Flash Flood Warning 23, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
900 PM HST Fri Mar 23 2018

.SYNOPSIS...
Unsettled weather conditions are expected 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.

&&

.DISCUSSION...
A strong high pressure center remains northeast of the islands
with a low north-northwest of the state. An upper level trough
will continue to move into the islands from the west deepening a
surface trough into a surface low south of the islands. Otherwise
tropical moisture will continue to stream northward across the
state with increasing showers from Kauai to Maui expected through
Saturday morning. The deep moisture and showers will continue to
spread eastward to the Big Island on Saturday with moderate to
heavy rainfall and some thunderstorm activity.

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.

&&

.AVIATION...
With surface and upper air troughs approaching the state from the
west and a good supply of low level moisture, broken to overcast
multilayered cloudiness and precipitation, and possibly isolated
thunderstorms, will be affecting the islands over the next day or
two. Most of the inclemency, including widespread MVFR conditions
and pockets of IFR, will be moving through Kauai and Oahu initially
and will then move down the chain thereafter.

AIRMETs for mountain obscuration are currently in effect for
Kauai, Oahu, and Molokai; and eastern portions of the Big Island.

Temporary light in-cloud icing expected for Oahu, Kauai, Molokai,
and surrounding waters through tonight, and then likely expanding
to the east later.

&&

.MARINE...
The nearshore buoys indicate the northeast swell continues to
slowly decline this evening, with surf along east facing shores
expected to remain below High Surf Advisory levels. The current
northeast swell will continue to subside this weekend, and 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.

Even though winds and seas are expected to remain generally
below Small Craft Advisory (SCA) levels overnight, mariners
should be aware of the threat of inclement weather overnight and
through this weekend, with locally strong winds and high seas due
to possible thunderstorms.

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.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Saturday afternoon for Niihau-Kauai-
Oahu-Molokai-Lanai-Kahoolawe-Maui.

Flash Flood Watch from 6 AM HST Saturday through late Saturday
night for Kona-South Big Island-Big Island North and East-Kohala-
Big Island Interior.

High Wind Warning until 6 AM HST Sunday for Big Island Summits.

Winter Storm Watch from 6 AM HST Saturday through late Saturday
night for Big Island Summits.

&&

DISCUSSION...Bohlin/Jelsema
AVIATION...Kinel
MARINE...Jacobson