Flash Flooding and Heavy Rain in Hawaii, March 6 to 10, 2009

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST FRI MAR 6 2009

.SYNOPSIS...
MODERATE TRADE WINDS WILL PERSIST THROUGH THE WEEKEND AND INTO NEXT
WEEK. HOWEVER...THE ATMOSPHERE WILL BECOME INCREASINGLY UNSTABLE ON
SATURDAY...INTENSIFYING SHOWERS...AND ALLOWING A SLIGHT CHANCE OF
THUNDERSTORMS FOR MOST OF THE ISLANDS. CONDITIONS WILL BECOME LESS
SOGGY ON SUNDAY...BUT STILL REMAIN UNSETTLED THROUGH WEDNESDAY.

&&

.DISCUSSION...
RIDGING AT THE SURFACE TO THE NORTH OF THE MAIN HAWAIIAN ISLANDS
KEPT MODERATE TRADE WINDS BLOWING ACROSS THEM. ALTHOUGH HIGHER CLOUD
LAYERS OBSCURED LOWER FEATURES IN SATELLITE IMAGES...RADAR DATA
REVEALED INCREASED SHOWER ACTIVITY EAST OF THE ISLANDS.

SYNOPTIC ANALYSIS STILL SHOWED A WEAK SURFACE TROUGH TO THE WEST OF
THE STATE. ALTHOUGH GUIDANCE GENERALLY SUPPORTED SLIGHT TROUGHING
TO CONTINUE...THE ECMWF NOTABLY PREDICTS THIS FEATURE TO DEEPEN INTO
A SIGNIFICANT LOW AND MOVE NORTHEASTWARD ACROSS THE ARCHIPELAGO.
THIS SOLUTION SEEMS OUT OF STEP WITH THE CONSENSUS...BUT IF IT DOES
BECOME MORE LIKELY...THEN MUCH OF THE WIND FORECAST FOR THE BIG
ISLAND WILL REQUIRE REVISION FOR SOUTHWESTERLY WINDS AT LEAST FROM
SATURDAY THROUGH THURSDAY...WHILE THE REST OF THE ISLANDS WOULD
EXPERIENCE STRONGER TRADES AT FIRST...FOLLOWED BY NORTHERLIES TO
NORTHWESTERLIES THROUGH THE REST OF THE WEEK. HOWEVER...AT PRESENT
THE GFS SOLUTION OF TRADES OVER THE ISLANDS THROUGH THE PERIOD LOOKS
MORE REALISTIC.

ALOFT...THE ELEVATED BASE OF THE INVERSION WAS ONLY ONE OF SEVERAL
UNUSUAL PHENOMENA. RIDGING IN THE MIDDLE ATMOSPHERE EXTENDED FROM
SOUTHWEST TO NORTHEAST JUST TO THE NORTHWEST OF THE ISLANDS...
REINFORCING THE HIGH AT THE SURFACE. HOWEVER...THE MID-LEVEL RIDGE
WAS LOCATED FAR ENOUGH TO THE NORTHWEST OF THE STATE THAT A NARROW
BUT SIGNIFICANT MID-LEVEL TROUGH MANAGED TO WEDGE ITSELF INTO THE
INTERVENING SPACE. THE UPLIFT...MOISTURE...AND COLD POOL ASSOCIATED
WITH THIS LOW HELPED TO CREATE THE STREAM OF MID-LEVEL AND HIGH
CLOUDS COVERING THE AREA...AND ALSO WEAKENED THE INVERSION OVER THE
ISLANDS. ON THE OTHER HAND...THERE WAS SO MUCH MOISTURE AND WARM
ADVECTION AT MID-LEVELS IN THE HILO SOUNDING THAT CONVECTION FROM
LOWER LEVELS WAS SUPPRESSED. IT REMAINS TO BE SEEN WHETHER THIS
MOISTURE ALOFT CAN CONVECT INTO THE DRIER AIR ABOVE IT.

MOIST CLOUDS FLOWING OVER THE SUMMITS OF THE BIG ISLAND CONTINUED TO
PRODUCE RIME ICE...AND PERHAPS SOME SNOW AS WELL. WIND AT UPPER
ELEVATIONS REMAINED JUST BELOW THE ADVISORY LEVEL...AND MAY RISE
FURTHER LATER IN THE WEEK IF THE JET STREAM SHIFTS CLOSER TO THE
SUMMITS AGAIN.

THROUGH SATURDAY NIGHT...THE TROUGH ALOFT HAS THE POTENTIAL BOTH TO
INTENSIFY WINDWARD TRADE SHOWERS...AND TO PERMIT AIR-MASS SHOWERS TO
DEVELOP LEEWARD AS WELL. ACCUMULATION FROM SUCH SHOWERS WILL VARY
CONSIDERABLY...BUT MAY BE HEAVY AT TIMES. A SHORT WAVE ON THE TROUGH
WILL BRING INCREASED VORTICITY AND COLDER TEMPERATURES OVER THE AREA
FROM SATURDAY INTO SATURDAY NIGHT...SO THE POTENTIAL FOR HEAVY RAIN
AND THUNDERSTORMS WILL PEAK DURING THIS TIME. ANOTHER SHORT WAVE MAY
ROTATE AROUND THE TROUGH FROM MONDAY INTO TUESDAY...AND IF GUIDANCE
CONTINUES TO SUPPORT THIS EVENT...THEN WE WILL HAVE TO RAISE POPS
ACCORDINGLY.

GUIDANCE STILL AGREES FAIRLY WELL THAT PROGRESSIVE MID-LEVEL TROUGHS
WILL PASS WELL NORTH OF THE STATE...LEAVING THE SITUATION ACROSS THE
ISLANDS LARGELY UNCHANGED THROUGH AT LEAST NEXT THURSDAY. BY FRIDAY
HOWEVER...GUIDANCE BUILDS THE RIDGE WEST OF THE STATE TO THE POINT
WHERE IT MAY SQUEEZE THE TROUGH OUT TO THE NORTHEAST...IMPLYING MORE
STABLE TRADE-WIND CONDITIONS.

&&

.MARINE...
A NEW NORTHWEST SWELL MAY BRING SURF HEIGHTS TO NEAR THE ADVISORY
LEVEL OF 15 FEET ALONG SHORES FACING NORTH BY EARLY MONDAY.

WINDS IN THE CHANNELS AND AROUND THE BIG ISLAND REMAIN STRONG ENOUGH
TO REQUIRE A SMALL CRAFT ADVISORY THROUGH SUNDAY EVENING.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY UNTIL 6 AM HST SUNDAY FOR BIG ISLAND
SUMMITS.

SMALL CRAFT ADVISORY UNTIL 6 PM HST SUNDAY FOR MAALAEA BAY-
PAILOLO CHANNEL-ALENUIHAHA CHANNEL-BIG ISLAND LEEWARD WATERS-BIG
ISLAND SOUTHEAST WATERS.

&&

RYSHKO