Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 241345

Area Forecast Discussion
National Weather Service Honolulu HI
345 AM HST WED AUG 24 2016

Deep tropical moisture moving over the state will maintain very
humid conditions with localized heavy downpours and a chance of
thunderstorms through today. Drier trade wind weather will begin
to overspread the state from east to west tonight and Thursday,
with typical trade wind weather then expected Thursday night
through early next week.


Currently at the surface, a weak trough of low pressure extends
from around 80 miles west of the Big Island northwestward through
eastern sections of Oahu and out over the Kaiwi channel. This
trough is resulting in frequent heavy showers over the island of
Oahu, with more scattered shower activity over the other islands.
Main short term concern for today revolves around the potential
for flooding.

Deep layer ridging will begin to build well north of the islands,
and this should allow some drier air to work its way into the Big
Island by late in the day. Model solutions still show very high
precipitable water values of 1.8 to 2.2 inches remaining in close
proximity to the surface trough over the smaller islands through
the day, so some locally heavy rainfall will be possible here.
Additionally, the atmosphere will remain unstable given surface
dewpoints in the middle 70s, so afternoon convection is likely in
all areas. As a result, a slight chance of thunder will remain in
the forecast for all islands through the day. Given the recent
heavy rainfall and elevated stream levels, the flash flood watch
will remain in effect through 6 pm this evening.

Tonight and Thursday,
The trade winds will begin to overspread the state from east to
west as strong high pressure builds well to the north-northeast of
the island chain. This will shift the deepest tropical moisture
over Kauai tonight and west of the state during the day on
Thursday. As a result, we should see some enhanced showers over
Kauai tonight, with more of a typical windward/mauka shower
pattern developing across the other islands. On Thursday, more
typical trade wind weather will settle over the entire island
chain with showers favoring windward and mauka locales.

Thursday night through next Tuesday,
High pressure well north of the state will keep a moderate trade
wind flow in place through the period, with clouds and showers
focused primarily across windward and mauka locales. Both the 00z
GFS and 00z ECMWF show the potential for some tropical cyclone
activity moving into the central pacific by early next week.
Although the model solutions show any tropical systems remaining
well to the east of the islands through Tuesday, we will need to
closely monitor the areas of disturbed weather currently in the
eastern Pacific, for tropical development.


Pockets of heavy showers and thunderstorms prevailed across Kauai,
Oahu, and the east facing side of the Big Island overnight. This
activity is still ongoing at the time of this writing. AIRMET Sierra
for mtn obsc is currently in place for these three places that will
likely be extended to 20z. We expect an active afternoon as this
slow-to-clear air mass gets added help from daytime heating. So
AIRMET Sierra can easily be extended into the afternoon as well as
expanded to cover the other islands. Otherwise, VFR rules.

We anticipate a drier and more stable air mass to start spreading
across the islands from the east this evening, that will result in
the cessation of cb and heavy showers and a return to a more typical
trade wind pattern.


High pressure will build well to the north of the state today,
with moderate to locally strong trades building into the eastern
islands tonight and into the western islands on Thursday. As a
result, small craft conditions are expected to develop over the
windier waters around Maui and the Big Island tonight, with winds
remaining at advisory levels here through at least Thursday
night and likely right through the upcoming weekend. No
significant surf is expected through the week.


Flash Flood Watch through this afternoon for all Hawaii islands.

Small Craft Advisory from 6 PM this evening to 6 AM HST Friday
for Maalaea Bay-Pailolo Channel-Alenuihaha Channel.



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