000 
fxhw60 phfo 241346 
afdhfo 


Area forecast discussion 
National Weather Service Honolulu HI 
346 am HST Sat Feb 24 2018 


Synopsis... 
moist and somewhat unstable conditions will persist across 
portions of the state through the weekend. East to southeast 
winds will focus rainfall over windward and southeast slopes. The 
greatest threat for heavy showers and thunderstorms will be over 
The Big Island tonight, then shift to the western end of the 
island chain over Kauai and Oahu on Sunday. Drier conditions will 
push back over the state early next week as potentially strong 
east to east- southeast winds develop across the entire state. 


&& 


Discussion... 
light to moderate showers continue to spread across the eastern 
coast of The Big Island this morning. Satellite imagery shows 
bands of heavier showers and thunderstorms lingering 200 miles 
west of Kauai. Elsewhere scattered light showers are forming over 
typical windward and mountain slopes. The threat for flash 
flooding has diminished over The Big Island and the Flash Flood 
Watch for Hawaii County was cancelled. The Winter Storm Warning 
for The Big Island summits was also cancelled with the morning 
forecast package. 


The large scale weather picture continues to show an upper level 
closed low roughly 900 miles northwest of Kauai and a strong 1040 
mb high center far to the northeast of the state. The interaction 
between these two features continues to draw tropical moisture 
northward over the Hawaiian islands keeping US in moist and 
moderately unstable low level environment. Morning upper air 
balloon soundings in Lihue confirm a weak high level subsidence 
inversion setting up around the 10000 to 12000 foot level. Model 
guidance indicates this weak inversion should help to keep showers 
mostly confined to windward and mountain areas today for the 
smaller islands. While deeper moisture and instability will 
linger over The Big Island today with elevated chances for shower 
activity along the eastern and southeastern slopes. 


The interaction between the ridge and a surface trough west of 
Kauai has increased winds over The Saddle and lower slopes of The 
Big Island. A Wind Advisory remains in effect for sustained winds 
forecast to range from 15 to 30 mph with locally higher gusts up 
to 50 mph for The Big Island through the afternoon hours. 


In the short term, we continue to see the upper low drifting 
slowly northwest away from the state as the strong high pressure 
ridge gradually builds in a more stable atmosphere from the 
northeast. On Sunday, forecast guidance continues to show a shift 
in shower activity towards the west with increasing rainfall 
chances over Kauai and portions of Oahu as an upper level 
negatively tilted trough sweeps across the western edge of the 
state. Kauai will see the best chances for heavier showers and 
thunderstorms, Oahu will also see some shower and thunderstorm 
enhancement. A Flash Flood Watch may be needed for these Western 
Islands. 


A drier trend develops next week as the strong ridge builds in 
over the state. Enough moisture and instability will linger over 
the windward sections of The Big Island for continued wet trade 
wind showers, however the rest of the state will see a drying 
trend from Monday into Tuesday. Stronger winds are expected on 
Tuesday afternoon and Wednesday where additional wind advisories 
may be needed as wind speeds ramp up across the state. 


&& 


Aviation... 
high pressure northeast of the state and a trough of low pressure 
to the west, will keep a gusty east to southeast flow in place 
across the island chain through tonight. A moist and unstable 
airmass will remain over the island chain, particularly the east and 
southeast areas of The Big Island, where widespread MVFR and isolated IFR 
conditions are expected. Across the smaller islands isolated MVFR 
conditions will be possible mainly in windward areas. Afternoon 
shower development will also be possible over interior and leeward 
sections of the islands as a result of daytime heating, bringing 
some brief MVFR conditions which should end during the early 
evening hours. 


Airmet Sierra remains in place for mountain obscuration across 
east to southeast facing slopes of The Big Island, and NE thru southeast facing 
sections of Kauai and east Maui. No other airmets are in effect. 


&& 


Marine... 
an unstable airmass will continue the possibility for isolated 
thunderstorms over all coastal waters today, before shifting to 
the western half tonight and tomorrow. 


Winds have backed off a touch in the waters near The Big Island, 
so the Gale Warning has been cancelled with the morning package. A 
Small Craft Advisory (sca) remains posted for most waters, 
particularly on the east sides of the islands due to the strong 
winds and elevated seas. Strong high pressure far to the north- 
northeast will maintain this breezy east to southeast wind pattern 
through the remainder of the weekend, before an uptick in winds 
during the first half of the new work week. Its possible the Small Craft Advisory 
will need to be expanded next week as the winds increase, and 
additional gale force winds are possible. 


A long and rather broad fetch of easterly winds on the south side 
of the high will continue to produce advisory level surf along 
east facing shores into early next week. The latest model guidance 
shows surf could reach warning levels as early as Monday night or 
Tuesday. This combined with high astronomical tides could cause 
some coastal inundation issues along some low lying areas exposed 
to east. 


The current small northwest swell will continue to lower 
gradually today. Otherwise surf along north and west facing 
shores will be well below normal for this time of year during the 
Sunday through Thursday time period. A series of small, mainly 
background south swells can be expected through the weekend and on 
into the middle of next week. 


See the latest Oahu surf discussion (srdhfo) for additional 
details on surf and swell. 


&& 


Hfo watches/warnings/advisories... 
high surf advisory until 6 am HST Monday for Kauai windward-Oahu 
koolau-Olomana-Molokai windward-Maui windward west-windward 
Haleakala-south Big Island-Big Island north and east. 


Wind Advisory until 6 PM HST this evening for Kona-south Big 
Island-Big Island north and east-kohala-Big Island interior. 


Small Craft Advisory until 6 am HST Monday for Kauai northwest 
waters-Kauai windward waters-Oahu windward waters-Kaiwi Channel- 
Maui County windward waters-Alenuihaha Channel-Big Island 
windward waters-Big Island leeward waters-Big Island southeast 
waters. 


&& 






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