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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1654Z Apr 20, 2015)
 
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EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1254 PM EDT MON APR 20 2015

VALID 12Z THU APR 23 2015 - 12Z MON APR 27 2015

...MEAN FLOW PATTERN...

THE CENTER OF A DEEP AND EXPANSIVE VORTEX SPINNING OVER SOUTHEAST
CANADA WILL GRADUALLY EDGE EAST THIS WEEKEND...WHILE ANOTHER DEEP
VORTEX SHOULD SET UP IN THE GULF OF ALASKA BY EARLY NEXT WEEK. 
ACROSS THE LOWER 48...MULTIPLE RELATIVELY WEAKER MID-LEVEL SYSTEMS
WILL IMPACT THE WESTERN U.S...BEFORE POSSIBLY PROGRESSING
DOWNSTREAM OVER THE CENTRAL AND EASTERN U.S..


...MODEL EVALUATION/PREFERENCES...

ALTHOUGH MODELS ARE SIMILAR WITH EDGING THE VORTEX EAST OVER
SOUTHEAST CANADA AND SETTING UP ANOTHER VORTEX OVER THE GULF OF
ALASKA EARLY NEXT WEEK...THERE IS STILL A LARGE AMOUNT OF SPREAD
AND UNCERTAINTY WITH THE EVOLUTION OF SMALLER SCALE FEATURES
IMPACTING THE LOWER 48.

IN PARTICULAR...DETERMINISTIC GUIDANCE HAS YET TO LATCH ON TO AN
EXACT SOLUTION WITH A BROAD TROF SWINGING THROUGH THE
SOUTHWEST/FOUR CORNERS REGION THURSDAY INTO FRIDAY...ESPECIALLY AS
IT WEAKENS WHILE MOVING DOWNSTREAM OVER THE CENTRAL PLAINS AND
EASTERN U.S. THIS WEEKEND.  ALSO...THERE ARE STILL DRASTIC
DIFFERENCES WITH THE EVOLUTION OF A CLOSED LOW APPROACHING FROM
THE EASTERN PACIFIC LATE NEXT WEEK.  A HANDFUL OF MODELS PREFER TO
WEAKEN THE ENERGY WHILE LIFTING IT INTO BRITISH COLUMBIA...WHILE
OTHER PIECES OF GUIDANCE SUGGEST THE LOW DIGS FARTHER SOUTH INTO
THE NORTHWEST BEFORE IT WEAKENS.

GIVEN THE CURRENT SPREAD IN DETERMINISTIC GUIDANCE...A LARGE
AMOUNT OF NOISE IN ENSEMBLE MEMBERS...AND EACH MODEL HAVING ITS
OWN RUN-TO-RUN CONTINUITY ISSUES...THE WPC FORECAST TRIED TO STAY
CLOSE TO CONTINUITY AND WAS LARGELY BASED ON THE 00Z ECMWF
ENSEMBLE MEAN AND NAEFS MEAN.


...SENSIBLE WEATHER HIGHLIGHTS...

THE EXPANSIVE AND DEEP VORTEX EDGING EAST OVER SOUTHEAST CANADA
WILL SET UP AN ACTIVE AND AT TIMES 'CHILLY' APRIL WEATHER PATTERN
FROM THE GREAT LAKES TO THE NORTHEAST.  OUT WEST...AN ACTIVE
SPRING WEATHER PATTERN WILL ACCOMPANY THE BROAD TROF SWINGING
INLAND OVER THE SOUTHWEST/FOUR CORNERS REGION LATE THIS WEEK...AND
IT SHOULD BE THE FOCUS FOR HEAVIER AND MORE ORGANIZED
PRECIPITATION AS IT PROGRESSES EAST OF THE CONTINENTAL DIVIDE AND
MOISTURE RETURNS FROM THE GULF OF MEXICO THIS WEEKEND.  LATE IN
THE PERIOD...HEIGHT FALLS FROM THE DEEP VORTEX SETTING UP IN THE
GULF OF ALASKA SHOULD BRING STEADY PRECIPITATION INTO THE PACIFIC
NORTHWEST EARLY NEXT WEEK.


GERHARDT