Flash Flood Watch 3, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
1008 AM HST Tue Feb 26 2019

.SYNOPSIS...
A slow-moving band of low level moisture currently draped across
Maui County will be the focus for unsettled weather tonight and
Wednesday. For today, expect continued windward rainfall on Maui
and Molokai with decreasing shower activity across windward and
north slopes of Oahu. A strong upper level trough will pass over
the state tonight, triggering heavy showers and thunderstorms
within the moisture band across Maui County and the Big Island.
On Wednesday, building high pressure to the northwest of the
islands will produce gusty north winds and cool conditions as the
focus for unsettled weather shifts to the Big Island. Cool
northerly winds and light showers on north facing slopes are
expected Thursday into the weekend.

&&

.DISCUSSION...
Somewhat wet conditions are currently in place for the central
islands while rather dry weather prevails on Kauai and the Big
Island this morning. A shallow band of low level moisture with
precipitable water (PW) values around an inch is draped over Maui
County and gradually diminishing on Oahu. Although PW in the band
is near seasonal normal, the feature was quite an efficient rain
maker overnight, dropping nearly 1.5 inches of rainfall across
some windward areas and north facing slopes of Oahu and Molokai in
the past 12 hours. Moisture along the trailing edge of the band
remains hung up on Oahu but has cleared Kauai, where limited
shower activity is occurring in a building northeast flow. On the
Big Island, mostly clear conditions prevailed ahead of the
moisture band. The atmosphere remains stable this morning, thanks
to a mid level ridge overhead, but a potent mid to upper level
trough fast approaching from the northwest is rapidly eroding the
ridge.

Wet conditions will persist for mainly windward and north facing
slopes of Maui County and, to a lesser degree, on Oahu, while
drier weather continues elsewhere. The moisture band will move
little today, keeping showers focused in current areas. Afternoon
sea breezes will likely produce some afternoon clouds and spotty
showers for leeward areas of mainly Maui County. Though not
seemingly likely at this time, there is a low chance that the
approaching shortwave could trigger some heavy rainfall along the
band this afternoon. Across the rest of the state, moderate
northeast winds on Kauai will produce showers on windward slopes,
while the Big Island experiences interior afternoon clouds and a
few showers.

Unsettled weather is due along the band of low level moisture
tonight and Wednesday as the potent mid to upper level trough
nears then moves over the state. Heavy showers and thunderstorms
are expected to develop on and around Maui County this evening.
Some of these thunderstorms could be strong, since 0-6 km shear
will be quite high. The low level band will be nudged
southeastward tonight as strong high pressure northwest of the
state pushes a shallow front toward the islands. By late tonight,
the focus for heavy showers and thunderstorms will shift to the
Big Island and remain into Wednesday. While there is a chance for
flooding for the eastern end of the state, the duration of this
event may be limited. We will take a close look at the need for a
Flash Flood Watch today. Away from the moisture band, expect
building northeasterly winds and periods of showers along windward
and north facing slopes. A few of these showers cloud be briefly
heavy.

There is potential for winter weather for the highest elevations
of the Big Island and possibly Maui. The probability of wintry
precip appears lower across the highest elevations of Haleakala,
as temperatures should remain above freezing, keeping any precip
in the form of rain. A Winter Storm Watch for the Big Island
summits continues as there remains the potential for tonight and
Wednesday. In addition, a High Wind Warning is in place for the
Big Island summits tonight through Wednesday.

Model solutions are in fairly good agreement through the extended
periods, with some minor differences noted late in the week into
the first half of the weekend. Both the GFS and ECMWF show a deep
closed low northeast of the islands Wednesday night lifting
northeastward through the end of the work week, while high
pressure northwest of the state weakens as it shifts southward and
closer to the island chain. Locally breezy northerly winds are
expected to continue Wednesday night, with the winds then slowly
easing through the remainder of the work week. The models show
another strong upper trough digging north of the islands over the
weekend, leading to another closed low developing northeast of the
state Sunday through early next week. The gradient will remain
fairly loose across the islands on Saturday, with moderate to
breezy northerly winds then filling in once again Saturday night
through early next week, as a new high builds northwest of the
state.

As for weather details, given the dry low level airmass building
in over the island chain with surface dewpoints falling well down
into the 50s and perhaps into the upper 40s, along with precipitable
water values at or below 1 inch, showers should remain light and
confined mainly to north facing slopes and coasts through the
period. It will remain quite cool through the period as well, with
highs generally in the 70s and lows in the 50s and 60s.

&&

.AVIATION...
A band of moisture draped across the central portion of the
state and strengthening north northeast winds will focus clouds
and showers over windward and north facing slopes of Oahu, Molokai
and Maui at least through the morning. AIRMET Sierra is in effect
for tempo mountain obscurations across windward sections of Oahu,
Molokai and Maui. The rest of the state is partly cloudy to mostly
sunny.

Cloud and shower coverage may increase over the Big Island today
with the onshore flow, and potentially again late tonight for the
central and eastern part of the state as the moisture band becomes
enhanced by a passing front.

&&

.MARINE...
A surface cold front, located northwest of Kauai this morning, is
moving toward the east-southeast. As this feature moves down the
island chain from late tonight through Wednesday, the winds will
shift out of the north, and strengthen. The seas will also
increase ahead of the front, reaching the Small Craft Advisory
(SCA) criteria. Seas are near the SCA criteria near Kauai and
will increase to the SCA near Oahu and Maui County windward waters
starting this evening. The SCA conditions are then expected to
spread to the Big Island windward waters by late tonight. Also, a
slight chance of thunderstorms has been included in the forecast
for tonight across the waters around Maui County and the Big
Island due to an upper- level trough destabilizing the atmosphere
as it passes over the region.

The winds are forecast to shift out of the north-northwest from
late Wednesday through Thursday as a surface low pressure area
deepens to a gale force system far northeast of Honolulu. The
stronger winds and elevated rough seas will likely result in SCA
conditions for additional Hawaiian Waters from Wednesday night
through Thursday. Winds and seas are forecast to trend down from
late Thursday night through Friday as the low pressure system
tracks away from the area.

Forerunners from a large long-period northwest swell have arrived
at the buoys northwest of Kauai. These initial waves will soon
begin to spread down the island chain. This swell will gradually
build from tonight through the day Wednesday, and it is expected
to peak Wednesday night. Surf will likely reach the High Surf
Advisory (HSA) thresholds along exposed north and west facing
shores of Kauai County starting this afternoon. Therefore, an HSA
has been issued starting at noon HST today for north and west
facing shores of Kauai and Niihau. The HSA will likely be needed
for north and west facing shores of Oahu and Molokai, and for
north facing shores of Maui starting tonight. The gale force winds
associated with the low pressure system to our northeast will
send a large short-period north swell down across the state from
late Wednesday through Thursday. Surf heights may approach the
High Surf Warning criteria along some north and west facing shores
starting late Wednesday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory from noon today to 6 PM HST Wednesday for
Niihau-Kauai Windward-Kauai Leeward.

High Wind Warning from 6 PM this evening to 6 PM HST Wednesday
for Big Island Summits.

Winter Storm Watch from 6 PM HST this evening through Wednesday
afternoon for Big Island Summits.

Small Craft Advisory from midnight tonight to 6 PM HST Wednesday
for Alenuihaha Channel-Big Island Windward Waters.

Small Craft Advisory until 6 PM HST Wednesday for Kauai
Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters.

Small Craft Advisory from 6 PM HST today to 6 PM HST Wednesday
for Kauai Channel-Oahu Windward Waters-Kaiwi Channel-Maui County
Windward Waters.

&&

DISCUSSION...Wroe/Jelsema
AVIATION...Foster
MARINE...Kinel