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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 201358
Area Forecast Discussion
National Weather Service Honolulu HI
358 AM HST Thu Sep 20 2018
A weak surface trough just north of Kauai will move slowly
westward through tonight, which will keep winds light across the
state. This will allow local afternoon sea breezes and nighttime
land breezes to develop. Showers will continue over windward Maui
and the Big Island this morning. Clouds and showers are expected
to develop over interior and mauka sections of the state this
afternoon as the sea breezes become established. Trade winds will
return by Friday, and will persist through this weekend. An area
of low pressure is expected to develop northwest of the islands
early next week, which will produce light southeasterly winds.
Rain chances may also increase over the western end of the island
chain by the middle of next week.
The southern end of a weak north to south oriented surface
trough, which is evident just north of Kauai, continues to drift
slowly westward. The close proximity of this feature to the state
is causing the winds to remain light. The local nighttime land
breezes that developed last evening over most of the islands
continue to dominate the flow early this morning. Abundant
moisture across the eastern half of the state continues to aid in
the development of showers, which are streaming into the windward
sections of the Big Island and Maui. Elsewhere, the land breezes
are expected to keep the remainder of the state west of Maui
mostly dry this morning.
The forecast guidance indicates the light wind regime will persist
today. As a result, we anticipate local afternoon sea breeze
circulations will develop over most islands. In addition, the
higher moisture values may spread westward, which could aid in the
development of showers over interior and mauka areas, as well as
over some leeward sections. Daytime heating combined with
lingering moisture over the eastern end of the state may allow
scattered showers and a few thunderstorms with locally heavy
rainfall to develop over the Kona slopes and interior sections of
the Big Island this afternoon. By this evening, we expect the
local nighttime land breezes will begin to clear out most of the
showers from interior and leeward areas of the state. Isolated to
scattered showers may continue over some windward facing slopes
As the surface trough shifts away from the region, we expect
surface high pressure will build north and northeast of the state.
This will tighten the pressure gradient across the area, which
will cause the trade winds to return. In addition, a drier air
mass will also filter in from east to west across the state. This
will result in brief trade showers over some windward and mauka
areas, but most leeward sections of the smaller islands will be
relatively dry from Friday through this weekend.
Beginning early next week, an area of low pressure is forecast to
develop northwest of the islands. This will cause the pressure
gradient to relax over the region, which will result in light
winds once again. In addition, the winds will shift out of the
east-south or southeast. The forecast models continue to show
some variations in the location of the area of low pressure and
its associated convergence band. In addition, the areal coverage
of the moisture field east and southeast of the low is uncertain
at this time. Depending on the proximity of these features to the
state, there could be an increase in rainfall chances over the
western end of the island chain starting around the middle of next
week. However, it is still too early to forecast with any
confidence that this will occur. Regardless of the chances of
rain, expect very warm and muggy conditions to develop starting
early next week. Note that with the lack of any significant
volcanic emissions on the Big Island, we are not including any
volcanic haze in our forecast for the smaller islands next week.
Low-level E to SE winds near the Big Island and Maui will increase
in strength today as a trough moves NW through the island chain.
This moist flow will deliver clouds and showers to E and SE
facing slopes and adjacent waters, bringing periods of MVFR VIS/CIG.
Additionally, there is a slight chance of a thunderstorm over the
Big Island slopes this afternoon. AIRMET SIERRA for mountain
obscuration is currently posted for windward portions of the Big
Island, where MVFR conditions have prevailed for most of the
Lighter winds from Molokai to Kauai have allowed overnight land
breezes to clear skies over land, while low cloud coverage over
the water has been increasing. VFR conditions will likely persist
through the morning. Afternoon sea breezes and increasing moisture
will bring clouds and showers to interior and upslope areas of
all islands, perhaps prompting an expansion of AIRMET SIERRA as
areas of MVFR VIS/CIG develop. A drier trade wind flow is
expected to begin nosing in from the E tonight and Friday.
A surface trough will move across the island chain from east to
west today. In its wake, easterly trades will strengthen beginning
this afternoon across the eastern islands and tonight across the
western end of the state. Small Craft Advisory (SCA) conditions
are expected to expand from east to west across the typically
windy waters around the Big Island and Maui this afternoon and
tonight, with these conditions holding in place through Saturday.
Winds will likely shift around to the southeast late Sunday
through early next week as an area of low pressure forms several
hundred miles northwest of Kauai.
Surf will remain below advisory levels along all shores through
early next week. The current south swell will slowly subside
today through Friday. As trade winds rebuild today through
Friday, rough, short-period surf will briefly rise above the
summer average along east facing shores, then decline during the
weekend. Elsewhere, no significant swells are expected.
Small Craft Advisory until 6 PM HST Saturday for Alenuihaha
Channel-Big Island Leeward Waters-Big Island Southeast Waters.
Small Craft Advisory from 6 AM Friday to 6 PM HST Saturday for
Small Craft Advisory from 6 PM this evening to 6 PM HST Saturday
for Pailolo Channel.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office