Heavy Rain in Hawaii, May 12, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
850 PM HST Thu May 11 2017

.SYNOPSIS...
A strong area of high pressure to the northeast of the state will
keep a moderate to breezy trade wind flow in place through the
middle of next week. Clouds and showers will favor windward and
mauka areas, but will occasionally spread leeward due to the
strength of the trades. Wetter trades are expected for the second
half of the weekend and early next week as an upper trough
develops just east of the islands.

&&

.DISCUSSION...
Currently at the surface, a 1038 mb high is centered around 1400
miles to the north-northeast of Honolulu, with a surface
trough located around 825 miles to the west of Kauai. The gradient
between these two features is driving moderate to locally breezy
trade winds across the island chain this evening. Infrared
satellite imagery shows partly to mostly cloudy skies in place
across the state, with radar imagery showing a mix of lingering
convective showers from daytime heating and trade winds showers
moving into windward areas. Main short term concern for the next
several days revolves around rain chances.

The surface high well to the north-northeast of the state will
strengthen to 1040 mb while remaining nearly stationary through
Saturday. Meanwhile the surface trough west of the islands will
slowly dissipate heading into the weekend. The high will then
settle southeastward while weakening slightly Sunday through mid
to late next week. Overall, the gradient resulting from the strong
high to the northeast of the islands will keep a moderate to
locally breezy trade wind flow in place right through the middle
and latter part of next week. Aloft, weak upper level troughing
will remain in place across the area through the weekend, with
upper troughing then amplifying east of the state early next week.
Upper level ridging is then expected to build in from the west by
the middle and latter part of next week.

Overall, expect a slightly wetter than normal trade wind shower
pattern to persist into the weekend due to weak upper troughing
remaining over the island chain. Shower coverage and intensity
will likely pick up late Sunday through Tuesday as the atmosphere
further de-stabilizes due to the amplification of the upper
trough just to the east of the state. A more typical trade wind
shower regime should return by the middle and latter part of next
week as upper ridging begins to build in from the west. Clouds and
showers will favor windward and mauka areas through the period,
with showers occasionally spreading leeward due to the strength of
the trades.

&&

.AVIATION...
A strong high pressure system far to the north-northeast of
Hawaii will continue to produce moderate easterly trade winds
across the islands through the weekend. An upper level trough
over the state will help produce enhanced showers near the
islands for the next few days. Expect brief periods of MVFR
ceilings with scattered showers along eastern slopes of all
islands.

AIRMET Tango in effect for portions of Maui and the Big Island
for low level turbulence due to moderate trade winds.

&&

.MARINE...
The latest surface analysis showed weak low pressure west of the
region near Midway and 1038 mb high pressure centered north-
northeast of the state. The latest guidance has initialized well
with these features and indicates the weak low drifting northward
and weakening as high pressure strengthens across the eastern
Pacific over the next few days (up to 1040 mb in 24 hrs). As a
result, fresh to strong trade winds across local waters will
continue into next week. A small craft advisory for the channels
east of Molokai, Maalaea Bay, the windward Maui County waters, and
the waters south of the Big Island has been extended through
Sunday.

Surf along the north facing shores will remain small into next week
with no significant swells expected. Mainly trade wind swell wrap at
select spots will be the only real source supporting surf through
the weekend. For the extended, a gale that has developed east of
Japan is forecast to slowly track northeastward tonight into Friday,
then stall and meander over an area south of the western Aleutian
islands (west of the Date Line) over the weekend. WAVEWATCH III
supports a small northwest (310 deg) swell associated with it
filling in locally through the second half of next week.

Surf along east facing shores will steadily rise through the weekend
as a large band of fresh to strong trade winds become established
south of the previously discussed strengthening high upstream of the
state. This combined with fresh to strong onshore winds locally will
translate to rough surf along east facing shores each day. Surf will
near the advisory level (8 ft faces) over the weekend into next week
for east facing shores.

A small reinforcing south swell is expected to fill in Friday and
Friday night from recent southern Pacific activity. This will
support moderate south shore surf into the weekend each day. For the
long range, a more active pattern is forecast to setup across the
southern Pacific east and southeast of New Zealand that will likely
correspond to an uptick in surf along south facing shores through
the second half of the month, beginning around the 19th or next
Friday. More on this in later packages as conditions evolve over the
coming days.

For more detailed information on surf for Oahu, see the latest
Collaborative Nearshore Swell and Wind Forecast that was issued
Wednesday afternoon.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Sunday for Maui County
Windward Waters-Maalaea Bay-Pailolo Channel-Alenuihaha Channel-
Big Island Leeward Waters-Big Island Southeast Waters.

&&

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Gibbs