Flash Flood Warning 12, NWS Honolulu, HI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 PM HST SUN JUL 13 2014

.SYNOPSIS...
A TROUGH PASSING JUST SOUTH OF THE BIG ISLAND WILL BRING AN INCREASE
IN SHOWERS AND MUGGY CONDITIONS TO THE STATE FOR THE NEXT FEW DAYS.
WHILE THE BULK OF THE RAIN WILL BE FOCUSED ACROSS WINDWARD
SECTIONS...SOME SHOWERS WILL SPILL OVER TO LEEWARD AREAS AS WELL.
MOISTURE WILL BE MOST ABUNDANT NEAR THE BIG ISLAND...WHERE THERE IS
ALSO A SLIGHT CHANCE OF THUNDERSTORMS THROUGH MONDAY. HIGH PRESSURE
STRENGTHENING FAR NORTHEAST OF THE AREA WILL BRING A RETURN TO
BREEZIER TRADES MID WEEK...AS WELL AS A RETURN TO MORE TYPICAL TRADE
WIND WEATHER.

&&

.DISCUSSION...
WATER VAPOR IMAGERY SHOWS AN UPPER LEVEL RIDGE OVER THE STATE...
EXTENDING FROM A HIGH TO OUR SOUTH. THE SUBTROPICAL JET IS NEAR AND
TO THE NORTH OF US...WHICH IS HELPING TO BRING A STEADY STREAM OF
HIGH CLOUDS OVER THE ISLANDS. AT THE SURFACE...A TROUGH IS LOCATED
JUST SOUTH OF THE BIG ISLAND. THIS TROUGH...THE REMNANTS OF FORMER
TROPICAL CYCLONE FAUSTO IN THE EASTERN PACIFIC...IS MOVING TO THE
WEST-NORTHWEST AT 15 MPH. OTHERWISE HIGH PRESSURE IS LOCATED FAR
NORTHEAST OF THE STATE. A STALLED AND WEAKENING SURFACE FRONT IS
LOCATED NORTH OF THE ISLANDS...WHICH IS HELPING TO WEAKEN THE
SURFACE RIDGE TO OUR NORTH. 00Z SOUNDINGS SHOW THE MAJOR CHANGE IN
AIRMASS ACCOMPANYING THE TROUGH....WITH A STABLE INVERSION NEAR
5KFT AND A PRECIPITABLE WATER AROUND 1 INCH AT LIHUE BUT NO
SIGNIFICANT INVERSION AND A PW OF 2.1 INCHES AT HILO. EARLY
AFTERNOON MIMIC TOTAL PRECIPITABLE WATER IMAGERY SHOWS THE TROPICAL
MOISTURE NEAR THE BIG ISLAND EXTENDING SEVERAL HUNDRED MILES
UPSTREAM TO THE EAST.

THE 00Z HILO SOUNDING SHOWS A CONVECTIVELY UNSTABLE AIRMASS...WITH
OVER 700 J/KG OF MIXED-LAYER CAPE. A COUPLE THUNDERSTORMS DEVELOPED
ACROSS THE INTERIOR SLOPES OF THE BIG ISLAND THIS AFTERNOON...AND WE
HAVE ALSO RECEIVED A NUMBER OF REPORTS FROM RAIN GAGES OF RAINFALL
RATES IN EXCESS OF 1-2 INCHES PER HOUR. MOISTURE CONTINUES TO STREAM
INTO THE BIG ISLAND...AND THE POTENTIAL FOR HEAVY RAIN WILL CONTINUE
DUE TO THE ABUNDANT MOISTURE. THE THUNDERSTORMS WERE INITIALLY AIDED
BY AFTERNOON HEATING...BUT WITH ENHANCED LOW-LEVEL CONVERGENCE NEAR
THE SURFACE TROUGH AND THE POTENTIAL FOR ADDITIONAL DESTABILIZATION
FROM CLOUD TOP COOLING OVERNIGHT...WILL ALSO CARRY A MENTION OF
THUNDERSTORMS THROUGH THE NIGHT. SIMILAR CONDITIONS ARE EXPECTED ON
MONDAY...WITH THE MOIST AND UNSTABLE AIRMASS SPREADING DOWN THE
ISLAND CHAIN. FOR NOW HAVE CONFINED THE MENTION OF THUNDER TO THE
BIG ISLAND THROUGH MONDAY...DUE TO ITS PROXIMITY CLOSER TO THE
TROUGH. THIS WILL NEED TO BE REEVALUATED AS THE SITUATION CONTINUES
TO UNFOLD.

IN ADDITION TO RAINFALL...THE INCREASE IN MOISTURE WILL LEAD TO
MUGGY CONDITIONS ACROSS THE STATE WITH SURFACE DEWPOINTS CLIMBING
INTO THE LOW 70S. WINDS WILL DIMINISH SOMEWHAT BEHIND THE TROUGH AND
TURN SLIGHTLY MORE EAST-SOUTHEAST THAN NORMAL. HOWEVER...THE TROUGH
ITSELF WILL WEAKEN AS IT PASSES SOUTH OF THE STATE...AND WILL HAVE
INCREASINGLY LESS INFLUENCE. ONE THING TO NOTE IS THAT THE TROUGH
HAS HELPED TO MAINTAIN STRONGER WINDS FOR THE LAST COUPLE DAYS. THE
STALLED FRONT TO OUR NORTH WEAKENED THE SUBTROPICAL RIDGE...AND
NORMALLY WOULD HAVE WEAKENED THE PRESSURE GRADIENT AND THE WINDS
OVER THE STATE. HOWEVER...THE LOWER PRESSURE WITH THE TROUGH TO THE
SOUTHEAST HAS MAINTAINED THE GRADIENT ACROSS THE STATE...AND KEPT
WINDS AT NEAR STEADY LEVELS. THE TROUGH WILL PAST WEST OF THE STATE
BY MONDAY...ALTHOUGH THE ASSOCIATED MOISTURE WILL LINGER INTO
TUESDAY BEFORE CLEARING OUT.

BY THE MIDDLE OF THE WEEK...THE FRONT NORTH OF THE STATE WILL
DISSIPATE AND THE SUBTROPICAL RIDGE WILL BUILD BACK BEHIND IT. THE
12Z GFS AND ECMWF ARE SIMILAR SHOWING THE HIGH FAR TO THE NORTHEAST
STRENGTHENING AND EXPANDING THROUGH THE SECOND HALF OF THE WEEK...
WITH IT GRADUALLY MIGRATING WESTWARD AS WELL. THIS WILL LEAD TO A
STRENGTHENING IN TRADE WINDS DURING THE SECOND HALF OF THE WEEK.

ONE OTHER FEATURE TO NOTE ARE THE PERSISTENT HIGH CLOUDS THAT
CONTINUE TO AFFECT THE STATE. THE SUBTROPICAL RIDGE WILL GRADUALLY
SHIFT EASTWARD THROUGH MID-WEEK...BUT WILL STILL BE IN A POSITION TO
PULL HIGH-LEVEL MOISTURE NORTHWARD ACROSS THE ISLANDS. THEREFORE WE
CAN EXPECT VARYING DEGREES OF HIGH CLOUDS TO REMAIN IN PLACE FOR THE
NEXT SEVERAL DAYS.

&&

.AVIATION...
ISOL THUNDERSTORMS WITH TOPS TO FL350 HAVE POPPED UP ALONG THE
WINDWARD AND SE BIG ISLAND SLOPES THIS AFTERNOON. LOWER CLOUDS AND
SHOWERS WITH EMBEDDED TEMPO MVFR CONDITIONS AND ISOL IFR CONDITIONS
CONTINUE TO COVER THE BIG ISLAND FROM UPOLU POINT TO CAPE KUMUKAHI
TO SOUTH POINT AND AN AIRMET SIERRA IS IN EFFECT FOR MTN OBSC.
LIKEWISE...THERE IS AN AIRMET SIERRA IN EFFECT FOR N THRU E SECTIONS
OF MAUI AND MOLOKAI. EXPECT THESE AIRMETS TO STAY IN EFFECT THROUGH
AT LEAST TOMORROW MORNING AS THE SURFACE TROUGH AND ASSOCIATED
ENHANCED MOISTURE MOVE WEST. OAHU AND KAUAI WILL LIKELY SEE THE
ISSUANCE OF AN AIRMET SIERRA OVER THEIR N THRU E SECTIONS TONIGHT.

&&

.MARINE...
EVEN WITH THE WEAKENING RIDGE FAR NORTH OF THE STATE...THE TROUGH
PASSING JUST TO THE SOUTH HAS MAINTAINED ENOUGH OF A PRESSURE
GRADIENT TO WARRANT ONCE AGAIN EXTENDING THE EXISTING SMALL CRAFT
ADVISORY THROUGH TONIGHT. AS THE TROUGH MOVES WEST OF THE BIG ISLAND
TONIGHT...THE GRADIENT SHOULD RELAX ENOUGH TO ALLOW WINDS TO DROP
WINDS BELOW THE 25 KNOT ADVISORY THRESHOLD. ANY DECREASE IN WINDS
WILL BE A BRIEF ONE THOUGH...AS BUILDING HIGH PRESSURE FAR NORTHEAST
OF THE STATE WILL HELP TO STRENGTHEN WINDS ENOUGH TO LIKELY REQUIRE
A SMALL CRAFT ADVISORY DURING THE SECOND HALF OF THE WEEK.

A SOUTH SWELL GRADUALLY BUILDING THROUGH THE DAY WILL BRING
INCREASED SURF ALONG SOUTH FACING SHORES THROUGH TUESDAY...WITH
EXPECTATIONS THAT SURF WILL APPROACH BUT REMAIN BELOW ADVISORY
LEVELS. THE LANAI AND BARBERS POINT BUOYS CONTINUE TO BE DOMINATED
BY ENERGY AROUND 14 SECONDS...BUT BUOY 51004 HAS SOME LOW ENERGY
AROUND 19 SECONDS. ANOTHER PULSE OF SOUTH SWELL WILL ARRIVE AROUND
WEDNESDAY...SUPPORTING MODERATE SURF ALONG SOUTH FACING SHORES INTO
NEXT WEEKEND.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
SMALL CRAFT ADVISORY UNTIL 6 AM HST MONDAY FOR MAALAEA BAY-PAILOLO
CHANNEL-ALENUIHAHA CHANNEL-BIG ISLAND LEEWARD WATERS-BIG ISLAND
SOUTHEAST WATERS.

&&

PUBLIC/MARINE...BRAVENDER
AVIATION...EATON