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Temp-2.4 C
RH95 %
WindSSW 44 mph
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Hawaiian Islands Synoptic Discussion and Guidance

644
FXHW60 PHFO 011944
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
944 AM HST Wed Mar 1 2017

.SYNOPSIS...
Moist and unstable conditions will maintain a threat for heavy
flooding rainfall and thunderstorms through tonight. A negatively
tilted upper trough just west of Kauai will move over the islands
later today and tonight, then lift northeast on Thursday, leaving
the islands under a humid southwesterly flow capable of generating
spotty heavy rainfall. Showers are then expected to diminish
across the state Thursday night through Friday night as a more
stable airmass moves over the area, with a cold front then
expected to move into the northern half of the island chain this
weekend increasing the chance for showers once again.

&&

.DISCUSSION...
Water vapor imagery shows a negatively-tilted upper trough to the
west and northwest of Kauai, with an embedded low centered just
west of Kauai. The upper low is inducing a surface low centered
around 150 miles west of Kauai. Satellite imagery shows a overcast
skies associated with a convergence band stretched across the
state. Radar shows numerous showers and thunderstorms streaming
northward across the islands, particularly across Oahu through the
rest of the morning, and Maui County and possibly the Big Island
through the afternoon and tonight.

The upper trough just west of Kauai will slowly push eastward
across the island chain through tonight, while the surface low
west of Kauai lift to the north. The surface low in combination
with high pressure anchored well northeast of the area, will keep
a south to southeasterly boundary layer flow in place across the
state, assisting in transporting deep tropical moisture northward
and across the island chain. Strong forcing for ascent provided by
the upper level trough moving through, favorable upper level jet
dynamics, along with plenty of deep layer moisture will continue
to support bands of heavy showers and thunderstorms across the
islands through the rest of today, then shifting to the eastern
half of the state tonight. With the ongoing potential for heavy
rain and saturated grounds, a Flash Flood Watch is in effect
statewide through tonight. While some remnant showers are likely
to remain over the eastern half of the state, the overall threat
for flooding will begin to trend down on Thursday.

In addition to the heavy rainfall threat, with the upper level
trough moving through the area, there appears to be the potential
for strong to severe thunderstorms. 700-500 mb lapse rates are
forecast to steepen into the 6.5 to 7 C/KM range, 0-6 km bulk
shear magnitude values increase to 25 to 35 knots, surface based
cape values rise to between 800-1200 J/KG, and 500 mb winds
strengthen to between 35 and 50 knots. This suggests there is the
potential for damaging winds and possibly hail with the strongest
thunderstorms that develop within the convergence band. A myriad
of short term products are in effect for hazards relating to
flooding and strong thunderstorms, please see our website for the
latest information.

Winds have increased to advisory levels across the Big Island
summits and with heavy snow and poor visibilities expected at
times, the Winter Storm Warning has been upgraded to a Blizzard
Warning, which is in effect through late tonight.

Thursday through Friday night,
The upper level trough will lift northeastward and away from the
islands on Thursday, while low and mid-level ridge will build over
the island chain Thursday night through Friday night. The trailing
end of the convergence band is forecast to remain over the eastern
end of the state through Thursday, resulting in some remaining
locally heavy showers. Showers should further diminish Thursday
night through Friday night as more stable conditions build over
the island chain.

Saturday through Tuesday,
Medium range guidance is in fairly good agreement through the
weekend, with solutions then diverging early next week. Both the
GFS and ECMWF show a weakening cold front moving into the western
islands over the weekend. The GFS shows the front stalling out
over the western islands early next week, with a potent shortwave
trough then moving through the area Monday night and next Tuesday,
increasing showers across the entire island chain. The ECMWF on
the other hand shows the weakening cold front stalling out over
the eastern islands, with a potent upper trough approaching from
the west on Tuesday. Given the differences in the model solutions,
will utilize a blended forecast approach until details become more
clear. As a result, we should see increasing showers across the
western islands this weekend, and due to the uncertainty in the
forecast next week, we will need to keep showers in the forecast
statewide.

&&

.AVIATION...
A deep-layer low just W of Kauai has an associated convergence
band that is straddling Oahu and Maui county. While the band is
moving slowly E, individual cells within the band are moving
rapidly NE at speeds between 30 and 40 kt. These +TSRA and +SHRA
are producing areas of strong to severe wind gusts, LIFR VIS/CIG
as well as icing and turbulence - with Oahu and Molokai currently
most threatened. AIRMETs are in effect for numerous hazards, and
most will continue to remain posted today. Gradually improving
conditions are expected later tonight and Thursday as the low
lifts N, and the convergence band moves E toward the Big Island
and weakens. SIGMET SIERRA for EMBD TS also intersects local air
space.


&&

.MARINE...
A surface low just west of Kauai will continue to track northward
through Thursday as an upper level trough moves over the state.
Hazardous conditions including gusty winds, hail, and thunderstorms
with frequent lightning can be expected through tonight over the
coastal waters, mainly from the Kaiwi Channel eastward. Mariners
should tune into NOAA Weather Radio or the website for updates to
Special Marine Warnings (SMW).

A Small Craft Advisory (SCA) is posted over coastal waters from
Oahu eastward for strong south winds ahead of the low. Winds
should begin to ease as the surface low lifts north. SCA is posted
through late tonight.

Swells and surf will remain relatively quiet until this weekend. A
developing gale to storm force low, about 1700 NM miles northwest
of Kauai, should direct a moderate west-northwest swell towards
the islands. Guidance shows the swell peaking late Saturday into
Sunday, producing near to low end advisory level surf for the
exposed north and west facing shores. This will need to be
monitored as the swell nears the islands later this week.
Elsewhere, no significant surf expected.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through late tonight for all Hawaii islands.

Blizzard Warning until 6 AM HST Thursday for Big Island Summits.

Small Craft Advisory until 2 AM HST Thursday for Oahu Windward
Waters-Oahu Leeward Waters-Kaiwi Channel-Maui County Windward
Waters-Maui County Leeward Waters-Maalaea Bay-Pailolo Channel-
Alenuihaha Channel-Big Island Windward Waters-Big Island Leeward
Waters-Big Island Southeast Waters.


&&

$$

DISCUSSION..Foster
AVIATION...Birchard
MARINE...Eaton

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office