Preliminary Forecasts
Issued by NWS

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FXUS02 KWNH 240542
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
141 AM EDT WED AUG 24 2016

VALID 12Z SAT AUG 27 2016 - 12Z WED AUG 31 2016

WORKING FROM WEST TO EAST ---
IN SHORT --- 45/35 OF THE 23/12Z ECENS/NAEFS MEANS RESPECTIVELY
AND 20 PCT OF THE DETERMINISTIC ECMWF THROUGH THE END OF DAY 5
SHOULD WORK. FOR DAY 6-7 --- TRANSITION TO A 55/35 OF THE SAME
MEANS AND 10 PCT OF THE ECMWF TO HOLD THE PATTERN DOWN. EVEN IF
THERE REMAINS NO `PERFECT PROG` SOLUTION AT DAY 7 ACROSS THE
SOUTHEAST. WILL USE THIS FOR THE D4-5 AND D6-7 QPFS AND WITHIN
REASON TAKE ANOTHER LOOK AT THE 24/00Z HI-RES 27KM ECMWF QPF TO
MAKE SURE THINGS LOOK HALF WAY DECENT.

THE FOLLOWING IS --- UNCERTAINTY ASSESSMENT AND ADDITIONAL
INFORMATION OUTSIDE THE NATIONAL PACKAGE MODEL BLEND.

THE 23/12Z CYCLE HAS SETTLED DOWN SOME ACROSS WEST CENTRAL CANADA
THROUGH DAY 5 AND THOUGHT THE DETERMINISTIC ECMWF/UKMET HAVE THE
RIGHT IDEA. IE...HOW TO EJECT THE ENERGY THROUGH THE GULF OF
ALASKA AND DOWNSTREAM. AN ISSUE SEVERAL DAYS AGO --- SEEMS MUCH
MORE AGREEABLE NOW AS THE DETERMINISTIC GUIDANCE (OUTSIDE OF THE
23/12Z CANADIAN) HAS SOME SHORTWAVE RIDGING CREATING SPACE BETWEEN
SYSTEMS ACROSS THE CANADIAN ROCKIES AT DAY 5. THIS DOES ALLOW THE
LEAD SYSTEM TO MIGRATE DOWNSTREAM AND SHEAR OUT ---OR AT THE VERY
LEAST MIGRATE DOWNSTREAM AND EAST OF THE CANADIAN DIVIDE TO STOP
THE SOLUTION ERRORS THAT WANTED TO CARVE COLD ADVECTION WEST OF
THE DIVIDE. EVEN THE 23/12Z GFS LOOKED `USE-ABLE` ALOFT FOR DAY 5
INTO DAY 6 ACROSS THE PACIFIC NORTHWEST AND NORTHERN DIVIDE.

OVER THE NORTHERN PLAINS AND GREAT LAKES...THE DAY 3-4 WAVE
EXITING NORTHERN COLORADO AND CENTRAL WYOMING LIFTS NORTHEASTWARD
INTO THE UPPER GREAT LAKES AND WESTERN ONTARIO AND THE MID-LEVEL
FRONT DOES DRAG ENOUGH OF A SURFACE BOUNDARY INTO THE CENTRAL
PLAINS AND MIDWEST --- TO ACCEPT A GENERAL GFS/ECMWF COMPROMISE
FOR DAY 5-6 FROM WESTERN KANSAS TO NEW ENGLAND.

THEN THE QUESTION OVER THE SOUTHEAST ARISES --- AND WHICH CONCERNS
ME. HOW DO YOU TRANSITION FROM SOMETHING THAT HAS SOME GFS BASIS
ACROSS MID-LATITUDE NORTH AMERICA THROUGH DAY 5 --- TO A SOLUTION
THAT SEEMS TO BE VERY `NON-GFS BASED` WITH PROBABLY THE KEY
SYNOPTIC-SCALE FEATURE IN THIS MEDIUM RANGE PERIOD (IN THE
SOUTHEAST ATLANTIC BASIN). BASED ON RUN-TO-RUN CONTINUITY ALONE
--- THE DIFFERENCES BETWEEN THE 23/12Z, 23/18Z AND 24/00Z GFS ARE
SO VASTLY DIFFERENT --- WHY IS THIS EVEN A CONSIDERATION? BUT
THAT`S ME.

IN GENERAL ---THE 23/12Z GUIDANCE CONTINUES TO MOVE THE AREA OF
DISTURBED WEATHER AT A 4-5 DEGREE-PER-DAY RATE IN A
WEST-NORTHWESTWARD MANNER--- AND THIS WILL LIKELY BE THE CASE
THROUGH THE EARLY PART OF THE UPCOMING MEDIUM RANGE --- BECAUSE
THE 594DM RIDGE AXIS REMAINS OVER THE CAROLINAS AND SOUTHERN
APPALACHIANS WITH A STATIONARY BOUNDARY DRAPE TO ITS IMMEDIATE
SOUTH. THE RIDGE WILL BE VERY SLOW TO MOVE THROUGH DAY 5.

THE MAIN THING THIS PACKAGE WANTS TO ACCOMPLISH --- IS GIVE THE
LOCAL OFFICES SOME DECENT GRIDS TO WORK WITH. EVEN IF IT IS DAY 6
OR 7. I COULD EVEN CONSIDER WHAT THE 23/12Z CANADIAN AND THE
23/18Z HWRF (FOR INVEST 99L) OFFER SINCE BOTH DEVELOP ENERGY ALONG
THE STATIONARY BOUNDARY --- BASICALLY SURFACE-BASED WAVES THAT ARE
NOT PART OF THE `DISTURBANCE`. THEY MAY ENHANCE RAINFALL ALONG THE
STATIONARY BOUNDARY AND RE-SHAPE IT AHEAD OF WHATEVER FORMS
BETWEEN SOUTH FLORIDA AND THE BAHAMAS.

VOJTESAK






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