574 
FXHW60 PHFO 191949
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
949 AM HST Thu Sep 19 2019

.SYNOPSIS...
A moderate trade wind pattern will persist through tonight, with
clouds and showers focused over typical windward and mountain
areas. Increasing amounts of clouds and showers are expected 
through the weekend and into Monday. An upper level low
approaching from the northeast will help to enhance the showers.

&&

.DISCUSSION...
High pressure far to the north-northeast of the islands is helping
to maintain a trade wind pattern over the islands today which
should continue into tonight. Soundings at Lihue and Hilo 
overnight showed trade wind inversions around 6500 and 9200 feet 
respectively. Additionally, they measured precipitable water (PW) 
values of 1.52 and 1.63 inches respectively. Climatological PW 
values for September are 1.38 and 1.47, so statewide we are seeing
values above normal for this time of year. 

Water vapor imagery shows an upper level low about 950 miles
northeast of Oahu this morning. A shortwave developing along the
west side of this low is expected to swing towards the islands
tonight and tomorrow. This will begin a period of unsettled
weather over and near the islands that is expected to persist into
Monday.

The shortwave coming through late tonight and tomorrow will be
accompanied by 500 mb temperatures dropping to colder than normal
temperatures and 700 mb temperatures dropping from warmer than 
normal to around normal tomorrow morning. Combined with above
normal PW values, cannot rule out some heavier showers or even a
rumble or two, particularly over the southern half of the state.
That all being said, these pieces will not be coming together at
the ideal time of day given our diurnal fluctuations. Am seeing a
general agreement with the high resolution and global models for
an uptick in PoPs tomorrow morning so will likely be boosting 
those values by the afternoon package.

An upper level low is expected to develop along the shortwave by
Saturday morning, with the low to the northeast of Maui County,
and then moving to the west through Sunday night. The ECMWF keeps
the upper low more than 450 miles west of Kauai on Sunday and 
Monday, before lifting it to the north on Tuesday. The GFS keeps 
the upper low more than 450 miles west of Kauai on Sunday, and
starts moving it to the north Sunday night and Monday. 

By Saturday night, the low will be closer to the northern end of
the state, helping to draw in much higher PW values from the
south, with values exceeding 2 inches. Expect those higher PW
values to spread across the state from east to west, maintaining a
wet pattern through Monday. Still need to work out the details,
but will likely be making some modifications to the PoPs and
associated fields for Saturday through Monday by the afternoon
package.

A return to a trade wind pattern is expected for the middle part
of the new week.

&&

.AVIATION...
There are no AIRMETs currently in effect.

A surface high far NE of the area will maintain locally strong 
east low level winds over the area. The low level winds will push
some clouds and showers over windward (east) facing sections of 
the islands and produce ISOL MVFR ceilings and visibilities. VFR 
conditions will prevail over most leeward areas, but afternoon sea
breezes may produce ISOL MVFR ceilings over the leeward slopes of
the Big Island of Hawaii and Maui.

&&

.MARINE...
Fresh to locally strong east southeast winds will slowly turn more
easterly today as a deep low pressure system west of the state 
lifts northward. A Small Craft Advisory remains in effect for the
typical windy areas around Maui and the Big Island through 
Friday. Guidance shows a return of light to moderate southeast 
flow this weekend due to an approaching surface trough, which 
will support leeward land and sea breeze conditions and more 
showers.

A series of small swells from the southwest and south are expected
through the rest of this week. The largest of these looks to be 
today and Friday as a long-period south-southwest swell and 
shorter-period west-southwest swell combine across the area. A 
small northwest swell is possible late in the weekend into early 
next week. A small swell from Tropical Cyclone Kiko is expected
later today along east facing shores before dropping over the 
weekend. 

Surf heights are expected to remain below advisory levels through
the period.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Friday for Pailolo Channel-
Alenuihaha Channel-Big Island Leeward Waters-Big Island 
Southeast Waters.


&&

$$


DISCUSSION...M Ballard
AVIATION...Donaldson
MARINE...Morrison