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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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