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Hawaiian Islands Synoptic Discussion and Guidance

634
FXHW60 PHFO 171940
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
940 AM HST Mon Jun 17 2019

.SYNOPSIS...
Moderate easterly trade winds will shift out of the east-southeast
today into Tuesday, then return to a more typical easterly
direction on Wednesday. Showers will continue to be focused across
windward slopes, though we will likely see increased clouds and a
few showers over leeward terrain each afternoon through tomorrow.
Winds will decrease again and shift out of the southeast across
most islands on Friday and the weekend.

&&

.DISCUSSION...
Easterly trade winds are veering out of the east-southeast from
Maui to Kauai today. This wind shift is occurring as a longwave
trough, which has bee n present for the last couple of weeks,
deepens several hundred miles northwest of the state. This will
invigorate a surface trough near Midway Atoll and cause the trade-
wind-generating surface high to drift far northeast of the state.
Expect the east-southeasterly winds to hold from Kauai to Maui
through the day tomorrow.

Showers will continue to be favor windward slopes, though the
shift to an east-southeasterly flow will likely lead to localized
leeward sea breezes that could trigger a few showers over leeward
terrain during the afternoon hours through Tuesday. A pocket of
slightly enhanced moisture is riding along the windward coasts of
Maui and Molokai and will reach Oahu during the afternoon, giving
these islands the highest potential for leeward showers today. As
the pocket of moisture advances westward tomorrow, the greatest
chances for afternoon leeward showers will be on Kauai. Though
weakened a bit, mid level ridging should keep the atmosphere stale
and prevent heavy showers from forming.

Trades will shift back to a more typical, easterly direction
Tuesday night, and modest rainfall will be confined to windward
slopes as a stable and somewhat dry air mass fills in from the
east.

Trades are expected to weaken and shift southeasterly over
portions of the island chain on Friday and persist through much
of the weekend. For now, the GFS and ECMWF are maintaining
somewhat dry and stable conditions over the state, so we are not
expecting any significant rainfall.

&&

.AVIATION...
Moderate to fresh east southeast flow over the islands will usher
in scattered low clouds and showers across windward and southeast
facing slopes. Leeward areas will be partially blocked by the
mountains and upstream islands, resulting in localized sea breezes
to set up today. Sea breezes will bring interior clouds and a few
showers this afternoon before the trades build back in by late
this afternoon. No AIRMETs are in effect at this time, but
mountain obscurations will be possible later today as cloud cover
fills in. Mountain wave turbulence will generally be light, but
low level wind shear may a problem on approach/takeoff into PHOG.

&&

.MARINE...
The combination of a trough to our west and high pressure far
northeast of the state will lead to fresh to locally strong east
to east-southeast winds across area waters through midweek. Due
to the southeasterly component in the wind flow, we will likely
see winds accelerate due to terrain, across the Maui and Big
Island windward zones. The Small Craft Advisory (SCA) has been
expanded to include the windward zones of Maui and the Big Island.
As the trough to our west strengthens Thursday into the weekend,
winds are expected to gradually weaken and veer southeast by
Saturday.

The current south-southwest swell will continue to decline through
the first half of the week. We should see a slight bump Tuesday
into Wednesday as a new small long-period south-southwest swell
fills in. Minimal surf is expected towards the latter half of the
week with only background south swells expected.

The current northwest swell will continue to lower through the
first half of the week. Additional pulses of tiny northwest swell
are expected thereafter. Also, surf along east facing shores will
begin to rise early this week as trade winds become more
established, especially for the east and central isles. A short-period
northeast swell is possible towards the end of the week due to a
fetch of strong northeast winds expected to develop west of the
California coast during the middle of the week.

&&

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

Small Craft Advisory until 6 AM HST Tuesday for Maui County
Windward Waters-Big Island Windward Waters.

&&

$$

DISCUSSION...Wroe
AVIATION...Foster
MARINE...Kino

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



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