortwave expected to begin amplifying near 40N/170W tonight, with
a closed upper low developing by Thu night moving southeast in the
general direction of Hawaii. Deterministic models are now in
agreement that the upper low should move just north of Hawaii Sat
night/Sun, with an associated cold front passing through the state
from northwest to southeast Fri night-Sun. While the track of the
upper low will result in a cold front that moves fairly quickly,
models show the potential for deeper tropical moisture to be
pulled northward into the system starting Sat (PWs approaching 2
inches). This would result in the potential for heavy rainfall Sat
into Sun for the islands from O`ahu east to Hawai`i. The ECMWF
indicates the potential for 3-4 inch rainfall totals, with locally
higher amounts, while the GFS is lighter, with rainfall totals in
the 1-2 inch range. These differences appear to be primarily
related to differences in arrival timing of the deeper tropical
moisture ahead of the system, with the GFS slower than the ECMWF.
ECENS probabilities show an approximately 30 percent chance of
exceeding 2 inches of rainfall over portions of the described
areas, with a 15-20 percent chance of exceeding 3 inches. Thus,
while some uncertainty remains as to the exact magnitude of
rainfall, the potential is there for a round of heavy rains
along/ahead of the cold front. In addition to the rainfall
potential, 20-30 kt and gusty southerly winds should develop ahead
of the system starting on Sat across the state.

As the upper low and surface frontal system move east of the state
by the middle of next week, models show Hawaii being left in a
region of relatively weak flow (aloft and at the surface) into
late Thu, with perhaps some resumption of weak trades by Thu as a
surface high begins to move north of the state.


Ryan