Drought in Hawaii, December 1 to 20, 2016

Area Forecast Discussion
National Weather Service Honolulu HI
941 PM HST Wed Nov 30 2016

.UPDATE...
Strong sustained winds and wind gusts continue to be reported at
some mesonet sites around the state this evening, and we have
extended the wind advisory through 6 AM Thursday morning. No other
changes to the existing forecast.

&&

.SYNOPSIS...
A strong high far northeast of the area will lead to breezy trade
winds through Thursday, while a trough aloft will keep the
atmosphere unstable through Friday. Windward areas will remain
quite wet, with showers spreading leeward over the smaller
islands. Locally heavy rains with a slight chance of thunderstorms
are forecast from late Thursday through Friday. Another trough
aloft will approach from the northwest Friday, then linger west of
the state through the weekend. This trough will usher in a period
of southeast winds and wet weather through the weekend and into
early next week.

&&

.PREV DISCUSSION.../ISSUED 831 PM HST Wed Nov 30 2016/

Breezy trade winds prevail over the Aloha State on this Wednesday
evening, with active showers moving over windward areas of Kauai,
Maui and the Big Island. Showers have been less active during the
past few hours over Molokai and Oahu. Rainfall amounts have
remained light to moderate due to fast movement of the showers,
with gauges reporting six-hourly amounts generally one half inch
or less, with a few spots receiving up to one inch. Surface
analysis shows a 1033 mb high centered far northeast of the
state, with a tight pressure gradient over the islands. Aloft,
water vapor imagery shows an upper level low centered a couple
hundred miles southwest of the Big Island, with a ridge centered
well to our north and northeast. The 00Z soundings show an
unstable but not especially moist profile. While there is no
significant inversion evident, PW values range from 1.1 inches
at Lihue to a slightly above normal reading of 1.39 inches at
Hilo. MIMIC-TPW satellite imagery shows a similar airmass around
the state, with estimated PW values around 1.1 inches upstream of
the smaller islands, and values around 1.3-1.5 inches around and
upstream of the Big Island. An area of greater moisture with
estimated PW values up to 1.7 inches is lurking a few hundred
miles southeast of the Big Island.

Evening mesonet obs continue to show sustained winds or wind gusts
near advisory levels at a few locations from Oahu through the Big
Island. The existing wind advisory expires at 10 pm, and will
evaluate obs and incoming model guidance to see if this will need
to be extended through the remainder of the night.

Otherwise, through the overnight we expect rather wet conditions
with frequent showers to continue windward, and some scattered
showers spilling over into leeward areas of the smaller islands.
Some locally moderate or even briefly heavy rains will be
possible, but radar trends this evening suggest that fast movement
of the showers should prevent excessive rainfall accumulations
from occurring at any given location.

Thursday through Friday, the upper low near the Big Island is
forecast to drift slowly to the north, while another longwave
trough digs from the mid-latitudes to our northwest. Unstable
conditions aloft will remain in place, while models show the
greatest lower/mid level moisture convergence focused over the
eastern end of the state. There will be a slight chance of
thunderstorms from late Thursday into Friday for the entire
state, with the apparent highest chances over the eastern islands.
Chances for heavy rainfall still appear greatest over the Big
Island, and a flash flood watch remains posted there from Thursday
afternoon through Friday. There also appears to be sufficient
deep moisture/lift and cold air aloft present to generate some
potentially significant snowfall amounts over the Big Island
summits, and a winter storm watch remains posted from Thursday
afternoon through Friday. If current model trends continue, this
watch will probably need to be upgraded to a winter storm
warning with the Thursday morning forecast package.

There continues to be greater uncertainty as we head into the
weekend. Both GFS and ECMWF show an amplifying longwave trough
digging west of the state but diverge with its strength. The new
01/00Z GFS remains stronger and develops a cutoff low aloft that
eventually develops into a Kona low at the surface. The previous
30/12Z ECMWF is weaker with the upper level trough but still
develops an inverted surface trough west of the state. While
there continue to be significant differences with the strength of
the developing system, the models do remain in agreement with it
remaining just west of the state. This would put us into a
southeasterly or southerly flow and in the favorable quadrant
of the developing system for widespread and potentially heavy
rainfall. The interactions between the existing low near the
state and the digging trough are difficult for the computer models
to handle, and some changes are likely over the next few model
cycles. However, right now it appears that the greatest chance
for widespread rain will be Sunday into Monday, with yet another
surge of moisture/rain possible later next week as well.

&&

.AVIATION...
High pressure northeast of the state will keep breezy to locally
strong trades in place through Thursday, with clouds and showers
favoring windward and mauka areas. Due to the strength of the
trades, however, showers will occasionally reach leeward areas of
the smaller islands and West Maui. MVFR cigs/vsbys will
occasionally impact windward areas through 02/00Z. Elsewhere,
predominantly VFR conditions are expected.

AIRMET Sierra remains in effect for mountain obscuration across
windward sections of Kauai, Maui, and the Big Island. These
conditions will likely continue through the night.

AIRMET Tango remains in effect for low level turbulence over and
downwind of the terrain. These conditions are expected to continue
through the night.

&&

.MARINE...
A strong high to the northeast will maintain strong to gale force
northeast to east trade winds over the coastal waters overnight.
A Gale Warning is in effect for the windiest zones around the Big
Island and Maui through tonight, with a Small Craft Advisory for
the rest of the coastal waters. A small craft advisory will also
replace the gale warning once it is discontinued. Winds will start
to weaken Thursday/Thursday night, then shift out of the southeast
starting Friday as a surface trough develops west of the area.

The strong trade winds will produce rough surf along east facing
shores. The nearshore CDIP buoys continue to run near 10 feet, and
surf observations are still at/above advisory levels. While local
winds will start to diminish on Thursday, the upstream fetch
remains long and wide with stronger winds remaining in place
upstream. Wavewatch III guidance shows seas remaining elevated,
and we have extended the high surf advisory through Friday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 6 AM HST Thursday for Oahu-Molokai-Lanai-
Kahoolawe-Maui Windward West-Maui Leeward West-Maui Central
Valley-Windward Haleakala-Leeward Haleakala-South Big Island-Big
Island North and East-Kohala-Big Island Interior.

High Surf Advisory until 6 PM HST Friday for east facing shores
of Kauai Oahu Molokai Maui and the Big Island.

Flash Flood Watch from Thursday afternoon through Friday
afternoon for Big Island.

Winter Storm Watch from Thursday afternoon through Friday
afternoon for Big Island Summits.

Gale Warning until 6 AM HST Thursday for Maalaea Bay-Pailolo
Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

Small Craft Advisory until 6 PM HST Thursday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maui County Leeward Waters-Big Island
Windward Waters.

&&

UPDATE/DISCUSSION/MARINE...Jacobson
AVIATION...Kinel