Flash Flood Warning 10, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
328 AM HST Fri Feb 17 2023

.SYNOPSIS...
Light to moderate rain may be locally heavy at times over Kauai
and Oahu while periods of heavy rain will result in much greater
flash flood potential over windward and southeast-facing slopes
of the Big Island and Maui. Showers will decrease in coverage over
western areas by tonight, but will continue over favored portions
of Maui and the Big Island well into next week.

&&

.DISCUSSION...

Satellite imagery shows an upper low centered just west of Kauai,
the attendant surface low orphaned and drifting westward as it spins
down. Deep southeasterly flow around the eastern flank of this low
has drawn north a plume of tropical moisture characterized by PWATs
around 2". Most of the overnight period has featured steady light to
moderate stratiform rain with a few embedded convective elements and
12-hour rainfall totals as of 2am generally about 1.5" or less
everywhere. Thunderstorms have been common in the vicinity of Kauai
where steep lapse rates exist near the center of low, but this
unstable mid-level airmass will only tend to drift further from the
islands and modify which will significantly limit thunderstorm
potential going forward. Of note from last night, a 47kt wind gust
was recorded at Barking Sands on Kauai, the result of enhanced
downsloping easterlies around a mesoscale low that was observed on
radar. This was highly localized and transient and is unlikely to
materialize elsewhere.

With the low-level flow finally veering to the southeast, rain has
filled in over the southeast slopes of the Big Island, though rates
mostly remain under 1"/hr at press time. It is worth noting that as
of 2am HST this morning, hyperactive convection in the hi-res suite
has caused those models to overestimate 12-hour rainfall over
southeast and windward sections of the Big Island by an average
factor of around 3-5x and locally as much as 10x. The same can be
said for the HREF.

IR imagery during the last few hours does show a distinct erosion of
cold cloud tops occurring around 300 miles east of the Big Island
while cloud top cooling is observed within the moisture plume
itself. This is visual confirmation of model depictions of upper-
level divergence developing in response to an amplifying
downstream ridge, and it will offer some background synoptic
support to ongoing rainfall. However, the lack of a large scale
mechanism for organization, such as a progressive shortwave aloft
or a cold front, means heavy rain potential will be almost
entirely reliant on orographic forcing. For this reason, shadowed
southeast flow downstream of the Big island is making for a rather
tenuous Flood Watch for Kauai, Oahu, and Molokai. Some island
interaction was observed overnight and probably represents the
most realistic flooding scenario for these islands. If said threat
fails to materialize today, then consideration will be given to
canceling the watch for western portions of the state.

For windward & southeast Big Island and Windward Maui, rain will
be essentially constant through at least Saturday. Rainfall
intensity has underachieved thus far, though this is at least
partially due to delay in low-level winds veering to
southeasterly. However, poor lapse rates and weak to moderate mid-
level static stability are also contributing factors, and they
will not improve during the course of the event. Nonetheless, with
the orographic contribution now in full swing and the imminent
the onset of diurnal heating, an increase in rates is forecast
during the next several hours. Strengthening southeasterlies are
also forecast to develop today as surface high pressure builds to
the northeast. The convergent lead edge of the building high may
give a boost to showers over east-facing zones. The GFS and the
high-res HRRR both key in on this potential this morning. Forecast
rainfall amounts have been lowered for the balance of the event
and the footprint of the heaviest rain through the near term has
been refined to focus more over interior and upslope zones. The
Flood Watch itself remains in effect and is especially good shape
over the eastern end of the state with several waves of rain
expected during the next 48 hours.

The deep southeast flow pattern will be a fixture through the middle
of next week. Even as large scale forcing diminishes and showers
become less organized by late this weekend, the resident band of
tropical moisture will stagnate over or near the area. Consensus
among the global models remains in tact that a deep mid-latitude
trough will develop to the west early next week. This will in
turn re-activate the moisture band further west (i.e. around
Kauai) and strengthen low-level southeasterlies bringing another
round of rain to windward and southeast-facing slopes of the Big
Island and Maui.

&&

.AVIATION...
A low pressure system southwest of Kauai will continue to move
westward and away from the state over the next 24-hours. However,
in its wake, trailing moisture and instability will lead to
numerous showers and isolated thunderstorms areawide. Periods of
MVFR conditions, due to lowered cloud bases and reduced
visibility, will likely accompany any of the heavier showers or
storms.

Moderate southeasterly flow will persist through the forecast
period.

AIRMET SIERRA remains in effect for mountain obscuration for the
Big Island, Maui, Molokai, and Oahu above 2000 feet due to low
clouds and showers.

AIRMET TANGO remains in effect over all islands for tempo
moderate turbulence between FL120 and FL420.

AIRMET ZULU remains in effect above all islands for moderate rime
ice in precipitation in layer FL120 to FL250.

&&

.MARINE...
Strong high pressure will remain anchored northeast of
the state through the middle of next week. This will keep
persistent strong to near-gale force east to southeast winds in
place over most windward waters through the middle of next week,
with lighter winds over the leeward waters. A Small Craft Advisory
(SCA) remains in effect through 6 PM Saturday for all windward
waters due to winds/seas or a combination of the two. This SCA
will likely need to be extended for most windward waters through
the middle of next week.

No significant northwest swells are expected during the next 7
days, with only small surf expected along north and west facing
shores. East shore surf will build today as east-southeast winds
strengthen. A High Surf Advisory remains in effect from 6 AM
this morning until 6 PM Saturday, and this advisory may need to
be extended through the middle of next week as winds over and
upstream of the islands will continue to support large and rough
surf. Elevated early morning high tides may bring some seawater
onto vulnerable coastal roadways this weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Saturday afternoon for all Hawaiian Islands.

High Surf Advisory through Saturday afternoon for east-facing
shores of Kauai, Oahu, Molokai, Maui, and Big Island.

Winter Storm Warning through Saturday afternoon for Big Island
Summits.

Small Craft Advisory until 6 PM HST Saturday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Channel-Oahu Windward Waters-
Kaiwi Channel-Maui County Windward Waters-Pailolo Channel-
Alenuihaha Channel-Big Island Windward Waters-Big Island Southeast
Waters.

&&

DISCUSSION...JVC
AVIATION...Thomas
MARINE...Jelsema