Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 190153

Area Forecast Discussion
National Weather Service Honolulu HI
353 PM HST Mon Feb 18 2019

A wet weather pattern is setting up over Maui County and the Big
Island over the next couple of days as tropical moisture moves up
from the southeast. An increase in showers, some heavy at times
along with scattered thunderstorms is expected over the eastern
islands through Tuesday night. Showers will be less likely and
lighter over the west end of the state. Decreasing shower trends
are forecast for most of the island chain late in the week and on
into the weekend as we gradually transition back into a trade wind
weather regime.


A broad upper level low is centered just to the west northwest of
Kauai with an associated surface low centered a few hundred miles
north northwest of Kauai. Bands of showers associated with weak low
level troughs are seen on satellite imagery in the island vicinity.
One band is centered to the south Kauai. Showers from this band
have remained offshore. A broader area of showers is currently
located south and east of the Big Island. This area of enhanced
moisture will be the main weather player for the next couple of
days especially for Maui County and the Big Island.

For the eastern half of the state, shower activity will gradually
increase, with wet weather expected over Maui County and the Big
Island. This wet weather pattern developing over the eastern islands
will arrive in the form of a deep tropical moisture plume moving
in from the southeast. Expect frequent showers, locally heavy at
times, with scattered thunderstorms developing under this band
from tonight through Tuesday. A Flash Flood Watch is in effect
from through Tuesday night. Temperatures aloft will be cold enough
for snow showers on the highest summits of Mauna Kea and Mauna
Loa where a Winter Storm Warning is in effect beginning tonight. A
Wind Advisory is also in effect for the Big Island summits.

For the western end of the state, some showers are possible under
a light and variable wind regime through Tuesday night. Showers
would tend to favor interior and mauka areas but other areas could
see some shower activity as well. Most of these showers are
expected to be on the lighter side.

On Wednesday the upper low will weaken and lift out to the
northeast and be replaced by more of a zonal flow across the area.
The low level trough however is expected to linger near or just
east of the Big Island.

Wednesday night through Friday, some minor differences show up in
model solutions. Both models show weak high pressure building in
to our north and northwest. This will allow light northeast trades
to fill back in from Maui County westward. However the position
of the low level trough is closer to the Big Island in the GFS
than the ECMWF which shows the trough axis just east of the Big
Island. Thus the GFS solution keep abundant moisture over the Big
Island while the ECMWF is a tad drier over them with the higher
moisture being to the east.

For next weekend, both models show a slighter stronger high
building in northwest of the area with a slight uptick in trade
wind speeds expected especially over the west end of the state.
The GFS remains the wetter solution with leftover moisture from
the low level trough getting picked up in the trades providing for
increased windward and mauka shower activity. The ECMWF remains
the drier solution. Thus a typical trade wind shower regime is
currently forecast with details on shower frequency still in question.


The air mass over the Big Island and Maui County is gradually
becoming moist and unstable. The cool relatively stable air mass
over Kauai and Oahu will hold overnight. By Tuesday afternoon
deep tropical moisture should encompass the whole of the state,
but the greatest instability will remain east of Oahu.

A band of layered clouds with embedded deep convection to the
south and east of the Big Island this afternoon will spread across
the Big Island this evening and across Maui County by mid morning
Tuesday. Lightning, heavy downpours, and IFR conditions are
associated with this feature.

The AIRMET for mountain obscuration for eastern slopes of the Big
Island late will likely be expanded to included the whole of the
Big Island by late this evening and possibly Maui shortly

Meanwhile, across the smaller islands this afternoon, a weak sea
breeze regime has been able to establish itself. Ragged clouds and
isolated showers will favor island interiors through early
evening then east and southeast slopes and coasts after sunset.
Expect layered high clouds to precede the band of active
showers moving in from the southeast.


A trough will begin to move into the waters around the Big Island
tonight and Tuesday, then linger in this vicinity through late
this week. Winds for the most part will be out of the east to
southeast at 10 to 15 knots through Tuesday and then become
variable winds Tuesday night through Wednesday night. The one
exception, is for areas east and southeast of the Big Island
along the edge of the coastal waters, this area may see moderate
to strong south to southeast winds Tuesday through Tuesday night.
Weak high pressure is expected to develop north of the state late
this week and we should see the return of light northeast winds.

As the current north-northeast swell continues to decline
tonight along north and east exposures, a new short-period east
swell will begin to build along east facing shores of the Big
Island on Tuesday, and late Tuesday into Tuesday night for the
smaller islands. The swell is expected to peak on Wednesday and
gradually decline Thursday through rest of the week. Due to the
new incoming east swell, the High Surf Advisory (HSA) for east
facing shores was extended through Wednesday night. The HSA for
north facing shores of Kauai and Niihau has also been extended
through tonight.

A Small Craft Advisory (SCA) is in effect through tonight for
most windward marine zones and channels due to seas at or above
the 10 foot SCA threshold. Although many marine zones will drop
below SCA thresholds by Tuesday, a new easterly swell may keep
SCA level seas in place for some eastern and windward waters
Tuesday through Wednesday night.


High Surf Advisory until 6 AM HST Tuesday for Niihau.

High Surf Advisory until 6 AM HST Thursday for Kauai Windward-
Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

Flash Flood Watch through Tuesday afternoon for Molokai-Lanai-
Kahoolawe-Maui-Kona-South Big Island-Big Island North and East-
Kohala-Big Island Interior.

Wind Advisory until 6 AM HST Wednesday for Big Island Summits.

Winter Storm Warning until 6 AM HST Wednesday for Big Island

Small Craft Advisory until 6 AM HST Tuesday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters-
Alenuihaha Channel-Big Island Windward Waters.



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