371
FXHW01 KWNH 180837
PMDHI
HAWAII EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
437 AM EDT FRI SEP 18 2026
VALID 00Z SAT 19 SEP 2026 - 00Z SAT 26 SEP 2026
...PERIODS OF FLASH FLOODING/EXCESSIVE RAINFALL WITH THE REMAINS
OF NORBERT TODAY/FRIDAY AND WITH THE REMAINS OF T.D. 15-E MID TO
LATE NEXT WEEK...
THE MODEL GUIDANCE GENERALLY SHOWS AGREEMENT OVER THE NEXT WEEK.
PRECIPITABLE WATER VALUES REMAIN ABOVE AVERAGE, WHICH SHOULD LEAD
TO HEAVIER THAN AVERAGE RAINFALL THROUGH THE PERIOD. NORBERT'S
REMNANTS MOVE WESTWARD, LOWERING THE HEAVY RAIN/EXCESSIVE RAINFALL
THREAT WITH TIME TODAY/FRIDAY INTO SATURDAY. THIS WEEKEND INTO
MONDAY IN NORBERT'S WAKE, ABOVE AVERAGE MOISTURE REMAINS, LEADING
TO WINDWARD/MAUKA SHOWERS WITH HEAVIER RAINFALL THAN USUAL BEING
POSSIBLE.
FROM MONDAY INTO WEDNESDAY, RIDGING ACROSS THE NORTH-CENTRAL
PACIFIC GETS SMALLER IN SCALE AS AN UPPER LEVEL TROUGH DIGS TO
THE WEST OF THE INTERNATIONAL DATELINE AND UPPER LEVEL TROUGHING
STRENGTHENS/AMPLIFIES ACROSS THE NORTHEAST PACIFIC. TO SOME
DEGREE, THE SURFACE RIDGE RESPONDS, AND THERE SHOULD BE AT LEAST
MODEST WEAKENING OF THE TRADE WINDS AFTER NORBERT PASSES BY. BY
THURSDAY, AN UPPER LEVEL LOW CUTS OFF NEAR THE PAPAHANAUMOKUAKEA
MARINE NATIONAL MONUMENT/NORTHWEST HAWAIIAN ISLANDS AND RIDGING
FROM THE WESTERN SUBTROPICAL PACIFIC BRIDGES OVER THE TOP OF THE
CLOSED LOW TO THE NORTH OF HAWAI'I. THERE IS DISAGREEMENT IN THE
GUIDANCE ON EXACTLY WHERE THE CLOSED LOW CUTS OFF -- THE 00Z GFS
APPEARS TO BE A REASONABLE COMPROMISE FOR LOCATION.
THE COMBINATION OF THE CLOSED LOW IN NORTHWEST HAWAII AND THE REMAINS
OF T.D. 15-E (WHICH FLARED UP BRIEFLY ON THURSDAY) TEAM UP TO
INCREASE RAINFALL AMOUNTS STEADILY FROM LATE TUESDAY THROUGH
NEXT FRIDAY. EXCESSIVE RAINFALL/FLASH FLOODING IS AGAIN POSSIBLE.
PRECIPITABLE WATER VALUES ARE FORECAST TO BE HIGHER THAN WHAT WAS
SEEN WITH NORBERT -- THE 00Z GFS/ECWMF ADVERTISE PRECIPITABLE
WATER VALUES REACHING 2.25-2.5".
ROTH
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