Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Honolulu HI
836 PM HST Tuesday Aug 23 2016
deep tropical moisture moving over the state will maintain very
humid conditions with localized heavy downpours and a chance of
thunderstorms through Wednesday. Drier trade wind weather will
begin to overspread the state from west to east Wednesday night
and Thursday, with typical trade wind weather then persisting
through early next week.
currently at the surface, a weak surface trough extends from
around 80 miles west of The Big Island northwestward and across
the island of Oahu. This surface trough is resulting in frequent
heavy showers over the island of Oahu, with more scattered shower
activity over the other islands. Main short term concern for
tonight revolves around rain chances and the potential for
Rest of tonight,
the weak surface trough over Oahu will remain nearly stationary
overnight as the low level flow shifts around to the southeast as
an upper level trough well northwest of the islands edges eastward.
This will keep a very moist and unstable airmass over the islands,
with mimic total precipitable water values ranging from 2.0 to 2.4
inches. As a result, expect the unsettled weather to continue,
particularly over Oahu and Kauai where the weak surface trough
resides. Additionally, with the moist unstable airmass in place,
an isolated thunderstorm can't be ruled out through the overnight
hours as well.
deep layer ridging will begin to build well north of the islands,
and this should allow some drier air to move into The Big Island
by late in the day. Model solutions still show very high
precipitable water values of 1.8 to 2.2 inches remaining over the
smaller islands through the day, so some locally heavy rainfall
will be possible. In addition, with the moist unstable airmass
remaining overhead, will add a mention of thunderstorms to the
forecast as well.
Wednesday night and Thursday,
the trade winds will begin to overspread the state from east to
west as strong high pressure builds well to the north-northeast
of the island chain. This will shift the deepest tropical moisture
over Kauai Wednesday night and west of the state during the day on
Thursday. As a result, we should see some enhanced showers over
Kauai Wednesday night, with more of a typical windward/Mauka
shower pattern across the other islands. On Thursday, more typical
trade wind weather will settle over the entire island chain with
showers favoring windward and Mauka areas.
Thursday night through next Tuesday,
high pressure well north of the state will keep a moderate trade
wind flow in place through the period, with clouds and showers
focused primarily across windward and Mauka locales.
moist, humid, and unstable conditions will continue during the
overnight hours with the bulk of the activity expected over Oahu
and Kauai. Currently, airmet Sierra for mtn obscuration is in
effect for the mountains of Kauai and Oahu, especially the ko olau
range. This airmet will likely stay in place through tonight, and
possibly, beyond sunrise Wednesday. Airmet Sierra for the NE
through east facing slopes of The Big Island is still in effect, but,
will be lowered in the next couple of hours or by 10z should the
improving trend continue.
An isolated thunderstorm is present 56 nm south of hnl at 0530
UTC with a top of 48k feet. Movement is nearly stationary at the
moment, but it is sending out a thick layered mid and high clouds
across Oahu. Light icing can be expected between 150 and fl250
with these clouds.
Otherwise, VFR conditions rule.
no marine advisories are in effect, but a slight chance of
thunderstorms will continue across the marine area through
Moderate to locally fresh east-southeast winds will prevail
tonight across windward waters and the Alenuihaha Channel. Fresh
to strong trade winds will gradually fill in late Wednesday and
Thursday then continue into the weekend as high pressure rebuilds
north of the state. Small Craft Advisory winds are expected across
the typically windier waters during this time. No significant surf
is expected through the week.
Flash Flood Watch until midnight HST tonight for Big Island.