Prognostic Meteorological Discussion
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FXUS02 KWBC 041543
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1142 AM EDT FRI SEP 04 2015

VALID 12Z MON SEP 07 2015 - 12Z FRI SEP 11 2015


THE MEDIUM RANGE FORECAST TODAY FOLLOWS A FAMILIAR SCRIPT IN THAT
THE FORECASTS FOR DAYS 3 AND 4/MONDAY AND TUESDAY ARE SIMILAR OVER
NORTH AMERICA FOR MANY MODELS AND ENSEMBLES AND CANNOT GO TOO
WRONG ON THE SCALE THAT IS USEFUL FOR THE FORECAST.  BY DAY
5/WEDNESDAY...THE SPREAD INCREASES AND BY LATER IN THE WEEK/DAYS 6
AND 7...USE OF THE ENSEMBLE MEANS IS THE BETTER WAY TO SMOOTHE OUT
SOME OF THE LARGE DIFFERENCES IN THE OPERATIONAL RUNS.  THAT
SAID...THE OVERNIGHT CANADIAN AND UKMET WERE TOSSED AFTER DAY 4
DUE TO EACH BEING MORE OF AN OUTLIER TO THE OVERALL PATTERN/MEANS
WHILE THE 06Z GFS/00Z ECMWF BOTH FIT WELL WITHIN THE ENVELOPE OF
SOLUTIONS THRU ABOUT DAY 6 BEFORE THEY BECOME TOTALLY OUT OF
PHASE.  THE MAIN SOURCE OF APPARENT ERROR TODAY APPEARS OVER THE
NORTH PACIFIC AS SOME LARGE CHANGES ARE OCCURRING THERE.

HURRICANE IGNACIO PLAYS A ROLE IN THE MEDIUM RANGE FORECAST SINCE
IT BECOMES TIED INTO THE WESTERLIES AND MOVES SOUTH OF ALASKA INTO
BRITISH COLUMBIA ON DAYS 4 AND 5/TUESDAY AND WEDNESDAY. THIS
SYSTEM WILL OF COURSE BE EXTRATROPICAL BY THEN AND BOTH THE 06Z
GFS/00Z ECMWF HAVE SIMILAR PATHS ALTHOUGH AT SLIGHTLY DIFFERENT
SPEEDS FOR BOTH.  AT PRESENT...IT DOES NOT APPEAR TO BE
SIGNIFICANTLY INFLUENCED BY A SEPARATE TROUGH THAT REMAINS OUT OF
PHASE TO THE NORTHEAST OF THE SYSTEM.  AND IT ALSO DOES NOT APPEAR
TO HAVE THAT GREAT OF AN IMPACT ON THE OVERALL FLOW OVER WESTERN
CANADA INTO THE US.

WHAT DOES APPEAR TO BE A MORE CONSISTENT SIGNAL IS THE
DEAMPLIFYING TROUGH THAT RIDES OVER THE TOP OF THE LARGE EASTERN
RIDGE AND EVENTUALLY SUPPRESSES THE RIDGE IN THE EAST.
HOWEVER...THE ASSOCIATED COLD FRONT WITH THIS SYSTEM THAT CROSSES
THE NORTHERN HALF OF THE NATION APPEARS TO HAVE SLOWED DOWN OVER
THE PAST SEVERAL RUNS...WITH THE EASTERN WARMTH SLOW TO BREAK
WHILE THE FRONT MAY ACTUALLY STALL NEAR THE EAST COAST BY THE END
OF THE FORECAST.  THIS MAY BE IN PART TO THE POSSIBLE SIGNAL OF
TROUGHING OCCURRING BY DAYS 6 AND 7/THURSDAY AND FRIDAY OVER THE
CENTER OF THE COUNTRY AS THE FIRST TROUGH LIFTS FARTHER NORTHWARD
INTO EASTERN CANADA MAINTAINING THE EAST COAST RIDGE.

THESE INTERACTIONS OCCUR AS A DEEP AND POORLY FORECAST SERIES OF
UPPER LEVEL SYSTEMS MOVES ACROSS ALASKA AND INTO THE NORTH
PACIFIC. AT PRESENT IT LOOKS LIKE THE NORTHERN STREAM MAY SUPPRESS
THE SOUTHERN STREAM BUT WHATEVER OCCURS WILL  HAVE A LARGE
INFLUENCE ON THE DOWNSTREAM FLOW OVER WESTERN CANADA AND THE US.
IT IS POSSIBLE THAT A BUILDING RIDGE WILL TAKE HOLD OVER THE
NORTHEAST PACIFIC/WESTERN CANADA AND AID IN THE DOWNSTREAM
TROUGHING OVER THE CENTRAL US.

PRECIPITATION IS LIKELY TO FOCUS ALONG THE TRAILING FRONT
EXTENDING FROM THE PARENT LOW IN CANADA SOUTHWARD INTO THE CENTRAL
PLAINS/MISSISSIPPI VALLEY. A WAVE OF LOW PRESSURE MAY DEVELOP
ALONG THE FRONT AND MOVE NORTHEASTWARD ON DAYS 3 TO 5...BRINGING
THE POSSIBILITY OF SIGNIFICANT QPF AMOUNTS FROM THE MISSISSIPPI
VALLEY ACROSS MIDWEST. SHOWERS AND THUNDERSHOWERS WILL ACCOMPANY
THE FRONT AS IT MOVES SLOWLY EAST AND SOUTHWARD WITH A LARGE UPPER
LEVEL LOW BRINGING SCATTERED SHOWERS/STORMS TO THE SOUTHEAST AND
FLORIDA.  THE DEVELOPMENT OF THE NEXT TROUGH OVER THE CENTER OF
THE COUNTRY LATE IN THE FORECAST COULD ALSO ENHANCE PRECIPITATION
DOWNWIND...WHILE THE POSSIBILITY OF A STATIONARY FRONT  LATE IN
THE FORECAST MAY ALSO FOCUS QPF ON THE EAST COAST.

KOCIN



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