NWS Honolulu, HI wrote its last area forecast discussion before the first event at Feb 15, 4:06 AM HST, 1 hour 54 minutes earlier.
Area forecast discussion, Feb 15, 4:06 AM HST, 1 hour 54 minutes before
Area Forecast Discussion
National Weather Service Honolulu HI
406 AM HST Wed Feb 15 2023
.SYNOPSIS...
A kona low about 300 miles north of the Big Island will move
steadily toward the west over the next several days, strongly
affecting island weather. Locally breezy and cool north winds will
gradually weaken today, then shift to the south tonight. A
significant moisture surge will move over the islands Thursday with
southeast winds, and remain in place through at least Saturday. This
moisture will fuel what is expected to be a prolonged period of
rainfall, with heavy showers and thunderstorms potentially affecting
both windward and leeward areas of all islands, with impactful
flooding possible. A Flood Watch is in effect from Thursday morning
through Saturday afternoon.
&&
.DISCUSSION...
The center of a kona low is about 300 miles N of the Big Island this
morning - after having moved very little overnight. Widespread
layered clouds with embedded showers and thunderstorms prevail to
the N and E of the low's center, while scattered to broken showery
low clouds prevail S and W of the center, and upwind of the islands
from Kauai to Maui. The gradient between the low and a strong
surface high about 1000 miles NW of Kauai is supporting moderate to
locally breezy N to NE winds over Kauai and Oahu, while winds are
diminishing over Maui County and the Big Island. Morning soundings
show that the island atmosphere remains unstable, with relatively
low PWAT between 1 and 1.2". Radar is again peppered with scattered
to numerous showers, especially N and NE of Kauai and Oahu, with
some heavier rain cores embedded. These showers have been
persistently dampening large portions of Kauai and Oahu overnight,
especially windward, but showers spilling over the Koolau have been
affecting urban Honolulu as well as other leeward areas. Fewer
showers are approaching Maui County in a diminishing NW to N flow.
Winds over the Big Island have weakened, which has allowed land
breezes and mostly clear skies to develop in most areas. However,
some briefly heavy showers and an isolated thunderstorm formed
overnight along weak eddies in the near shore waters S and W of the
island, and some soaking showers managed to move ashore near
Hawaiian Ocean View Estates and Milolii.
The short-term forecast anticipates light and variable winds over
the Big Island today as the kona low passing to the N cuts off the
winds. A mostly sunny morning will be followed by afternoon sea
breezes that spawn upslope clouds and showers, and possibly some
thunderstorms. Light to moderate winds will shift to the SE and S
tonight, beginning what is expected to be an extended period of deep-
layer moisture advection over the area, with a significant surge
arriving from the SE early Thursday morning.
The same general pattern with respect to the wind flow and moisture
advection will play out on the other islands as the low moves W,
with some timing differences. Moderate to breezy N to NE winds on
Oahu (Kauai) will diminish this morning (evening), becoming light
and variable by the afternoon (evening). This could allow sea breeze
circulations to drive heavy shower and thunderstorm formation over
the interior and/or S portions of Oahu and Maui County this
afternoon. The atmosphere will remain unstable due to the mid-level
reflection of the passing kona low bringing steep low- and mid-level
lapse rates. By tonight, the low will be passing N of the Kauai, and
SE to S winds will be developing over all islands. Moisture will
surge NW over the smaller islands on Thursday.
As the kona low moves W and further away from the islands, model
guidance has been persistently indicating that a slow-moving low-
level convergent band will set up over the island chain, affecting
all islands from Thursday until at least Saturday. Strong E to SE
winds will feed deep moisture into the band, with the potential for
heavy rain to become terrain-anchored, or repeatedly train over the
same areas. Difficult to know how windy it will get over land in
this regime, as the smaller islands typically lie in the shadow of
the Big Island in SE flow, but there could be periods of gusty
winds, especially in showers.
The mid-level low and associated instability will shift W of the
islands Friday and Saturday, but strong divergence aloft and PWAT
surging to nearly 2" means that there will be a slight chance for
thunderstorms, which could also bring strong and erratic winds. A
Flood Watch remains in effect for all islands starting Thursday
morning. Now is the time to prepare and/or adjust outdoor plans as
heavy rainfall could affect both leeward and windward areas.
Deep moisture and cold temperatures aloft mean wintry weather for
the Big Island summits and upper slopes, and a Winter Storm Watch is
in effect starting Thursday. Periods of freezing rain, sleet, heavy
snow and gusty winds will produce hazardous driving conditions and
dangerously low visibility for several days.
Long range guidance indicates that the convergent band will be slow
to move through the weekend, with the potential for another kona low
to develop early next week. This would keep the islands in an
extended period of unsettled weather although there may be some
breaks.
&&
.AVIATION...
A low pressure system north of the state will slowly track westward
over the next 24-hours. This feature will bring unsettled weather to
the islands for the next several days.
Moderate north northeasterly trades will give way to mainly light
winds by this afternoon and allow for the development of sea
breezes.
This pattern will likely bring scattered shower and isolated
thunderstorm to all islands. MVFR ceilings and visibility may
accompany some of the more robust shower and storm activity due to
low cloud bases and reduced visibility.
AIRMET Tango remains in effect for moderate turbulence between FL250
and FL350 across the state due to the subtropical jet over Hawaii.
The jet will remain over the islands until at least Thursday.
AIRMET Sierra remains in effect for the north through east sections
of Maui, Oahu, Kauai, and Molokai above 2500 feet due to low clouds
and showers.
&&
.MARINE...
A kona low with gale force winds, currently about 220 nm northeast
of Maui, will slowly pass over offshore waters north of the state
through Wednesday night, then drift to the west Thursday and Friday.
Fresh to strong northeasterly winds, which have already eased around
the Big Island and Maui County, will lower around Oahu today, and
Kauai tonight as the low passes to the north. A Small Craft Advisory
(SCA) remains in effect for waters around the Maui County Windward
Waters, Oahu, and Kauai due to a combination of the winds and large
seas. The large seas are mainly short period northeast swell from
the developing low. This SCA may be dropped Wednesday night as winds
and seas briefly diminish.
However, as the low moves off to the west and a strong surface high
pressure system develops to the northeast on Thursday, fresh to
strong southeast winds and large wind waves will develop, likely
requiring a new SCA for most waters into the weekend. In addition to
the hazardous winds and seas, heavy rainfall and thunderstorms will
be possible.
A High Surf Advisory (HSA) remains in effect for east-facing shores
of Kauai and Oahu through Wednesday afternoon due to short period
northeast swell. East shore surf will briefly subside Wednesday
night, but as southeast winds increase Thursday and Friday, large
and rough surf will return, possibly reaching the advisory level
again. The current, small northwest swell will subside tonight.
Another northwest swell will build Wednesday, peak below advisory
levels Wednesday night, and decline Thursday.
South facing shores will experience minimal surf this week.
&&
.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch from Thursday morning through Saturday afternoon for
all Hawaiian Islands.
High Surf Advisory until 6 PM HST this evening for east facing
shores of Kauai and Oahu.
Winter Storm Watch from Thursday morning through Saturday
afternoon for Big Island Summits.
Small Craft Advisory until 6 PM HST this evening for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-Oahu
Windward Waters-Kaiwi Channel-Maui County Windward Waters.
&&
DISCUSSION...Birchard
AVIATION...Thomas
MARINE...Wroe