Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Honolulu HI
400 am HST Monday Apr 27 2015
a slow-moving front will spread modest rain from Maui to The Big
Island through tonight. Light southerly winds ahead of the front
will turn into breezy trade winds behind it. Active showers will
continue over windward slopes through much of the week...but Lee
areas should start to dry out by Tuesday.
a slow-moving frontal cloud band became more diffuse over Maui
County during the past six hours. However...in the process this
feature shed its Load of water vapor onto north slopes of Maui and
Molokai...where orographic forcing produced persistent rain.
Although the rain should diminish as the remnants of the front move
toward The Big Island...saturated soil will increase the risk of
flooding over north Maui through much of the morning. Convergence
between southeast and more southerly or southwesterly winds
supported limited shower development ahead of the front...mainly in
combination with convergence produced by land breezes just offshore
from The Big Island.
Slowly dissipating cloud layers will obscure the sky and drop a few
sprinkles on both sides of the main rain band. Further to the
northwest...relatively clear conditions in the immediate aftermath
of the frontal cloud band will yield to cold-air cumuli...and then a
nearly solid stratus overcast...at least until the air mass modifies.
Guidance agreed well in predicting the shallow trough aloft
currently supporting the weakening front to shift southeast and
weaken further through Wednesday. Solid ridging aloft building in
from the northwest will support the rapid development of the surface
high behind the front...bringing moderate to strong winds shortly
after frontal passage. These winds will have more of an easterly
component than is usual in such cases...so one could say simply that
moderate to strong trade winds will return to nearly all of the
state by this evening...and then persist through most of the week.
As the frontal cloud band falls apart...it will shed its Load of
water vapor onto the trades. The winds in turn will carry this moist
air mass over the state again...maintaining continued rainfall over
windward slopes. Lee areas should experience some drying by
mid-week...but a few showers blooming windward will continue to
drift leeward before dissipating. Thus...conditions will remain on
the wet side of usual through the forecast period...especially over
Maui County and The Big Island...where most of the frontal remnants
will reside at first.
By the weekend...most solutions predicted a weak middle-level trough to
erode the ridge somewhat from the west...reducing support for the
surface ridge as well. Also...by that time most of the frontal
remnants should pass the state...leading to more moderate...slightly
drier trade-wind conditions.
a new northerly swell expected to arrive today likely will build
surf heights to the thresholds for a high surf advisory along north-
and east-facing shores. This swell will continue to arrive into
Tuesday...and then will lower gradually on Wednesday. Strengthening
trade winds will keep short-period choppy surf elevated along
east-facing shores through most of the week.
Strong high pressure building to the north will increase trade winds
over the area through mid-week. Winds and seas will exceed
thresholds for a Small Craft Advisory across most of the area by
around Monday morning.
high surf advisory until 6 PM HST Tuesday for Niihau-Kauai
windward-Kauai leeward-Oahu North Shore-Oahu koolau-Olomana-
Molokai-Maui windward west-Maui Central Valley-windward
Small Craft Advisory until 6 am HST Tuesday for Kauai northwest
waters-Kauai windward waters-Kauai leeward waters-Kauai Channel-
Oahu windward waters-Oahu leeward waters-Kaiwi Channel-Maui
County windward waters-Maalaea Bay-Pailolo Channel-Alenuihaha