Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 232028

Area Forecast Discussion
National Weather Service Honolulu HI
1028 AM HST Fri Mar 23 2018

Unsettled weather conditions are expected today through the
Saturday night as a series of low pressure troughs move through
the state. Heavy rainfall and thunderstorms will be possible at
times as these disturbances move through. Conditions are expected
to improve across the area on Sunday. A light wind regime will
allow for daytime sea breezes and overnight land breezes Sunday
through the middle of next week. This will allow for daytime
shower development over interior and leeward sections of the
islands, with showers hugging the coast at night. Light trade
winds may return late next week.


Currently at the surface, a trough of low pressure is beginning to
develop just south of the state, with another trough which has
lingered for several days now around 500 miles west of Kauai.
Meanwhile aloft, a strong shortwave trough can be seen in water
vapor imagery around 900 miles west-northwest of Kauai, and this
feature is tracking southeastward at around 35 mph. Satellite
imagery shows mostly cloudy to overcast conditions over the
smaller islands, with partly to mostly cloudy skies over the Big
Island. Radar imagery shows some light showers over the islands of
Kauai and Oahu, with mainly some light showers moving into
windward sections of all islands as well. Main short term concern
revolves around the potential for heavy rainfall and severe
weather over the next couple of days.

Rest of today,
Model solutions are in reasonably good agreement showing the
shortwave trough northwest of the state tracking southeastward
through the day, resulting in a sharpening up of the low pressure
troughs to the west and south of the state. The best forcing for
ascent is expected to hold off until tonight, so expect the shower
activity to remain rather light and focused primarily across
Kauai and Oahu through the day. Across Maui County and the Big
Island a few afternoon showers will be possible as well, but the
better rain chances are expected to hold off until tonight and
Saturday here.

Tonight through Saturday night,
The GFS and ECMWF are in good agreement showing a pair of low
pressure troughs shifting across the state. The first trough south
of the state is expected to sharpen up this evening with a weak surface
low developing just south of Oahu and Maui County by daybreak
Saturday. The models then show this weak low tracking eastward
near or over the Big Island Saturday afternoon into Saturday
evening, before exiting east of the state after midnight Saturday.
The other trough currently west of the state is expected to
approach from the west tonight, then shift eastward across the
smaller islands Saturday into Saturday evening.

Overall, we expect a fairly wet night on Kauai and Oahu, with
showers increasing across Maui County this evening as well. The
most persistent shower activity is then expected to shift from
west to east across the island chain Saturday and Saturday night,
exiting to the east of the state after midnight. There will likely
remain some lingering shower activity over the islands in
association with the secondary trough of low pressure Saturday
afternoon in Saturday evening however.

As far as flooding potential goes, the combination of deep
tropical moisture surging northward in advance of the pair of low
pressure troughs along with strong forcing for ascent, could
result in some heavy rainfall and thunderstorms across the
islands. The Flash Flood Watch remains in effect from 6 PM this
evening through 6 PM Sunday. The Watch may be able to be shifted
back to 6 AM Sunday as details on how the weather situation will
evolve become more clear.

In addition to the potential for heavy rainfall, there is a threat
for severe weather mainly across the Big Island as the lead
trough of low pressure moves through. Model solutions show plenty
of deep layer shear of 45-55 knots in the 0-6 km layer to support
organized convection. The limiting factor will be surface based
instability, with Most Unstable (MU) CAPE values in the 500 to
1000 J/KG range. The other factor that lowers confidence about
severe weather occurring is that the model solutions have shifted
quite a bit from the previous run regarding location where the
severe weather potential would be highest. That said, based on the
latest guidance, the highest probability of severe weather appears
to be across the Big Island, with damaging straight line winds of
60+ mph and isolated tornadoes being the main threats.

Finally, the summit weather on the Big Island will likely go down
hill beginning tonight. H6 winds are expected to increase
significantly tonight, and a High Wind Warning will likely be
issued this afternoon and run through 6 AM Sunday. Additionally,
with the deep moisture moving through, there is the potential for
heavy snow at the summits. This is a little uncertain given summit
level temperatures around or a little above freezing. Nevertheless
the potential appears to be there for a heavy snow evening, so a
Winter Storm Watch will likely be issued later this afternoon.

Sunday through next Friday,
Conditions should improve across the islands on Sunday as a
surface low intensifies north of the state. Over the islands a
weak wind regime will remain in place through the first half of
the week, with light trades returning by late next week. As a
result, we should see a more convective weather pattern featuring
daytime shower development over the interior and leeward sections
of the islands, with shower activity remaining offshore or near
the coast at night.


There is an upper level trough moving in from the west which is
already impacting western portions of the state with widespread
rain and cloud cover stretching from western Oahu to Kauai and
west. Expect MVFR ceilings/visibilities and mountain obscurations
for Kauai and Oahu through the morning with these conditions
gradually spreading ESE encompassing Molokai Lanai and Maui
through this evening and overnight tonight. Isolated periods of
IFR conditions may occur. AIRMET Sierra is in effect for mountain
obscurations for Oahu and Kauai due to the clouds and showers.

Temporary light in-cloud icing expected for Oahu and Kauai and
surrounding waters through at least the rest of the day.


The current large northeast swell continues to gradually decline,
and this is expected to continue through the weekend. A High Surf
Advisory remains posted for the east facing shores through this
afternoon, but may need to be extended through tonight. Will be
monitoring the nearshore buoys and surf reports today.

A Small Craft Advisory (SCA) remains posted for the windier areas
near Maui County and the Big Island through this afternoon. Winds
are expected to continue to trend down, so expect the SCA to be
cancelled with the afternoon package. Even though winds and seas
are expected to be below SCA levels starting tonight, mariners
should be aware of the threat of inclement weather this weekend.

An area of low pressure is expected to move across the state this
weekend, producing an extended period of rain, with a chance of
thunderstorms and locally heavy rainfall. The current forecast
continues to show a belt of localized south to southwest winds
approaching 25 knots developing along the eastern flank of this
low. Should this pan out, we may need to re-post an SCA Saturday
for portions of the waters adjacent to the western islands. Once
this low begins to depart the state, expect westerly winds to
spread across the islands from Saturday night through Sunday,
followed by light and variable winds early next week.

Once the current northeast swell subsides this weekend, no other
significant swells are expected through early next week. A modest
north-northeast swell is expected from Saturday night into Monday.
A small northwest swell is also expected from Sunday night
through Tuesday, followed by another small northwest swell during
the middle of next week. Small south-southwest and southwest
swells are also expected from today into early next week.


Flash Flood Watch from 6 PM HST this evening through Sunday
afternoon for all Hawaii islands.

High Surf Advisory until 6 PM HST this evening for Kauai
Windward-Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-
Windward Haleakala-Big Island North and East.

Small Craft Advisory until 6 PM HST this evening for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.



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