EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1141 AM EDT THU MAY 16 2013

VALID 12Z SUN MAY 19 2013 - 12Z THU MAY 23 2013

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


...OVERVIEW...

THE TRANSITION FROM WINTER TO SUMMER CONTINUES AS EVIDENCED BY THE
RESURGENT AMPLIFIED/BLOCKY PATTERN. SEVERAL CLOSED LOWS ARE
FORECAST TO EVOLVE THROUGH THE MEDIUM RANGE... NORTH OF HAWAII...
ALONG THE PAC NW COAST... OVER THE UPPER MIDWEST... AND IN THE
CENTRAL ATLANTIC. SUNY ALBANY CSTAR* RESEARCH HAS SHOWN THAT THE
CLIMATOLOGICAL MAXIMUM IN CLOSED LOW FREQUENCY IN THE NORTHERN
HEMISPHERE OCCURS IN JUNE... SO THIS PATTERN IS FAIRLY TYPICAL.

...MODEL PREFERENCES...

THE GFS/ECMWF/UKMET ALL CLUSTERED FAIRLY WELL TOGETHER THROUGH
ABOUT MON/D4... WHEN THE 00Z-06Z GFS RUNS BECAME TO QUICK TO LOWER
HEIGHTS IN THE PAC NW COMPARED TO THE CONSENSUS AND PERHAPS TOO
QUICK TO PUSH THE UPPER LOW OVER THE UPPER MIDWEST NORTHWARD.
ONGOING WPC CONTINUITY FAVORED A BIT SLOWER PROGRESSION VIA THE
ENSEMBLE MEANS AND THIS WAS BEST REPRESENTED BY A 00Z ECMWF/UKMET
BLEND EARLY IN THE FORECAST BEFORE INCORPORATING ABOUT HALF
WEIGHTING OF THE ENSEMBLE MEANS DAYS 6-7... WHICH REMAINED IN GOOD
AGREEMENT THROUGH THU/D7. FELT LITTLE REASON TO STRAY FAR FROM
THEIR IDEA BUT CAPITALIZED ON THE NEARBY DETERMINISTIC MODELS FOR
ADDED DETAIL.


...SENSIBLE WEATHER HIGHLIGHTS...

THE SLOWLY-MOVING SURFACE BOUNDARY HAS BEEN ADVERTISED AS A POTENT
WEATHER MAKER FOR THE PLAINS... AND THE MODEL GUIDANCE HAS NOT
BACKED DOWN... FOCUSING CONVECTIVE PRECIPITATION FOR THE UPPER
MIDWEST. GFS/ECMWF/UKMET FOCUS SIGNIFICANT RAINFALL... 2-4
INCHES... FOR A LARGE PORTION OF THE UPPER/MID MISSOURI AND UPPER
MISSISSIPPI RIVER BASINS DAYS 3-5... WITH A SECONDARY RAINFALL
MAXIMA... 1-3 INCHES... CENTERED ON THE CENTRAL APPALACHIANS AND
SOUTHERN MID ATLANTIC STATES BY THE END OF THE PERIOD. ECENS
PROBABILITY OF AT LEAST 1 INCH OF QPF DAYS 3-7 EXCEED 50 PERCENT
OVER A VERY LARGE AREA FROM THE HIGH PLAINS SOUTHWARD TO THE
MID-MS VALLEY AND EASTWARD TOWARD THE EAST COAST... WHERE GEFS PW
ANOMALIES EXCEED +1 STD DEVIATION. SPC CONTINUES TO ADVERTISE THE
THREAT FOR SEVERE WEATHER ALONG THE BOUNDARY AND AHEAD OF THE
UPPER HEIGHT FALLS FROM THE CENTRAL PLAINS INTO THE MID-MS VALLEY
SUN-MON/D3-4 IN THE WARM AIRMASS. IN THE PAC NW... THE GUIDANCE
ALSO GENERATE CONSIDERABLE PRECIPITATION FOR NORTHERN/COASTAL
WASHINGTON INTO COASTAL OREGON WITH AN EXTENDED ONSHORE FLOW BUT
ALSO INTO THE IDAHO PANHANDLE AND WESTERN MONTANA AS THE CUTOFF
LOW EASES INTO WA/OR. UPPER RIDGING AND SFC HIGH PRESSURE OVER
SOUTH CENTRAL CANADA SHOULD HELP IN GRADUALLY INCREASING PW VALUES
ALONG THE DIVIDE BEFORE THE BEST FORCING AND SFC BOUNDARY
APPROACHES FROM THE WEST.


FRACASSO


* CSTAR... COLLABORATIVE SCIENCE... TECHNOLOGY... AND APPLIED
RESEARCH PROGRAM