PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK 
NWS CLIMATE PREDICTION CENTER CAMP SPRINGS MD 
300 PM EDT SUN AUG 31 2008 
 
30-DAY OUTLOOK DISCUSSION FOR SEPTEMBER 2008 
 
NWP GUIDANCE FOR THE FIRST COUPLE OF WEEKS OF SEPTEMBER INDICATES ANOMALOUS 
TROUGHING DOMINATING THE CENTRAL PART OF THE NATION... WIGTH A RIDGE ALONG THE 
WEST COAST.  THIS PATTERN HAS BEEN PRETTY WELL INDICATED BY THE ALL MODEL 
GUIDANCE FOR THE PAST FEW DAYS... WITH THE TROUGH SLOWLY SHIFTING EASTWARD 
DURING THE PERIOD.  THIS EXPECTED CIRCULATION PATTERN WILL RESULT IN CHANGES TO 
THE 0.5 MONTH LEAD OUTLOOK ISSUED ON 19 AUGUST.  THAT FORECAST WAS LARGELY 
BASED ON LONG TERM TRENDS... WHICH WERE STRONGEST IN THE WESTERN HALF OF THE 
NATION.  THE EVOLVING PATTERN DOES NOT CONFLICT WITH THAT FORECAST... AND THE 
BULK OF THAT FORECAST WAS LEFT UNTOUCHED. 
 
THE UPDATED TEMPERATURE FORECAST MAINTAINS EXPECTATIONS FOR ABOVE AVERAGE 
TEMPERATURES IN THE SOUTHWEST AND BELOW NORMAL TEMPERATURE ALONG SOUTHERN 
ALASKA.  HOWEVER, SEA SURFACE TEMPERATURES HAVE WARMED TO NEAR NORMAL LEVELS 
ALONG THE PACIFIC NORTHWEST COAST... SO THE TILT TOWARD BELOW AVERAGE 
TEMPERATURES IN THAT REGION WAS REMOVED.  ENHANCED PROBABILITIES FOR BELOW 
AVERAGE TEMPERATURES WERE ADDED TO A LARGE PART OF THE CENTRAL PART OF THE 
NATION... EXTENDING FROM THE NORTHERN PLAINS SOUTHEASTWARD THROUGH PARTS OF THE 
SOUTHEAST.  THIS OUTLOOK IS CONSISTENT WITH THE EXPECTED TROUGHING OVER THE 
REGION DURING THE EARLY AND MIDDLE PART OF THE MONTH... WITH SOIL MOISTURE ALSO 
PLAYING A ROLE IN THE SOUTHEAST AS THIS REGION IS EXPECTED TO SEE VERY WET 
CONDITIONS DURING THE EARLY PART OF THE MONTH IN ASSOCIATION WITH HURRICANE 
GUSTAV. 
 
THE OUTLOOK FOR PRECIPITATION DURING SEPTEMBER HAS ALSO SEEN SOME FAIRLY 
SIGNIFICANT REVISIONS, MAINLY WITH REGARDS TO INCREASED PROBABILITIES FOR ABOVE 
MEDIAN RAINFALL IN PARTS OF THE SOUTHEAST.  HURRICANE GUSTAV IS EXPECTED TO 
DROP AT LEAST 10-15 INCHES OF RAIN ACROSS PARTS OF LOUISIANA AND ARKANSAS 
DURING THE FIRST FEW DAYS OF THE MONTH, LEADING TO A SHARP INCREASE IN THE 
PROBABILITIES FOR ABOVE AVERAGE PRECIPITATION IN THIS REGION.  THE AREA WITH AN 
ENHANCED LIKELIHOOD OF ABOVE MEDIAN PRECIPITATION WAS ALSO EXTENDED FARTHER 
NORTH IN THIS REGION TO ACCOUNT FOR THE EFFECTS OF GUSTAV.  ELSEWHERE, DRIER 
THAN AVERAGE CONDITIONS IN THE PACIFIC NORTHWEST WERE EXTENDED TO THE 
WASHINGTON AND OREGON COASTS, AS THE BUILDING RIDGE INTO THAT REGION WILL FAVOR 
GENERALLY DRY CONDITIONS DURING THE FIRST HALF OF THE MONTH. 
 
THE MESSAGE FROM AUGUST 21 FOLLOWS BELOW: 
 
ENSO-NEUTRAL CONDITIONS CONTINUED DURING JULY 2008, AS SEA SURFACE TEMPERATURES 
(SSTS) IN THE CENTRAL EQUATORIAL PACIFIC OCEAN REMAINED NEAR-AVERAGE.  SSTS IN 
THE EASTERN PACIFIC ARE ABOVE AVERAGE... BUT THE ACTUAL SSTS ARE NOT WARM 
ENOUGH TO SUPPORT CONVECTION.  THE ATMOSPHERIC CIRCULATION OVER THE WESTERN AND 
CENTRAL TROPICAL PACIFIC CONTINUES TO REFLECT SOME ASPECTS OF LA NINA. ENHANCED 
LOW-LEVEL EASTERLY WINDS AND UPPER-LEVEL WESTERLY WINDS PERSIST IN THIS REGION, 
WHILE CONVECTION REMAINS GENERALLY SUPPRESSED OVER THE CENTRAL PACIFIC.  THE 
CONSOLIDATION OF NUMERICAL AND STATISTICAL NINO 3.4 FORECASTS INDICATES ENSO 
NEUTRAL CONDITIONS THROUGH THE NORTHERN HEMISPHERE FALL. THE JUST CONCLUDED LA 
NINA IS NOT EXPECTED TO HAVE ANY IMPACTS ON U.S. TEMPERATURE OR PRECIPITATION 
DURING SEPTEMBER. 
 
BELOW NORMAL SST ANOMALIES ALONG THE PACIFIC COAST OF NORTH AMERICA FROM OREGON 
TO ALASKA TILT THE ODDS TOWARD BELOW AVERAGE AIR TEMPERATURES FOR THE COASTAL 
AREAS FROM THE SOUTHERN COAST OF ALASKA TO WESTERN WASHINGTON AND OREGON. AN 
ENHANCED PROBABILITY OF ABOVE NORMAL TEMPERATURES IS FORECAST FOR THE 
SOUTHWESTERN U.S. THIS IS DUE MOSTLY TO LATE SUMMER-EARLY FALL TEMPERATURE 
TRENDS IN THESE REGIONS AS INDICATED IN THE CAS AND OCN TOOLS. 
 
LITTLE DEFINITIVE GUIDANCE IS PROVIDED BY THE TRADITIONAL STATISTICAL TOOLS FOR 
THE SEPTEMBER PRECIPITATION FORECAST WITH THE EXCEPTION OF A LARGE PART OF THE 
PACIFIC NORTHWEST... WHERE THE OCN FAVORS BELOW NORMAL PRECIPITATION. IN 
ADDITION, THE EXPECTED ACTIVE HURRICANE SEASON INCREASES THE PROBABILITY OF 
ABOVE MEDIAN PRECIPITATION ALONG BOTH THE GULF COAST AND THE SOUTH ATLANTIC 
COAST DURING THE MONTH. 
 
FORECASTER: MIKE HALPERT 
 
NOTES: AREAS OF UNCERTAINTY THAT FORMERLY WERE LABELED CL (CLIMATOLOGICAL 
PROBABILITIES) ARE NOW LABELED AS EC (EQUAL CHANCES). 
 
THE NEXT MONTHLY OUTLOOK...FOR OCTOBER 2008...WILL BE ISSUED ON THU SEP 18 2008 
 
THESE OUTLOOKS ARE BASED ON DEPARTURES FROM THE 1971-2000 BASE PERIOD. 
 
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