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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 231937
Area Forecast Discussion
National Weather Service Honolulu HI
937 AM HST Sun Sep 23 2018
Low pressure northwest of Kauai will lift to the northeast over
the next couple of days, bringing a weakening cold front and
increasing moisture towards the western islands. Showers will
increase mainly over the western islands Monday night through
Tuesday. Weak, south winds and higher than normal dewpoints will
continue muggy conditions into the middle of the week.
A 1001 mb low was analyzed about 775 miles northwest of Kauai
overnight, with a surface trough extending to the south of the
low. This feature will be the focus of our weather over the next
few days. The analysis also showed a northeast to southwest
oriented ridge over Kauai, which supports the light southeasterly
winds we are seeing spreading across the islands.
Dewpoints across the islands are in the upper 60s to low 70s, and
as winds weaken, this will begin to feel more uncomfortable.
Dewpoints are expected to hover around 70 into mid week, before
starting to climb into the low to mid 70s. This will continue to
make atmosphere feel muggy for several days.
The GFS remains the most aggressive with the trough or weakening
front moving near or over Kauai Monday night and Tuesday. All the
models have slowed the increase in precipitable water (PW) since
yesterday morning, but all the models suggest PW reaching 2 inches
over Kauai at times Monday night through Tuesday. Although 500 and
700 mb temperatures remain near normal, that increase in PW is
significantly above the 1.42 inches which is normal for Lihue for
September, with 2 inches being about 2 standard deviations above
normal. While there have been some isolated thunderstorms noted
near the low over the last few days, it would appear that the
bulk of the instability will stay closer to the center. However
with the amount of PW moving over Kauai, it won't be surprising to
have some locally heavy rainfall move overhead.
While the main focus of the moisture will be over Kauai, other
areas will see an increase in PW as well, which will likely bring
an increase in showers to some areas.
The models are still in some disagreement with the handling of the
situation during the second half of the week. The GFS keeps some
lingering parts of the trough over the western end of the island
chain, which would prevent trade winds from building back in too
strongly. Meanwhile, the ECMWF has a stronger high build in behind
the trough, with more trade winds making their way back in. Will
continue to watch for better consensus before making any
significant changes to the grids in the long term.
Winds will veer to the southeast across the state today as a low
pressure trough to the west slowly approaches the state, with
seabreezes developing across many areas this afternoon.
Clouds and passing showers will remain mostly confined to the
windward and mauka areas the rest of this morning, with
isolated/brief MVFR conditions possible. Some clouds and a few
showers will form over interior/leeward areas along the seabreezes
this afternoon. Otherwise, VFR conditions will prevail. The more
widespread clouds and showers associated with the trough are
expected to remain west of Kauai through today.
No AIRMETs are currently in effect, and none are anticipated
An area of low pressure far northwest of Kauai will continue to
move slowly northeast and east through the middle of the week.
Surface winds will then weaken and shift more southeasterly today
and become southerly by Monday night through Wednesday. This will
also bring an increase in showers, especially across the western
coastal waters throughout the week.
Surf will remain below advisory levels along all shores through the
middle of the week. Rough, short-period surf along east facing
shores will gradually decline throughout the work week as the
winds ease and shift. A north-northwest swell is expected late
this week, which may increase surf to near the advisory level
along north facing shores. Small, long-period southwest and south
swells will give surf along south facing shores a small boost
throughout the week, but remain well below advisory level.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office