Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Honolulu HI
1000 am HST Monday Jul 28 2014
high pressure far northeast of the state will maintain moderate to
locally fresh trade winds across the islands. Showers carried by the
trades will affect mainly windward and Mauka areas for the next
several days. A surge in moisture from the southeast could bring an
increase in showers and more humid conditions late in the week.
water vapor imagery shows an upper level trough north of the
state...with a ridge to the southeast. At the surface...high
pressure is located far northeast of the state...with a trough far
to the north. 12z soundings from Lihue and Hilo show inversions of
5-6kft and precipitable waters of 1.3-1.4 inches...which is normal
for this time of year. Early morning mimic total precipitable water
imagery shows drier air over 100 miles east of The Big Island
extending well to the east and northeast.
Rainfall amounts have been on the low side overnight. A few
locations on windward Big Island received up to a tenth of an inch
of rain in the last 12 hours...but most locations that saw rain only
received a few hundredths. One more pocket of moisture is located
east of The Big Island and Maui that will keep showers going into
early afternoon...at least across the eastern islands. However drier
low-level air will spread across the state during the
afternoon/evening...leading to drier conditions later in the day.
The surface high will remain nearly stationary far northeast of the
state through much of the week. While the high does not
strengthen...we are already starting to see a tightening of the
pressure gradient across the islands due to low pressure well to our
south. Overall we expect a typical trade wind setup with showers
focused mainly across windward and Mauka areas...at least for the
first half of the week. GFS forecast soundings show inversion
heights dipping even lower for the next day or two. With no
noticeable sources of moisture immediately upstream...this will help
to limit rainfall chances for the next couple days. Winds may be
locally breezy at times.
By the end of the week...we may see a surge in tropical moisture
from the southeast that could bring an increase in showers and muggy
conditions. Extended models over the last few days have been back
and forth with timing and how far north to bring the moisture...so
it is difficult to nail down specifics at this point. However...they
seem to be converging on a solution that would bring an increase in
tropical moisture to Big Island late Thursday/Friday...and spread
across the rest of the state through the weekend.
morning showers linger over windward Big Island...bringing tempo
MVFR ceilings and isolate MVFR visible to phto. VFR should begin to prevail by
this early this afternoon for windward Big Island. All other
terminals continue with prevailing VFR conditions. Passing showers
embedded within the trade wind flow may bring isolate MVFR ceilings
throughout the day...mainly for windward and Mauka areas.
Early morning soundings from both sites show an inversion between
5.5-6.5 kft. With the moderate trades expected to increase slightly
and the inversion...an airmet for turb in the Lee of the terrain may
be necessary later today. Otherwise...no other airmets are expected.
an ascat pass last night showed a wide area of 25 knot winds through
the Alenuihaha Channel. Models show the gradient continuing to
tighten over the next couple days as low pressure passes far south
of the area. Therefore the current Small Craft Advisory for the
typically windier areas near Maui and The Big Island will likely
remain in effect at least through Wednesday.
A series of small southerly swells may bring minor bumps on south
facing shores...with a slight increase on Thursday night. Short
period trade wind swell will continue across east facing shores...
possibly increasing a bit in the coming days as trades strengthen.
No other significant swells are expected through the next several
Small Craft Advisory until 6 PM HST Wednesday for Maalaea Bay-
pailolo and alenuihaha channels-Big Island leeward and southeast