EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1152 AM EDT FRI MAY 17 2013

VALID 12Z MON MAY 20 2013 - 12Z FRI MAY 24 2013

***HEAVY RAINFALL POSSIBLE FOR THE UPPER MISSOURI AND MISSISSIPPI
RIVER BASINS***


...OVERVIEW...
AMPLIFIED FLOW OVER THE CONUS AND MULTIPLE CLOSED LOWS WILL BE THE
MAIN THEME THROUGHOUT THE MEDIUM RANGE PERIOD.  A LONGWAVE TROUGH
DIGGING IN OVER THE WESTERN U.S. THIS WEEKEND WILL LEAD TO A
MID-LEVEL LOW CLOSING OFF OVER THE MIDDLE MISSOURI VALLEY BY DAY
3/MONDAY.  THIS CUT-OFF SYSTEM SHOULD REMAIN RELATIVELY STAGNANT
THROUGH THE MIDDLE OF THE WEEK WHILE THE ASSOCIATED SURFACE COLD
FRONT SLOWLY EDGES EAST/SOUTHEAST OVER THE MIDDLE MISSISSIPPI
VALLEY/SOUTHERN PLAINS.  AFTER MID-WEEK...THE MID-LEVEL LOW CENTER
SHOULD BECOME MORE PROGRESSIVE AND IS EXPECTED TO FILL IN WHILE IT
SLIDES EASTWARD AND EVENTUALLY GETS ABSORBED INTO A TROUGH
AMPLIFYING OVER THE LOWER GREAT LAKES/NORTHEAST LATE IN THE
PERIOD.  ON THE OPPOSITE SIDE OF THE NATION...ANOTHER RELATIVELY
STAGNANT CLOSED VORTEX WILL SET UP OVER THE PACIFIC NORTHWEST BY
THE MIDDLE OF THE WEEK.  THIS FEATURE IS NOT EXPECTED TO BUDGE
MUCH LATE NEXT WEEK THANKS TO A DOWNSTREAM RIDGE BUILDING INTO THE
GREAT PLAINS.   


...MODEL PREFERENCES...
DETERMINISTIC GUIDANCE WAS IN VERY GOOD AGREEMENT OVER THE LOWER
48 FOR THE DAY 3-7 FORECAST.  IN REGARDS TO THE CLOSED LOW OVER
THE MIDDLE MISSOURI VALLEY...SOME SPREAD IN THE ENSEMBLE MEMBERS
SUGGEST THERE IS STILL SOME UNCERTAINTY WITH THE EXACT TIMING OF
PROGRESSING THE LOW EASTWARD DURING THE MIDDLE OF THE WEEK. 
ALSO...AS EXPECTED FOR LATE IN THE MEDIUM RANGE
PERIOD...DETERMINISTIC MODELS VARY IN THE EVOLUTION OF INDIVIDUAL
IMPULSES OF ENERGY ROTATING AROUND THE DEEP CLOSED VORTEX SETTING
UP OVER THE PACIFIC NORTHWEST.

STRONG AGREEMENT AMONG DETERMINISTIC MODELS LED TO A COMBINATION
OF THE 00Z ECMWF/06Z GFS FOR THE DAY 3-5 FORECAST.  THE 00Z GFS
WAS A TAD ON THE FASTER SIDE OF MODEL SPREAD WITH PROGRESSING THE
CLOSED LOW OUT OF THE MIDDLE MISSOURI VALLEY...SO THE SLOWER 06Z
GFS WAS USED INSTEAD.  DO TO THE DETAIL DIFFERENCES WITH THE DEEP
VORTEX SETTING UP OVER THE PACIFIC NORTHWEST...A BLEND OF THE VERY
AGREEABLE 00Z GEFS/ECMWF ENSEMBLE MEANS WAS USED FOR THE DAY 6-7
FORECAST. 





GERHARDT