EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
144 AM EDT MON MAY 20 2013

VALID 12Z THU MAY 23 2013 - 12Z MON MAY 27 2013

...OVERVIEW AND MODEL PREFERENCES...

THE MODELS CONTINUE TO FCST 500 MB TROUGHS ALONG BOTH WEST/EAST
COASTS AND A RIDGE OVER THE CENTRAL STATES AND A CLOSED
ANTICYCLONE OVER NORTHERN MEXICO THROUGH THE WEEKEND INTO EARLY
NEXT WEEK.  THE DEEPER LOW IN THE PAC NW IS FORECAST TO YIELD 500
MB HEIGHTS ABOUT 1 TO 2 STD DEVIATIONS BELOW NORMAL AND ABOUT 1
STD DEVIATION BELOW NORMAL IN THE NORTHEAST.  RIDGING ALONG 100W
BY LATE IN THE WEEK SHOULD AVERAGE AROUND 2 STD DEVIATIONS ABOVE
NORMAL ALL THE WAY FROM THE RIO GRANDE VALLEY TO SOUTH CENTRAL
CANADA AND BEYOND. 

THE CLOSED LOW OVER THE PAC NW WED-FRI IS FCST BY THE
MODEL/ENSEMBLE MEAN MAJORITY TO EJECT NORTH INTO CANADA OVER THE
WEEKEND. BETTER THAN AVERAGE AGREEMENT IS SEEN AMONG THE GUIDANCE
THROUGH FRI/D5 BEFORE SMALLER DETAILS AMONG THE MODELS AMPLIFY.
THE QUESTION BECOMES HOW DEEP WILL THE ENERGY BY THAT ROTATES
THROUGH THE BASE OF THE TROUGH IN THE PACIFIC NORTHWEST INTO
NORCAL OVER NEXT WEEKEND INTO THE EARLY PART OF THE WEEK. OPTED TO
SPLIT THE DIFFERENCE AMONG THE SOLUTIONS GIVEN THE UNCERTAINTY IN
THE PHASING OF THE STREAMS ACROSS THE NORTHEAST PACIFIC.

IN THE EAST... THE MODELS/ENSEMBLES SHOW AN UPPER TROUGH MOVING
FROM THE GREAT LAKES AND ACROSS THE NORTHEAST OVER THE
WEEKEND....THEN DEPARTING EARLY NEXT WEEK AS UPSTREAM RIDGING IN
THE NORTHERN PLAINS BUILDS INTO THE UPPER MS VALLEY. 

MANUAL PROGS USED A BLEND OF THE 18Z GFS/18Z GEFS MEAN/12Z ECMWF
AND 12Z ECENS MEAN HEIGHTS/SEA LEVEL PRESSURES...AS WELL AS THE
TEMPERATURES/PROBABILITY OF PRECIP FCSTS.


...SENSIBLE WEATHER HIGHLIGHTS...

BELOW NORMAL TEMPERATURES ARE EXPECTED INVOF THE SLOW-MOVING
UPPER-LEVEL TROUGH AND CUTOFF LOW IN THE PACIFIC NORTHWEST TO
NORTHERN ROCKIES...UNTIL THE PRIMARY LOW EJECTS INTO CANADA NEXT
WEEKEND. WET CONDITIONS ARE EXPECTED OVER NORTH CENTRAL MT AS
850-700 MB MOISTURE TRANSPORT AND CONVERGENCE OCCUR THU...WITH
COVERAGE WANING NEXT WEEKEND AS THE UPPER LOW MOVES INTO CANADA
AND THE MOISTURE TRANSPORT/LOW LEVEL CONVERGENCE WEAKENS. BELOW
NORMAL TEMPS ALSO DEVELOP WITH THE UPPER TROUGH ACROSS NY/NEW
ENGLAND NEXT WEEKEND. 

ABOVE NORMAL TEMPS SHOULD PERSIST ACROSS THE NORTHERN AND SOUTHERN
PLAINS AND SOUTHERN ROCKIES AS AREAS OF 700 MB TEMPS 1-2 STD
DEVIATIONS ABOVE NORMAL PERSIST THROUGH NEXT WEEKEND INTO MON 27
MAY PER THE ENSEMBLES.  SOUTHERLY FLOW AHEAD OF THE TROUGH AND SFC
LOW/WARM FRONT DEVELOPMENT SHOULD FOCUS PRECIP OVER THE CENTRAL
PLAINS LATE IN THE WEEKEND AND INTO THE NORTHERN PLAINS/MID-UPPER
MS VALLEY EARLY NEXT WEEK WHERE CONVECTION IS LIKELY AS INDICATED
IN THE GFS/ECMWF/MULTIPLE ENSEMBLE MEMBERS WITHIN A LOW-MID LEVEL
WARM ADVECTION PATTERN UNDERNEATH UPPER DIVERGENCE MAXIMA/LOW
LEVEL CONVERGENCE MAXIMA. 

PETERSEN