165 
FXHW60 PHFO 120205
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI 
405 PM HST Sat Jan 11 2020

.SYNOPSIS...
Wet and windy weather will continue for another couple of days, as 
an upper level disturbance lingers west of the islands. Strong 
easterly trade winds will continue through the weekend, then shift 
to the east-southeast early next week as a surface low develops 
northwest of Kauai. The potential for localized flash flooding will 
continue through the weekend, especially over windward and mountain 
areas, where soils remain saturated. Some drying is possible around 
Tuesday and Wednesday, but another disturbance may bring a return of 
wet weather, but lighter winds, Thursday into Friday. A drier trade 
wind flow may develop by next weekend. 
  
&&

.DISCUSSION...
The combination of strong high pressure centered to the NNE of the 
islands and a developing low NW of Kauai will keep strong E trade 
winds over the islands through the weekend. As the low tracks N 
early next week, winds will remain elevated as they veer to the ESE. 
A convergence band associated with the low is expected to keep wet 
weather in place over the islands until Tuesday/Wednesday, when the 
convergence band may shift just W of the islands, bringing the 
potential for a brief drying trend. Latest guidance brings an upper 
trough and associated front (with lighter winds) over the islands 
around Thursday, with the potential for a drier, cooler breezy NE 
flow to develop by next weekend.  

In the meantime, a closed low aloft centered just W of the islands 
is supporting an unstable island atmosphere, while the strong trade 
winds continue to stream an abundant amount of low-level moisture 
over the islands from the E. A Flash Flood Watch remains in effect 
for all islands as localized heavy downpours could quickly lead to 
flash flooding given how saturated the ground is, especially in 
windward areas. In some areas, like windward Big Island today, 
showers are not necessarily advecting in from the ocean. Instead, 
the terrain-anchored heavy rainfall appears to be developing in 
response to orographic lift supplied by the strong trades 
interacting with the dynamic large-scale lift provided by the low 
aloft. This persistent rain is resulting in dangerously high water 
levels in windward Big Island's Honolii Stream and Wailuku River. 
Water levels in Oahu's Lake Wilson are also very high, although a 
break in rainfall on Oahu has slowed the rate of water rise today. 

The best chance for flash flooding through the weekend will remain 
over windward and mountain areas, where soils are saturated and 
streams are running high from the incessantly steady rainfall over 
the past few days, but streams statewide (even those in leeward 
areas) can be expected to be running higher than normal. Moisture is 
sufficiently deep to impact the summits of Maui and the Big Island, 
and a Winter Storm Warning is now in effect for Mauna Kea and Mauna 
Loa, where near- to below-freezing temperatures, accumulating snow 
showers and low visibility are occurring. 

As the surface low develops NW of Kauai, and the low-level flow is 
expected to veer to the ESE. While this flow normally places the 
smaller islands in the wind shadow of the Big Island, it remains 
uncertain how much veering will occur, and thus how much reduction 
in wind speed is realized over land. Guidance indicates a surge in 
east to east-southeast wind speeds Sunday night and Monday, which 
may require another Wind Advisory for portions of the state. 

The low aloft is expected to lifts slowly N over the next couple of 
days, while a surface low develops in response. While mid-level 
temperatures gradually warm over the islands early next week as this 
occurs (leading to a reduction in the instability), a low-level 
convergence band over the area is expected to keep wet weather in 
place through Monday, especially over the W end of the state. The 
convergence band may slide sufficiently W of the area to allow a 
brief drying trend Tuesday/Wednesday, but confidence in those 
details are low. An upper trough and surface front bring the 
potential for wet weather by Thursday, but latest guidance shows a 
cool, breezy and drier NE trade wind flow developing by next 
weekend as the deep moisture moves E of the islands. 

&&

.AVIATION...
Strong high pressure system north of the state will continue breezy 
to windy trade winds. A west to east oriented trough aloft and 
subtropical jet will continue to produce wet and unsettled weather 
across the region with heavy rain and the chance for thunderstorms 
expected through the next 24 hours. Periods of MVFR and IFR 
conditions will occur due to low ceilings/visibility in developing 
shower bands, especially along north through east facing sections of 
each island. 

AIRMET Sierra for Tempo Mountain Obscuration will continue for north 
through east sections of Kauai, Oahu, Molokai, Maui, and the Big 
Island. These conditions will likely remain for the next 24 hours.

AIRMET Tango remains in effect for Tempo Moderate Turbulence over 
and immediately south through west of all island terrain. Another 
AIRMET Tango was issued for Tempo Moderate Turbulence aloft between 
FL200 and FL350 due to strong jet stream winds. A third AIRMET Tango 
remains in effect for strong surface winds greater than 30 KT for 
the area around Kauai, but will be dropped for all other areas. The 
surface wind AIRMET may need to be reissued tomorrow as winds come 
back up.

&&

.MARINE...
Strong high pressure far north of the state will maintain strong to 
near gale force trade winds through the middle of next week with a 
slight uptick in wind speeds possible Sunday night into Monday. 
Small Craft Advisories (SCA) are in effect for all Hawaiian waters 
due to a combination of strong winds and large seas. A Gale Watch is 
also in effect for the Alenuihaha Channel Sunday night through 
Monday. Winds should begin to decrease during the second half of 
next week as a front passes north of the state, causing the high to 
weaken and shift further east.

An unstable airmass over the islands will continue to bring the 
chance for locally heavy downpours and a risk for thunderstorms 
through at least Sunday afternoon and possibly into Monday. 

The High Surf Warning for east facing shores of Kauai, Oahu, 
Molokai, Maui and the Big Island has been downgraded to a High Surf 
Advisory. Although surf heights have dropped a little over the last 
day, we should continue to see elevated surf through the middle of 
next week. As the winds begin to decrease during the second half of 
the week, surf should drop below advisory levels by Friday.

The current long-period northwest swell will continue to gradually 
decrease over the next few days, with a smaller reinforcing 
northwest swell filling in on Sunday. These swells should drop to 
background levels by Tuesday with minimal swells expected through 
most of next week. A front passing north of the state late next week 
may produce a moderate short-period north swell towards the end of 
next week. Otherwise, no significant swells are expected through 
next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Sunday afternoon for all islands.

High Surf Advisory until 6 AM HST Thursday for East facing shores of 
Kauai Oahu Molokai Maui and the Big Island.

Winter Storm Warning until 6 PM HST Sunday for Big Island Summits.

Small Craft Advisory until 6 PM HST Monday 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-Maalaea Bay-Pailolo 
Channel-Big Island Windward Waters-Big Island Leeward Waters-Big 
Island Southeast Waters.

Gale Watch from Sunday evening through Monday afternoon for 
Alenuihaha Channel.

Small Craft Advisory until 6 PM HST Sunday for Alenuihaha Channel.

&&

$$

DISCUSSION...Birchard
AVIATION...TS 
MARINE...Kino