Heavy Rain in Hawaii, May 17 to 20, 2013

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
955 AM HST FRI MAY 17 2013

.SYNOPSIS...
A HIGH NORTHEAST OF THE AREA WILL MAINTAIN LOCALLY BREEZY TRADE
WINDS TODAY. THE TRADES WILL FOCUS SHOWERS OVER MAINLY WINDWARD
AREAS. A TROUGH APPROACHING FROM THE NORTHWEST WILL RAPIDLY WEAKEN
THE WINDS STARTING TOMORROW. MOISTURE AND INSTABILITY ASSOCIATED
WITH THE TROUGH WILL BRING UNSETTLED WEATHER THROUGH MOST OF NEXT
WEEK.

&&

.DISCUSSION...
WE UPDATED THE GRIDS EARLIER TO ADJUST POPS OVER MAINLY LEEWARD
AREAS. NO OTHER CHANGES ANTICIPATED TO THE SHORT TERM PORTION OF THE
FORECAST AT THIS TIME. THE SERIES OF UPPER TROUGHS IN OUR VICINITY
WILL CONTINUE TO PROVIDE SOME ENHANCEMENT TO LOCAL SHOWERS THIS
WEEKEND...BUT THERE ARE SOME TOUGH QUESTIONS TO ANSWER AS TO THE
PLACEMENT AND TIMING OF ANY HEAVIER SHOWERS BASED ON THE AMOUNT AND
LOCATION OF THE FORCING PROVIDED BY LOW LEVEL SYNOPTIC FLOW OR
MESOSCALE ISLAND CIRCULATIONS. WILL LIKELY BE MAKING SOME
ADJUSTMENTS TO THE WEEKEND TIME FRAME AND BEYOND AFTER WE STUDY THE
17/12Z ECMWF SOLUTION AND UPDATED MULTI-MODEL ENSEMBLE SOLUTION.

&&

.PREV DISCUSSION...
A SFC HIGH PRESSURE REMAINS CENTERED NE OF THE ALOHA STATE AND
KEEPING A TRADE WIND WEATHER REGIME ACROSS THE AREA THROUGH THE NEXT
COUPLE OF DAYS. THE TRADES REMAIN MODERATE TO LOCALLY BREEZY WITH
SHOWERS FAVORING WINDWARD AND MAUKA AREAS. SOME OF THE LARGER
SHOWERS WILL BE PUSHED OVER THE MOUNTAIN RANGES AND INTO LEEWARD
AREAS AT TIMES...ESPECIALLY AT NIGHT.

AN UPPER LVL LOW LINGERING OVER THE ISLAND CHAIN WILL KEEP ENHANCED
SHOWER ACTIVITY TODAY AND THROUGH EARLY SAT...THEN QUICKLY PUSH E
AS A DEEPENING UPPER TROUGH MOVES ACROSS THE N CPAC DURING THE
WEEKEND. MODELS STILL SHOW FAIR AGREEMENT REGARDING THE OVERALL
SOLUTION FOR THIS TROUGH...WHICH DIGS FURTHER S TOWARDS THE ISLAND
CHAIN AND DESTABILIZES THE AIRMASS BY BRINGING COLDER TEMPS ALOFT
AND INCREASING DEEP MOISTURE. AT THE LOW LVLS...A SFC TROUGH
REFLECTION SEEMS TO DEVELOP ACROSS THE ISLANDS. WITH THIS SYNOPTIC
SETUP IN PLACE IT IS POSSIBLE THAT A HYBRID WIND REGIME WILL
ESTABLISH...WITH LIGHT SE FLOW OVER WINDWARD AREAS OF THE BIG ISLAND
AND MAUI...AND A WEAK NE FLOW OVER THE W HALF OF THE ISLAND CHAIN.
IF THIS PATTERN MATERIALIZES...EXPECT WINDS TO BECOME WEAK...
ESPECIALLY OVER OAHU AND MAUI COUNTY...ALLOWING FOR SEA BREEZES TO
DEVELOP EACH DAY FROM SUN THROUGH AT LEAST THE MIDDLE OF NEXT WEEK.
SHOWERS WILL BECOME MORE WIDESPREAD ACROSS THE ISLANDS WITH HIGHER
CHANCES OF RAIN OVER INTERIOR AND MOUNTAIN AREAS.

THE WEAK SE FLOW AROUND THE BIG ISLAND NEXT WEEK MAY COMBINE WITH
THE LIGHT WINDS ELSEWHERE AND BRING WIDESPREAD VOG ACROSS THE
SMALLER ISLANDS...WHICH SHOULD ALSO REMAIN IN PLACE THROUGH AT LEAST
THE MIDDLE OF NEXT WEEK.

&&

.MARINE...
THE FRESH TRADE WINDS ARE EXPECTED TO CONTINUE TODAY. THEREFORE...
THE SMALL CRAFT ADVISORY FOR WINDIER WATERS NEAR MAUI AND THE BIG
ISLAND WILL BE EXTENDED UNTIL 6 PM HST THIS EVENING. CONDITIONS
SHOULD DROP TO BELOW ADVISORY LEVELS LATER TONIGHT.

BUOYS SO FAR ARE RUNNING RIGHT ABOUT...OR JUST SLIGHTLY OVER...WHAT
WAS EXPECTED. WILL NOT MAKE ANY CHANGES TO FORECAST SURF HEIGHTS AT
THIS POINT.

&&

.AVIATION...
VFR COND WILL PREVAIL ACROSS THE ISLAND CHAIN TODAY. BRIEF MVFR CIGS
AND MTN OBSC ALONG THE WINDWARD SIDES OF THE ISLANDS DUE TO PASSING
LOW CLOUDS AND SHRA.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
SMALL CRAFT ADVISORY UNTIL 600 PM HST THIS EVENING FOR THE PAILOLO
AND ALENUIHAHA CHANNELS...MAALAEA BAY...AND WATERS SOUTH OF THE BIG
ISLAND.

HIGH SURF ADVISORY FOR SOUTH FACING SHORES OF ALL ISLANDS THROUGH
THE WEEKEND.

&&

DISCUSSION...R BALLARD
AVIATION...FOSTER