Prognostic Meteorological Discussion
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000
FXHW01 KWNH 261237
PMDHI

HAWAII EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
837 AM EDT WED APR 26 2017

VALID 00Z THU APR 27 2017 - 00Z THU MAY 04 2017

THE INITIAL SYNOPTIC PATTERN SUPPORTS BROAD UPPER RIDGING
EXTENDING FROM THE HAWAI`IAN ISLAND CHAIN EASTWARD TOWARD 135W
LONGITUDE. AS TIME PROCEEDS FORWARD, AMPLIFIED FLOW UPSTREAM WILL
TREK EASTWARD WITH THE BASE OF THE TROUGH REACHING THE STATE BY
FRIDAY MORNING. AT THIS POINT THE FLOW REGIME BECOMES RATHER
BLOCKED IN NATURE AS A CLOSED LOW SAGS SOUTHWARD FROM THE
MID-LATITUDES. THE CIRCULATION CENTER IS FORECAST TO CONTINUE ITS
SLOW MIGRATION SOUTHWARD OVER THE WEEKEND WITH THE UPPER LOW
GENERALLY OVER 25N/155W BY 30/1800Z. THERE IS ONE NOTABLE OUTLIER
WHICH IS THE 00Z UKMET GIVEN ITS DISPLACEMENT TO THE SOUTHWEST OF
THE MODEL CONSENSUS. EVENTUALLY THE NEGATIVE HEIGHT ANOMALY WILL
REACH HAWAI`I BY LATE SUNDAY/EARLY MONDAY WITH GRADUAL SHEARING
ALONG WITH AN EVENTUAL SHIFT EASTWARD TOWARD THE MIDDLE OF NEXT
WEEK.

AT THE SURFACE, WITH AN ANTICYCLONE CENTERED NORTHEAST OF THE
REGION, MODERATE TRADE WINDS ARE TO BE EXPECTED BEFORE A SHIFT IN
THE PATTERN IS EVIDENT. WITH THE ASSOCIATED COLD FRONT APPROACHING
FROM THE NORTHWEST, A LARGE CHANGE IN THE SURFACE PATTERN IS
FORECAST WITH NORTH-NORTHEASTERLY FLOW PREVAILING WITHIN THE COOL
SECTOR OF THE CYCLONE. EXPECTED WINDS WILL BE IN THE 15 TO 20 KNOT
RANGE WITH THIS COLD ADVECTION MOVING FROM NORTH TO SOUTH ACROSS
THE STATE. GRADUALLY WINDS WILL BECOME MORE CYCLONIC ACROSS THE
REGION AS THE VERTICALLY STACKED SYSTEM MOVES DIRECTLY OVERHEAD.
AS THE SURFACE GRADIENTS WEAKEN, A DECREASE IN THE WIND FIELD IS
LIKELY THROUGH THE REMAINDER OF THE PERIOD.

IN TERMS OF PRECIPITATION, TRADE WIND DRIVEN SHOWERS ARE POSSIBLE
INITIALLY BUT CONDITIONS WILL GRADUALLY BECOME WETTER AS THE
AFOREMENTIONED UPPER LOW MOVES CLOSER TO THE REGION. EXPECT AN
INITIAL INCREASE IN PRECIPITATION COVERAGE AS THE COLD FRONT
PASSES THROUGH. HOWEVER, RAINFALL AMOUNTS SHOULD BE ON THE LIGHTER
SIDE GIVEN ONLY MODEST PRECIPITABLE WATER VALUES ACCOMPANYING THE
FRONTAL PASSAGE. EVENTUALLY STRONG FORCING FROM THE UPPER LOW
ITSELF COUPLED WITH INCREASING MOISTURE ADVECTION SHOULD LEAD TO
MORE EXTENSIVE RAINFALL COVERAGE BY THE END OF THE WEEKEND INTO
EARLY NEXT WEEK. BASED ON THE AVAILABLE GUIDANCE, THE BEST THREAT
FOR HEAVIER AMOUNTS WILL BE ACROSS THE MORE SOUTHERN ISLANDS. SOME
FAIRLY HEFTY CONVECTION IS LIKELY GIVEN THE ANOMALOUS UPPER LOW
COUPLED WITH ITS RATHER SLOW FORWARD MOTION.


RUBIN-OSTER

$$




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