EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1200 PM EDT THU SEP 04 2014 VALID 12Z SUN SEP 07 2014 - 12Z THU SEP 11 2014 ...SIGNIFICANT RAINFALL LIKELY ACROSS THE DESERT SOUTHWEST... ...OVERVIEW... FLOW PATTERN ALOFT AT HIGH LATITUDE WILL BE IN THE PROCESS OF 'RESHUFFLING' FROM A ZONAL WAVE PATTERN TO A HIGH-AMPLITUDE PATTERN THIS MEDIUM RANGE PERIOD. ...MODEL PREFERENCES/UNCERTAINTY ASSESSMENT... THE 4/00Z ECENS...GEFS AND NAEFS CONTINUED TO MAINTAIN A RATHER STABLE FORECAST...WHILE THEIR DETERMINISTIC COUNTERPARTS---GENERATE THE LARGE RUN-TO-RUN SPREAD OVER SOUTH CENTRAL CANADA...THE DESERT SOUTHWEST...THE CENTRAL MIDWEST AND ALONG THE MID-ATLANTIC COAST. THOUGHT A BLEND OF THE MEANS WILL GIVE THE BEST OVERALL FORECAST...AND GIVE THE UNIQUE REGIONAL WEATHER DIFFERENCES ANOTHER DAY TO SHAKE OUT. REGIONAL SYNOPTIC HIGHLIGHTS INCLUDED.... THE CONSIDERABLY COLDER CANADIAN HIGH PRESSURE MIGRATING SOUTHEASTWARD OUT OF THE YUKON AND BEAUFORT SEA. THE GFS/ECMWF GENERALLY AGREE ALONG THE LEADING EDGE OF THIS CANADIAN AIRMASS BOUNDARY WITH RESPECT TO TEMPERATURES...BUT VARY SIGNIFICANTLY WITH THE PRECIPITATION DISTRIBUTIONS. MUCH OF THE STRUGGLE APPEARS TO BE VERY SHALLOW NATURE OF THE SURFACE COLD POOL...AND ITS RAPID SPREAD INTO THE CENTRAL STATES. CAN SEE WHERE RESIDUAL 'NORBERT' MID-LEVEL MOISTURE COULD BE INFLUENTIAL IN GENERATING AN 'OVER-RUNNING' ENVIRONMENT. UNFORTUNATELY...THE TIMING OF THE LEAD FRONT AND WIND SHIFT...WILL NOT NECESSARILY BE A PERFECT PROG...AND THEREFORE THE FLIP-FLOP NATURE OF THE HEAVIER PRECIPITATION WILL CONTINUE TO BE A CHALLENGE. WOULD RECOMMEND STAYING CLOSER TO THE ECENS/GEFS TO HANDLE THE POPS...WITH LITTLE CONFIDENCE IN THE DETERMINISTIC RAINFALL DISTRIBUTIONS ACROSS THE MID-MISSOURI VALLEY AND MID-MISSISSIPPI VALLEY. MID-LEVEL MOISTURE ASSOCIATED WITH 'NORBERT' WILL COME IN THE FORM OF A SERIES OF PREDECESSOR RAINFALL EVENTS (OR PRE's) THAT FANS OUT ACROSS THE WESTERN MOUNTAINS OF MEXICO AND MIGRATE DOWNWIND INTO THE FOUR CORNERS. THE DETERMINISTIC GFS/ECMWF HAVE THEIR OBVIOUS DIFFERENCES...AND THE PRECIPITATION FORECAST WILL FOLLOW THE OFFICIAL NHC TRACK. LAST BUT NOT LEAST...WILL HOLD ONTO A CAA WEDGE IN THE EAST AND ALLOW SOME SFC WAVE REFLECTION AT THE SFC TO CARRY DOWNWIND INTO THE WESTERN ATLANTIC. THIS KEEPS THE COOLER...AND WETTER WEATHER CONFINED TO THE COASTAL SECTIONS OF THE CAROLINAS. VOJTESAK