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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 160644

Area Forecast Discussion
National Weather Service Honolulu HI
844 PM HST Sun Sep 15 2019

Moist and unstable conditions associated with an upper disturbance
and surface trough of low pressure in the vicinity of the islands
will keep the potential for heavy showers in the forecast through
Monday. A return of drier air and more stable conditions is
anticipated from east to west Tuesday through the second half of
the week.


Water vapor imagery showed a broad upper low centered a few hundred
miles north of the islands with plenty of middle- to upper-level
moisture pooling northward across the region to its east. This upper
feature combined with a surface trough north of Kauai and plenty of
deep tropical moisture continues to fuel spotty heavy showers this
evening. Recent satellite-derived precipitable water (PW) imagery
reflected this with pockets of higher moisture near or around two
inches, which is well above average. In addition to the heavy
showers, a few thunderstorms managed to develop early this evening:
one over the northern Koolaus (upslope from Laie) and one over the
Kauai leeward waters or just west of Niihau. The latest rainfall
summary showed peak accumulations over the Koolaus in the past few
hours (through 7 pm HST) with totals ranging from 1.25 to 1.6

The short-term guidance (through Tuesday) remains in good agreement
and depicts a similar pattern continuing, which will keep the rain
chances in the forecast. The better instability and deep tropical
moisture are forecast to hold over the western end of the state.
Although upper heights will begin to rise as the upper low to the
north drifts westward, a thunderstorm or two can't be ruled out
through the day Monday. Model PWs will hold around two inches over
these areas. Light to moderate east-southeast winds will
continue, which will give way to localized sea breezes,
potentially triggering leeward afternoon and early evening
showers Monday. Conditions will gradually begin to improve over
the eastern end of the state by Tuesday, as drier air and more
stable conditions move in from the east.

Guidance remains in decent agreement Tuesday night through the
second half of the week and reflects a return of drier and more
stable conditions statewide. The low-level flow (up to 10,000 ft)
is forecast to shift back to a more typical easterly direction
and strengthen. Model PWs should drop back to and around normal
through this period. Clouds and showers will focus over windward
and mountain areas, especially through the overnight and early
morning periods as pockets of higher moisture move through from
east to west.

Despite some differences between the various model solutions, a
return of light winds and increasing moisture (two inch PWs) will be
possible next weekend as a trough associated with a tropical
disturbance to the south moves through.


A low pressure system north of Kauai will continue to move away
from the islands and easterly trade winds will increase. Elevated
amounts of tropical moisture will keep low clouds and showers in
the forecast with periods of MVFR/IFR conditions expected. Drying
trends are forecast to spread from east to west across the islands
from Monday to Tuesday with the increasing trade winds.

AIRMET Sierra remains in effect for Mountain Obscuration over
north through east sections of Oahu.

AIRMET Zulu remains in effect for light icing FL180-280.


Trade winds will continue to steadily increase tonight over the
far western half of the state as the surface trough northwest of
Kauai continues to move west and away from the state. High
pressure far north of the state is expected to strengthen
slightly and slowly move east through the week. This will lead to
moderate to locally strong east to east-southeast trades through
most of this week. A Small Craft Advisory (SCA) remains in effect
for waters around Maui and the Big Island through at least late
Monday afternoon. This SCA may need to be extended.

A series of small swells from the southwest, south, and southeast
is expected this week. The largest of these swells looks to be
towards the end of the week as a south-southwest swell fills in
late Thursday into Friday. Surf heights are expected to be below
advisory levels with this swell.

Small surf is expected along north, east and west facing shores
through the new week. Expect a small east swell from Tropical
Cyclone Kiko as early as Wednesday along exposed east facing
shores and likely continuing through the weekend.


Small Craft Advisory until 6 PM HST Monday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Windward Waters-
Big Island Leeward Waters-Big Island Southeast Waters.




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

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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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