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Hawaiian Islands Synoptic Discussion and Guidance

055
FXHW60 PHFO 180650 CCA
AFDHFO

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
850 PM HST Tue Sep 17 2019

.SYNOPSIS...
A brief period of more stable weather conditions remains in the
forecast through Thursday. More unstable wet weather will spread
from east to west across the state from Friday afternoon through
Sunday as a deepening upper level low approaches the islands from
the northeast. A high pressure ridge will build back in over the
region from Monday to Tuesday with weather trending back toward
more stable and drier conditions through the first half of next
week.

&&

.DISCUSSION...
The satellite picture this evening shows a complicated large scale
weather pattern with a high pressure center far to the north of
the region, a narrow upper trough (TUTT) just north of the
state, and two tropical disturbances 500 and 700 miles southwest
of the Kauai. Cirrus clouds continue to stream across the Hawaiian
Islands from west to east. Tonight's radar imagery shows only
isolated showers developing across the islands indicating a higher
degree of atmospheric stability statewide. Upper air balloon
soundings this afternoon show a weak trade wind inversion at
Lihue near the 8000 foot elevation level, due to higher moisture
levels and lingering instability, with a stronger inversion at
Hilo near the 7000 foot level. The inversion at Lihue will likely
strengthen overnight as more stable and drier air moves in from
the east.

Large scale surface winds will continue to flow from east to west
on Wednesday with a slight southeasterly direction in some areas.
These wind directions are modified by the position of the high
pressure ridge over the region interacting with the large low
pressure system southwest of Kauai. Wind directions will become
more northeasterly on Thursday as this low pressure system drifts
northward and the high pressure ridge builds back in over the
state. Expect a brief period of more stable weather conditions
with light to moderate winds are forecast for the Hawaiian Islands
through Thursday. Wind speeds will remain light enough for
afternoon sea breezes to form along the more sheltered western
slopes of each island.

The weather forecast this weekend will trend towards wet, humid
and unstable conditions as the large scale pattern changes. An
upper level low will deepen and approach the state from the
northeast, drifting over the islands by Friday afternoon. This
cold core low will add instability to the forecast along with
increasing clouds and showers from Friday into Saturday. The wind
pattern shifts back to a more east to southeast direction from
during this period as an upper level low deepens over the
islands, and a low pressure trough deepens along the tropical
convergence zone.

Medium range weather model solutions this weekend show this upper
low drifting slowly westward as the trough in the deep tropics is
drawn northward into the islands, spreading another plume of deep
and unstable tropical moisture over the entire state. Precipitable
Water (PW) levels associated with this tropical moisture plume
range form 2.0 to 2.5 inches. These PW levels are similar to the
moisture plume that brought periods of locally heavy showers to
the western half of the state this past weekend. Winds will
diminish on Sunday as the upper low drifts westward across the
state. Locally heavy showers and a few thunderstorms may develop
across the state. More specific location and timing of rainfall
impacts for each island will depend greatly upon the final
strength and location of these two low pressure systems. Stay
tuned for more details on this event as the forecast time period
grows shorter.

In the extended range forecast starting next week Monday, models
are indicating the upper low will begin to weaken and drift
northward away form the state. Higher moisture levels and
instability will keep enhanced showers in the forecast for one
more day. By Tuesday the long range model guidance shows a high
pressure ridge building back in across the eastern half of the
state with more stable trade winds returning. Enhanced showers
with higher amounts of tropical moisture will linger over Kauai
and Oahu on Tuesday with drier and more stable trends spreading
statewide from next week Tuesday into Wednesday.

&&

.AVIATION...
Pretty much a trade wind type situation for the main Hawaiian
Islands overnight and into Wednesday, and a dry one at that,
meaning VFR conditions prevailing. Any showers that comes in will
affect mainly the windward and mountain areas of the larger
islands. Also, a shower or two may still pop up along the Kona
coast this evening. Brief MVFR ceilings and vis are expected with
these showers. The trade winds are at moderate speed and does not
warrant AIRMET Tango. Little change is expect on Wednesday. A
broad area of low pressure between the islands and the dateline
and high pressure to the NNE of here, will maintain a slight south
component to the trade wind flow.

&&

.MARINE...
Moderate to locally fresh, east to east-southeast winds will hold
through Wednesday as a tropical disturbance to the southwest of
the islands lifts northwestward. A shift back to a more typical
east to northeast direction is expected Thursday through Friday,
possibly strengthening to Small Craft Advisory speeds for a short
time around Maui and the Big Island. Guidance shows a return of
light to moderate southeast flow late Saturday through Sunday due
to an approaching trough from the east-southeast, which would
support localized land and sea breeze conditions near the coasts.

A series of small swells from the southwest and south are expected
through the rest of this week. The largest of these looks to be
towards the end of the week as a long-period south-southwest
swell and potential short-period west-southwest swell combine
across the area. Surf heights are expected to be below advisory
levels with these swells.

Small surf is expected along north, east, and west facing shores
through the week. Expect a small swell from Tropical Cyclone Kiko
around Thursday along exposed east facing shores before dropping
over the weekend. Exposed west shores may see some of the
aforementioned west-southwest swell late Thursday into Friday. A
small northwest swell is possible late in the weekend into early
next week.

&&

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

&&

$$

DISCUSSION...Bohlin
AVIATION...Lau
MARINE...Foster

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