Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Honolulu HI
1000 am HST Tuesday Sep 1 2015
very humid conditions with spotty heavy showers...mainly each
afternoon...will continue through most of the week. Winds will turn
southerly for much of the week...then become light and variable late
in the week. The extended forecast will depend heavily on the track
of Hurricane Jimena well northeast of the state...but a brief return
to drier easterly winds is possible early next week.
Hurricane Ignacio is located within 300 miles northeast of the
state...with outer rain bands associated with it apparent on radar
approaching the windward coastal waters. Water vapor imagery shows a
high west of the state with a trough to the northeast. A surface
high is located far north of the state...though with Ignacio
interrupting the gradient between US and the high. 18z soundings
from Hilo and Lihue show precipitable waters around 1.8 inches...
which are slightly drier than the 2.0 inches from the 12z
soundings. Early morning mimic total precipitable water imagery is
too moist compared to the soundings...but does show qualitatively
slightly drier air just northeast of the islands between US and
The drier air seen by mimic northeast of the state is likely what
has moved south over US...and will soon be replaced by higher values
once again from the north. A number of stations on Kauai received
1-2 inches of rain overnight...with areas on Oahu generally around a
quarter inch. We are already seeing showers flare up near
Kauai/Oahu...with more moving in from the north. Sent an update
earlier this morning to increase rainfall chances and also add
thunder over the coastal waters. We already had a mention of thunder
during the afternoon across interiors of the islands...due to the
abundant moisture/instability. Low-level winds are relatively
weak...but the overall flow is from the north from Kauai to Maui
and west near The Big Island. These winds are expected to continue
backing through the day as Ignacio continues moving northwest well
north of the state. Winds from this direction will help to enhance
convergence along sea breezes over south and southeast sections of
the islands...and the update focused greatest afternoon convection
from south to southeast sections of the islands /mainly southeast
Maui and Big Island...which started the day mostly sunny/.
Aside from the heavy rain and thunderstorm threat...another concern
will be the very humid conditions. The islands are locked under a
tropical airmass with dew points hovering in the middle 70s to even the
upper 70s at times. While maximum temperatures will be near to slightly above
normal...the humidity will create heat index values in the very
uncomfortable range of the upper 90s...and overnight temperatures will
struggle to get below the upper 70s.
Ignacio is forecast to lift north of the state Wednesday through
Friday...bringing an extended run of very humid conditions and
spotty locally heavy showers. Similar temperatures and humidity conditions
will hold into Saturday as light southerly winds offer no relief.
The atmosphere will remain moist through a deep layer as
precipitable water holds in the 1.75 to 2 inch range. The atmosphere
will continue to be somewhat unstable with no inversion as an upper
level trough remains over or just west of the islands. As a
result...locally heavy showers will be possible...and we will have
to watch for the possibility of a developing convergence zone
producing more organized heavy rainfall over portions of the state
on Wednesday and Thursday. Heavy showers were left out of the
forecast after Thursday as ridging aloft begins to work its way over
the islands from the east.
Drier easterly flow may develop early next week...but confidence
remains low in the extended. This is mainly due to uncertainty
regarding the evolution of Hurricane Jimena...which is forecast to
gradually track far east and northeast of the state.
see tc significant meteorological information series Tango for tropical cyclone Ignacio.
Hurricane Ignacio continues to move northwest approx 200 nm NE of phog. The
wind surrounding the islands will be greatly influenced by the
passing of Ignacio. Winds over the east end of the state...near The
Big Island...have turned SW to S behind Ignacio. Over the western
end of the state...near Kauai moderate NE winds continue ahead of
Ignacio. Elsewhere...there are light north to variable winds.
Expect scattered showers to stream from the north over the Western
Islands. Some showers have the potential to carry MVFR to isolate IFR
conditions. The airmass over the islands remains unstable and very
moist. This will provide the potential for spotty heavy showers and
isolated thunderstorms over the islands as daytime heating occurs.
Cloud tops will generally be between 080-120 with towering cumulus tops between
150-fl250. Any thunderstorms may have cumulonimbus tops between fl350-400 and
also have the potential for icing and turb.
the current large swell from Hurricane Ignacio is leading to seas of
10-12 feet at the Hanalei/mokapu/Pauwela cdip buoys. The direction
of this swell will also back with time...as Ignacio continues
tracking northwest. Current readings are easterly at Kauai and
northeasterly at Maui. This swell will diminish through tonight and
Wednesday as it backs northwesterly. At the same time...a new longer
period east swell associated with Hurricane Jimena will build
today/tonight. This swell will keep surf elevated near warning
levels through the weekend.
A Small Craft Advisory is also in effect through tonight for
hazardous seas due to the swells from Ignacio and Jimena. This
advisory will also need to be extended as seas remain elevated.
Winds are expected to remain below advisory levels this week.
Long-period swells from the south-southwest may produce
advisory-level surf along south-facing shores this weekend. Please
refer to the latest collaborative nearshore swell and wind
forecast /srdhfo/ for more details on the swells.
high surf warning until 6 am HST Wednesday for Kauai windward-
Oahu koolau-Olomana-Molokai-Maui windward west-Maui Central
Valley-windward Haleakala-south Big Island-Big Island north and
Small Craft Advisory until 6 am HST Wednesday for Kauai
northwest waters-Kauai windward waters-Kauai Channel-Oahu
windward waters-Kaiwi Channel-Maui County windward waters-
Pailolo Channel-Alenuihaha Channel-Big Island windward waters-
Big Island southeast waters.