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Hawaiian Islands Synoptic Discussion and Guidance

669
FXHW60 PHFO 200131
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
331 PM HST Wed Jun 19 2019

.SYNOPSIS...
Moderate to locally breezy trade winds will focus showers mainly
across windward areas through Thursday. Winds will weaken and
shift to the southeast from Friday through the weekend, leading to
daytime sea breezes, spotty interior showers, and more muggy
conditions.

&&

.DISCUSSION...
Water vapor imagery shows an upper level ridge over the state,
with a high far to the northeast and low to the north. At the
surface, high pressure is located far to the northeast with a
ridge extending southwestward from it, passing several hundred
miles north of us. A surface trough has been nearly stationary a
couple hundred miles northwest of Kauai. 00z soundings from Hilo
and Lihue show strong inversions below 6kft. Precipitable waters
range from 1.04 inches in Hilo to 1.34 inches inches in Lihue,
which is similar to the moisture gradient apparent in MIMIC total
precipitable water imagery. A loop of TPW shows drier air moving
across the state from the east, with similar values near Big
Island/Maui, spreading across Molokai/Lanai toward Oahu.

Rainfall amounts today have been limited (mostly less than a
tenth of an inch over the past 12 hours) but with decent coverage
across windward sections, especially Kauai and Big Island. With
drier air moving in from the east and stable conditions remaining
in place, we do not expect any significant rainfall totals tonight
either. Showers will remain focused primarily across windward
sections.

Over the next day or so, a new low will develop and cutoff
northwest of the state. This low will reinvigorate the surface
trough to our northwest, with a surface low developing along it.
While this weather system will be far enough away to limit direct
impacts, it will shift the surface ridge closer to the state.
Initially winds may weaken a touch on Thursday, then moreso from
Friday into the weekend as the direction veers toward the
southeast. The atmosphere over the state will remain relatively
stable with an established inversion. Combined with dry low-level
air, the expectation of limited rainfall will continue through the
work week.

Conditions change a bit this weekend as the deep layer trough/low
shift closer to the state. Forecast soundings show inversions
weakening and lifting to 8-10kft near Kauai while remaining lower
and more stable farther east. Combined with deeper moisture, this
could lead to a showery period especially across Kauai this
weekend, especially Friday night/Saturday. Fewer showers are
expected farther east.

Little change is expected in the overall pattern nearby as we head
into next week. The upper trough lifts northward leaving a ridge
in place across the state, keeping the atmosphere stable across
the islands. The surface ridge also remains in place near or just
north of us, keeping the pressure gradient weak and generally from
the east or southeast. This will lead to somewhat of a dry
convective pattern, with onshore winds during the day leading to
cloud buildups across the interiors of the islands. Differences
emerge between the GFS and ECMWF by this point, with the ECMWF
quicker to shift the ridge north and bring back a weak trade wind
pattern. Either way it looks like winds will be on the lighter
side through the extended period.

&&

.AVIATION...
A 1032 MB surface high will continue to sit stagnant far to the
northeast of the State. This will allow for moderate to locally
breezy easterly trade winds to persist across the islands this
evening into tonight. As typical with this type of pattern,
scattered showers will be most concentrated on the windward side
of the islands, especially by late tonight. In addition, a stray
shower or two could spill onto the leeward side.

No AIRMETs are currently in effect.

&&

.MARINE...
A surface high far northeast of the area will maintain locally
strong trade winds over the coastal waters through tonight. Winds
will weaken and shift out of the southeast starting tomorrow as a
trough deepens west of the offshore waters. The southeast flow
will be partly blocked by the Big Island, resulting in light winds
around the smaller islands Friday through the weekend.

Surf will remain well below the advisory threshold along all
shores through at least early next week. Surf along east facing
shores may rise over the weekend as a short-period, northeast
swell builds.

&&

.FIRE WEATHER...
Fuel conditions are drier than normal, with the Keetch-Byram
Drought Index (KBDI) already over the red flag warning threshold
of 600. The relative humidity and wind approached but didn't reach
the other warning thresholds today (45 percent and 19 mph
sustained for two hours at HNL). While we're still not expected
to reach those thresholds on Thursday (wind will likely be the
limiting factor), conditions will be dry and somewhat gusty,
which bring increased fire weather concerns. Decreasing winds and
increasing RH vales from Friday onward will diminish those
concerns.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Thursday for Pailolo Channel-
Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

&&

$$

DISCUSSION/FIRE WEATHER...Bravender
AVIATION...Thomas
MARINE...Donaldson

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



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