Area Forecast Discussion
Issued by NWS Honolulu, HI
FXHW60 PHFO 211345
Area Forecast Discussion
National Weather Service Honolulu HI
345 AM HST Tue Feb 21 2017
Breezy trade winds can be expected through the work week as a
series of high pressure areas build north of the state. Clouds
and showers will favor windward and mauka areas, with showers
spilling over into leeward locations from time to time. Winds
will diminish over the weekend as an area of low pressure
develops northwest of the island chain. Rain chances may increase
statewide early next week, as deep tropical moisture lifts into
the area from the south.
Currently at the surface, a 1030 mb high is centered around 850
miles north-northeast of Honolulu, with breezy trades in place
across the island chain. Infrared satellite imagery shows mostly
cloudy conditions across windward areas, with partly cloudy skies
in most leeward locales. Meanwhile radar imagery shows scattered
light showers or some mist moving into windward areas, with rain
free conditions over leeward locales. Main short term concern
revolves around the breezy to windy conditions through mid week.
Today through Wednesday,
A series of high pressure areas will build north of the island
chain through the period, maintaining breezy to locally windy
trades. A bit of an increase in windward and mauka showers is
expected today through Wednesday as an upper level trough moves
overhead. Showers will favor windward and mauka areas through the
period, but with the increase in inversion heights, some light
showers will push into leeward areas from time to time. With
precipitable water (PW) values remaining at or below 1 inch
however, rainfall amounts are expected to remain light.
Additionally, due to very cold temperatures aloft along with some
mid-level moisture, thunderstorms with snow showers and sleet may
develop over the Big Island summits each afternoon.
Breezy trades are expected through the morning hours today. An
increase in the winds is then expected later this afternoon
through Wednesday as a front well north of the state stalls out,
and a re-enforcing area of high pressure builds north of the
islands. Winds may approach Wind Advisory levels in some of the
typically windier areas of Maui County and the Big Island from
late this afternoon through around noon on Wednesday, but
confidence in reaching advisory level wind speeds is too low to
issue a Wind Advisory at this time. We will continue to monitor
observations through the period and issue a Wind Advisory if
Wednesday night through next Monday,
High pressure north of the state will keep moderate to breezy
trade winds in place through Friday, with the trades diminishing
over the weekend as a stacked area of low pressure (surface and
aloft) develops to the northwest of the island chain. A dry
airmass will remain in place across the state for the tail end of
the work week and through the weekend, with PW values remaining
at or below 1 inch. As a result, showers should remain limited
primarily to windward and mauka areas through Sunday. There may
be an increase in showers statewide by early next week as some
tropical moisture begins to work its way northward from the
Intertropical Convergence Zone (ITCZ). Confidence remains low
regarding the exact details at this time given continued
disagreement amongst the reliable model guidance.
VFR conditions will prevail across the main Hawaiian Islands today.
An upper level trough will bring some instability to the skies
above the Big Island summits this afternoon with an isolated
thunderstorm (ts). The trough is not expected to do much for the
lower levels except increase the trade showers a bit that will
favor the windward and mountain areas of all islands. These
showers will cause brief MVFR ceilings but not a factor with the
vis. AIRMET Sierra is not expected with the coverage of the showers
limited to scattered.
The morning soundings showed a thermal inversion based at 71
hundred feet at LIH sloping to 68 hundred feet at ITO. The mid
level instability which is responsible for the isol ts the past
couple of days over the Big Island summits has turned more stable
overnight. We believe this is temporary and the instability at the
mid layers will return this afternoon with the approaching upper
level trough. So isol ts will be in the FA package for the Big
Island summits at 16z.
A stagnant strong surface high pressure system sitting north of the
islands will maintain breezy and gusty trade winds well beyond
tonight. The models are suggesting the trades to be a bit stronger
this afternoon and evening than last night. In any case, AIRMET
Tango for low level turb below 8k feet will be extended.
Strong high pressure passing by around 740 nm north-northeast of
Honolulu will keep strong trade winds in place through Wednesday.
The trades have diminished below gale force across the typically
windy waters around Maui and the Big Island, so a Small Craft
Advisory (SCA) is in effect for all waters through Wednesday.
Moderate to breezy trade winds will continue through the remainder
of the work week, with the SCA likely needing to be extended in
time for portions of the coastal waters. The trades are then
expected to diminish over the weekend.
The current north-northwest swell which helped produce high surf
across north facing shores yesterday continues on a downward trend
early this morning. Combined seas at the NOAA buoys to the
northwest of the islands are down to 6 feet at 10 seconds, while
larger seas just northeast of the islands are down to around 8 to
9 ft at 11 seconds at buoy 51000. Based on the NOAA buoy data,
and expected steady decline through the day today, have canceled
the High Surf Advisory.
A pair of long period swells from distant sources will produce
somewhat elevated surf along north and west facing shores for the
latter half of the week. Rough, short period trade wind seas will
also maintain surf just below advisory levels along east facing
shores through Thursday. The first northwest swell is expected to
arrive Wednesday and peak in the double overhead range Wednesday
night and Thursday. The second northwest swell will likely be
larger and may lead to advisory level surf Friday night and
Small Craft Advisory until 6 PM HST Wednesday for all Hawaiian