Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Honolulu HI
900 PM HST Sat may 23 2015
Synopsis...a weak front currently near the southern Big Island will
focus showers mainly across the windward Big Island before pushing
south of the state on Sunday. Cool and rather dry northeasterly
winds will continue to spread over all of the smaller islands
tonight...and will be across the entire island chain on Sunday.
These cool and dry conditions will persist through the Holiday
weekend...confining light showers mainly to northeast facing slopes.
Lighter winds are expected during the new work week.
a rare late may front just south of The Big Island continues to move
slowly toward the south this evening. Elsewhere...the circulation
around a nearly stationary 1026 mb surface high near 31n 172w...or
about 1030 miles northwest of Lihue...is bringing cool and dry
northeasterly winds into the islands north of this front. As a
result...the islands are experiencing a very atypical late may
Aloft...a northeast to southwest oriented upper level trough is
evident just east of The Big Island. An upper level anticyclone is
centered more than 800 miles west of Lihue. The general flow high
above the islands is from the northwest.
Satellite imagery and radar reflectivity data Show Low clouds and
showers lingering near the windward and southeastern Big Island...
including the adjacent coastal waters. It appears that the most
significant rainfall has pushed south of The Big Island at this
time...but additional showers may develop along the upslope and
coastal areas of the windward Big Island later this evening or
tonight. Additional widespread low clouds remain near windward Maui
early this evening. Elsewhere...patches of mainly stable
stratocumulus clouds with a few light showers are being carried down
toward the islands west of Maui. The showers that do occur will
likely affect windward and north facing slopes of the smaller
islands. Most leeward areas are expected to remain dry.
The cool and relatively dry northeasterly flow already in place
across the western half of the state will fill in across all of the
smaller islands tonight. Dew points from Kauai to Molokai are
already hovering in the upper 50/S to low 60/S...which indicates the
atmosphere is unusually dry for late may. Expect somewhat cool
conditions tonight with overnight low temperatures to be around 3 to
5 degrees fahrenheit below normal by daybreak Sunday morning.
The front is forecast to stall and dissipate just south of The Big
Island on Sunday. The moderate northeasterly low-level flow will
continue through Monday due to the surface high pressure located
far northwest of the islands. As a result...rainfall will remain
modest and confined primarily to northeast facing slopes...in
addition to the upslope sections of Kona each afternoon.
The forecast models suggest that the upper level trough that is east
of the island chain may shift westward toward the state late Tuesday
or Wednesday. This should weaken the pressure gradient across the
area. Therefore...the current forecast indicates a light trade wind
pattern will develop starting Tuesday. A weak surface trough may
also develop in the vicinity of the islands. In that case...a
diurnal convective nighttime land breeze and afternoon sea breeze
pattern could develop over the state. We will monitor future model
output to determine if we need to add this type of weather regime to
the forecast for next week.
high pressure north of the state will keep a north to NE trade wind flow in
place through Sunday...with clouds and showers focused primarily
across north through east sections of the islands. Best chances for MVFR
ceilings/visibilities will be at phto overnight...although some brief MVFR
ceilings will be possible at phli as well. At the other terminals...VFR
conditions should prevail.
Airmet Sierra remains in effect for north through east sections of The Big
Island due to mountain obscuration. These conditions will likely
continue through the night.
moderate to fresh northeasterly winds will persist behind a slowly
southward moving front currently near The Big Island. The Small
Craft Advisory /sca/...which had been posted for the Kauai leeward
waters...has been allowed to expire early this evening. Border line
Small Craft Advisory conditions may continue near leeward Kauai...as well as some of
the waters adjacent to Maui County and The Big Island as the front
continues its push southward tonight. The latest forecast keeps
winds below Small Craft Advisory conditions over all Hawaiian waters from Sunday
through the middle of next week.
An extended period of south swells...which have been generated by
the fetch associated with multiple surface lows in the southern
hemisphere during this past week...will maintain somewhat elevated
surf along south facing shores of all islands through next week.
Overlapping south swells are expected to cause surf heights to
gradually approach the high surf advisory criteria along most south
facing shores through early next week. Based on the current
forecast...surf may reach the high surf advisory threshold along
most south facing shores Tuesday and Tuesday night. Surf heights are
expected to subside somewhat along south facing shores around
Elsewhere...small swells from the north northwest and west northwest
are expected to maintain a late season bump in surf heights along
most north and west facing shores of the state through early next
week. Also...a small short-period north northeast swell arriving
Sunday is expected to boost surf heights slightly along exposed
north and east facing shores of the island chain through Monday.
Surf heights are forecast to remain well below the high surf
advisory criteria along north...west and east facing shores through
mid-week. See the latest Oahu surf discussion /srdhfo/ for more