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Hawaiian Islands Synoptic Discussion and Guidance

610
FXHW60 PHFO 252144 AAA
AFDHFO

Area Forecast Discussion...UPDATED
National Weather Service Honolulu HI
1144 AM HST MON JUL 25 2016

.UPDATE...
Updated to drop the flash flood watch as mentioned below, and to
adjust the current weather grids. Across the state the highest
rainfall chances will still seen be over Kauai/Oahu in an area
of weak moisture convergence, but widespread showers or heavy rain
is not anticipated.

&&

.SYNOPSIS...
Warm and muggy conditions will linger across the state behind
Tropical Depression Darby. Light winds will allow for some
afternoon showers, but drier conditions will spread across the
state from east to west. More typical trade wind weather will then
return for the remainder of the week as high pressure builds well
northeast of the island chain.

&&

.DISCUSSION...
As of the 11 am HST advisory package, Darby is tropical
depression. An exposed low level center can be seen west of Kauai
with deep convection centered north of Oahu. Otherwise, at the
surface high pressure is located far northeast of the state. 12z
soundings show an unstable profile at Lihue, with a weak low-
level inversion at Hilo. The Hilo sounding also shows some steep
mid-level lapse rates with an elevated inversion around 20kft.
Precipitable waters are 1.8-1.9 inches. Early morning MIMIC total
precipitable water imagery shows drier air upstream east of Big
Island, with PW values less than 1.3 inches over the eastern
offshore waters.

The latest CPHC forecast is for Darby to continue weakening and
move away from the state. A band of deep tropical moisture remains
across the state, leading to our humid conditions and making the
potential for additional heavy rain a concern. The first area of
concern is across Kauai/Oahu, where a flash flood watch is
currently in effect. Relatively light south to southeast winds
will allow for some island-scale sea breezes, leading to some
interior showers during the afternoon. Model forecast soundings
show the airmass stabilizing pretty quickly from east to west
today, and an 18z AMDAR ascent sounding out of Honolulu already
shows a strengthening low-level inversion. This weak stabilization
should inhibit deep convection, which is also depicted by the 12z
WRF ARW and NMM runs (some showers interior but nothing very
strong). With this stabilizing trend, should be able to drop the
flash flood watch for Kauai/Oahu at noon. The second area of
concern is across the interior of the Big Island. The Hilo
sounding showed pretty steep mid-level lapse rates, and at least
initially there is plenty of moisture available for convection to
fire across the upper slopes. The limiting factor will be the
drier air moving in from the east combined with some mid-level
stabilization, though most of the stabilization occurs tonight. We
currently have a slight chance of thunder across the interior
slopes, which seems reasonable for now.

From tonight through mid-week, Darby or the remnants thereof will
continue to move northwest away from the state, placing us
increasingly under the influence of strengthening high pressure
far northeast of the state. Forecast soundings/cross sections show
the airmass stabilizing significantly from east to west
tonight/Tuesday. The large subtropical high and strengthening
trade winds will bring in drier air, with dewpoints possibly
dropping into the mid/upper 60s for the second half of the week.
We will see a more typical trade wind distribution of clouds and
showers, though with less rainfall due to the lower than normal
inversion heights. We should see a surge in moisture late in the
week/next weekend, in part due to remnant moisture from the
remains of Georgette that gets picked up in the trades. So, after
a drier than normal work week, it looks like we transition to
more of a wet trade wind pattern for the weekend.

&&

.AVIATION...
Tropical Depression Darby continues to move away from the islands.
Southerly flow remains over Kauai and Oahu, with easterly flow
found near the Big Island and Maui County.

Satellite and observations show significant improvements in clouds
and showers, so have dropped AIRMET Sierra for mountain
obscuration. VAD wind profile shows winds have weakened, combined
with a lack of a substantial inversion, low level turbulence is
not expected to be an issue. As such, AIRMET Tango has also been
dropped. No AIRMETs are in effect or expected, with VFR conditions
expected to prevail.

&&

.MARINE...
As of the 11 am advisory package, Tropical Depression Darby has
moved west of the coastal waters and will continue to track away
from the area and weaken. High pressure far northeast of the
state will exert more influence across the islands. Strengthening
trade winds associated with the high may lead to small craft
advisory conditions as early as tonight for the typically windier
channels near Maui and the Big Island. As the high strengthens
during the week, small craft conditions may expand to additional
areas.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

Discussion/Marine...Bravender
Aviation...MBallard

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office