Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 020604
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
830 PM HST SUN MAR 1 2015

.SYNOPSIS...
SHOWERS AND THUNDERSTORMS AROUND MAUI AND THE BIG ISLAND WILL
DIMINISH SLOWLY THROUGH THE FIRST HALF OF THE WEEK. A VERY MOIST AIR
MASS SURGING FROM THE SOUTH...AND COOL AIR MOVING IN OVERHEAD AND TO
THE NORTH...WILL SPREAD STRONGER SHOWERS OVER THE STATE AGAIN FROM
THURSDAY INTO THE WEEKEND. TRADE WINDS WILL PERSIST OVER MOST OF THE
ISLANDS THROUGH THE FORECAST PERIOD.

&&

.DISCUSSION...
DEEPLY STACKED TROUGHING PREVAILED AROUND THE BIG ISLAND. AT THE
SURFACE...THE TROUGH REPRESENTED THE TAIL END OF A MOSTLY DISSIPATED
FRONT. ALOFT...LOOSE...BAGGY TROUGHING EXTENDED OVER MOST OF THE
STATE FROM THE DEEPER VORTEX LOCATED MOSTLY OVER THE NORTHEAST
PACIFIC. THE MOST SIGNIFICANT DYNAMICS NOW OCCURRED JUST TO THE
NORTHEAST OF THE OFFSHORE WATERS...BUT SHARP TEMPERATURE AND
HUMIDITY GRADIENTS KEPT CONDITIONS UNSTABLE OVER MAUI AND HAWAII
COUNTIES.

THESE DIFFERENCES IN AIR MASS OCCURRED IN TWO DIFFERENT WAYS.
VERTICALLY...DESPITE ITS POOR DYNAMICS...THE SHALLOW MID-LEVEL
TROUGH OVERHEAD INCLUDED AN IMPRESSIVE COLD POOL. SIMILARLY...THOUGH
TO A LESSER EXTENT...THE AIR MASS AT LOW LEVELS OVER MOST OF THE
ISLANDS STILL RETAINED SOME OF ITS COOL AND DRY POLAR CHARACTER. THE
EXTREMELY MOIST AIR MASS OVER THE SOUTHEAST END OF THE STATE
CONTRASTED SHARPLY WITH BOTH THE AIR ABOVE AND TO ITS NORTHWEST.
SHORT WAVES FLOWING AROUND THE EDGE OF THE LOW ALOFT ALSO
CONTRIBUTED TO THE THUNDERSTORM DEVELOPMENT OVER MAUI AND THE BIG
ISLAND...BUT SIMPLE THERMODYNAMIC INSTABILITY SEEMED LIKE THE MOST
SIGNIFICANT INGREDIENT THIS EVENING.

THAT BEING THE CASE...THE CONVECTION LIKELY WILL CONTINUE THROUGH
THE EARLY PART OF THE WEEK EVEN IF THE TROUGH ALOFT FILLS
CONSIDERABLY AS GUIDANCE SUGGESTED. THUNDERSTORMS AS SUCH WILL
DEVELOP FURTHER SOUTHEAST WITH TIME...BECOMING LIMITED MAINLY TO
AROUND THE BIG ISLAND ON MONDAY...BUT TOWERING CUMULI MAY CONTINUE
TO PRODUCE LOCALLY SIGNIFICANT SHOWERS THROUGH THE FIRST HALF OF THE
WEEK.

BY WEDNESDAY...GUIDANCE PREDICTED THE TROUGH ALOFT TO REFORM AND
SHARPEN...PROVIDING MARGINALLY INCREASED DYNAMIC SUPPORT AGAIN.
ALTHOUGH CONSENSUS WAS SOMEWHAT INDIFFERENT...SOME SOLUTIONS SHOWED
A SIGNIFICANT SURGE OF WATER VAPOR FROM THE DEEP TROPICS AS WELL.
THE AIR MASS OVER MOST OF THE STATE LIKELY WILL MODIFY BY THAT
TIME...BUT STILL SHOULD REMAIN SUBSTANTIALLY COOLER AND DRIER THAN
THIS SURGE FROM THE SOUTH...ESPECIALLY IF ANOTHER WEAK FRONTAL
REMNANT REACHES THE STATE AS MOST SOLUTIONS SHOWED. THUS...BY
THURSDAY...CONVECTION...OR AT LEAST SIGNIFICANT SHOWER
DEVELOPMENT...SHOULD START SPREADING NORTH ACROSS THE STATE
AGAIN...EVEN THOUGH WINDS SHOULD REMAIN NORTHEASTERLY OVER THE
SMALLER ISLANDS.

DEPENDING ON HOW STRONG THE MOISTURE SURGE FROM THE SOUTH TURNS OUT
TO BE...INCREASED SHOWERS MAY WELL CONTINUE THROUGH THE SECOND HALF
OF THE WEEK AND INTO THE WEEKEND. SOME ADJUSTMENTS MAY BE NEEDED
WITH TIME...BUT FOR NOW THIS WET TRADE-WIND PATTERN LOOKED FAIRLY
SOLID.

&&

.AVIATION...
AIRMET SIERRA IS POSTED FOR MTN OBSC OVER THE BIG ISLAND. CLOUD TOPS
TO FL350 AND LOCALIZED IFR CONDITIONS CAN BE EXPECTED IN
THUNDERSTORMS.

VFR CONDITIONS WILL PREVAIL OVER THE SMALLER ISLANDS WITH SHOWERS
MOVING OVER THOSE ISLANDS FROM THE NORTH-NORTHEAST. TEMPO MVFR IS
POSSIBLE IN THESE SHOWERS.

AIRMET TANGO FOR TEMPO MOD TURB BETWEEN FL250 AND FL350 REMAINS OVER
MAUI AND THE BIG ISLAND DUE TO A STRONG SW TO NE JET STREAM.

&&

.MARINE...
A TROUGH...THE SOUTH END OF A LARGELY DISSIPATED FRONT...LANGUISHED
NEAR THE BIG ISLAND...KEEPING WINDS LIGHT AND VARIABLE THERE...BUT
MODERATE TO STRONG FROM THE NORTHEAST AROUND KAUAI...WHERE A SMALL
CRAFT ADVISORY REMAINED IN EFFECT. MORE WIDESPREAD WINDY CONDITIONS
ARE EXPECTED BY MID-WEEK.

A LARGE NORTH-NORTHWEST SWELL SHOULD PRODUCE SURF ABOVE THE
THRESHOLD FOR A HIGH SURF ADVISORY LATE TONIGHT AND MONDAY...SO A
HIGH SURF ADVISORY REMAINED IN EFFECT. SMALLER NORTH-NORTHWEST
SWELLS ARE LIKELY TO ARRIVE LATER IN THE WEEK.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
HIGH SURF ADVISORY FROM MIDNIGHT TONIGHT TO MIDNIGHT HST MONDAY
NIGHT FOR NIIHAU-KAUAI WINDWARD-KAUAI LEEWARD-OAHU NORTH SHORE-
OAHU KOOLAU-OLOMANA-MOLOKAI-MAUI WINDWARD WEST-MAUI CENTRAL
VALLEY-WINDWARD HALEAKALA.

WINTER WEATHER ADVISORY UNTIL 6 AM HST MONDAY FOR BIG ISLAND
SUMMITS.

SMALL CRAFT ADVISORY UNTIL 6 AM HST MONDAY FOR KAUAI LEEWARD
WATERS.

&&

$$

RYSHKO




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