Current Conditions
Temp7.4 C
RH70 %
WindNNE 13 mph
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 161959

Area Forecast Discussion
National Weather Service Honolulu HI
959 AM HST Mon Sep 16 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
this evening. A return of drier air and more stable conditions is
anticipated from east to west Tuesday through the second half of
the week. Increasing moisture and rain chances will become a
possibility once again over the upcoming weekend.


At the surface, a 1026 mb high is centered about 1400 miles north
of Hawaii. A passing trough to the north between the islands and
the high is allowing for the otherwise moderate to fresh trade
winds to veer east southeasterly across the smaller islands this

Water vapor imagery shows an mid to upper low centered about 400
miles north northwest of Hawaii that will gradually be lifting
northwest over the next day or so. Southwest winds along the
eastern flank of the low is currently drawing scattered to broken
midlevel clouds across the central and eastern parts of the state
as shown on visible satellite. A few low clouds are anchoring
along the windward slopes as well. MIMIC Total Precipitable Water
(PW) shows a swath of PW near 2 inches across the state and 12Z
soundings show a weak inversion. Dew points are mainly in the
upper 60s to low 70s at the sea level observation stations.

The upper low will keep conditions unstable today, especially
across the western half of the state. Localized sea breezes may
set up leeward and along the northwest coasts allowing for clouds
and showers to develop this afternoon into the early evening
similar to yesterday. Can't rule out a few of these showers
becoming heavy at times. Conditions will gradually begin to
improve over the eastern end of the state by late tonight into
Tuesday, as drier air and more stable conditions move in from the

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. Added instability
associated with an upper low setting up over the state combined
with this moisture will support increasing rainfall chances.


Elevated moisture will keep low clouds and showers around for
today and into the overnight hours. Brief periods of MVFR
conditions are possible, particularly for windward areas. Winds
are in the light to moderate category out of the east-southeast. A
drying trend with increasing trades is on tap as we head into
Tuesday thanks to the upper low north of Kauai moving away from
the state.

AIRMET Zulu remains in effect for light icing 160-FL280.


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 easterly trades through most of this
week. Winds may veer to the southeast across the far western
Hawaiian waters near the middle of the week as a low pressure
system tracks south to north, west of the state. The Small Craft
Advisory (SCA) remains in effect for waters around Maui and the
Big Island through this afternoon. We will continue to monitor the
winds today to see if the SCA needs to be extended. Nonetheless,
borderline SCA level winds are expected for Monday night into

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 combination of a long-period
south-southwest swell and potential short-period southwest swell
moves in 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 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. Exposed west
shores may see some of the aforementioned southwest swell on
Friday. A small northwest swell will be possible late in the
weekend into early next week.


Small Craft Advisory until 6 PM HST this evening 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