Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Honolulu HI
400 am HST Friday may 22 2015
a dissipating front will be moving down the island chain between
today and Saturday...bringing with it some scattered showers. In the
wake of the front...light to moderate northeast winds will settle
in...lingering into next week.
a light but convergent cyclonic westerly wind flow just above the
surface has taken over the main Hawaiian islands with northwest winds over
Kauai...westerly over the central islands and S to SW over The Big
Island. Radar is painting isolated showers ahead of the front
covering a wide area of our surrounding waters. But the most
threatening of these showers are close to reaching the North Shore
of Oahu and the leeward kohala coast of The Big Island. To some
degree an offshore flow from these two places is slowing down these
showers from coming ashore but eventually they will make it.
Thus...this has tromped ME to include isolated to scattered showers
for the surrounding water with emphasis to these two areas for
the next couple of hours.
A weakening/dissipating front is located 90 miles northwest of Kauai this
early Friday morning...moving southeast at 15 miles per hour with a possible slowing
down after reaching Kauai. The very latest European model (ecmwf) and GFS have
slowed down the front up to 6 hours. So...for Kauai...instead of 11
am or noon...it will be middle afternoon. For Oahu...it is early this
evening. The NAM solution on the other hand...is tad faster with the
original time line of about noon for Kauai. The NAM has the front
slowing down slightly upon reaching Oahu late this afternoon...Maui
County overnight...and The Big Island early Saturday morning. NAM is
the accepted solution.
Remnants of the front are expected to reach The Big Island Saturday
where it will linger through Sunday morning. The front will usher in
not only a moderate northeasterly wind flow but a slightly drier air
mass as well.
No upper level support for the front as the upper trough will be
running out ahead of it. So no excessive rainfall expected. The
depth of the moisture associated with the front will be confined to
within 12k feet of the surface.
The low that helped to push the front down the chain is forecast to
linger 900 miles NE of the islands through at least the middle of
next week while high pressure builds northwest of the islands. This
juxtaposition of features will lead to unseasonable north/NE winds for
the most of the forecast period with light to moderate speeds.
As the low takes a dip to the S early next week...winds may become
light enough to allow local land/sea breezes to dominate the island
weather pattern. Otherwise...shallow bands of moisture rotating
around the low may favor the north and east facing slopes...but the
atmosphere will remain relatively stable through the period. The low
will exit to the NE by the second half of next the week allowing the
return of light to moderate trade winds.
a weakening cold front will move through Kauai around
22/2200z...through Oahu by 23/0400z and into Maui County tonight.
Expect scattered showers to develop along and in advance of the cold
front as it shifts southeastward...but predominantly VFR conditions
are expected at all taf sites through 23/12z.
No airmets are in effect and none are anticipated through the
morning hours. Mountain obscuration could become a problem this
afternoon across Kauai behind the cold front.
winds and seas will remain below Small Craft Advisory criteria for
all waters into next week. An extended run of S swell...generated by
several lows in the deep S Pacific over the last week...will bring
a slight rise in the surf for the S facing shores into the middle of
next week. Surf heights are expected to remain below advisory levels
for the bulk of the time...but overlapping swells Sunday/Monday
could bring advisory level surf to exposed S facing shores. A couple
of small swells from the north-northwest and west-northwest are expected over the weekend
and early next week...with resultant surf well below advisory levels.