697 
FXHW60 PHFO 281352
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
352 AM HST Mon Jan 28 2019

.SYNOPSIS... 
Gusty north-northeast winds along with cool and dry conditions will 
prevail through a good portion of the week. Any showers that do 
develop will focus over windward and northern slopes each day. 
Rainfall accumulations will be minimal with the dry air in place. 
Increasing moisture with a return of a more typical trade wind 
pattern will be possible by the upcoming weekend. 

&&

.DISCUSSION...
Water vapory imagery showed a potent shortwave trough digging east-
southeastward skirting the eastern end of the state ushering in an 
anomalously cold pool of mid- to upper-level air and strong west-
northwest winds aloft. The mid-level moist layer that brought the 
layered clouds and frozen precipitation to the Big Island summits 
yesterday afternoon has shifted east of the state with much drier 
air filling in across the area behind it early this morning. Wind 
observations at the summits quickly surged to warning levels with 
gusts up to around 90 mph early yesterday evening, however, have 
since lowered below criteria. The potential for warning-level 
winds, however, will continue through tonight, especially later 
today as the upper flow veers to the north and increases as the 
upper low cuts off to the east and a ridge builds west of Kauai.

The latest short-term guidance is lining up well the current pattern 
and shows the aforementioned layer of mid-level moisture continuing 
eastward away from the Big Island today with drier air filling in on 
the back side of the upper trough. Despite the dry air, this amount 
of instability in place combined with a sufficient amount of low-
level moisture and gusty northerly winds will be enough to continue 
to generate brief showers periodically, mainly over northern and 
eastern slopes of Maui County and the Big Island. Any showers that
do develop should be brief with limited accumulations. Otherwise,
dry and cool conditions will prevail with dewpoints remaining in 
the 50s and daytime highs only reaching the upper 70s. 

At the surface, gusty northerly winds will prevail as the 
attendant surface reflection (broad trough/low) remains parked 
east-northeast of the Big Island and high pressure remains located
north-northwest of the islands. Periods with advisory-level winds
at the lower elevations will be possible, especially as the 
gradient tightens today through Tuesday as a surface low/trough 
pivots southward near the Big Island. In addition to the winds, 
this low/trough may usher in a better source of moisture 
(precipitable water (PW) increasing to 1.35") Tuesday night 
through midweek. The current forecast reflects this and depicts 
increasing rainfall chances for northern/windward slopes. 

Forecast uncertainty rises quickly later in the week and upcoming 
weekend due to model inconsistencies shown from cycle to cycle. The 
GFS and ECMWF solutions are now both depicting the surface trough to 
the east dampening out with a return of a more typical trade wind 
pattern by the weekend. This is a significant change from previous 
cycles where both were advertising the surface feature to the east 
shifting westward over the islands as a shortwave trough and cold 
front approach the region from the northwest. This scenario would 
have resulted in light winds across portions of the state.
Additional changes and fine tuning of the forecasts in later
packages will be likely for this upcoming weekend period. 

&&

.AVIATION...
A surface low pressure system to the northeast and high pressure 
cell to the northwest of the islands will maintain a tight 
pressure gradient over the islands resulting in another day of 
moderate to strong northerly winds. Therefore, AIRMET Tango, for 
low level turbulence at or below 9k feet, south through west of 
all mountain ranges of all islands, continues on.

At the upper levels, the low to the northeast of the islands has 
nudged the jetstream to over the islands. This will lead to a
deep layer of light to moderate high level turbulence, between 
fl200 and fl400. Not much change is expected through tonight as 
the upper low remains nearly stationary NE of the area.

The patch of middle and high clouds has cleared the islands. But
it left behind some instability over parts of the Big Island. This
resulted in a brief flare up of a couple of thunderstorms over the
Hamakua coast earlier in the evening. Spotty heavy showers
continues to come and go along parts of the Hamakua coast and 
adjacent waters as the air mass there continues to stabilize as
mid level ridging slides in. Tops of these showers are as high as
20k feet right now. We anticipate these tops to be much lower,
down to around 6k feet by this afternoon. Little in the way of 
showers elsewhere. Cloud tops are estimated around 8k feet as
well, but like the Big Island, mid level ridging will bring down
the low level inversion to 6k feet.

&&

.MARINE...
An area of low pressure east and northeast of the state along 
with a series of reinforcing highs building northwest of the 
island chain, will keep moderate to strong north-northeasterly 
winds in place across the marine area through Thursday. These 
strong winds in combination with a series of northwest swells will
produce Small Craft Advisory (SCA) conditions for most marine 
zones through much of the work week. Moderate trade winds may 
return by the end of the work week or during the upcoming weekend.

A series of new, moderate to large, northwest swells will move 
through the islands this week. The long period forerunners of the
first swell have arrived at buoys 51001 and 51101, however this 
swell is running 6-9 hours later than predicted by the Wave Watch
III guidance. This swell will continue to fill in this morning,
peak around the High Surf Advisory (HSA) level across north and 
west facing shores of Niihau and Kauai this afternoon and tonight,
then lower on Tuesday. The second northwest swell is expected to 
build Tuesday night and Wednesday, then peak close to warning 
levels late Wednesday night and Thursday. This swell will then 
slowly lower through the end of the work week. Another northwest 
swell may bring advisory level surf to north and west facing 
shores over the upcoming weekend.

A north to northeast swell generated by a gale low northeast of 
the state will produce rising surf along exposed shores over the 
next couple of days. Surf is expected to peak above the HSA
threshold for east-facing shores tonight through Wednesday, then 
slowly decline through the rest of the week.

&&

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

Wind Advisory until 6 AM HST Tuesday for Lanai-Kahoolawe-South 
Big Island-Big Island North and East.

High Wind Warning until 6 AM HST Tuesday for Big Island Summits.

Small Craft Advisory until 6 AM HST Wednesday for all Hawaiian 
waters-

&&

$$

DISCUSSION...Gibbs
AVIATION...Lau
MARINE...Jelsema