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Hawaiian Islands Synoptic Discussion and Guidance

009
FXHW60 PHFO 021325
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 AM HST MON MAR 2 2015

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

&&

.DISCUSSION...
DEEPLY STACKED TROUGHING LINGERED 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 GULF OF ALASKA.
THE MOST SIGNIFICANT DYNAMICS NOW OCCURRED JUST TO THE NORTHEAST OF
THE OFFSHORE WATERS...BUT SHARP TEMPERATURE AND HUMIDITY GRADIENTS
KEPT CONDITIONS UNSTABLE OVER NORTHEAST THROUGH SOUTH PARTS OF THE
OFFSHORE WATERS...AND TO A LESSER EXTENT 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 THE NORTHWEST
HALF OF THE STATE 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 THE OFFSHORE
WATERS...BUT SIMPLE THERMODYNAMIC INSTABILITY SEEMED LIKE THE MOST
SIGNIFICANT INGREDIENT THIS MORNING.

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...ESPECIALLY ONCE SOLAR HEATING
BECOMES IMPORTANT AGAIN LATER THIS MORNING. THUNDERSTORMS AS SUCH
WILL DEVELOP FURTHER SOUTHEAST WITH TIME...BECOMING LIMITED MAINLY
TO AROUND THE BIG ISLAND TODAY...BUT TOWERING CUMULI MAY CONTINUE TO
PRODUCE LOCALLY SIGNIFICANT SHOWERS THROUGH THE FIRST HALF OF THE
WEEK.

THE RISK OF AFTERNOON CONVECTION ALSO MOTIVATED EXTENDING THE WINTER
WEATHER ADVISORY OVER THE BIG ISLAND SUMMITS THROUGH THIS AFTERNOON.
WITH THE COLD POOL FIRMLY IN PLACE...ANY CONVECTIVE SHOWERS FORMING
OVER THE BIG ISLAND SLOPES WILL PRODUCE MOSTLY FROZEN PRECIPITATION
THERE.

BY WEDNESDAY...GUIDANCE PREDICTED THE TROUGH ALOFT TO REFORM AND
SHARPEN ALONG A ZONAL ORIENTATION...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 THE NORTHWEST
END 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 WILL REMAIN NORTHEASTERLY.

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. RECENT RUNS OF THE GFS HAVE
PREDICTED THE AREA OF MAXIMUM POPS TO RETREAT TO THE SOUTH AGAIN A
LITTLE MORE QUICKLY THAN EARLIER RUNS DID...SO CLOUDS AND SHOWERS
HAVE NOT BEEN INCREASED FOR THE WEEKEND OVER EARLIER THINKING.
HOWEVER...FURTHER ADJUSTMENTS MAY BE NEEDED WITH TIME.

&&

.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. 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 TODAY...SO A HIGH SURF ADVISORY
REMAINED IN EFFECT. THE SWELL ALSO BUILT SEAS TO THE THRESHOLD FOR A
SMALL CRAFT ADVISORY OVER MOST OF THE COASTAL WATERS. SMALLER
NORTH-NORTHWEST SWELLS ARE LIKELY TO ARRIVE LATER IN THE WEEK.

&&

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

WINTER WEATHER ADVISORY UNTIL 6 PM HST THIS EVENING FOR BIG
ISLAND SUMMITS.

SMALL CRAFT ADVISORY UNTIL MIDNIGHT HST TONIGHT FOR KAUAI
NORTHWEST WATERS-KAUAI WINDWARD WATERS-KAUAI LEEWARD WATERS-
KAUAI CHANNEL-OAHU WINDWARD WATERS-OAHU LEEWARD WATERS-KAIWI
CHANNEL-MAUI COUNTY WINDWARD WATERS-MAUI COUNTY LEEWARD WATERS-
PAILOLO CHANNEL-ALENUIHAHA CHANNEL-BIG ISLAND WINDWARD WATERS.

&&

$$

RYSHKO



Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office