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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 272002 CCA

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
1002 AM HST Tue Oct 27 2020

Unsettled wet weather will prevail over the next several days as
deep low pressure to the north pushes a front across portions of
the island chain. While heavy showers and thunderstorms could
develop anywhere, the greatest threat for heavy flooding rainfall
will be over the western half of the state through Wednesday. The
low will remain north of the state Thursday and Friday, likely
sending bands of showers across the state. The low will lift away
to the northeast during the weekend, decreasing the threat for
heavy showers.


Mostly cloudy skies and spotty showers are observed this morning.
The leading edge of a slow-moving and diffuse front sits roughly
100 miles northwest of Kauai, while a sharp upper level trough is
centered just west of the islands. A jet stream aloft running
along the eastern flank of the upper trough is producing
instability and broad lift that is generating thick high clouds
over the entire state. At the lower levels, easterly winds around
the Big Island quickly veer southeasterly near Maui and become
variable across the western half of the state. This flow is
producing showers across the eastern slopes of the Big Island and
within areas of weak convergence southwest of Kauai and north of
Oahu and Maui County.

Mostly cloudy skies will persist today, and since forecast models
do not show a distinct surface feature to focus rainfall, heavy
showers and thunderstorms could flare just about anywhere. A
Flash Flood Watch remains in effect for Kauai and Oahu, where
recent rainfall has led to greater saturation of soils.

The potential for spotty heavy showers will continue state-wide
overnight, but the greatest chances for flooding rainfall are
expected to be along the front, which will push down to near or
over Kauai. The GFS model is also hinting at deeper mid level
moisture being pulled over the state, which could lead to winter
weather on the high summits of the Big Island.

On Wednesday, the GFS and ECMWF depict the front moving slowly
across Kauai and Oahu, while a persistent jet stream aloft
maintains unstable conditions. The highest chances for heavy
rainfall will be along the front, but the remainder of the state
will continue to have a threat for spotty heavy showers and

On Wednesday night and Thursday, the front may move to Maui
County, with widespread rainfall diminishing across Kauai and
Oahu. The GFS and ECMWF show drier air aloft moving over the
island chain, which may limit active showers to along and just
ahead of the front. The Flash Flood Watch for Kauai and Oahu
currently runs through Thursday afternoon, though the flood threat
could be diminishing on these islands and possibly increasing on
Maui County. For now, we will leave the watch as is due to

On Thursday night and Friday, the front will likely weaken and
stall over Maui County as the parent surface low drops down to
within 350 miles north of the islands. Showers will likely remain
concentrated along the weakening front, but moderate to
potentially breezy westerly winds will also deliver some showers
to leeward slopes. These winds could become gusty over some
windward communities due to downsloping off of terrain. In
addition, there will be a noticeable drop in humidity behind the

During the weekend, the low will lift off to the northeast, but
the islands will remain under southwest to northwest winds with
bands of showers likely dropping down from the north from time to


Fairly nondescript surface gradient across most of the island
chain will favor light southerly or variable winds through today.
The opaque cirrus canopy will limit the intensity of any sea
breezes and inland cloud/shower development should be modest. Dry
low-levels in this morning's Lihue and Hilo soundings combined
with morning radar trends support prevailing VFR for the remainder
of the afternoon. The potential exception will be Big Island
where deeper moisture pivoting up from the south will favor
increased coverage of showers and low clouds later this afternoon.

Mid-level southwesterlies will consolidate in advance of the front
tonight through early Wednesday morning bringing deeper moisture
and stronger ascent for Kauai through Molokai. More widespread
convection will bring MVFR/localized IFR and the potential for
isolated thunder during this time. Light rime icing will remain
possible statewide.

No AIRMETs in effect.


A ridge near Oahu will bring southeast flow across the area with
the strongest winds near Maui and the Big Island. A front
currently northwest of Kauai will move into the coastal waters
Wednesday and push east through Thursday. West-northwest winds
will follow behind the front and southerly winds ahead of the
front. An upper level disturbance linked with the front will keep
heavy rain and thunderstorms in the area through Thursday.

A current northwest swell will diminish through Wednesday. A short
period northwest swell will bump surf up along exposed north and
west facing shores Wednesday night and peak Thursday night and
Friday. A larger long-period northwest swell is expected to arrive
late Saturday and may produce advisory level surf along north and
west facing shores early next week.

A small northeast swell will linger through tonight. A series of
small, long period south swells are anticipated through the week.


Flash Flood Watch through Thursday afternoon for Niihau, Kauai,
and Oahu.




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