Drought in Hawaii, August 1 to 31, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
841 PM HST Wed Jul 31 2019

.SYNOPSIS...
Moderate to breezy trades will hold overnight into Thursday, then
increase Thursday through Saturday as Hurricane Erick passes south
of the state. In addition to the windy conditions, deep tropical
moisture expected to move over the islands north of Erick will
lead to increasing rainfall chances, initially over the eastern
portion of the state beginning Thursday, then across the smaller
islands Thursday night through Saturday. The best chance for
flash flooding will remain over the Big Island through this time.
Additional impacts will include large surf for the exposed
coasts. Later this weekend through early next week, Tropical
Storm Flossie will approach and could bring additional impacts to
the islands.

&&

.DISCUSSION...
The latest surface analysis showed the subtropical ridge axis
positioned well north of the state, which supports the moderate to
breezy trades over the region. Afternoon observations across the
windier locations showed peak gusts reaching the 30 to 40 mph
range. Outside of the typical windward showers moving through
this evening, mostly quiet and stable conditions continue. The
afternoon upper air soundings at Lihue and Hilo reflected this
showing plenty of mid- to upper-level dry air in place above a
subsidence inversion positioned between 8,000 and 10,000 ft.
Precipitable water (PW) values were around the seasonal average
from both soundings (1.3 to 1.4 inches). Peak afternoon (past six
hours ending at 7 PM HST) rainfall accumulations ranged from
around a quarter to a little over half of an inch.

The short-term guidance remains in good agreement and depicts the
potential for a wet and windy period across the islands as Erick
passes to the south late Thursday through Saturday. The threat for
heavy rainfall will initially setup over the eastern end of the
state Thursday afternoon through early Friday, then potentially for
the smaller islands Friday through Saturday. Guidance reflects this
and depicts a surge of tropical moisture moving up the island chain
through this time with widespread PWs exceeding two inches. Although
the bulk of the rainfall coverage will focus over windward locales,
some will manage to spill over into leeward sections of the smaller
islands due to the elevated inversion and strong easterly winds. The
greatest chance for flash flooding will remain over the windward and
southeast slopes of the Big Island through this time (Flash Flood
Watch Thursday afternoon through Saturday for the Big Island).

In addition to the increasing moisture and rainfall chances,
easterly winds are forecast to strengthen in response to a
tightening pressure gradient between Erick passing to the south and
the subtropical ridge to the north. Wind speeds across the
typically windier locations will likely near, if not exceed,
advisory levels late Thursday through Friday night.

This weekend through early next week, the focus will shift to
Tropical Storm Flossie, which is currently located just under 2000
miles east-southeast of Hilo, Hawaii. The National Hurricane Center
in Miami, Florida, is issuing advisories on Flossie and forecasts
the system to near the eastern end of the state late Sunday through
Tuesday on a weakening trend. Although forecast confidence is
steadily rising, it remains too early to determine or pin down any
impacts at this point.

&&

.AVIATION...
With a ridge of high pressure remaining north of the islands,
trade winds will persist in the breezy range. However, locally
strong conditions will become more frequent, especially over the
Big Island and Maui County, as ERICK approaches the area from the
southeast. Brief to TEMPO MVFR weather, in lower clouds and
passing showers, will be mainly concentrated over windward and
mauka sections, though leeward areas will likely see brief periods
of MVFR from time to time as well.

AIRMET Sierra remains in effect for mountain obscuration over the
northeast part of the Big Island. And AIRMET Tango also continues
in effect for areas south thru west of terrain on all islands
below 8000 feet due to TEMPO moderate turbulence.

&&

.MARINE...
Hurricane Erick, about 350 miles southeast of the Big Island of
Hawaii, is forecast to move west through the southern offshore
waters from Thursday morning through Saturday evening. A Hurricane
Warning is in effect for the offshore waters and a Tropical Storm
Warning is in effect for the coastal waters south of the Big
Island. A Small Craft Advisory (SCA) is in effect for most other
areas of the Hawaii Coastal Waters southeast of Oahu. The SCA may
be expanded later this evening.

A High Surf Warning is in effect for east facing shores of the Big
Island for large surf generated by Erick. The Big Island will
block much of the swell from Erick from reaching the smaller
islands, but the fresh to strong trade winds will maintain rough
surf along east facing shores. As Erick passes south of the Big
Island, southeast swells from the system may affect the smaller
islands for a short time.

Tropical Cyclone Flossie is expected to cross from the East
Pacific into the Central Pacific basin as a hurricane late Friday
or Friday night. The current forecast track has Flossie tracking
further north than Erick. This will allow less shadowing for the
smaller islands and may bring higher surf later this weekend into
early next week along east facing shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from Thursday afternoon through Saturday
morning for Big Island.

High Surf Warning until 6 PM HST Thursday for South Big Island-
Big Island North and East.

Tropical Storm Warning for Big Island Leeward Waters-Big Island
Southeast Waters.

Small Craft Advisory until 6 PM HST Thursday for Oahu Windward
Waters-Kaiwi Channel-Maui County Windward Waters-Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Windward Waters.

&&

DISCUSSION...Gibbs
AVIATION...Kinel
MARINE...Donaldson