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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 180139
Area Forecast Discussion
National Weather Service Honolulu HI
339 PM HST Sat Feb 17 2018
Moist and unstable conditions will spread across the entire state
overnight and Sunday as a strong upper level trough approaches
and a weak surface trough currently near Maui drifts westward. The
resulting unstable conditions will bring a threat of heavy,
flooding rainfall and thunderstorms through the Monday holiday. A
somewhat wet, though more stable, southeasterly flow will likely
develop by Tuesday.
Moist and unstable conditions are in place across the eastern half
of the state this afternoon. The afternoon Hilo sounding, recent
aircraft data from Kona and Kahului, and satellite imagery show no
inversion and ample precipitable water (PW) values of 1.25 to 2
inches. Persistent clusters of thunderstorms have been pulsing
around Maui and the Big Island through much of the day, and a few
brief thunderstorms have flared across Maui County this
afternoon. That said, conditions remain ripe for heavy rainfall
over the entire eastern half of the state, and a Flash Flood Watch
remains in effect, as well as a Winter Weather Advisory for the
high summits of the Big Island.
Rather dry conditions have managed to hang on over the western
half of the island chain so far today, though instability is
increasing. Recent aircraft data out of HNL and Lihue and
satellite imagery showed the low level inversion eroding and
precipitable water values creeping up. As the upper level trough
advances, we could see an isolated heavy shower or thunderstorm
pop off this afternoon.
Moist and unstable conditions will spread to the entire island
chain tonight and Sunday. The approaching upper level trough will
wipe out the inversion everywhere this evening, and later tonight,
500 mb temps will drop to -13 to -16C, leading to mid level lapse
rates greater than 7C/km. Meanwhile, the surface trough and
associated moisture currently over the eastern half of the state
will spread westward. As a result, the Flash Flood Watch that
was already in effect for Maui and the Big Island has been
expanded to all islands, and the Winter Weather Advisory for the
Big Island summits will continue. In addition, we will have to
monitor the potential for strong thunderstorms capable of
generating hail and gusty winds.
The threat for heavy showers and thunderstorms is expected to
continue into the Monday holiday. The GFS and ECMWF models both
show a compact upper level low developing just southwest of Kauai
Sunday night then swinging over the islands on Monday. Although
500 mb temperatures will be on an upward trend, significant
instability will persist and will be easily strong enough to
support thunderstorms. The models are in fairly good agreement in
showing a sharpening of the surface trough over the western end
of the island chain during this time. While this could shift the
focus of heavy showers to a zone of convergence to the northeast
of the state, there should still be enough moisture to trigger
heavy showers and thunderstorms over the islands, especially
during the afternoon hours.
The upper level trough lifts to the northeast on Tuesday,
allowing the atmosphere to become more stable through midweek.
East to southeast winds and lingering shower activity is expected.
A surface trough near Maui will move slowly westward this
evening. The airmass over the state will become increasingly moist
and unstable. Thick layered clouds and embedded heavier showers
will linger over the Big Island and waters south of Maui will
spread north and westward covering the remainder of the state by
morning. Isolated thunderstorms over the eastern half of the state
this afternoon will spread as far west as Oahu by late this
evening. AIRMET Sierra for tempo mountain obscuration along
eastern exposures of the Big Island and Maui may need to be
expanded this evening to include Molokai and Oahu.
An upper low will bring the chance of thunderstorms and heavy
rain through Monday. A surface trough near Maui will move slowly
west over the weekend bringing moderate southeast winds east of
the trough and northeast winds to the west. East to southeast
winds will prevail next week.
A series of small west-northwest swells are expected over the
next few days with resultant surf remaining below advisory
levels. A longer period swell is expected Tuesday night which
will produce near advisory level surf Wednesday. High pressure in
the East Pacific will send a short period swell into east facing
shores starting Monday. This could produce advisory level surf
along east facing shores next week.
See the Oahu Surf Discussion (SRDHFO) for additional details on
surf and swell.
Flash Flood Watch from 6 PM HST this evening through Monday
afternoon for Niihau, Kauai, Oahu, Molokai, Lanai, and Kahoolawe.
Flash Flood Watch through Monday afternoon for Maui and the Big
Winter Weather Advisory until 6 PM HST Monday for Big Island
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office